FastHenry has proven to be a powerful numerical engine with which to calculate inductance in superconducting structures. The filaments model used by FastHenry makes it notoriously difficult to model uneven multidirectional current flow along arbitrarily curved structures. InductEx can model complex 3D structures for FastHenry, but the limitations imposed by rectangular uniaxial filaments can result in very inefficient models. We have developed a new 3D numerical engine which we call TetraHenry, and which uses tetrahedral volume elements to discretize complex geometries. Volume Loop basis functions are used to discretize the volume integral equation to obtain the Method of Moments matrix equation. The Fast Multipole Method used in FastHenry has been modified to support the conventional SWG basis functions. We demonstrate the validity and capabilities of this method by extracting the inductance from several complex superconducting structures and comparing the results to those obtained with FastHenry.

This work was supported financially by the South African National Research Foundation, grant number 78789.

Adiabatic quantum-flux-parametron (AQFP) [1] is one of the most promising devices as a candidate to realize a practical reversible computer, because of its extremely high energy efficiency. In previous work [2], we proposed a logically and physically reversible logic gate using AQFP logic and numerically demonstrated reversible computing with an assumption that thermal noise does not induce non-adiabatic state changes while switching. However, it is not well understood yet how the phase difference of a Josephson junction in AQFP logic behaves at finite temperature and if thermal noise makes difference on energy dissipation. Here we study energy dissipation in AQFP logic at finite temperature through numerical analyses using the Monte Carlo method. We investigate the average and deviation of the energy dissipation in both numerical analyses and theoretical estimations. Finally, we discuss the minimum energy dissipation required for adiabatic switching operations.
References:
[1] N. Takeuchi, D. Ozawa, Y. Yamanashi, and N. Yoshikawa, Supercond. Sci. Technol. 26, 035010 (2013).
[2] N. Takeuchi, Y. Yamanashi, and N. Yoshikawa, Sci. Rep. 4, 6354 (2014).

The present study was supported by a Grant-in-Aid for Scientific Research (S) (No. 26220904) from the Japan Society for the Promotion of Science (JSPS).

Adiabatic quantum-flux-parametron (AQFP) logic is one kind of novel superconductor logic offering extremely high energy efficiency for building high-performance computing systems. An 8-bit AQFP-based carry-lookahead adder with more than 1000 Josephson junctions has already been demonstrated. However, the lack of logic-level simulation tools is an obstacle for designing large-scale AQFP integrated circuits. In this paper, we present a logic simulation model for AQFP logic. We made a functional model based on finite-state machine approach using a hardware description language, which enables the simulation of large-scale AQFP circuits using commercially available logic simulation tools. We have developed a library for logic simulation and designed a relatively complex AQFP processor, which is composed of thousands of logic gates. We also include timing and switching-energy information in our logic simulation models for timing analysis and energy estimation. Since the library is based on a parameterized approach, it can be easily modified for different fabrication technologies and low-level circuit parameters.

This work was supported by a Grant-in-Aid for Scientific Research (S) (No. 26220904) from the Japan Society for the Promotion of Science (JSPS).

We studied Shapiro steps induced by rf current applied to an underdamped Josephson junction at resonance frequency, when the resonance is provided by the in parallel connected junction capacitance and external LC circuit. As far as the resonance excitation of the pulse component of Josephson phase yields from both the force impact of the rf current and impact of Josephson oscillation process including parametric mechanism as well, their competition determines specific character of the phase-locking manifestation. In particular, this competition is responsible for substantial change in amplitude of the pulse component within Shapiro step and hence asymmetric position of the step in IV curve of the junction. Analytic theory is developed and compared with results of numerical simulation.

A SQUID is a high sensitive magnetic sensor. The SQUID has been widely studied for various applications. In recent, the research and development of HTS SQUID is in progress. The HTS SQUID has an important advantage that it is easy to use and it can operate at the temperature of liquid nitrogen. On the other hand, maximum sensitivity is inferior as compared with an LTS SQUID. Additionally, it has a tendency that the line width of the HTS SQUID is wider than LTS SQUID. Due to the width, it is considered that a common simulation method is unsuitable for HTS SQUID simulation. In this study, we have been proposed a new simulation method suitable for the HTS SQUID. In our study, the FEM is employed to simulate the electromagnetic behavior of the HTS SQUID and coupled with the equivalent circuit of the Josephson junction to consider the quantum behavior. By using the proposed simulation method, the numerical simulation of HTS SQUID is performed taking into account both of the electromagnetic and quantum behavior. In the paper, the characteristic of SQUID that is affected by the amount of the external magnetic flux are shown.

Information transfer in single-flux quantum logic circuits is based on the exchange of transient voltage pulses between individual cells. This particular feature stimulates to consider the adaptation of biologically inspired computing paradigms. We propose ways of constructing digital processing elements using superconductive electronic circuits. Besides architectural aspects, cells and their basic behavior are described and illustrated. The possible computational models are discussed.

We have numerically investigated the self-excited motion of vortices and the self-oscillation of the magnetic moment in current-carrying superconducting nanostrips with holes. A superconducting nanostrips has a circular hole whose center is shifted from the center of a superconducting nanostrip, and therefore, a nanostrip has two bridges (i.e, a narrower bridge and a wider bridge) at the edges of a hole. The self-oscillation is induced by a dc transport current that is larger than the critical current with no applied magnetic field. The self-excited motion of vortices proceeds as follows: (1) a vortex is generated at a narrower bridge in a nanostrip and the vortex exit from a strip, (2) an antivortex is trapped in a hole and a circular current flows around the hole, (3) the circular current induce an antivortex at a wider bridge and the antivortex exit from a strip, (4) the trapped vortex in the hole is zero and the situation return to the process (1). The self-excited motion of vortices results in the coherent self-oscillation of the magnetic moment of a superconducting nanostrip.
On the basis of the time-dependent Ginzburg-Landau equations, we have numerically investigated the motion of vortices and self-oscillation in superconducting nanostrip. The time scale of the vortex motion is on the order of 1–10 ps, and the frequency of the self-oscillation is on the order of 0.1–1 THz. A possible application of the self-oscillation phenomena to a THz oscillator will be discussed.

We thank stimulating discussion with H. Asai, S. Kawabata, and S. Kohjiro. This work has been supported by JSPS KAKENHI grant (No. 25420350).

We investigated the dependence on the circuit parameters of the operation of a single-bit comparator for currents using a quasi-one-junction superconducting quantum interference device (QOS). A single-bit comparator using a QOS has high sensitivity for probing currents flowing through a readout circuit of multiple superconducting detectors. We demonstrated that the QOS comparators have high current sensitivity and wide operating margin of bias currents in our preceding study [1]. However, it is quite difficult to design the QOS comparators suitable for the readout applications due to complicated procedures in analyses. We conducted numerical analyses of the operation of the QOS comparators in order to obtain a guideline in designing the QOS comparators. We used JSIM_N for simulations of the QOS comparators and took into account noise in investigation of the operation. We assume that the critical current density of a Josephson junction (JJ) is supposed to be 2.5 kA/cm² in AIST standard process 2 (STP2). Our QOS comparator is composed of three Nb/AlO_x/Nb Josephson junctions (JJs) with the critical currents I_c of 40 µA, 45 µA, and 70 µA, respectively. A shunt resistor R_s is connected in parallel with each JJ and the I_cR_s product is 373 µV. Josephson transmission lines (JTLs) are connected both in the front side and the back side of a QOS comparator. A DC-to-SFQ converter is used as an input circuit of SFQ pulses for the QOS comparator assuming the practical circuits. An isolated bias line is connected to a QOS comparator to adjust the operating points. We found that the loop inductances in a QOS comparator affect the operation significantly. The circuit parameters around a QOS comparator can also affect the operation.
[1] S. Miyajima, et al., Jpn. J. Appl. Phys., 52, 033101, 2013

This work is partially supported by Grant-in-Aid for Young Scientists (B) (No. 26820130) from JSPS, and by VLSI Design and Education Center (VDEC), the University of Tokyo with the collaboration with Cadence Corporation.

The charge and spin transport through multi-terminal structures involving one (or more) superconducting leads is currently attracting a considerable amount of interests. Conversion of the unpaired electrons into the Cooper pairs or an opposite process of the Cooper pair splitting into individual electrons produce the spatially entangled charge carriers and can induce the exceptional nonlocal transport effects. One of peculiar effects is due to the crossed Andreev reflections, capable to induce the sign change of the nonlocal conductance/resistance. This effect has been already observed in three-terminal junctions, composed of two conducting (or magnetic) electrodes interconnected through a piece of the superconducting sample.
Here we consider a different three-terminal configuration, where two conducting leads are coupled with another superconducting reservoir through the quantum dot. Via the proximity effect the quantum dot
absorbs electron pairing and is converted into, a kind of, nanoscopic superconducting grain. For comparable couplings to external leads the nonlocal transport is of the same order as the local one. Furthermore, such nonlocal transport should be tunable experimentally in contrast with the three-terminal planar junctions, where it is usually 100 times weaker than the local one. For strong enough coupling to superconducting electrode the nonlocal resistance may undergo the sign-change (from negative to positive values) upon varying the applied bias. These sign-changes might eventually depend also on the electron correlations. We are going to describe several realistic systems (with such quantum dots as e.g. carbon nanotubes, semiconducting nanowires , self-assembled InAs nonoscopic islands) where the tunable sign-change of the nonlocal resistance can be fulfilled.

This work is supported by the National Science Centre under the contracts
DEC-2014/13/B/ST3/04451 (TD, KIW) and DEC-2012/05/B/ST3/03208 (GM, BRB).

Bi-SQUID is a well-known device for highly linear conversion of magnetic field into voltage. The device has already been realized using niobium fabrication process. A high-temperature superconductor (HTS) amplifier is now deemed as one of the highly demanded applications of the circuit. We analyze and discuss design issues of such a HTS bi-SQUID with bi-crystal Josephson junctions. As far as the junction fabrication technique implies the using of a bi-crystal thin film formed on a bi-crystal substrate, this puts rigid restrictions on possible design of the one-layer circuit. In fact, all Josephson junctions are bridges over the same bi-crystal boundary and therefore the bi-SQUID circuit cannot be realized without additional junction in one of its loops (or in the both). The additional junction is a bottom of some distortion of the voltage response linearity. Possible tradeoff for the circuit design is considered.

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

Magnetization and magnetic relaxation are specific to each material and these properties are useful for non-destructive evaluation and biochemical measurement in addition to basic characteristic evaluation of material. Thus, a measurement system for the evaluation of these magnetic properties, which is suitable for practical use, is required. In the previous study, we have developed a compact and highly sensitive rotating-sample magnetometer using high-Tc superconductor superconducting quantum interference device (HTS-SQUID) that can measure both magnetization and magnetic relaxation. Although this developed system could successfully detect a small magnetic signal from a sample, such as water, the magnetic signal was detected by a one detection coil. Therefore, the position of detection coil should be changed when the magnetic signal of magnetization or that at different time after magnetization is measured. This measurement method causes the difference of distance between the detection coil and a sample leading to a fluctuation of magnetic signal intensity. In addition, this method needs much measurement time for changing the detection coil position. For this reason, HTS-SQUID based rotating-sample magnetometer using multiple detection coils was developed in this study. The developed system consists of a rotating sample component, permanent magnets, eight detection coils directly coupled to an input coil and inductively coupled to a HTS-SQUID. The detection coil was fixed at position in the circle described by a rotating-sample with respect to each 45 degree and the coil that is used for measurement is selected by switching operation. One coil was used for the measurement of magnetization placed between the pole of permanent magnets and the other coils were used for the measurement of magnetic relaxation. To evaluate the basic characteristics of the developed system, a paramagnetic material of MnF2 which has sufficient magnetic signal intensity was measured. The magnetic signal could be detected at all detection coils and the magnetic signal intensity during magnetic relaxation decreased with increasing the time after magnetization. Therefore, the developed system can realize a fast and precision measurement of magnetization and magnetic relaxation.

This work is supported by the “Strategic Promotion of Innovative R&D” program funded by the Japan Science and Technology Agency (JST).

Despite the fact that active electrically small antenna based on Superconducting Quantum Array forms output voltage which is independent of the antenna size to wavelength ratio, a small size effect can appear and limit antenna linearity with the ratio increase. We analyze and discuss impact of the size effect on the antennas of both the transformer and transformer-free types. In the case of transformer antenna, this effect can be associated with a finite phase delay of the input signals coming to the cells which are coupled to opposite sides of the superconducting transformer. This delay is substantial for differential network. In transformer-less antenna, every quantum cell operates independently with input flux applied directly to the cell area. If the quantum cell has only one main loop as in the case of bi-SQUID, the size effect is negligibly small. However, this effect can not be neglected in cases of the differentially connected cells and the cells containing parallel arrays of Josephson junctions. Owing to the size effect, the flux distribution along the array-based cell can get an asymmetry which noticeably affects the linearity characteristics despite smallness of the asymmetry.

The author Xu Jie gratefully acknowledge financial support from China Scholarship Council.

We have developed robust HTS-SQUIDs for a sensor-mobile practical NDE system, which can be operated in unshielded environments. To take advantage of the HTS-SQUID’s high sensitivity in low-frequency range to detect deeper defects, a HTS-SQUID magnetometer is more suitable for the system than a HTS-SQUID gradiometer. Furthermore, it is necessary that a HTS-SQUID magnetometer should avoid both having flux trapping in its HTS film and change of critical current of the Josephson junctions in the SQUID due to the Earth’s field to realize stable operation and move of the SQUID in unshielded environments. In this study, we designed a HTS-SQUID magnetometer based on bicrystal SrTiO₃ (STO) substrate and YBa₂Cu₃O_7-x (YBCO) film, which have mesh structure with 5 µm width in the film to avoid the flux trapping in the Earth’s field. An alternative HTS film with cross shape was designed to cover the SQUID ring of the magnetometer in a flip-chip configuration in order to suppress the change of the critical current of the SQUID ring due to coupling flux by means of the Meissner effect of the HTS film. By adopting an active magnetic shielding scheme with a feedback circuit to cancel the Earth’s field coupling to the magnetometer, we developed a sensor-mobile NDE system integrating the covered HTS-SQUID magnetometer with a robot arm. Using the system, detection of deep-lying defects in aluminum alloy sample was demonstrated utilizing an eddy current technique while moving the magnetometer on the robot arm in an unshielded environment. Comparison with detection of the same kind using a HTS-SQUID gradiometer, which have the equivalent effective area with the magnetometer, revealed that the magnetometer showed better capability to detect deeper defects in conductive materials than the gradiometer.

The authors wish to acknowledge Dr. Keiichi Tanabe of ISTEC/SRL for offering the HTS-SQUID gradiometer.

Currently, one of the critical issues in the superconducting quantum interference device (SQUID)-based ultralow-field nuclear magnetic resonance/magnetic resonance imaging (ULF-NMR/MRI) is to increase a signal-to-noise ratio (SNR). A common way to obtain the high SNR is to utilize a strong pre-polarization field (B_p) in a moment. However, increasing the B_p strength has also yielded unwanted effects, such as flux trapping in the superconducting pickup coil and the induction of eddy currents on the walls of a magnetically shielded room. These side effects can deteriorate the NMR/MRI signals. An alternate method to increase the SNR is to utilize a dynamic nuclear polarization (DNP) method. The enhanced NMR/MRI signal can be achieved by the DNP technique. We performed a series DNP experiments by using the SQUID-based ULF-NMR/MRI system at less than Earth’s magnetic field. Experimental results showed that the DNP spectra for 2mM TEMPOL solution were nearly unchanged in those regimes and were composed of two peaks corresponding to the overlap of two main transitions with opposing enhancement factors. Here, we also introduced new method to remove the need for B_p. In combination with a circularly polarized pulse technique that enables us to make a precise nutation of a nuclear spin at the micro-tesla range field, we obtained the DNP-MR image without B_p at 34.5 μT.

We demonstrate a proton spin echo magnetometer, in which the interrogation time is not limited by T^*₂ and can be prolonged to T₂. Therefore, even under a severe field gradient, the precision of the measurement does not degrade. We devised a phase linearization method that enables accurate estimation of the precession frequency from a spin-echo train. With proton spins in deoxygenated tetrametyl silane (TMS) and a superconducting quantum interference device (SQUID)-detected NMR system at KRISS, an average field of around 5 μT was measured with an uncertainty of 0.42 nT in the presence of a field gradient of 12.8 μT/m. This implies that our system tolerated a 25 % variation in magnetic field over the sample area. The proton spin-echo magnetometer will be useful in measuring magnetic fields without compensating for residual field gradients.

This work was supported by World Class Laboratory (WCL) grant from Korea Research Institute of Standard and Science (KRISS).

SQUIDs are highly suited to investigate the magnetic properties of samples with small dimensions, such as nanoparticles, or to read out nano-electromechanical systems (NEMS). For this purpose we have developed different types of nanoSQUIDs based on Nb/HfTi/Nb junctions having a lateral size of about 200 nm x 200 nm. The normal conducting HfTi barrier of the junctions has a thickness of 30 nm. The nanoSQUIDs were designed as gradiometers, with an inner loop diameter of about 840 nm. Implemented gradiometric feedback loops, gradiometric transformers and rf filters enhance their practical use. At 4.2 K these devices are hysteresis-free and the white noise level is about 200 nΦ0/√Hz for series nanoSQUID gradiometers, or 115 nΦ0/√Hz for parallel nanoSQUID gradiometers.
When SQUIDs are used, for example for the characterisation of nanoparticles or the read out of NEMS, they need to be operated in high magnetic fields. Therefore, we characterise the nanoSQUIDs in high ac and dc magnetic fields. In ac magnetic fields of up to a few mT, which is orientated either perpendicular or in-plane to the gradiometric SQUID loop, the SQUIDs can be operated in the flux locked loop mode with a high level of linearity. For these measurements the nanoSQUIDs are read out in a two-stage configuration with a SQUID series array acting as a low-noise preamplifier. For the readout of NEMS, the field tolerance to a dc in-plane magnetic field is of considerable interest. We investigated for example the V-Phi-characteristic of these nanoSQUIDs versus different field strengths up to a few tens of mT. These measurements were performed in both single-stage and two-stage configuration.

This work was partly supported by the DFG under Grant No. SCHU1950/5-1 and within the European Metrology Research Programme EMRP ‘MetNEMS’ NEW-08, which is jointly funded by the participating countries within EURAMET and the European Union.

During the last years, there are continuous efforts to improve the magnetic field noise spectral density of SQUID magnetometer to meet the requirements for future demanding applications like e.g. in geophysics or ultra-low field magnetic resonance imaging. In order to e.g. increase the signal-to-noise ratio for the latter scenario, one needs to lower the system noise and to increase the strength of the prepolarizating field Bp.
This work therefore focuses on the further noise-improvement of SQUID magnetometer by using ultra-low noise SQUID current sensors fabricated in our recently presented cross-type Josephson junction technology as well as on all thin-film niobium-based pickup coils. Experimental results on such devices show exceptionally low magnetic field noise. Provisions to make these devices suitable for the above discussed in-field applications are discussed.

We are proposing a new device, called a Double-SQUID, whose function is similar to a SQUID, but where one of the junctions (or both) is replaced by a smaller SQUID with its own flux coupling circuit. The smaller SQUID is used as a Josephson junction with tunable critical current. The device voltage modulation by the magnetic field can be maximized when the critical current of the smaller SQUID is made equal to the critical current Ic3 of the single Josephson junction. This may be useful for high Tc superconducting (HTS) Josephson junction technology, which shows large scattering in the characteristic parameters of the devices. In this arrangement, the critical current of the junctions are only required to satisfy the triangular relation |Ic1-Ic2|<=Ic3<=Ic1+Ic2. Accounting for the scattering of the critical currents, we define the domain of nominal values for Ic1/Ic3 and Ic2/Ic3 where the critical currents for the single junction and smaller SQUID can be made equal by tuning the magnetic flux in the smaller SQUID.
A direct injection scheme has been chosen for tuning the smaller SQUID. The devices have been designed for the ion damage barrier technology and fabricated. Results will be presented and discussed.

Some studies try to correlate seismic events of Earth to other physical phenomena that can be observed beforehand to be used as precursors of earthquakes. One of such physical phenomena is the magnetic field of the Earth. Currently the INTERMAGNET magnetic field observatory network that consists of about 150 nodes spread all around the world is based on magnetometers with magnetic field sensitivity of 1 nT with a 1 Hz sampling rate. So far, there is no solid report which demonstrates that the variations of the magnetic field of the Earth can be used as a precursor for earthquakes. One of the main reasons for this may be that the sensitivity levels and/or sampling rates of the observatories are not adequate. For much better sensitivity levels (<1 pT) and sampling rates (>100 Hz), SQUID-based systems are natural candidates. Such systems are already installed in the Low Background Noise Underground Laboratory (LSBB) in France and at the South African National Space Agency (SANSA) in South Africa. The systems are based on LTS SQUIDs cooled with liquid helium and HTS SQUIDs cooled with liquid nitrogen, respectively. We plan to install a cryogen free, mobile SQUID-based magnetic observatory in Turkey which will be networked with the available SQUID systems at a later stage. Hence, with the increased number of SQUID nodes, it will be possible to extract more meaningful information related to seismic events by correlating the data from many stations. In this work, we present the investigation results of several sites for the future installation of a SQUID-based magnetic observatory. This was done by using a fluxgate magnetometer with reference noise levels to the LSBB station in France and to the IZN INTERMAGNET node in Turkey.

SQUID can greatly improve the exploration depth of Transient Electromagnetic Method (TEM) for its frequency independent high sensitivity. However, the later signal of TEM (tens of fT), which corresponds to deeper information, is often polluted by the earth magnetic field fluctuation (usually several nT). Stacking can decrease this influence but exploration efficiency is also decreased.
In this paper, we bring out a new method based on two SQUID receivers. One SQUID is placed in the center of the transmitting loop while the other SQUID sensor is placed about 1km far away from the first sensor, where the transmitting field can be neglected. The two SQUID receivers is GPS synchronized and the data is combined to compensate the earth magnetic field fluctuation.
In our field test, a 100*100 transmitting loop is used and two SQUID receivers is applied, the exploration efficiency is greatly improved ( stacking number decrease 2 times to get same SNR decay curves).

We present simulations of rapid single flux quantum (RSFQ) circuits with the aim of characterising and fabricating a superconducting Josephson comparator circuit. Superconducting RSFQ circuits offer very high speed (order of 100 GHz) as a result of the flux quanta being transmitted by picosecond-duration voltage pulses. The devices are inherently low-power dissipation due to operation in the superconducting state. A Josephson comparator can be configured as a current amplifier for application in high resolution electrical measurements at cryogenic temperatures, with intended application in quantum electrical standards. The Josephson comparator is one of the building blocks of RSFQ electronics and is used as a fast analog-to-digital converter. When an SFQ pulse V(t) enters the decision element, a 2π phase difference is induced across the comparator. In order for the comparator to return to its initial state one of the two Josephson junctions in the comparator must switch. The threshold current, Ix, inserted between the two junctions determines which of the junctions will switch. There exists a region of Ix over which the switching probability is determined by a sum of the switching events of both of the junctions – this region of uncertainty is defined as the grey zone. The grey zone dependence on bias currents and loop inductance is discussed. We also present simulations of the comparator subject to the addition of thermal noise at typical measuring temperatures, e.g. 4.2 K.
Superconducting nanobridges are used as the Josephson junction in our devices. Fabrication of individual nanobridges has been carried out using both niobium sputtering and aluminium deposition techniques. Sub-50 nm junction sizes have been achieved. Preliminary measurements of IV characteristics of the nanobridges have been performed.

We gratefully acknowledge A. Blois for the niobium device fabrication. This work was supported by the UK NMS Electromagnetics and Time Programme, and a UCL Impact Studentship.

Superconducting mesa-structures fabricated from metal-oxide YBa₂Cu₃O_x, composite ferromagnet La_0.7Sr_0.3MnO₃/SrRuO₃ (LSMO/SRO), and Au/Nb counter electrode were studied experimentally. It was shown that in our heterostructures the composite LSMO/SRO ferromagnet exhibits a noncollinear magnetic state which is important for generation of spin-triplet component of the superconducting order parameter. Superconducting current was observed at magnetic fields H raised up to 2000 Oe which is greater than saturation field of manganite LSMO (less than 300 Oe) and by two orders greater than the value of magnetic field corresponding to penetration of one magnetic flux quantum. Microwave measurements of integer and half-integer Shapiro steps under a few fixed levels of relatively low external magnetic field H<30 Oe showed that the second harmonic of current-phase relation reaches the 50% of the first one. Fourier analysis of I_C(H) dependencies allow us extracting the components of fractional periods in I_C(H) function that also confirm the deviation from the sinusoidal current-phase relation. The obtained experimental data are discussed taking into account existing theoretical models which predict a huge enhancement of the second harmonic of spin-triplet component of superconducting current, particularly due to changing by external magnetic field the directions of magnetization in composite bi-layer ferromagnet inserted between two spin-singlet superconductors.

The work was supported by Russian Academy of Science, Scientific School Grant SH-4871.2014.2, RFBR project 14-07-00258а, 14-07-93105.

Since the discovery of the first Fe-based superconductors in 2008, extensive effort has been directed to understand the novel properties of these exotic materials, in particular the symmetry pairing. Josephson junctions offer ways to investigate the fundamental properties of iron pnictides. To prepare hybrid Josephson junction along the crystallographic c-axis a newly developed surface polishing as well as standard thin film technologies are used. We use single crystals of Ba(Fe_1-xCo_x)₂As₂ prepared by a self-flux method as a base electrode. We tested different junction barriers, including normal metals (Au, Ti), insulating material (SiO₂) as well as subsequently oxidized thin metal films (Ti, Al). A conventional superconductor was used as counter electrode. For the characterization of the Josephson effects we present temperature dependent I‑V characteristics which may be fitted within the RCSJ‑model. Interesting I_cR_N - T dependencies suggest a possible 0-π transition, which may imply the symmetry of the order parameter in the pnictide electrode. Additionally, we show the variation of the critical current as a function of the applied magnetic field and the formation of Shapiro steps under microwave radiation.

We thankfully acknowledge financial support by the DFG within SPP 1458 (project. no. SE664/15-2), the EC within project IRON-SEA (project no. FP7-283141), the German Academic Exchange Service and the Landesgraduiertenförderung Thüringen.

Josephson junctions containing normal (N) and ferromagnetic (F) materials in the weak link region are currently the subject of intense research. The interest in such structures is due to the possibility of their use as a control elements of superconductor memory compatible with the RSFQ logic. At present there are many implementations of such controls. Among them, the tunnel structures containing one ferromagnetic layer in the weak link region, are of greatest interest. Anisotropy of their properties necessary for operation of the cell is achieved in such devices by complicating the structure of the weak-coupling area.
In this work we study theoretically the properties of S-F/N-sIS tunnel devices in the frame of the quasiclassical Usadel formalism. We assume that the scale of the structure is much less than the Josephson penetration depth. We demonstrate that in the S-F/N-sIS device it is possible to achieve its separation into two regions which have positive (0 segment) and negative (π segment) critical current densities. We prove by numerical calculations that this separation is accompanied by a new phenomenon, namely the violation of a spatial uniformity of the superconductive film and its decomposition into domains with an order parameter phase difference equal to π. The effect is sensitive to thickness of the s layer and widths of F and N films in the direction along the sIS interface. We found that in the crossover regime the S-F/N-sIS device may have two metastable states and can be switched between these states by current induced in the s film. Finally, the state of the system can be nondestructively read by current injected through the S electrodes.

This work was supported by the Ministry of Education and Science of the Russian Federation (14Y26.31.0007, MK-1841.2014.2), by the RFBR (14_02_90018_bel_a, 14_02_31002_mol_a, and 15_32_20362_mol_a_ved), by the Dynasty Foundation, and the Scholarship of the President of the Russian Federation

The superconducting proximity effect in the artificial layered ferromagnet-superconductor (FS) heterostructures leads to many striking effects [1,2], notably the re-entrant, periodically re-entrant and solitary superconductivity. Note the solitary superconductivity was firstly predicted for pure symmetric FSF trilayers [3,4]. In this work we investigate the more realistic case of dirty both S and F metals for asymmetrical F₁F₂S trilayer. In contrast to the standard approach to the FS proximity effect theories, we take into account the superconducting pairing interaction in ferromagnet layers. Actually, an electron-electron interaction in a ferromagnet is in existence, but it is suppressed by strong exchange field. It is known that under certain conditions at the antiparallel (AP) mutual orientation of the magnetizations, the effective exchange field of the F layers may be partially weakened. We show that in this case the pairing interaction can be revealed itself and lead to the solitary superconductivity. Note that, in this mode the critical temperature Tc has a strong dependence on mutual orientation of the magnetizations of the F layers.
We consider and analyse the model of the spin valve for asymmetrical trilayer F₁F₂S setup, basing on solutions of the Usadel-like equations and corresponding boundary value problem.
Changing varios parameters of F₁F₂S system (thicknesses of layers, boundary transparencies, angle between magnetizations, and so on), we show that (i) existence of the pairing interaction in a ferromagnet can essentially change the phase diagrams of FS systems; (ii) the regime of FS systems with the solitary superconductivity is more promising for spin valve applications.
[1] Izyumov Yu.A., Proshin Yu.N., Khusainov M.G. Physics-Uspekhi, 45, 109 (2002)
[2] Buzdin A. Rev. Mod. Phys., 77, 935 (2005)
[3] Khusainov M.G. et al.JETP Lett. 90, 359 (2009)
[4] Proshin Yu.N., Khusainov M.M., Minnullin A. Physica Status Solidi C, 11,1080 (2014)

The work is partially supported by Russian Government Program of Competitive Growth of Kazan Federal University. MA is also thankful to the RFBR (13-02-01202) for partial support.

We present progress in the fabrication of circuits containing superconducting nanowires. An inert-gas focused ion beam (FIB) is used to directly pattern nanowires with widths <20nm. Such nanowires can exhibit either a Josephson-type non-linearity [1] or quantum phase-slip [2] [3], depending on details such as the choice of superconductor. Parasitic two-level systems are generally present in superconducting circuits as a result of conventional fabrication and processing, and these lead to noise and loss of coherence in quantum systems [4]. Here we examine the use of a Ne focused ion beam to produce nanowires integrated into superconducting circuits with a minimised density of parasitic two-level systems.
[1] E. M. Levenson-Falk et al., Applied Physics Letters 98, 12 (2011)
[2] O. V. Astafiev et al., Nature 484, 7394 (2012)
[3] C. H. Webster & J. C. Fenton et al., Physical Review B 87, 14 (2013)
[4] J. Burnett et al., Nat. Commun. 5, 4119 (2014)

The authors gratefully acknowledge funding from the UK EPSRC, grant references EP/J017329/1 (JB and JCF) and EP/K024701/1 (JS and PAW), and Carl Zeiss SMT (JS and PAW).

The current-crowding cryotron (yTron) is a nanoscale three-terminal superconducting device which can be used as an inline nondestructive superconducting current sensor. The yTron uses the flow of current through its gate to modulate the critical current of its channel. The device was fabricated by etching a "Y"-shaped pattern in a single-layer thin film of niobium nitride. The channel and gate terminals are represented by the two upper arms of the Y, and the two terminals join at a sharp intersection which leads to a common ground. Each arm was 200 nm wide, merging to a common ground 400 nm wide. The gate current was able to modulate the channel by effectively redistributing the current flow around the intersection point of the terminals. When the gate was off, the channel current bent sharply around the intersection on its way to the common ground, causing a localized increase of current density (current crowding). Adding current to the gate, however, reduced the total current crowding at the intersection, which increased the critical current of the channel. The components of the gate current which were tangential to the intersection boundary (and thus produced current crowding) opposed those of the channel currents, resulting in a total superposition which had less current crowding at the intersection. As a result, by measuring the critical current of the channel (e.g. by current biasing until a hotspot forms), we were able to infer the magnitude of current flowing into the gate, all without significantly perturbing the current in the gate. We have demonstrated this indirect current measurement by using the yTron to nondestructively read out the number of discrete fluxons (n) trapped in a loop. We were able to successfully resolve adjacent fluxon states (n, n+1, etc), and readout those values several thousand times without changing n, demonstrating its potential use as a nondestructive superconducting memory readout element. This device has potential applications for superconducting sensors as a digital current sensor and amplifier, as well as readout for superconducting memories.

The authors would like to thank Faraz Najafi and Andrew Dane for helpful discussions. Additionally, they would like to thank James Daley and Mark Mondol of the MIT Nanostructures Laboratory (NSL) for technical support. This work was supported by the Air Force Office of Scientific Research (AFOSR).

Josephson junctions with ferromagnetic materials have a potential to be elements of high-density memories and enhance the performance of the superconducting integrated circuits. We focus on superconductor-ferromagnetic-insulator-superconductor (SFIS) Josephson junctions and studied the characteristics for fabricating memory devices. In this study, we evaluated uniformity of series of junctions, the critical current density (J_c) of junctions and the field response. They are important for circuit applications because they are strongly affected by the magnetic property of F layer.
Nb-PdNi-AlO_x-Nb SFIS Josephson junctions are fabricated and evaluated. PdNi with 11-at% Ni was used for the F layer and the thicknesses of the F layer ranged from 0 to 12 nm. AlO_x layers were formed on the surface of 7-nm-thick-Al layers by a 10-minute exposure to O₂ gas (2.0 Pa).
The standard derivations, 1σ spreads, of the critical-current (I_c) for 100 junctions (20 um□) and the characteristics of single junctions were evaluated at 4.2 K. The 1σ spreads ranged from 0.43% to 1.7%, which were much smaller than 2% required for integrated circuits. The field dependence of Ic of junctions (40 um□) fits a theoretical curve. Otherwise the J_c decreased with increasing the thickness of F layers. A junction without F layer and that with a 12-nm-thick F layer have J_c of 3.0 kA/cm² and 0.01 kA/cm², respectively. For circuit applications, 1 kA/cm² or higher J_c is required. The magnetization of an F layer and the shift of a field dependence were observed with a field (~1 mT) and thermal assistance (~150 K). We will consider the oxidation conditions and materials for an F layer to obtain higher J_c and a weaker F layer which respond to a weaker field.

This work is supported by ALCA-JST project entitled "Superconductor Electronic System Combined with Optics and Spintronics" and JSPS KAKENHI (26420306, and 26220904).

Background suppression plays a crucial role in experiments searching for rare events, like neutrino-less double beta decay (0nDBD) and Dark Matter interactions.
Bolometers, that are among the most competitive devices in this field, would largely benefit from the development of ultra-sensitive light detectors, as the combined read-out of the bolometric and light signals enables the particle identification.
The CALDER collaboration is developing cryogenic light detectors that will match the requirements of next generation experiments: noise lower than 20 eV, large active area, wide temperature range of operation, high radio-purity and ease in fabricating hundreds of channels. For this purpose, we are exploiting the superb energy resolution and the natural multiplexed read-out provided by Kinetic Inductance Detectors (KIDs). These devices can be operated in a phonon-mediated approach, in which KIDs are coupled to a large insulating substrates in order to increase the active surface from a few mm² to 5 cm².
We will explain the working principle of KIDs-based light detectors, present the results obtained with the first prototypes based on aluminum sensors, and discuss the impact of this project on the most advanced bolometric experiments searching for 0nDBD and Dark Matter interactions.

This work is presented on behalf of the CALDER collaboration. The CALDER experiment is funded by the European Research Council under the European Unions Seventh Framework Programme (FP7) / ERC grant agreement n. 335359

We have developed microwave kinetic inductance detector (MKID) array working on a compact He4 refrigerator (equilibrium temperature ~3 K) for Fourier transform terahertz spectroscopy. The target performance of each pixel is the frequency coverage of 1-5 THz, the response time of less than 100 micro-second and the noise equivalent power (NEP) of the order of 10^-14 [W/√Hz]. MKIDs using rectangular spiral resonators (spiral-MKIDs), whose resonant frequencies are around 3 GHz, were designed [1]. The spiral-MKIDs have two promising potentials: a microwave resonator with a high quality factor, and a terahertz antenna with a broad bandwidth, in only one pixel.
To achieve good electrical performance at 3 K, we selected the NbN compound superconductor and fabricated the spiral-MKIDs. An NbN film has a relatively high transition temperature of about 13 K, and is expected to provide an effective terahertz detection. The resonance characteristics of the spiral-MKIDs were evaluated by measuring one of the scattering-matrix elements, S21. We confirmed that the spiral-MKIDs with 40 nm-thick films have loaded quality factors of the order of 10^4 at 3.4 K, and the NbN material is suitable for the MKIDs working on the He4 refrigerator [2].
For the evaluation of optical performance, the terahertz spectral response of the detector was obtained by using a Fourier transform spectrometer. We have detected broadband terahertz radiation up to 9 THz based on the Cooper-pair breaking process, and confirmed that the sensitivity has a sharp increase around 1 THz, a value that is in agreement with the gap frequency of the superconducting NbN. We also measured the responsivity, noise and time constant to estimate the detector NEP. The latest results, including the terahertz spectroscopy of sample polymers, will be given in this presentation.
[1] K. Hayashi et al., Physics Procedia 45 (2013) 213.
[2] S. Ariyoshi et al., Appl. Phys. Express 6 (2013) 064103.

This work was supported by the Development of Systems and Technology for Advanced Measurement and Analysis of the Japan Science and Technology Agency, and partly supported by the Grant-in-Aid for Scientific Research B (No. 25286083) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

[1] DOI 10.1109/TASC.2014.2363413

Recently, a number of materials have been suggested for use in superconducting nanowire single-photon detectors (SNSPD) that are superconducting in their amorphous state. WxSi1-x was not only the first suggested amorphous material; it is also the material with the highest reported system detection efficiency for an SNSPD of about 93% to date [1]. Amorphous WxSi1-x is also a favorable material for the fabrication of X-ray sensitive SNSPD (X-SNSPD) [2], due to the good X-ray absorption properties of tungsten. Furthermore, as in the case of conventional SNSPD, the amorphous structure eases the deposition of films with good material properties on a variety of substrates and the fabrication of more complicated structures, such as multilayers.
We have prepared a series of WxSi1-x films of different compositions and different thicknesses ranging from 5 nm to 200 nm. The films were deposited by co-deposition in a magnetron sputtering system onto various substrates held at ambient temperature. The sputtering conditions were optimized to achieve highest Tc for the resulting films. We have determined superconducting and normal-state parameters of these films from magneto-conductivity measurements. The knowledge of these parameters is necessary to design optimized structures of SNSPD and X-SNSPD for a particular application. We will discuss the influence of film thickness and wire width on the detector performance, with an emphasis on X-ray sensitive detectors. We find that it may be necessary to use multilayer structures to obtain X-SNSPD with good detection efficiencies that offer an advantage over existing technologies.
[1] F. Marsili et al., Nature Photonics, 7, 210-214 (2013).
[2] K. Inderbitzin et al., IEEE Trans. Appl. Supercond., 23, 2200505 (2013).

The understanding of the detection mechanism in superconducting-nanowire single-photon detectors (SNSPD) has improved significantly, because of advanced detection models and better experimental methods. However, it is not yet possible to predict the detection characteristics of an SNSPD of known geometry and material. The technique of quantum detector tomography (QDT) allows one to discriminate between the results of different detection models much better than it is possible with conventional experimental methods [1]. One model that involves the entry of vortices from the edges [2] and another, recently revised model based on unbinding vortex pairs [3] are both consistent with experimental data from QDT.
Results will be presented from QDT measurements on a NbN bridge for two orthogonal polarizations of the incoming photons: with the electric field parallel and perpendicular to the length of the bridge, respectively [4]. We observe higher absorptivity and higher internal detection efficiency for parallel polarization. Differences in the local absorptivity for the two polarizations allow us to extract the local detection efficiency with a resolution of about 10 nm. The variations in the local detection efficiency are explained by a threshold current that depends on the position across the wire, where the photon is absorbed. The position-dependence of the threshold current has also been calculated within the detection models. Results are consistent with the vortex-edge model [2], but not with the vortex-pair model [3]. We will also apply our results to published data [5] and discuss possible directions in which the current detection models could be developed further.
[1] J. J. Renema et al., Phys. Rev. Lett., 112, 117604 (2014).
[2] A. Engel et al., IEEE Trans. Appl. Supercon., 25, 2200407 (2015).
[3] A. N. Zotova and D. Y. Vodolazov, Supercond. Sci. Technol., 27, 125001 (2014).
[4] J. J. Renema et al., submitted to Nano Letters.
[5] V. Anant et al., Opt. Express, 16, 10750 (2008).

A d-dot is a nano-scaled composite structure that consists of a d-wave superconductor embedded in an s-wave matrix. Since the phase of the superconducting order parameter in the d-wave superconductor depends on direction [1], phase difference appears at a corner junction between d-wave and s-wave superconductors. Compensating this phase difference, a spontaneous half-quantized vortex (SHQV) appears under zero external magnetic field [2]. Because of broken time-reversal symmetry, there are two stable states. Since we can make a superposition of these two states lowering the size of the d-dot and then increasing the transition probability between two states, such d-dot may work as a quantum bit (qubit) in quantum computers [3].
There are many candidates for qubits, such as flux qubits, charge qubits and the Cooper pair box. However we believe that d-dot’s have considerable potential as qubits, because d-dot’s have advantages in scalability and stability. Fujii et al. made d-dot’s that consist of YBa2Cu3O7-δ (YBCO) and Pb [4]. But YBCO has an orthorhombic structure, because CuO chains break a tetragonal symmetry. Therefore there are twin boundaries (TBs) between two kinds of orthorhombic domains.
In this study, we investigate the effects of TBs on SHQV’s in d-dot’s, using the Ginzbrug-Landau (GL) equations and the finite element method [2]. We consider two- and three-dimensional systems. In order to incorporate effects of TBs into the GL equations, we introduce anisotropic effective mass into the Gor’kov equations, and derive anisotropic two-components GL equations. We show how TBs affect SHQV’s using this numerical method.

[1] C. C. Tsuei and J. R. Kirtley, Rev. Mod. Phys. 72, 969(2000)
[2] M. Kato et al., Superconductors – Materials, Properties and Applications. InTech (2012) 319
[3] T. Koyama et al., Physica C 426-431 (2005) 1561–1565.
[4] M. Fujii et al., Physica C 426 (2005) 104–107.

We would like to thank Y. Higashi and M. Umeda for beneficial discussions. This work is supported by the much supports of Program for Leading Graduate Schools of Ministry of Education, Culture, Sports, Science and Technology in Japan (MEXT) and JPSP KAKENHI Grant Number 26400367.

We report on the first detector tomography experiment performed on WSi nanowire superconducting single photon detectors (SSPDs). Since their first demonstration in 2001, SSPDs found applications in fields as relevant as quantum optics and quantum communication. They consist of a very narrow wire patterned in a meander shape from a thin superconducting film usually made of NbN, NbTiN or TaN. The absorption of a photon creates a cloud of quasiparticles that can produce a transition to the normal state resulting in a measurable voltage pulse. Despite the efforts and progress in improving device design, film quality and device coupling with light, the highest reported system detection efficiency (SDE) was not more than 60% for 1550nm wavelength photons.
Recently, an SDE as high as 93% has been achieved by employing an amorphous superconducting material, WSi. Contrary to NbN SSPDs, the WSi detectors show a strong evidence of saturation in the internal detection efficiency which suggests that the detection mechanisms involved in the two materials may be different in nature. Recent experimental evidence shows that the detection mechanism in NbN SSPDs is dominated by quasiparticle diffusion while no clear experimental evidence for the detection mechanism in WSi is available. Recent theoretical calculations suggest that the detection in WSi SSPDs is governed by a different mechanism: a resistive hot spot is formed upon photon absorption. The experimental confirmation and the deeper understanding of these differences would complete the picture of SSPD detection mechanism at the nanoscale. To address this open question, we resolved the response of a simple WSi detector, a bow-tie constriction, by means of detector tomography. The preliminary result shows that, for a given detection probability, the relation between bias current and photon energy is linear in the energy range between 0.75 and 2 eV, similarly to NbN. Further ongoing work to extend this energy range should allow us to conclude whether or not the photon response in WSi is affected by quasi-particle diffusion.

The authors would like to acknowledge the research programme of the Foundation for Fundamental Research on Matter (FOM), which is financially supported by the Netherlands Organization for Scientific Research (NWO) and by NanoNextNL.

We have developed a turn-key, cryogen-free adiabatic demagnetization refrigerator (ADR) cryostat with <10 mKrms temperature stability and fully automated operation for a variety of applications using superconducting detectors. The ADR is cooled using a two-stage pulse tube cryocooler with base temperature ~2.7 - 3.0 K. The cryocooler consists of a remote, electrically isolated rotary valve, a coldhead that is mechanically decoupled from the cryostat using a bellows at 300 K and OFHC copper rope thermal links at the first and second stages of the coldhead, and a water-cooled compressor. The two-stage ADR is launched from the coldhead base temperature, and includes a 4 T superconducting magnet, a GGG stage that provides cooling to ~0.5 K, and an FAA stage that provides final cooling to <50 mK. A vanadium permendur magnetic shield surrounds the magnet to eliminate stray fields in the experimental area.
Regeneration of the ADR takes around 1.5 hours and is fully automated, after which the ADR temperature can be regulated at temperatures up to 250 mK. Local and remote control of the set point temperature are possible. All temperatures, magnet control and operating parameters, and compressor parameters are logged.
We have built several cryostats configured with customizable feedthrough options at room temperature, internal wiring, SQUID readouts, IR windows, and snouts (horizontal or angled) for a variety of superconducting detector applications such as X-ray absorption spectroscopy at the synchrotron using superconducting tunnel junction (STJ) detectors, and X-ray microanalysis in conjunction with an SEM using transition edge sensor microcalorimeters. The cryostat includes a large internal sample volume that can be configured with customized fixturing for a variety of superconducting detector or R&D applications.
We will present a more detailed overview of the cryostat design and operation, along with data recorded using the STJ and microcalorimeter spectrometers.

This work was supported in part by the U.S. Department of Energy under Grants DE-SC0004359 and DESC0006214, and by the U.S. Department of Commerce under Grants 70NANB4H3056 and SB1341-13-CN-0037.

We have been developing superconducting time-of-flight mass spectrometry (TOF-MS) systems, which are composed of a superconducting strip ion detector (SSID) and a single-flux-quantum (SFQ) time-to-digital converter (TDC). Because the SSID has constant ions sensitivity even for high-mass molecules and the SFQ TDC can measure the time intervals with high time resolution, we can realize a high-resolution TOF-MS system to measure high-mass molecules. We have already demonstrated a 24-bit SFQ TDC with a 3 × 24-bit first-in first-out (FIFO) buffer using the AIST Nb standard process (STP2), whose time resolution and dynamic range are 100 ps and 1.6 ms, respectively. In the previous study, we installed the SSID and SFQ TDC into a cryo-cooler, and measured mass spectrums of high-mass molecules to demonstrate the operation of the superconducting TOF-MS system. In this study, we developed a current recycling technique of SFQ circuits to decrease the total bias current and to increase the circuit scale of the SFQ FIFO buffer, which results in the improvement of ion count rates. We will show that the size of the FIFO buffer can be increased even with smaller total bias current. Test results of pulse transfer circuits for a current recycling technique with different magnetic-coupling structures will be shown.

The circuits were fabricated in the clean room for analog-digital superconductivity (CRAVITY) of AIST with the standard process 2 (STP2). The AIST-STP2 is based on the Nb circuit fabrication process developed in ISTEC.

This work was supported by the National Basic Research Program of China (“973” Program) (Nos. 2011CBA00100 and 2011CBA00200), the National Natural Science Foundation of China (Nos. 11227904, 11074114 and 61371036….) and Nature Science Foundation for Distinguished Young Scholars of Jiangsu Province (No. BK2011013).

During the last decade research on nanowires as superconducting single photon detectors (SNSPD) has gained interest thanks to many basic experiments demonstrating their fascinating potentialities in several fields[1]. The search for more appealing and performing materials for SNSPDs is presently a hot topic. Several superconductors have been proposed and investigated as possible alternatives to NbN and NbTiN, which are presently the racehorses. Examples are WSi [2] and MoSi [3] which have several advantages like an amorphous morphology and a narrow-gap with a low quasi-particle diffusivity which permits to extend the application in the mid- and far-IR ranges. In this context, a strategy based on material hybridization of superconductors with ferromagnets has the potentiality to open a new direction in the development of SNSPD [4, 5].
We present results concerning the hybrid superconducting/ferromagnetic (S/F) single nanowires for SNSPDs. In particular, experiments under laser illumination show a single-photon response of S/F-based nanowires. The ferromagnetic overlayer also has the beneficial effect of an increase of the superconducting critical current density and a significant decrease of dark counts. We compare dark counts in hybrid S/F with pure S nanowires. The experimental results are explained in terms of the existing models based on the presence of magnetic vortices [6 ].The data analysis allows to discriminate between the various models, giving the possibility to infer more about the intriguing debate on the physical origin of dark counts.

[1] C. M. Natarajan, M. G. Tanner, R. H. Hadfield, Supercond. Sci. Technol. 25, 063001(2012)
[2] F. Marsili et al., Nature Photonics 7 , 210 (2013)
[3] Yu P Korneeva et al, Supercond. Sci. Technol. 27, 095012 (2014)
[4] N. Marrocco et al., Appl. Phys. Lett. 97, 092504 (2010)
[5] R. Arpaia et al., Supercond. Sci. Technol. 27 , 044027 (2014)
[6] H. Bartolf et al, Phys. Rev. B 81, 024502 (2010)

Superconducting nanowire single photon detectors (SNSPD) is a promising candidate for near-infrared detection, due to its high system detection efficiency (SDE), high speed, low dark count rates and low timing jitter [1]. Recently, high SDEs over 70% at 1550 nm have been demonstrated in the WSi [2], NbN [3] and NbTiN [4] SNSPDs by integrating cavity structures. However, a further improvement to realize more practical, high SDE detector is significant, due to the requirements of quantum information and communication implementation [1]. Interesting experiments, like the linear optical quantum computing and loophole-free Bell test would benefit from the high performance detectors.
In this presentation we report the high efficiency SNSPDs at 1550 nm wavelength, via fiber-coupling through the front-side illumination. The detectors were fabricated on a Si substrate with a multilayer TiO2/SiO2 as a distributed Bragg reflector to realize high optical absorptance. We fabricated 6-nm-thick NbN meander nanowires with width/spacing (55 nm/125 nm), active area of 18 μm * 18 μm. Our SNSPDs can achieve a saturated SDE > 50% with dark count rate (DCR) of 100 Hz at 2.28 K. The performance can be improved by optimization of coupling and adsorption efficiency.
References
[1] C. M. Natarajan,, M. G.Tanner, and R. H. Hadfield, Supercond. Sci. Technol. 25 (2012) 063001.
[2] F. Marsili, V. B. Verma, J. A. Stern, S. Harrington, A. E. Lita, T. Gerrits, I. Vayshenker, B. Baek, M. D. Shaw, R. P. Mirin, and S. W. Nam, Nature Photonics 7 (2013) 210.
[3] D. Rosenberg, A. J. Kerman, R. J. Molnar, and E. A. Dauler, Opt. Express 21 (2013) 1440.
[4] S. Miki, T. Yamashita, H. Terai, and Z. Wang, Opt. Express 21 (2013) 10208.

This work was funded by the NSFC (61401441 & 61401443), Strategic Priority Research Program (B) of the CAS (XDB04010200 & XDB04020100), National Basic Research Program of China (2011CBA00202).

Kinetic-inductance detectors (KID) combine high sensitivity with intrinsic frequency-division readout capability. This kind of detector is ideal to build large arrays, which can be read out via frequency-division multiplexing (FDM). In order to precisely measure the detuning of the resonators, a highly specialized data acquisition system (DAQ) has to be used.

We have developed a DAQ system which is based on a XILINX Kintex 7 FPGA with additional AD/DA converters. The analog signal chain is divided in a baseband with a bandwidth of 400 MHz and a RF band of 4 to 8.5 GHz which can be adaptable to different resonant frequencies of the detector array. High linearity and low noise require carefully selected analog components. To achieve a readout with video rate speed and a high frequency resolution, the digital signal flow and signal processing are crucial. Therefore clock synchronization is an important factor to achieve a dual-channel sampling rate of 1 GSps.

We will present the electrical characterization of the Kinetic-Inductance Readout Circuit (KIRC). Measurement results of large KID arrays will be shown and discussed in more detail.

The microwave kinetic inductance detector (MKID) is a promising terahertz (THz) wave detector array for Fourier transform THz spectroscopy. We previously proposed and investigated the use of MKIDs with a rewound spiral resonator (spiral-MKID) that simultaneously acts as a microwave resonator with high-Q and a wide-band THz antenna [1-3]. However, until now, there has been no 2D imaging system that uses spiral-MKIDs. In this work, we present our development of a real-time 2D THz imaging system using a 25-array spiral-MKID. The microwave resonant frequencies of the 25-array spiral-MKIDs were designed to be separated by 30 MHz at around 3.3 GHz. Frequency responses were analyzed using a 2.5-dimentional simulator (Sonnet EM). The 25-array spiral-MKIDs were fabricated using NbN thin films with thickness varying from 30 to 60 nm deposited on an m-plane sapphire substrate. The MKIDs were placed in a GM cryocooler and their microwave responses measured using a low-noise amplifier and a vector network analyzer. We observed 25 half-wavelength resonances around 3.3 GHz at 3.5 K in the MKIDs with a thickness of 36 nm. The optical responses were also confirmed from the thermal radiations between hot (300 K) and cold (77 K). Details of the system set-up and results of our 2D real-time THz imaging system will be presented at the conference.
References
[1] S. Ariyoshi, K. Nakajima, and A. Saito, et al., Appl. Phys. Express 6 (2013) 064103.
[2] K. Hayashi and A. Saito, et al., Physics Procedia 45 (2013) 213.
[3] A. Saito and K. Nakajima, et al., IEEE Trans. Appl. Supercond., 25 (2015) (to be published).

This work was supported by JST (development of systems and technology for advanced measurement and analysis) and the Kato Foundation for Promotion of Science. Part of this work was performed in the clean room of Yamagata University.

Superconducting nanowire networks of Bi2Sr2CaCu2O8 (Bi-2212) were fabricated by means of the electrospinning technique. The electrospinning technique enables the growth of long nanowires up to the millimeter range, while the diameter of the nanowires can be controlled by the processing parameters. The resulting materials are fabric-like structures of about 4 × 4 mm2 in size showing a large number of interconnects and junctions between the nanowires. The resulting nanowires are of granular nature with a grain size similar to the wire thickness of about 100-150 nm, and the width of the nanowires is about 100-200 nm as determined by electron microscopy. As these nanowire networks are a new class of superconducting materials, we studied the electric transport properties (resistance, I/V-characteristics) of such nanowire networks in applied magnetic fields (0-12 T). SQUID measurements of the susceptibility and magnetization hysteresis loops were performed as well. Possible applications of such nanowire networks are discussed.

Financial support by Volkswagen-Stiftung is gratefully acknowledged.

Superconducting radiation detectors are among the most sensitive and/or fastest detectors that can be used in a wide spectrum range. There are a many types of different superconducting detector types such as superconducting tunnel junctions, transition edge sensors, superconducting nanowire single-photon detectors, and kinetic inductance detectors (KID). In this study, we use a monolithic superconducting stripline detector (SSLD) fabricated by AIST standard process. SSLDs are promising devices as they require only one current bias point for many-pixel configurations, have fast response times and high spatial resolutions. Also, it is possible to fabricate them in a monolithic configuration together with their read-out electronics by using standard Nb foundry processes. The detector may operate in bolometer mode and/or KID mode based on the radiation intensity and rise time.
For practical applications it is important that the detector systems work in a liquid cryogen free system. We develop a test system by using a pulse tube cryrocooler originally implemented for SFQ circuits. In the system, we use a fiber coupled 1550 nm wavelength diode laser with about 100 mW continuous output power. The laser is modulated with an electro-optic modulator based on a Mach-Zehnder interferometric architecture up to 20 GHz. The laser signal is fed to the detector chip by using a hermetic fiber feedthrough. Fiber is terminated with a 1.7 mm focal length lens close to the detector. Lens is manipulated by using a 3 axis, closed loop nanopositioner for accurate alignment and focusing of the laser beam. In this study, we report about the details of the experimental system and optical test results.

This work is partially supported by TUBITAK with the project number 114E099 and JSPS KAKENHI with the grant number 23226019

The richness of the energy level structure and the diversity of the associated transitions are at the heart of atomic and quantum optics experiments. They are provided by the multiples degrees of freedom existing in these systems. On the opposite, studies on superconducting artificial atoms were focused on devices with only one degree of freedom, as for example the Camelback phase qubit [1] or tha multilevel anharmonic oscillator. In order to reach new regimes and increase the range of application of these systems, we report here the novel “Bridge Free Fabrication” process [2] and measurement of a superconducting device with two degrees of freedom (DoF) [3]. This device can be described by two anharmonic oscillators coupled via strong non-linear couplings terms [4]. We show independent coherent control of the two DoF and we use here one of the coupling terms to demonstrate a time resolved up and down coherent frequency conversion in the microwave domain. This device can be a new building block for superconducting electronics providing crucial benefits with regard to non destructive qubit readout.
[1] Quantum dynamics in a camel-back potential of a dc SQUID E. Hoskinson, et al. Phys. Rev. Lett. 102, 097004 (2009).
[2] Junction fabrication by shadow evaporation without a suspended bridge, F. Lecocq, et al, Nanotechnology 22 315302 (2011).
[3] Superconducting artificial atom with two internal degrees of freedom, F. Lecocq, et al , Phys. Rev. Lett. 108, 107001 (2012).
[4] Non-linear coupling between the two oscillation modes of a dc-SQUID, F. Lecocq, et al, Phys. Rev. Lett. 107, 197002 (2011).

This work has been supported by European SOLID Integrated Projet and French ANR-NSFC project.

Superconducting qubits based on Josephson junctions are widely used in solid-state quantum information experiments. These qubits exploits the non-linear current-phase relation of the Josephson effect. According to the microscopic description of the Josephson effect, the supercurrent is carried by pairs of Andreev bound states that form two-level systems (TLS) localized at the junction. Yet the current-phase relation exploited by Josephson qubits corresponds only to the ground state of these TLS. In this work, we probe all the states of unique Andreev pair TLS on the simplest Josephson junction: a single-atom contact. This contact is inserted in a superconducting loop coupled to a microwave resonator. This standard circuit-QED architecture allows us to measure the state of this TLS in a single shot, to manipulate it coherently, measure its excited state lifetime T1 = 2-4 µs, and its coherence time T2 = 20-180 ns. Besides, poisoning of the TLS by out-of-equilibrium quasiparticles is observed.

Superconducting Stripline Detectors (SSLD) are recently emerging devices as they have the potential to enable very large scale integration since the readout of the detectors can be achieved on the same chip in a monolithic configuration. SSLD's may operate in the kinetic inductance mode or resistive mode depending on the circuit parameters as well as input radiation. When the SSLDs work in the kinetic inductance mode, transient kinetic inductance variations on superconducting strip-line are the reason for voltage output whereas when the SSLDs work in the resistive mode, local loss of the superconductivity causes the stripline to generate voltage under constant current bias. In both cases, the generated voltages are converted to current with a series resistance to the stripline with expected output levels of about few µA. To detect and convert this analog signal to SFQ signals, a simple comparator circuit may be used. Comparator circuits developed in this study consists of a Quasi-one junction SQUID (QOS) designed in a conventional clocked fashion or clock-free fashion terminated with a D-flip flop circuit. As the sensitivity of the read-out circuit heavily depends on the circuit parameters of the SSLD, it is not practical to use an already available comparator circuits designed for some other detectors. In this study, we report the particle swap optimization results of a clocked and clock-free read-out circuit for two different SSLD parameters.

This work was partially supported by TUBITAK, under grants 111E191 and 114E099 and JSPS KAKENHI with the grant number 23226019. Authors would like to thank to Y. Tukel for developing the PSO algorithm for SFQ circuits used in this study.

Authors have been studying the current-voltage characteristics of superconducting tunnel junctions (or SIS junctions) and have shown that a very little density of quasiparticle states is present inside the superconducting gap, which play an significant role in the behavior of subgap current of the SIS junctions at low temperatures [1,2]. Furthermore, the quasiparticle density of states are strongly deviated from the prediction of the BCS theory at very low temperature, which is very similar to the behavior of the surface resistance at low temperature. Based on the result, we assumed that the conductivity might be affected by the quasiparticle states in the superconducting gap and the deviation of the surface resistance from the prediction of the Mattis-Bardeen theory might be explained by the contribution from a small density of quasiparticle states in the superconducting gap. Then, we extended the Mattis-Bardeen theory considering the contribution of quasi-particle states in the superconducting gap in order to obtain appropriate surface-resistances of the superconductor.
It is shown that anomalous behavior of the quality factor of the superconducting resonator at low temperature is well described by the extended Mattis-Bardeen equation into which a complex gap energy is incorporated. It is also shown that the residual number of quasiparticles in the thin film resonator at low temperature well agrees with the calculated one using a density of states which consists of the gap-broadened and localized quasipartcle states. The density of states used in this calculation is approximately same as the one obtained from the fitting of the dc I-V characteristics of a SIS junction. From these results we confirm that the quasiparticle states are really present in the superconducting gap.
[1] T. Noguchi, T. Suzuki, A. Endo, and T. Tamura, Physica C, vol. 469, pp. 1585-1588, 2009.
[2] T. Noguchi, M. Naruse, and Y. Sekimoto, IEEE Trans. Appl. Supercond., vol. 23, 1501404, 2013.

This work was supported in part by MEXT KAKENHI Grant Number 24111711. The authors thank Dr. Matthias Kroug for helpful discussions.

In the past years, we have been developing High-Tc Josephson nano-junctions made by ion irradiation. Based on commercial YBa2Cu3O7 thin films, this versatile and highly scalable technique opens a new route towards superconducting electronics in the temperature range between 20K and 80K. DC and RF applications have been explored, with very encouraging results.
In particular, we built a heterodyne High-Tc Superconductor (HTSc) receiver in the THz range, made of a Josephson mixer embedded in a broad-band antenna, operating at 50K-60K. High-frequency mixing properties of such device up to 420 GHz has been obtained, with interesting conversion gain. Accurate modeling of the detector was made using a three-ports model.
We now develop a new project using ion irradiation patterning of HTSc thin films. The goal is to make HTSc Superconducting Single Photon Detectors (SSPD), operating at higher temperature and higher speed than commercially available SSPDs. Meander line nanowires are patterned on ultra-thin (10 to 30 nm thick) YBa2Cu3O7 films, and polarized with a DC current close to the critical current. Incident photons locally destroy the superconducting state, and a voltage pulse is recorded. First DC characterization of nanowires (50 to 500 nm wide) will be presented.