Amorphous superconductors have emerged as a promising alternative material for superconducting nanowire single photon detectors (SNSPDs). Detectors based on amorphous superconductors share the higher detection efficiencies, lower dark counts and fast reset times of their nitride counterparts allowing for operation at telecoms wavelengths. The reduced superconducting energy gap results in larger hotspot formation allowing for detection of lower energy photons in the mid infrared. The amorphous nature allows for a more homogenous film to be grown resulting in a spatially uniform detection efficiency and higher internal quantum efficiency¹.
Amorphous WxSi1-x has been shown to have performance comparable or exceeding that observed in traditional Nb(Ti)N based devices. Without the use of optical cavities, detection efficiencies of up to 40% have been observed. By utilising an optical stack, detection efficiencies of up to 93% have been recorded². Recently, devices based on MoSi have also been shown to operate as photon detectors with promising results³, however detailed investigation is necessary to understand how the film growth conditions affect the detector properties.
This study of the MoSi system finds that the superconducting transition temperature reaches a maximum Tc of 7.5 K at a composition of Mo₈₃Si₁₇ for a 100 nm thick film. The transition temperature and amorphous character can be improved by cooling of the substrate during growth which inhibits formation of a crystalline phase. While X-ray diffraction can be used to confirm the absence of significant crystallinity, it is insufficient to quantify the extent to which the film is amorphous. We present transmission electron microscopy results which better describe the extent of crystallinity in the films. We observe that for 6 common substrates there is no variation in Tc making MoSi an excellent candidate material for SNSPDs integrated with advanced optical architectures.
[1] B Baek et al Superconducting a-W_xSi_1−x nanowire single-photon detector with saturated internal quantum efficiency from visible to 1850 nm APL, 98 25 251105 2011
[2] F Marsili et al Detecting single infrared photons with 93% system efficiency Nat. Photonics, 7 210 2013
[3] Y Korneeva et al Superconducting single-photon detector made of MoSi film SUST, 27 9 095012 2014

The effect of the superconducting film parameters, in relation to that of the substrate material and structure on the responsivity of bolometer detectors is investigated. It has already been reported that the growth of epitaxial cerium oxide (CeO2) buffer layer on crystalline substrate using MOD method results in the fabrication of c-axis oriented YBCO superconducting film with sharp normal-superconductor transition. CeO2 buffer layer was deposited on both crystalline strontium titanate (SrTiO3) and lanthanum aluminate (LaAlO3) substrates using acac MOD method. The thickness of the deposited CeO2 buffer layers was approximately 20 nm. Then, YBCO thin films have been deposited by RF magnetron sputtering on CeO2 buffered as well as on bare substrates for comparison. As was earlier reported, by using the buffer layer, the superconducting transition width of the fabricated films on the substrates is improved to less than 0.4 K which provides a proper ground for the fabrication of bolometer detectors by enhancing their responsivity. The films were patterned to a meander-line with four contact pads to make use of four-probe configuration for current biased measurements. Due to the positive effects of the buffer layer on the thermal parameters of the devices, the response of the devices was further improved apart from the enhancing effect of the sharpness of the normal-superconductor transition caused by the implementation of CeO2 buffer layer in the growth. We report on the response of the devices and compare their responsivity for the same device pattern on the substrates with and without the buffer layer.

We have recently presented results of submillimeter MgB2 Hot electron bolometer (HEB) mixers in terms of intermediate frequency (IF) bandwidth and noise temperature utilizing “as grown” MgB2 films (thickness down to 10 nm) deposited with the hybrid physical-chemical vapor deposition (HPCVD) method. Such devices offer a significant advantage over state of the art NbN HEBs in both IF bandwidth (> 8 GHz) and operating temperature (Tc = 36-38 K). New research developments towards thinner films has been achieved which can further advance the utility of MgB2 for HEB applications. It is well known that the HPCVD process suffers from discontinuities in very thin films (< 20 nm) due to initial island growth of the films. Thinner films (down to 3 nm) have been achieved by etching down a film which has been grown to a thickness (50-100 nm) which exceeds the threshold limit where island growth of the film leaves discontinuities in the films. The high deposition temperature continues to anneal the film as it is grown and the result is that a more continuous film is achieved through the entire thickness of the film, and hence etching a thicker film to the desired thickness allows for thinner films with better superconducting properties. This has a large impact on the HEB application as film thickness is directly related to the phonon escape time in the HEB, largely responsible for the IF bandwidth. In addition, the necessary local oscillator power to optimally pump an HEB mixer is directly related to the HEB volume. There is an additional benefit from thinner films in that the mean free path of electrons in HPCVD grown film is only limited by surface scattering and hence the sheet resistance is also increased. This allows for a further reduction of the device volume/number of squares while ensuring a good impedance match to the antenna. Here we take advantage of the new film developments to achieve a device made with film thickness < 10 nm. We present the data demonstrating a reduction in local oscillator power seen from the reduced thickness as well as that seen from optimized sheet resistance. We obtain an estimate of the device thermal conductance from DC measurements and compare this result with the pumping power needed to achieve optimal mixing.

Hot electron bolometers (HEB) are well-known as good performance THz mixers. In this paper, we have reconsidered the hypotheses of our initial hot spot modelling of YBaCuO micro- and nano-constrictions applied to phonon cooled HEB DC and THz mixer characteristics. A first (obvious) hypothesis is to assume a constant DC bias current, hence a non-uniform DC power dissipation along the constriction length due to the hot spot phenomenon. A second (usual) hypothesis is to consider the local oscillator (LO) radiofrequency (RF) power to be uniformly absorbed over the HEB constriction length (inserted between the THz antenna arms). We have substituted to this latter the major assumption that the RF current is now constant along the constriction, hence a non-uniform LO power absorption. The local electron temperature was obtained by solving the coupled electron and phonon thermal reservoir equations, which could be used to determine the local YBCO complex resistivity (from the superconducting transition), hence the locally dissipated LO power. A modified two-fluid model description allowed to obtain the mid-transition temperature shift (about -1% per THz) and transition broadening (about +20% per THz). Another outcome of the model was to obtain the constriction global THz impedance, hence the ability to introduce the power matching coefficient to the antenna. The device simulations provided first the DC current-voltage plots under LO pumping, which were in line with available experimental results (both low-Tc and high-Tc HEBs), and so confirmed the non-uniform LO power assumption. Secondly, we considered the mixer double sideband noise temperature Tn and conversion gain G for a typical medium size constriction of 400 nm width, 400 nm length and 35 nm thickness. At the optimal LO power of 9 microW, Tn (G) ranged from 1900 K (-10 dB) below 100 GHz to 4150 K (-15 dB) at 2.5 THz, in agreement with published measurements. Intermediate frequency bandwidth will be also discussed.

This work is being supported by the French National Research Agency (ANR) under grant # 2011-BS03-008-01.

Superconducting nanowires have become a very efficient means of detecting infrared single photons. Recent experiments and development in theory have elucidated large parts of the detection mechanism, and have shown that a combination of a photon-induced quasiparticle distribution and the passing of vortices through the wire are responsible for the detection event. Detailed optical experiments on the position dependence of the detection efficiency are however difficult, due to the diffraction limit of the illumination that exceeds the relevant device dimensions by an order of magnitude.

To probe the microscopic details of a perturbation in a superconducting nanowire, we have instead used a scanning tunneling microscope as a local injector of non-equilibrium quasiparticles in a 200 nm wide, 4 nm thick titanium nitride nanowire. We locally inject the quasiparticles with STM, and measure the critical current of the nanowire. The injection of quasiparticles significantly reduces the current carrying capacity of the nanowire. This reduction is strongly dependent on both quasiparticle energy and rate. As a function of position, we find both geometric and microscopic variations: quasiparticles injected at the edges of the nanowire are more efficient in reducing the critical current than those injected in the middle of the wire. The microscopic variations are correlated with variations in the local quasiparticle density of states.

I will relate these results to our current knowledge of the microscopic working mechanism of superconducting single-photon detectors, and discuss how our scanning current injector can be used as a diagnostic tool for nanodevices.

This work was supported by

1. National Basic Research Program of China (“973” Program) (Nos. 2011CBA00100 and 2011CBA00200).
2. National Natural Science Foundation of China (Nos. 11227904, 11074114 and 61371036….).
3. Nature Science Foundation for Distinguished Young Scholars of Jiangsu Province (No. BK2011013).

We use external magnetic field to probe the detection mechanism of the single photons by the superconducting nanowire single photon detector (SNSPD). In our experiment we use detectors based on NbN and which demonstrate the saturation of the photon count rate (as function of the current) even for photons with wavelength λ=1200 nm. We find that the hot belt model [1] (which assumes partial suppression of the superconducting order parameter across the whole width of the superconducting film after absorption of the single photon) does not explain very weak dependence of the photon count rate on the magnetic field for photons with λ=450 nm and observed noticeable decrease of the photon count rate (with increasing the magnetic field) in some range of the currents for photons with wavelengths λ=450-1200 nm. The last effect was not observed in the preceding experiments [2,3] and it was theoretically predicted in Ref. [4]. Found experimental results for all studied wavelengths λ = 450-1550 nm are well explained by the hot spot model [4,5] (which assumes partial suppression of superconducting order parameter in the area with size smaller than the width of the film) if one takes into account nucleation and entrance of the vortices to the photon induced hot spot and their pinning by the hot spot with relatively large size and strongly suppressed superconductivity inside it.

[1] L. N. Bulaevskii, M. J. Graf, V. G. Kogan, Phys. Rev. B 85, 014505 (2012).
[2] A. Engel, A. Schilling, K. Il'in and M. Siegel, Phys. Rev. B 86, 140506(R) (2012).
[3] R. Lusche, A. Semenov, Y. Korneeva, A. Trifonov, A. Korneev, G. Gol'tsman, and H.-W. Hubers, Phys. Rev. B 89, 104513 (2014).
[4] A. Zotova and D. Y. Vodolazov, Supercond. Sci. Technol. 27, 125001 (2014).
[5] A. D. Semenov, G. N. Gol'tsman, and A. A. Korneev, Physica C 351, 349 (2001).

We report on the generation of quantum metrological current by periodically modulating a single quantum electronic level coupled to two superconducting leads. Single quantum dot junctions are formed by inserting individual gold nanoparticles between superconducting aluminum leads. We demonstrate current quantization up to frequencies exceeding 200 MHz, conveyed by a single quantum level. Essential differences with previous experiments based on a normal metallic island rather than a quantum dot are highlighted and explained.
In particular, a striking dynamical transport feature associated to the hybridization of the single quantum level to the superconductor single particle density of states is demonstrated

We present transmission spectroscopy measurements of a circuit QED (Quantum Electrodynamics) system. Our superconducting transmon qubit is embedded in a 3D superconducting resonator, and the measurement is performed in a cryogen-free dilution refrigerator at 7 mK. The qubit transition frequency is fixed and far-detuned form the resonator frequency, so that the system is operated in a strong-dispersive regime. We have performed systematic study on the transmission spectrum of the resonator as a function of various measurement conditions such as microwave power, frequency detuning and qubit drive. We will try to present a unified understanding of the spectrum based on a comprehensive analytical model. Althought quite straighforward, these information is very useful in measurement and control of a superconducting qubit in circuit QED architecture.

In intrinsic Josephson junction (IJJ), strong coupling between superconducting layers has to be considered. Charge coupling and inductive coupling have been introduced to explain phenomena observed in IJJs [1, 2]. The inductive coupling causes global effects for an IJJ stack like synchronization of IJJs yielding terahertz electromagnetic wave radiation. The charge coupling is a short range effect because the charge neutrality of superconducting layer is broken due to the charge screening length shorter than the order of superconducting electrode thickness.
In this study, we measure the switching probability distribution of Bi₂Sr₂Ca_n-1Cu_nO_4+2n+\delta in both macroscopic quantum tunneling (MQT) region in low temperatures up to 0.4 K and thermal activation (TA) region in higher temperatures. We use three materials of n = 1 to 3 called Bi2201, Bi2212 and Bi2223, respectively. Thickness of the superconducting layer is increase with the increase of the number of n. The MQT rate for the second switch is much higher than that for the first switch in Bi2201 and Bi2212 [4]. This result is consistent with the previous reports of Bi2212 [3]. On the other hand, the MQT rates for the first and second switch is almost identical in Bi2223 [5]. These results show that the enhancement of the MQT rate is observed in the IJJs having short superconducting layer than 0.3 nm. The difference between Bi2212 and Bi2223 is the thickness of the superconducting layer. Our study shows that the charge coupling is the dominant mechanism for the coupled MQT potentially utilized for quantum bits operated at higher temperatures.

We thank Prof. Osamu Sakai for allowing us to use the electron-beam lithography facility.

The phenomenon of coherent quantum phase-slips (CQPS) is expected to result in behaviour in superconducting nanowires which is the exact charge-flux dual of the Josephson effect. Despite microwave-spectroscopy verification of CQPS for superconducting nanowires in a ring geometry, research efforts with a dc transport geometry and microwave irradiation, aimed at developing a new quantum current standard dual to the Josephson voltage standard have not yet resulted in clear Shapiro-step-like features. Stringent control of fabrication and careful experimental design are crucial for this work; we present our latest experimental work using sub-20nm NbN nanowires embedded in a controlled high-impedance environment.

The authors gratefully acknowledge financial support from the UK EPSRC, grant reference EP/J017329/1.

Lasers use population inversion between two ordered energy levels to produce stimulated emission. Here we demonstrate lasing using atomic tunneling states (TS) which are disordered in energy and are known as defects which produce microwave loss and decoherence in quantum computing. Sweeping a control electric field causes the TS to “traverse through” a pumping frequency for population inversion, and then through a microwave resonator frequency which induces stimulated emission. The emission and linewidth of the tunneling atom laser is studied as a function of the pump power. Characterization with the use of a probe tone shows a parameter region with negligible material loss. In another parameter regime the disordered states produce coherent amplification.

The latest breakthroughs in the field of microwave quantum photonics and the design of superconducting circuits has lead to a point where the coherent dynamics of superconducting qubits have been applied to a great range of activities like control and measurement of microwave fields. A particular development over the past few years merges concepts from the fields of quantum optics and optomechanics with superconductivity. The new developments based on Josephson junctions exhibit macroscopic quantum coherence and made it possible to implement quantum optics experiments on a chip. These work describes superconducting Josephson tunnel junctions and the effect produced in the nanoscale.

I would like to acknowledge the support of Professor Eduardo Schmidberg, who advised me in my research on optoelectronics and whom I consider as a great reference. I would also like to thank Professor Mario Camuyrano, who was my reference in the study of the microwaves.

In recent years, we have developed HTS-SQUID systems for various applications, such as non-destructive evaluation, mineral exploration and immunoassay. Our devices have two features: ramp-edge-type Josephson junctions and oxide multilayer structures. In this work, we propose a new design for a small biaxial gradiometric SQUID. It is comprised of two gradiometric SQUIDs with a baseline length of 30 microns. It would be useful for detection of magnetic signals from a sample surface with high spatial resolution. We report its successful fabrication. A novel technique of interface engineering is devised.
The biaxial gradiometric SQUID contains 4 junctions and 16 crossovers in a chip. 9 superconducting contacts between counter- and base-electrodes are also included. When the counter-electrode is deposited, the junctions and the superconducting contacts are simultaneously prepared. We developed a novel technique to realize the distinguishing preparation of the junctions and the contacts. Ion bombardment procedure for the ramp surfaces of the base-electrode prior to the counter-electrode deposition is crucially important. We prepared modified layers suitable to form the junctions and the contacts through the bombardment process. The preparation was achieved by controlling the ion incident angle against the ramp surface during bombardment. In a fabricated chip, proper operations of gradiometers crossing at right angles were independently confirmed without appreciable cross-talk by passing a current through feedback coils.

This work is supported by Strategic Promotion of Innovative R&D in JST.

Superconducting quantum interference device (SQUID) based microtesla nuclear magnetic resonance (NMR) was developed at the beginning of the 2000s. In the microtesla (or ultra-low field—ULF) regime, the use of SQUIDs and pre-polarization techniques mitigate the inherent loss in signal due to the much lower measurement fields. The SQUID’s gradiometer pick-up loop is also exposed to the relatively strong pre-polarization field. Common choices of gradiometer wire material are the type-II superconductors niobium (Nb) and niobium-titanium (NbTi), both of which trap magnetic flux during the pre-polarization. The trapped flux release randomly and lead to flux jumps in the SQUID signal during the subsequent measurement time leading to a significant increase in the low-frequency noise with a 1/f-shaped spectrum. We and others have reported on such 1/f noise. In this work we investigated 1/f noise of small Nb, NbTi and tantalum (Ta) wire-wound gradiometers exposed to magnetic fields from 5 mT to 80 mT. We also constructed a large Ta wire-wound second-order axial gradiometer (90 mm diameter and 90 mm baseline) with a parallel heating wire to investigate efficiency and timing of its thermo-cycling process after it was exposed to the pre-polarization field. We were able to thermally cycle the gradiometer by applying 30 mJ energy to the heater wire during 0.5 s of the pre-polarization pulse. We used this gradiometer in imaging experiments with heater current and without heater current to show the feasibility of this type of gradiometer.

The authors gratefully acknowledge the support from the Los Alamos National Laboratory LDRD office through grants 20130625ER and 20130121DR.

We have developed a magnetic prospecting system with HTS SQUID gradiometers for exploration of metal resources. The SQUID gradiometer consists of a flux transformer chip made of a YBCO thin film and a SQUID gradiometer chip which is stacked on the transformer chip. The transformer consists of parallel gradiometric pickup coils with the size of 20 mm square and the baseline of 20 mm, and a single-turn gradiometric input coil which matches the size of the pickup coil of the SQUID gradiometer chip. The mutual inductance of the input coil was estimated to be approximately 0.022 nH. The flux noise of the assembled HTS SQUID gradiometer was 4 and 9 mF0Hz1/2 at 1 kHz and 10 Hz, respectively. Their effective volume and the balance estimated by using a planar gradient coil and a Helmholtz coil were approximately 3 x 10-9 m3 and 1/500, resulting in the gradient field noise of 3.5 and 7 pT/m/Hz1/2 at 1 kHz and 10 Hz.
The magnetic prospecting system consists of the HTS SQUID gradiometers, fluxgate sensors, a GPS module, a gyro sensor and A/D converter unit controlled by wireless communication from a note PC. Two assembled SQUID gradiometers which measure dBz/dx and dBz/dy field gradients were cooled with liquid nitrogen in a cryostat of the magnetic prospecting system. The cryostat was suspended from the frame of the system and its attitude was self-controlled by gravity.
The magnetic prospecting system was tested in a field near an old mine. The magnetic field gradient was measured along a survey line crossing a boundary of a magnetite deposit described in a geological map. A magnetic anomaly which suggested existence of the magnetite was observed at a similar site using both the SQUID gradiometers and fluxgate magnetometers.

This work is a part of mineral exploration renovating program conducted by Japan Oil, Gas and Metals National Corporation (JOGMEC) and is fully funded by Ministry of Economy, Trade and Industry, Japan.

Fabrication of multilayer high-temperature (high-T_c) superconducting structures requires extremely smooth YBa₂Cu₃O_7-x (YBCO) film surfaces. Growth of atomically smooth YBCO films on SrTiO₃ substrates is challenging due to nucleation of islands with different atomic layer stacking sequences in the beginning of the film growth process that results in the formation of anti-phase boundaries and an increase in roughness of the film surface [1]. Chemical-mechanical polishing (CMP) was suggested as a treatment to decrease roughness in YBCO films. We recently demonstrated that CMP does not lead to degradation of the superconducting properties of the YBCO films and can be used for fabrication of multilayer structures [2]. In the present work, we demonstrate that YBCO films grown with pulsed laser deposition on SrTiO3 substrates have atomically smooth surfaces after polishing. Atomic force microscopy (AFM) images show single unit cell steps on the surface of polished YBCO film. Reflection high-energy electron diffraction (RHEED) patterns prove the two-dimensional structure of the YBCO film surface. We also demonstrate the effect of ion beam etching (IBE) used for patterning superconducting electrodes on the surface of YBCO films. IBE leads to amorphization of the film surface that requires additional annealing in order to recrystallize YBCO surface. Our investigation opens ways for fabrication of high quality multilayer high-T_c structures and devices.
[1] S. Bals, G. Rijnders, D.H.A. Blank and G. Van Tendeloo, Physica C 355, 255 (2001)
[2] Chukharkin M., Kalabukhov A., Schneiderman J. F., Öisjöen F., Snigirev O., Lai Z., and Winkler D. 2012 Appl. Phys. Lett., 101, 042602

The work was supported in part Swedish Research Council, Swedish Institute Visby program, and Knut and Alice Wallenberg foundation. We also acknowledge support from Swedish national research infrastructure for micro and nano fabrication (Myfab).

In order to address the challenge problems of multi-sensor information fusion and attitude projection in the measurement of aviation superconducting full tensor magnetic gradient, a new method based on GPS integrated navigation system is proposed in this paper. Firstly, we analyze all the factors which affect the multi-sensor information fusion, develop a signal synchronization algorithm based on digital phase locked loop, GPS Pulses Per Second (PPS) and resample technique, and then give the methods for implementation and calibration of microsecond level synchronous precision for superconducting full tensor magnetic gradient measurement in details. Consequently, the validity of space attitude information which is required by attitude projection can be effectively protected using our proposed techniques. Secondly, based on the configuration of hexagonal pyramid planar superconducting gradiometer and related attitude projection algorithm, we introduce a full tensor measurement error model which resulted from space attitude measurement error. Then, according to the field test requirements (range: 20 nT/m, resolution: 0.05 nT/m), attitude measurement accuracy of GPS integrated navigation system is deduced to guide the full tensor system design. Finally, field trials were conducted to validate the synchronization accuracy and the error model. The results show that the error model is in line with expectation, and the system synchronization accuracy is less than 15 us. Therefore, system measurement precision can be achieved when attitude slew rate is less than 1080 °/s. The data analysis shows that the signal jitter of SQUID readout circuit is the main error source, and synchronization precision of the customized DAQ which is less than 0.1 us, can be ignored.

Supported by R&D of Key Instruments and Technologies for Deep Resources Prospecting (the National R&D Projects for Key Scientific Instruments), Grant No.ZDYZ2012-1-02

This material is based on work supported by Agency for Defense Development under grant Basic Research ADD-13-01-01-04.

NanoSQUIDs with nano-bridge junctions can be miniaturized into the nano-scale to measure a single Bohr magneton. Recently, the Pb SOT nanoSQUID achieved the spin sensitivity comparable to the magnet-tipped ultra-soft cantilever in MRFM[1]. Therefore, it is possible to use a nanoSQUID as a spin sensor for the on-chip nanoMRI integration. Usually, a 2D Nb nanoSQUID allows a large parallel magnetic field, which enables the SQUID detection in the high magnetic field[2]. While a 3D nanoSQUID made of Al has been demonstrated a deep flux modulation because of the nonlinear current-phase relation in the 3D nano-bridge junctions[3]. Here, we are going to present the research progress made on both 2D Nb and 3D NbN nanoSQUID at SIMIT. The 2D Nb nanoSQUID is made of ultra-thin Nb film on the sapphire substrate for high field applications. The device shows a non-hysteresis current-voltage curve without the shunted resistance. The depth of flux modulation was about 10% at 4.2K. The featuring nano-bridge junction size is of ~10nm thickness and ~50nm width. The 3D NbN nanoSQUID was fabricated through a multi-step process. The device characterization gave the depth of flux modulation above 30% at 4.2 K.
References:
[1] D. Vasyukov, et al., Nat. Nano., Vol.8 (2013) 639.
[2] L. Chen, et al, Nanotechnology, Vol.21 (2010) 405504
[3] R. Vijay. et al, Appl. Phys. Lett., Vol.96 (2010) 223112.

This work is supported by the Strategic Priority Research program of the Chinese Academy of Sciences (Grant No. XDB04000000) and the National Science Foundation of China (Grant No. 61306151).

Measurements of weak magnetic signals in a background of environment noises require gradiometers as the pickup coils. Axial wire-wound pickup coils allow easy fabrication of long baseline axial gradiometers. For low-noise axial gradiometers, the field-to-flux transfer of the flux transformer should be high, with an optimized inductance matching with the input coil of the SQUID sensor. We designed and fabricated an axial gradiometer having features of i) small parasitic inductance caused by the superconductive connection structure, ii) high field-to-flux transfer by using parallel winding of pickup coil, and iii) low-noise SQUID sensor using double relaxation oscillation SQUID. In a pickup coil of 20 mm coil diameter, field senstivity of the SQUID system could be below 1 fT/Hz^0.5 in the white region, including the contribution of the thermal noise of the liquid He dewar.

This work was supported by IBS project of CAPP.

Challenging real world applications for SQUID based mobile magnetic anomaly detection, such as UXO detection and geological surveying, require sensor systems with high sensitivity at low frequencies, large dynamic range and that are compact, lightweight, reliable and robust. In addition, sensors systems should ideally be configured to measure the gradient tensor which provides far richer information about the target feature with greater immunity to background noise which improves the data quality and accuracy of target location and classification.
We have demonstrated a promising new concept tensor gradiometer formed primarily from a single planar gradiometer and which has the potential to meet the demands of high sensitivity mobile magnetic surveying. The sensor system uses a single HTS flip-chip planar gradiometer mounted on a plinth angled at 27° to plane orthogonal to the rotation axis. This arrangement is then rotated about the instrument’s z-axis at speeds up to 33 Hz whilst cooled to liquid Nitrogen temperatures. Fourier analysis of the output signal is used to extract the frequency modulated gradient signals. Here we discuss referencing approaches to correct for imperfect balance of the gradiometer in particular using a 3 axis vector magnetometer adjacent to the rotating gradiometer.

In multi-channel SQUID measurement system, it is time-consuming for adjusting device parameters to operation condition in all channels. In this paper, we presented an intelligent control system to determine the optimal working point of devices which is automatic and more efficient comparing the manual one. A best working point searching algorithm is introduced, which is the core component of the control system. In this algorithm, the bias voltage V_bias is changed automatically by a variable step to obtain the maximum value of the peak to peak voltage value V_pp. We choose this point as the best working point. Using above control system, more than 50 weakly damped SQUID magnetometers with area of 5*5 mm² or 5*10 mm² are adjusted and a 36-channel MCG system perfectly worked in a magnetically shielded room. The average white flux noise is under 10 microPhi0/sqrtHz.

This work was Supported by the "Strategic Priority Research Program（B）" of the Chinese Academy of Sciences (Grant No: XDB04020200).

Nano-SQUIDs (superconducting quantum interference devices) significantly shrink the SQUID washer by replacing the traditional tunneling junctions with the nano-bridge junctions. In such a design, the spin sensitivity of nano-SQUIDs, which is proportional to the radius of SQUID washer, are greatly improved. Additionally, nano-SQUIDs made of NbN can be applied in the high magnetic field. However, most 2D nano-SQUIDs made of NbN showed relatively low depth of the flux modulations. It was mainly due to the small nonlinearality in the current-phase relation of the nano-bridge junction made of thin film. By replacing the 2D nano-bridge junctions with a 3D structure, a deep flux modulation has been shown in Al nano-SQUID¹⁾. However, the fabrication method of 3D Al nano-SQUID is not compatible with NbN. Here, we present a fabrication process for 3D NbN nano-SQUID. By using of this process, we are able to manufacture NbN nanoSQUIDs with flux modulation above 30%.
Keywords: Nano-SQUID, nano-junctions, 3D, coherence length
Reference:
1.R. Vijay, E. M. Levenson-Falk, D. H. Slichter, and I. Siddiqia. Approaching ideal weak link behavior with three dimensional aluminum nanobridges[J]. Applied Physics Letters, 2010, 96(223112).

The authors appreciate the supports from the Strategic Priority Research program of the Chinese Academy of Sciences (Grant No. XDB04000000) and the National Science Foundation of China (Grant No. 61306151).

In multi-channel system of magnetocardiogram (MCG), crosstalk becomes more serious due to the mutual inductances between the pickup coils of adjacent channels. In this paper, we employ the external feedback method which rearranges the feedback coil to suppress the crosstalk interference. Based on this method, we design weakly damped SQUID which has an area of 3×5 mm² with different value of mutual inductance between feedback coil and flux-transformer circuit. Among these designs, the number of feedback coil turns range from 1 to 44 and the corresponding inductances vary from 0.5 nH to 660 nH. Furthermore, the SQUID has a loop inductance of 350 pH and a large junction shunt resistor of 15 Ω to achieve a low flux noise of 6 μФ₀/√Hz. Then several SQUID gradiometer modules with wire-wound pickup coil using superconducting connection are developed. These modules are different in order, baseline and diameter of sensing coil. In different magnetically shielded conditions, we set up several multichannel systems using such modules to record MCG signals with optimal signal-to-noise ratios.
Reference
[1] H. J. M. Ter Brake, F. H. Fleuren, J. A. Ulfman and J. Flokstra, Cryogenics 26, 670(1986).
[2] Chao Liu et al 2013 Supercond. Sci. Technol. 26 065002

This work was Supported by the "Strategic Priority Research Program（B）" of the Chinese Academy of Sciences (Grant No: XDB04020200)

We have developed dc superconducting quantum interference devices (SQUID) fabricated in an NbN/AlN/NbN technology. The NbN SQUIDs consist of NbN/AlN/NbN Josephson junctions and Pd/Ti shunt resistors. The NbN/AlN/NbN tunnel junctions were prepared by epitaxial NbN/AlN/NbN trilayers on MgO (100) substrates, they showed excellent tunneling properties with a large gap voltage of 5.6mV, a large IcRn product of 3.6mV and quality factor Rsg/Rn above 20 for the junctions with critical current density Jc of 100A/cm². The NbN SQUIDs were designed and fabricated on MgO substrates with a size of 1cm*1cm, the critical current of dc SQUID and the shunt resistance were 20uA and 5ohm respectively, the loop inductances of the SQUID were designed from 50pH to 200pH. The I-V and V-Ø characteristics and noise performance of the SQUIDs were tested at 4.2K in a liquid helium cryostat. The white flux noise of NbN SQUID measured in a flux-locked-loop mode was below 8uØ₀/Hz^1/2.

This work was Supported by the "Strategic Priority Research Program (B)" of the Chinese Academy of Sciences (Grant No: XDB04010400 and XDB04030000).

Abstract
MCG is now widely used in clinical research, but in the urban hospital environment it still faces many challenges, such as strong electromagnetic disturbances and its high cost etc. In this paper, we proposed a full tensor compensation technique for noise cancellation under unshielded MCG measurements. The full tensor module was composed of 8 SQUID magnetometers which were placed along the 5 independent 1^st order gradient tensor directions. The tensor components of the environmental field were obtained by the synthetic gradiometers with baseline of 2.9 cm. Three uniform environmental field components were also simultaneously acquired by the central three orthogonal magnetometers. The imbalance responses of the 2nd order gradiometer were effectively compensated. Using above technique, the noise level of about ±2 pT was achieved by the unshielded MCG system under urban hospital environment. This method will be a good choice for clinical MCG measurement system.

References
Uzunbajakau S A, Rijpma A P, ter Brake H J M, and Peters M J, “On Gradiometer Imbalance.” IEEE Transactions on applied superconductivity. 16, NO. 3, pp.1898-1907,2006
Vrba J and Robinson S E, “SQUID sensor array configurations for magnetoencephalography applications”, Supercond. Sci. Technol.15 R51–89, 2002
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This work was Supported by the "Strategic Priority Research Program（B）" of the Chinese Academy of Sciences (Grant No: XDB04020200).

Large Green Superconducting Computers based on LTS Josephson junctions and ultrafast-ultralow dissipation Single Flux Quantum Logic gates are planned to replace inside the Data Servers and Big Data Clouds established CMOS semiconductor computers based on FET logic which dissipate x100 times more energy/effective bit. Static and dynamic dissipation of the FET capacitor is the limiting factor, while the superconducting circuit integration density and the Josephson junction cut-off frequency have to be increased by a factor 10 to 50 from the state of art of Jc~10 kA/cm^2, externally shunted Nb-AlOx-Nb junctions. Superconducting Chip with 10^6, low spread self-shunted junctions, is such an issue.
In the French ANR ‘HyperSCAN’, instead of Nb, we addressed a reliable technology based on superconducting cubic refractory nitride films, i.e. niobium nitride (NbN) and NbTiN with Tc up to 15.5K and a sputtered TaxN junction barrier. Self-shunted patterned and planarized NbTiN-NbN-TaxN-NbN-NbTiN Josephson junctions and NbTiN stipe-lines are forming SFQ logic circuits on top of 8-inch silicon wafers taking advantage of the non-hysteretic TaxN barrier with cut-of frequency above 500GHz and of the accurate lithography of the CEA-LETI CMOS platform.
We have also demonstrated for the first time that an array of 24 NbN Single Photon Sensitive Nanowires (40-80 nm width and 4-5nm thick) integrated on chip with the SFQ circuit covered with SiN optical waveguides act as a fast wave tracker or as a Fourier spectrometer. Such superconducting LSI chip operating between 2K and 10K, is fully compatible with optoelectronics and THz front-end sensor arrays sampled by fast SFQ digitizing functions.
Such Refractory Nitride Superconducting-IC route will address useful applications such as Quantum Cryptography, IR Medical Imagers, THz Imagers and Green Computers in a shorter term than expected from HTS-IC still lacking of a reliable epitaxial Josephson junction.

Acknowledgments to ANR-Telecom “HyperSCAN”, CE-STREP “SINPHONIA” and to “SWIFTS-SNSPD” partners at CEA INAC, CEA LETI, University of Alpes-Savoie, TELECOM Paris-Tech, IPAG, Neel Institute CNRS and to Industrial Partners of those projects.

A practical automated DC voltage standard based on an array of high-temperature superconductor (HTS) Josephson junctions suitable for metrological, scientific and industrial applications has been developed and its performance has been characterized. In the new instrument the quantum voltage from 25 mV to 100 mV of the cryocooled up to liquid nitrogen temperatures HTS array is converted by a resistive divider into an output voltage up to 10 V with a relative uncertainty smaller than 5 × 10-8. This voltage standard consists of three units with dimensions of 450 × 500 × 185 mm. The results of the comparison with 10V niobium Josephson DC voltage standard will be presented at the Conference.

The work has been supported by the Ministry of Industry and Trade of the Russian Federation (Minpromtorg Russia) and in part by the grants of the Russian Foundation for Basic Research N15-02-05793 and N15-42-02469.

There are several topologies for readout circuits of RSFQ T-Flip Flop, which are divided into two main categories as destructive and non-destructive. This is while a relatively new category of RSFQ TFF is introduced as bi-directional circuits that uses an AC bias current as the clock signal. In this work we have investigated the effects of readout cell topology and critical parameters on the proper functionality and stability of the states of both uni-directional and bi-directional TFFs. It is observed that the instability and fluctuation in the state of the gate (memory of TFF) dictates the minimum time intervals between the clock pulses, limiting the ultimate operation frequency of the circuits. The absolute values of current levels of the junctions, which play an important role in the behavior of the cell, are studied and their variations over several consequent pulses are stabilized. We found the appropriate values of the circuit and junction parameters, resulting in the optimum point for having the best margins possible. We report on the investigated circuit topology and parameter optimizations of the readout circuit in order to reach the optimal and fastest possible RSFQ TFF cell.

Research on Josephson quantum standards is foreseeing a fundamental change in the Metrology of the volt, for both AC and arbitrary waveforms. The aim is a direct link to the SI from DC up to operating frequencies in the MHz range, fulfilling the requirements set today by digital instrumentation. Programmable voltage standards converting a digital code into a quantum-accurate value are already available in primary laboratories. Even more advanced standards to convert sub-nanosecond binary coded pulses into any arbitrary signal with quantum accuracy are now actively developed and tested.
In our contribution we will focus on the recent and ongoing activities on helium-free operation with both types of Josephson standards: programmable SNIS arrays and pulsed devices. Helium-free operation provides not only ease of use, but a way to reduce cable effects, now a crucial issue to overcome frequency-related limitations to the accuracy of the standard. Yet, thermalization problems not faced with helium cooling are still challenging. Some of our recent results indicate that a careful design of the setup allows operation with short cables without affecting the cooler effectiveness. If confirmed, cryogen-free Josephson standards will prove a benefit to speed up the synthesized quantum signal frequency rather than another obstacle to overcome.

We have improved the design of integrated quantum voltage noise source (IQVNS) that is a rapid single flux quantum (RSFQ) integrated circuit version of QVNS for Johnson noise thermometry (JNT) used in thermal metrology. IQVNS generates calculable quantum voltage noise, which is synthesized with a number of single flux quantum voltage pulses, for calibrating a JNT system. Previously, we proposed IQVNS to overcome problems with conventional QVNSs, for example, crosstalk among discrete components, by integrating subsystems, pseudorandom number generator (PRNG), variable pulse number multiplier (VPNM) and voltage multiplier (VM), on a single chip.
A drawback of our previous design was lax synchronization of low- and high-frequency clock signals separately fed from room temperature electronics: while PRNG was operated at ~2 MHz, VPNM was operated at ~10 GHz. This made the whole system complicated, possibly inducing a timing error and degradation of the calculability.
A key improvement was on-chip generation of the low-frequency clock by down-converting the high-frequency clock so that the whole circuit was synchronized with a single clock. To maintain the compatibility with our room temperature electronics, we used an 8.2-GHz master clock from which a 2.0-MHz clock was derived with a 4096-to-1 pipelined prescaler comprising 12 toggle-flipflops. For further robustness and wider timing margins, we employed counter-flow pipeline structures both in the chip and subsystem levels. Moreover, a DMUX stage used to generate complementary pulse trains was separated from the VPNM block and a shift register cell was inserted between them for timing buffer.
The new IQVNS circuit was implemented on a 5-mm chip with a RSFQ cell library, CONNECT, and was fabricated by using our 2.5-kA/cm² Nb/AlO_x/Nb-junction technology. We confirmed the correct operation of the IQVNS chips by low-speed testing and are proceeding to JNT measurements at the triple point of water.

This work was supported by JSPS KAKENHI Grant Number 25289126.

Low-Tc rapid single flux quantum (RSFQ) digital circuits, based on Josephson junctions and inductive lines enable to the operation of functional cells up to several hundreds of GHz, depending on the technology. Such superconducting digital electronics allows the design of circuits that perform functions from simple logical operations up to the ones of microprocessors for very high speed digital applications.
In this work, we designed RSFQ test circuits, fabricated with the FLUXONICS Foundry process, based on Josephson tunnel junctions composed of a Nb/Al-Al2O3/Nb trilayer with a critical current density of 1 kA/cm². We experimentally determined the maximum operating frequency of a few cells such as Toggle Flip Flop (TFF) or Complementary Toggle Flip Flop (TFFC), for different values of the Mc-Cumber parameter , in order to validate and optimize more complex functional blocks.

In 2004 D-Wave had down-selected Adiabatic Quantum Optimization/Quantum Annealing (QA) as the most promising and ready to be implemented at scale approach to harness quantum mechanical effects for computation. Ten years and about 40 chip mask generations later, D-Wave processors have become competitive with state-of-the-art classical algorithms running on modern CMOS computing hardware for some computation problems in the realms of optimization and sampling. This achievement by a relatively small venture-funded company required steady progress from early few-qubit all analog-controlled prototypes, through the synthesis of on-chip digital control circuitry based on SFQ logic and an analog fabric of qubits and couplers, to modern large-scale processors that are being used by research groups and customers to evaluate the performance and potential applications of QA. In the process, we moved from a simple two metal layer fabrication technology hosted at JPL's Micro Devices Laboratory to having our own 6 metal layer fully planarized process with 0.25 um minimum feature size. Recent fabrication development efforts have focussed on both improving device uniformity and improving qubit coherence within the context of a manufacturable process.
While some of the implementation problems we had to solve were unique to our particular approach to manufacturing QA processors, many others are common to a wider range of both classical and quantum computation technologies based on superconducting circuits. This poster summarizes some of those key insights that will be of interest to the broader superconducting electronics community.

Keywords: energy efficient rapid single flux quantum, ERSFQ, nSQUID, digital gates, low power, T flipflop, superconductor.
1. Kirichenko D, Sarwana S and Kirichenko A 2011 Zero static power dissipation biasing of RSFQ circuits IEEE Trans. Appl. Supercond. 21 776J.
2. Ren, V. K. Semenov, Y. A. Polyakov, D. V. Averin, and J. S. Tsai, “Progress towards reversible computing with nSQUID arrays,” IEEE Trans. Appl. Supercond., vol. 19, no. 3, p. 961, 2009.

The authors warmly thank CNR-SPIN (Italy) for the financial support.

The authors are grateful for funding from the NIH/NIBIB (R01EB009772) and from the National High Magnetic Field Laboratory under National Science Foundation DMR-1157490 where a portion of the work was performed. The authors would also like to acknowledge financial support from Agilent Technologies, Santa Clara, CA.

Matching issues of active electrically small antenna based on superconducting quantum array are analyzed and discussed. To keep the highly linear antenna characteristics, the active antenna ought to be loaded with much higher impedance than the normal resistance of the superconducting quantum array. At the same time, the interface in use is to be well matched with the next receiver device such as superconductor Analog-to-Digital Converter characterized by low input impedance of about normal resistance of overdamped Josephson junction. To provide optimal matching between the active antenna and the low-impedance device, a broadband filter realizing impedance transformation from 50…100 Ohm to 2…4 Ohm is developed and characterized.

This work was supported by Russian Ed.&Sci Ministry Grant 14.613.21.0022 (RFMEFI61314X0022).

A novel deign for multi-zeros superconducting L-band-pass filter is presented in the paper. This linear phase and high-selectivity filter consists of ten resonators with quasi-elliptic response by cascaded quadruplet (CQ) structure. By sharing two nodes, three CQ structure realized by cross-coupling lines are located on ten resonators evenly. The compact cross coupling structure contributes to the miniaturization of filter and product more transmission zeros for n-pole superconducting filters. The resonator is composed of a cross-finger and lumped clip in order to suppress second harmonic better. The filter is fabricated using double-sided DBa2Cu3O7 (DBCO) thin films on a piece of 44×13 mm MgO substrate. The experiments results show that this filter has a relative band width of 0.58% and the insertion loss is less than 0.2 dB. Return loss of the filter in passband is more than 20dB and measurable slope out of band reaches 70dB /MHz. Good agreement between simulations and measurements has been achieved, which prove effectiveness of our design using shared node structure for multi-transmission zeros superconducting filters.

This work was supported by the National Natural Science Foundation of China (Grant No. 61471208, Grant No. 61171028, Grant No. 61176119), the Specialized Resarch Fund for the Doctoral Program of Higher Education of China (Grant No. 20110031120033)

We have developed a high-temperature superconducting (HTS) dual-band bandpass filter (BPF) using shorted stub-loaded hair-pin resonators with controllable bandwidths and feeding structure. The proposed dual-band BPF is composed of four shorted stub-loaded hair-pin resonators with H-shaped waveguides between them. The resonator enables independent control of the first and second band resonant frequencies. The coupling coefficient of the first one was controlled by the H-shaped waveguides, which did not affect the coupling coefficient of the second one. A tapped-line coupling is mainly adopted to adjust the external coupling for the first one and a coupled-line coupling is adopted to adjust the external coupling for the second one. An electromagnetic simulator was used to design and analyze the filter. The filter was designed at 0.8 GHz with a 16-MHz (2%) bandwidth for the first band and at 2.0 GHz with a 40-MHz (2%) bandwidth for the second band. The filter was fabricated using YBa₂Cu₃O_y thin film on a CeO₂-bufferd Al₂O₃ substrate. The measured results agree well with the simulated ones.

This work was partly supported by a Strategic Information and Communications R&D Promotion Programme.

We have developed a high-order high-temperature superconducting (HTS) dual-band bandpass filter (BPF) using stub-loaded hair-pin resonators for future mobile communication system. The proposed dual-band BPF is composed of stub-loaded hair-pin resonators with H-shaped waveguides between them. The main advantage of the proposed filter was to enable independent control of the bandwidths of the first and second band. The coupling coefficient of the second one was controlled by the distance between the resonators, which did not affect the coupling coefficient of the first one. On the other hand, the coupling coefficient of the first one was controlled by the H-shaped waveguide, which did not affect the coupling coefficient of the second one. An electromagnetic simulator was used to design and analyze the filter. We designed a six-pole dual-band BPF and an eight-pole dual-band BPF. The two filters were designed at 2.2 GHz with a 44-MHz (2%) bandwidth for the first band and at 3.5 GHz with a 70-MHz (2%) bandwidth for the second band. The two filters were fabricated using YBa₂Cu₃O_y thin film on a CeO₂-bufferd Al₂O₃ substrate. The measured results of the two filters agree well with the simulated ones. Additionally, we measured the frequency response of the six-pole dual-band filter with wideband low noise amplifier (LNA). The gain of the LNA is 10 dB. The measured gain is around 10 dB from 1 GHz to 6 GHz. The measured return loss shows very little change from without LNA.

One of the most promising sources for pulsed THz radiation is coherent synchrotron radiation (CSR) [1]. Due to their response time in the picosecond range, direct THz detectors based on YBa₂Cu₃O_7-x (YBCO) hot-electron bolometers are ideally suited for the analysis of electron- beam instabilities occurring in the synchrotron storage ring and affecting the emitted CSR spectrum. Here, response times down to 16 ps (FWHM) have already been achieved with wideband antenna-coupled detectors [2]. We have developed a detection system combining this fast response time of the detectors with the single-shot spectral resolution of the individual CSR pulses. The detector array consists of an integrated four-pixel array which contains four narrow-band antennas with embedded detectors.
We present results of the design and optimization of narrow-band planar double-slit antennas for four discrete frequencies at 140 GHz, 350 GHz, 650 GHz and 1 THz. One of the design challenges was to achieve matching of the antenna with the detector. On the one hand, the decrease of the detector size, required for high sensitivity, simultaneously leads to larger detector impedances. On the other hand, broadband matching of the 50 Ω readout electronics to the impedance of the YBCO detector is needed to preserve a fast response time. Simulations and measurements with continuous-wave sources covering the whole frequency range of interest were performed. A comparison of the results showed an increase in antenna bandwidth for larger mismatches between antenna and detector impedance. At the same time, first measurements with pulsed CSR showed sufficient detector sensitivity.
[1] A. S Müller, et al., Proceedings of EPAC08, WEPC046, 2008
[2] P. Thoma, et al., Applied Physics Letters 101, 142601 (2012)

In the current research a comparison between two kinds of antennas will be presented. The first antenna is used as a part of a conventional detection system while the second is a part of a superconducting detection system, both used in the THz frequency range. The superconducting detection antenna was designed in bow-tie configuration based Josephson junction in high-temperature superconductors (HTSC) rendering a sensitive detection system. The conventional detection system was designed similar to the superconducting architecture, with a room temperature diode as a detector. Both systems were designed with a bandwidth of 150GHz about the central frequency of 1.05THz. The superconducting bow-tie antenna was designed with 200nm thin layer of YBCO films on 0.5mm thick MgO substrate. The YBCO layer was coated with a 50nm thick gold, in order to prevent oxygenation of the layer. The Josephson junction detector was placed in the center of the superconducting antenna. The comparison between the antennas was made through theory and electromagnetic simulations of the radiation curve. To increase the sensitivity of the detecting systems, and to achieve a wider antenna radiation curve, antenna arrays, based on the superconducting and conventional detectors, were compared. Our preliminary results, based on comparison between the Q factors, show significant improvement in the efficiency of the superconducting array.

High temperature superconducting (HTS) bandpass filters are promising pieces of technology due to their efficient use of their frequency resources because they have a low insertion loss and sharp skirt characteristics. However, transmit HTS filters have not yet been commercially used because they lack a high power-handling capability (PHC). Therefore, they must be improved to increase their PHC. We examined the transmit bandpass filter using a thick single-crystal HTS bulk disk resonators to improve the PHC, because bulk resonators should have a higher critical current in comparison with a conventional thin film resonator. When the number of the thin film resonators increase the maximum current density increase, therefore PHC should be decrease^[1]. However, the relationship between the PHC and the skirt characteristic caused by increasing the number of the bulk resonators was not experimentally clarified in the previous studies^[2]. Therefore, the aim of this study is to clarify the differences in the PHCs and skirt characteristics when the number of bulk resonators is increased. We designed 3-, 5- and 7-pole HTS filters using superconducting bulk resonators and a trimming mechanism with dielectric rods. MW-studio software was used to simulate the frequency responses, electromagnetic fields, and current densities of these HTS filters. We could estimate from the results of the simulations that the PHC of the 5-pole bulk filter was about 1.7 times higher than that of the 3-pole bulk filter. We fabricated these HTS filters and measured their frequency responses and PHCs at 20 K. The experimental results showed the filtering properties. The PHC of the 3- and 5-pole bulk filters were 41.7 and 114 W. Therefore, the 5-pole bulk filter had an approximately 2.7 times higher PHC than that of the 3-pole bulk filter. This dependence was different from that of the relationship of conventional filters made by thin film resonators. This reason has not yet been clear. However, it was an effective result for application purposes. The details of the results for the 7-pole bulk filter will be presented at the conference.

References
[1] R. R. Mansour et al., IEEE Trans. MTT, 44, 7, 1322-1388 (1996).
[2] T. Kato et al., IEEE Trans. Appl. Supercond., (2015); (To be published).

This work was supported by JSPS KAKENHI Grant Numbers 22560317, 24560393, Nippon Sheet Glass Foundation for Materials Science and Engineering, Casio Science Promotion Foundation, the Mazda Foundation, Kato Foundation for the Promotion of Science, and Asahi Glass Foundation. A part of this work was performed in the clean room at Yamagata University.

References
[1] W. W. Brey et al., Journal of Magnetic Resonance, 179, 2006, 290-293.
[2] T. Yamada et al., IEEE Trans. Appl. Supercond.,(2015);(To be published).

This work is supported by the Strategic Promotion of Innovative Research and Development of JST (development of next-generation NMR technology using HTS materials). A part of this work was performed in the clean room of Yamagata University.

We consider electron transport in hybrid structures consisting of superconducting and normal-metal electrodes and nanoscale islands. For voltages below the superconducting order parameter, transport is achieved through processes involving simultaneous tunnelling of several electrons. Study of these processes is of importance in quantum metrology [1,2] and information processing [3].
[1] V. Bubanja, Phys. Rev. B 83, 195312 (2011).
[2] V. Bubanja, J. Low Temp. Phys. 175, 564 (2014).
[3] V. Bubanja, Ann. Phys. (in press), http://dx.doi.org/10.1016/j.aop.2015.03.003

A superconductor-insulator-superconductor Josephson junction (JJ) is formed by two superconductor electrodes separated by a very thin insulating barrier of few atomic layers of thickness. If a constant voltage is applied on the junction, an alternating current with a frequency in the range of 1 to 1000 GHz will be generated in the system. This phenomenon has many technological applications, such as the fabrication of a volt standard, SQUIDs and other magnetic sensors, quantum computing (qubits), and the fabrication of ultra-fast superconducting microelectronics based on rapid single flux quantum logic. In the last years, conductive atomic force microscopy (CAFM) has become an important technique to study and characterize insulating barriers of tunnel junction devices, such as magnetic tunnel junctions or spin filters. However, little work has been done that uses CAFM for the electrical characterization of JJ. In this work, we have studied by CAFM the transport properties of ultra thin SrTiO3 (STO) layers grown over YBa2Cu3O7 (YBCO) electrodes for the development of High Temperature JJ. A phenomenological approach was recently developed, that allows to obtain critical information for this system using CAFM, e.g. the barrier thickness required to totally cover the superconducting electrodes and the physical properties of the barrier (its thickness distribution, the attenuation length and the barrier’s energy). Moreover, the model allows to evaluate and consider in a simple way, the influence of the image forces and the dielectric constant of the barrier in the tunnelling properties in these systems. A very good control of the barrier thickness with a low roughness (~ 0.5 nm) of the STO layer was achieved during the deposition process. However, due to the growth mechanism of the superconducting layer, an important density of outgrowths was found on the surface of the samples. The STO layers present an energy barrier of 0.9 eV at 1.25 V and an attenuation length of 0.23 nm, indicating its good insulating properties for the development of Josephson junctions of high quality.

The authors would like to acknowledge the CONICET/CNRS, PICS level 2, Fabrication and study of electronic devices based in oxides nanostructures, and the PEOPLE MARIE CURIE ACTIONS, International Research Staff Exchange Scheme, Call: FP7-PEOPLE-2012-IRSES, Coupling effects in magnetic patterned nanostructures, COEF-magNANO.

Anomalous modulation characteristics of superconducting Josephson current I_c through niobium tunnel junctions after applying the vertical field are measured. Niobium/aluminum-oxide/niobium tunnel junctions are fabricated by DC-magnetron sputtering method. Josephson current I_c through superconducting niobium tunnel junction is modulated by applying the parallel magnetic field (H_x, H_y) and the vertical field H_z to the junction plane using three pairs of Helmholtz coils. Without a vertical magnetic field to the junction plane, dependences of Josephson current I_c through square- and rectangular-shaped niobium tunnel junctions are Fraunhofer-type modulation both on H_x and H_y directions parallel to junction edges [1,2].
In this article, anomalous modulation characteristics of Josephson current I_c through square-shaped niobium tunnel junction have been measured by applying the perpendicular magnetic field H_z [3,4] more than 4000 A/m to the junction plane. The I_c-(H_x-H_y) dependences have no hysteresis on (H_x,H_y) values, however, I_c-H_z characteristics depends upon the changing history of the vertical field H_z. Electromagnetic field inside the junction area can be studied from the I_c-(H_x-H_y) dependence under the examined vertical H_z field during the changing history of H_z. Before applying the vertical field H_z, the I_c-(H_x-H_y) dependences are the product of the two Fraunhofer diffraction patterns in H_x and H_y direction parallel to each edge of the square junction and shows square-shaped main peak area. After applying and removing the vertical field H_z more than 4000 A/m, the I_c-(H_x, H_y) modulation pattern shows triangle-shaped main peak area, due to the remained trapping and shielding magnetic flux through niobium electrodes. This triangle main peak area has successfully been explained by assuming the trapped flux through the electrodes inside the junction area using planar circuit model.
[1] A.Nakayama, S.Abe, N.Watanabe, J. Appl. Phys., 111, 113907 (2012).
[2] A.Nakayama,S.Abe,N.Watanabe,Y.Okabe, Microelectronic Eng., 108, 163 (2013).
[3] A.Nakayama,S.Abe,N.Watanabe,Y.Okabe, Microelectronic Eng., 108, 93 (2013).
[4] N.Watanabe, S.Abe, A.Nakayama, J. Appl. Phys., 116, 033920 (2014).

This research is supported in part by the Ministry of Education, Science, Sports and Culture (Japan) [Grant number 21560372].

Superconducting Josephson tunneling junction has been one of the most promising candidates in achieving quantum computation for its easy fabrication, macroscopic size and easy coupling with the outside world. We have fabricated superconducting flux qubit using aluminum material by obique e-beam evaporation and have got the critical parameters of the Al/AlOx/Al tunnel junctions including the sub-gap current, the current density and the barrier layer thickness. The flatness of aluminum films with different thickness was obtained by using atomic force microscopy (AFM) and the results show that the tunnel junction, with better flatness of Al film, has lower leakage current.The superconducting current density of the tunnel junction was controlled by adjusting the oxygen pressure. Analyzing the tri-layer structure of aluminum tunnel junction by transmission electron microscopy (TEM), we have got the thickness of the barrier layer with different oxygen pressures ranging from 10 to 1000 Pa. According to the equation of j_s≈2ehß₀e^-d/£_d/m£_d ,we do numerical computation and numerical fitting .The numerical results and experimental ones are in good agreement for certain parameter values.

The work was supported by the National Natural Science Foundation of China (Grant Nos. 61371036, 11474154 and 11227904), the Natural Science Foundation of Jiangsu (BK2012013)

According to contemporary experiments, intrinsic Josephson junctions in mesa structures of high-temperature superconductors can radiate coherently at some applied bias currents. At these conditions the system switches to the coherent dynamic state at which voltages over intrinsic junctions oscillate coherently. We developed numerical models of one and two wide layers of intrinsic Josephson junctions with small irregularities of the critical current density at ends and took into account magnetic field induced by the bias current. Modeling of such systems was made on the base of the s-wave as well as the d-wave dynamic models of the intrinsic junction. We showed that the appearance of the coherent radiation at certain bias currents was explained by the excitation of resonant modes in the whole system consisted of one or two layers. We analyzed the width of the emission line and compare it with experimental data. We showed that the appearance of the coherent dynamic state exists in both s-wave and d-wave descriptions of wide intrinsic junctions. Finally, we showed that the developed model explained the appearance of the coherent dynamic state in single crystal mesa structures of intrinsic junctions.

Epitaxial Ho/Nb/Ho and Dy/Nb/Dy superconducting spin valves (SSVs) show a reversible change in the zero-field critical temperature of ~400 mK and an infinite magnetoresistance on changing the relative magnetization of the Ho or Dy layers. Unlike transition metal SSVs which show much smaller change in the critical temperature values, our results can be quantitatively modelled. However, the fits require an extraordinarily low induced exchange splitting which is dramatically lower than known values for rare-earth Fermi-level electrons implying that new models for the magnetic proximity effect may be required.

Yuanzhou Gu acknowledges financial support from King’s College, Cambridge

There is a increasing interest towards cryocooler operation of superconductive electronics, motivated by various reasons. Reliable cryocooler operation requires a very specific thermal design to cope with problems not faced with liquid coolants, like minimization of thermal gradients to allow uniform operation of the chip. In particular, when device operation requires a non negligible electrical power, dissipating the heat generated within a very small area may be extremely difficult. This is the case with arrays of Josephson junctions in voltage standard applications. It is then very important to exactly estimate the local junction temperature dependence on the device operating conditions. We devised a time-resolved technique to determine the temperature rise in SNS junctions from the observation of the junctions critical current during the temperature decay after heating with controlled power. In the work we'll discuss the method for estimation of junction thermal parameters and the application in the design of cryocooled ac voltage standards.

Complex future SFQ circuits require hundreds of thousands to millions of junctions with uniform and reproducible properties to allow for their correct operation. Even a very small fraction of junctions deviating from nominal values can cause a circuit to fail. To attain the required high levels of junction uniformity it is necessary to detect, analyze, and solve any source of junction defects in the fabrication process. Recently we have studied the junction uniformity and yield of Josephson voltage standard circuits fabricated at NIST by measuring over 25 million junctions (A. E. Fox et al., IEEE Trans. Appl. Supercond., v. 25, no.3, June 2015). This is possible by measuring Shapiro steps on current voltage curves of long arrays of junctions under microwave irradiation and estimating the number of single-junction defects within these long arrays from the characteristics of the quantized steps. Lately we have used this method to study the uniformity of junctions fabricated at MIT Lincoln Laboratory for SFQ digital circuits in the Cryogenic Computing Complexity (C3) program. The junctions tested have AlOx barriers, a critical current density of 10 kA/cm², and an area of 7 μm². Shunt resistors make these junctions strongly overdamped with an I_cR_n product of 68 μV and a characteristic frequency of 33 GHz. Diagnostics chips with arrays totaling 7500 junctions per chip were fabricated and measured. We were able to measure quantized steps, demonstrating good junction uniformity, and account for shorted junctions, which are undetectable by viewing only IV curves of arrays without microwaves. This procedure is a valuable tool for characterizing uniformity and yield of junctions in a fabrication process.

This work was funded by the Cryogenic Computing Complexity program of the Intelligence Advanced Research Projects Activity.

Quantum devices based on nanoscale Josephson junctions are an emerging topic attracting a lot of interest thanks to the countless application fields including quantum electronics, nanosized superconducting quantum interference devices (nano-SQUIDs) and AC voltage metrology. The downscaling of Josephson junctions must be pursued without affecting their fundamental properties, such as the critical current density and the characteristic voltage.
The 3D nanofabrication based on the focused ion beam (FIB) sculpting method [1] represents an alternative solution to the classical electron beam lithography, with a good control of the circuit dimensions and simplifying the fabrication by reducing the process steps. Nevertheless, the highly energetic process and the Ga⁺ ions implantation could lead to an uncontrollable change in the electrical properties of junctions.
In this contribution, we realised Nb-based Josephson junctions by FIB sculpting and studied the influence of the fabrication process on the electrical parameters. Furthermore, we propose two technological approaches to reduce the FIB effects on the junctions’ electrical response, such as a liquid anodization and a low-keV ions surfaces cleaning, evaluating positive and negative aspects of each technique.
[1] M. Fretto, E. Enrico, N. De Leo, L. Boarino, R. Rocci and V. Lacquaniti IEEE Trans. Appl. Supercond. vol. 23, 1101104 (2013)

The samples were fabricated at NanoFacility Piemonte, INRiM, a laboratory supported by Compagnia di San Paolo. The authors thank Mr. R. Rocci for his technical support.

We have developed high quality NbN junctions to realize superconducting devices operating at 10 K and/or high speed. We developed the fabrication technique of (100)-oriented NbN thin films on Si (100) wafers by DC magnetron sputtering method using TiN as a buffer layer. We found that the TiN films grow with (100) direction on the hydrogen-terminated Si (100) substrates at the substrate temperature above 600C [1]. The measured lattice constant of TiN buffer layer from the X-ray diffraction (XRD) analysis was 0.4242 nm, which is relatively close to 0.446 nm of NbN. Therefore, the NbN on TiN buffer layer epitaxially grow with (100) direction. The junctions consist of epitaxial NbN/AlN/NbN tunnel junctions using TiN buffer layer fabricated on single-crystal Si(100) substrates. The TiN, NbN and AlN films are prepared by DC magnetron sputtering in an ultra-high vacuum load-lock sputtering system. The NbN/AlN/NbN/TiN trilayer shows a single-crystal structure with (200) orientation without other orientations. The gap voltage and the ratio of R_sg/R_N were about 4.9 mV and 30 for the junctions with the Jc of 8 A/cm², respectively.

[1] R. Sun et al. IEEE Tran. on Appl. Supercon., 25 1101204 (2015).

This work is supported by a Grant-in-Aid for Scientific Research C (No. 26420334) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

We have developed NbN/AlN/NbN tunnel junctions on Si substrate with TiN buffer layers for the usage of qubits or large scale integrated circuit. For the large lattice mismatching between Si and NbN, it’s hard to fabricate junctions consist of NbN/AlN/NbN films with good crystallinity. So we introduced TiN as buffer layers to improve the quality of junctions on Si. The TiN buffer layers were deposited with magnetron sputtering method and XRD results revealed that the buffer layers had high (200) orientation and good crystalline structure. Tri-layers with the structure of NbN/AlN/NbN were deposited on the TiN buffer layers. Then films were processed by photolithography and etching into into junctions. I-V curve of the junctions were measured and the results exhibited Josephson effect.

This work was Supported by the "Strategic Priority Research Program (B)" of the Chinese Academy of Sciences (Grant No: XDB04010400 and XDB04030000).

Ion-irradiating technology for high temperature superconducting integrated circuits is a very promising approach for many implementations because of possibility to design the schemes with a high density of arbitrary situated Josephson elements. Recently developed in our group, a technological process [1] demonstrated ability to produce high quality Josephson junctions, particularly suitable for mixing and heterodyne detecting in sub-THz and THz frequency range [2,3]. One of well known problems for many potential applications of HTS Josephson junctions is their excessive noise level (and consequently broad generation linewidth) which are often higher than the ones theoretically expected [4,5]. Unlike the other types of HTS junctions, which noise characteristics were extensively investigated previously, there is lack of experimental as well as theoretical results reported on the ion-irradiated devices.
We present the results of our experimental and numerical investigation on noise characteristics of ion-irradiated Josephson junctions and arrays and discuss the results in the context of their potential implementation for heterodyne THz detecting.
[1] N Bergeal et al. “Using ion irradiation to make high-Tc Josephson junctions”, J. Appl. Phys. 102, 083903 (2007)
[2] M Manlou et al. “Toward terahertz heterodyne detection with superconducting Josephson junctions”, Appl. Phys. Lett. 101, 233505 (2012)
[3] M Manlou et al. “Josephson mixers for terahertz detection”, J. Appl. Phys. 116, 074505 (2014)
[4] Yu Divin et al. “Linewidth of Josephson oscillations in YBa2Cu3O7-x step edge grain boundary junctions”, Future directions in nonlinear dynamics in physical and biological systems, NATO ASI Series, 312, pp 343-346 (1993)
[5] G Kunkel et al. “Millimeter-wave radiation in high-Tc Josephson junctions”, IEEE Trans. Appl. Supercond., v. 7, 3339-3342 (1997)

MoRe-Si(W)-MoRe Josephson junctions with a resonance-percolation charge transport (see [1] about this) have been developed, fabricated and investigated, (here Si(W) is a silicon layer of thicknesses up to 30 nm doped by tungsten with 4 - 12 at.%.).
We have studied low-temperature (T=4.2 K) current-voltage characteristics of the junctions and revealed two novel important features, comparatively high characteristic voltages V_c=I_cR_n (I_c is the critical current of a Josephson junction and R_n is the normal-state resistance of the junction) up to 5 meV and even more which exceed theoretical estimates for a single Josephson junction and well-resolved Shapiro steps at voltage biases V_n= (h/2e)nf (n is an integer) in the presence of external microwave irradiation with the frequency f in the range from 0.5 to 20 GHz. Unusually high values of V_c and V_n in some samples can be explained by the presence of tens of junctions in series. We propose a simple theoretical formalism for charge transport across a set of resonance-percolating trajectories inside a nanometer-thick semiconducting films which is based on a distribution function of the transmission coefficient across a doped semiconducting interlayer.
The emergence of novel functionalities due to the disorder in doped nanometer-thick semiconducting films makes it possible to realize a trilayered junction with enhanced conductance properties and, at the same time, well separated metallic electrodes (see also [2]). We believe that it enables their use for various superconductive electronics applications including voltage standards.

[1] I.M. Lifshitz and V.Ya.Kirpichenkov.Sov.Phys.JETP50, 499 (1979).
[2] A.L. Gudkov, M.Yu. Kupriyanov, and A.N. Samus'. JETP114, 818 (2012).

We present the first results of an extensive NbN fabrication campaign by DC reactive magnetron sputtering carried out at the Institut d'Electronique Fondamentale (IEF, Orsay, France). Tens of samples have been produced varying deposition parameters such as N₂ concentration, total pressure and cathode polarization in our sputtering bench, in such a way to obtain superconducting NbN with extremely high normal state resistivity, up to 15000 μΩcm. Here we discuss the repeatability of sample production on large (4") substrates, and the characterization techniques in DC and RF that we have used (and that we are further developing). A prospect for the use of these thin (~tens of nm) and thick (~ hundreds of nm) metal films is also given.

A short normal metal (N) in between two superconducting (S) electrodes constitutes a proximity Josephson junction, a device capable of sustaining a dissipationless supercurrent. By means of a versatile nanofabrication technique based on a suspended metallic mask, we obtain very short (200 nm) Nb-Au-Nb junctions displaying a very high critical current (≈ 100 μA). The use of high-quality evaporated Nb thin films (T_c ≈ 8.5 K) makes the Josephson coupling observable up to above 6 K. The junctions are perfectly non-hysteretic down to about 1 K and hysteretic below, which makes them interesting in terms of detector applications. We analyze the experimental temperature-dependence of the critical current in terms of the device's Thouless energy and Nb-Au contact transparency. Further, a perpendicular magnetic field results in a Fraunhofer-like modulation of the critical current, which can be observed here in a single proximity Josephson junction due to the small length of N. Finally, a microwave (1÷26 GHz) signal coupled to the circuit induces a step-like behavior in the I-V curves (Shapiro steps), at integer and even fractional multiples of the characteristic voltage V=hf/2e. At low temperatures, the suppression of low-order Shapiro steps is evidenced and discussed.

We acknowledge the financial support from the ANR contract "Nanoquartets". Samples were fabricated at the Nanofab platform at CNRS, Grenoble. We thank A. Nabet and D. van Zanten for the help in the experiments.

The intrinsic Josephson junctions (IJJs) of Bi2Sr2CaCu2Oy (Bi2212) cuprate superconductors have been extensively investigated both theoretically and experimentally. In particular, the recent discovery of the macroscopic quantum tunneling (MQT) for IJJs in Bi2212 occurring below ~1 K motivates many studies on the complex phase escape in the multi-junction systems such as IJJs. Among them, a MQT-like behavior for the second switch (2nd SW), that is, a switch from the first resistive state to the second resistive state in the multiple-branched I-V curves, has been an interesting issue, since it was observed below a much higher temperature (roughly, ~10 K) than that for the 1st SW. If the MQT phenomena for the 2nd and higher order switches in IJJs were established, the mechanism of MQT phenomena in the multi-junction systems will be understood more deeply and a way to realize the implementation of quantum bits utilizing IJJs will be opend.
In this work, we study on the dissipation effects of the Josephson phase in higher order switching events, by considering multiple-retrapping processes after the escape events in the moderately damped regime. Small stacks of IJJs build in a narrow bridge of Bi2212 single crystal were fabricated by using focused ion beam (FIB) techniques. We found that the switching rate for the 2nd or 3rd SW as a function of bias current showed a slight convex curvature in a lower bias current and higher temperature region, in contrast to that for the 1st SW. Our analyses strongly suggest that this feature was successfully explained by the multiple-retrapping effects for the higher order switches. Thus, we conclude that the multiple-retrapping effects are quite important for higher order switching events, and that the finite dissipation after the escape events contributes to the phase retrapping rather than the self-heating.

This work was partly supported by Grant-in-Aid for Scientific Research from
JSPS and MEXT of Japan (Grant No. 21560020, 24560017) and
MEXT-Supported Program for the Strategic Research Foundation at Private Universities
(2013-2017).

MgB₂/BN/MgB₂ trilayers have been fabricated by using hybrid physical-chemical vapor deposition (HPCVD) method for the MgB₂ layers and chemical vapor deposition (CVD) method for the BN layers in the same reactor. The elemental composition and atomic chemical state of the films were determined by X-ray photoelectron spectroscopy (XPS). The surface morphology and the cross-section were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM) respectively. Resistivity and magnetization measurements indicated that the superconductivity of the MgB₂ films has not been deteriorated after depositing the BN films. Our results show that it is feasible to grow MgB₂/BN/MgB₂ trilayers in the same reactor sequentially, which has the advantage of reducing contamination during the growth. This therefore opens the door for fabricating all-MgB₂ Josephson junctions by using the BN film as the insulating layer, which is currently pursuing in our group and preliminary results will also be presented.

We would like to thank the support of the Ministry of Science and Technology of China under 973 Program 2011CBA00100

It has been frequently observed, that even at very low temperatures the number of quasiparticles above the superconducting gap is higher than predicted by standard BCS-theory. These quasiparticles can interact with the coherent degrees of freedom of two-level systems, such as superconducting qubits or two-level fluctuators in the amorphous oxide layer of a Josephson junction. The interaction of these systems with quasiparticles leads to decay and decoherence, with specific results, such as the time dependence, depending on the form of the interaction. We study various forms of the interaction, incl. quasiparticle tunneling and scattering, and the resulting decay laws.

We acknowledge support of the DFG under Grant No. SCHO 287/7-1 and GIF under Grant No: 1183/2011.

This work was supported by TUBITAK under the project number 114F165.