We propose a superconducting toroidal flux qubit design naturally protected from ambient noise. The superconducting toroidal design exhibits properties similar to an effective two-level scheme. A toroidal qubit laser based on this design is also considered.

AC acknowledges support by the Federal Ministry of Education and Research (PhoNa Project). EI acknowledges funding from the EC’s Seventh Framework Programme (FP7/2007-2013) under Grant No. 270843 (iQIT). FN is partially supported by the RIKEN iTHES Project, MURI Center for Dynamic Magneto-Optics, and a Grant-in-Aid for Scientific Research (S).

We present an experimental study on two transmons (i.e., small capacitively shunted Josephson junctions) coupled via a large inductance [1]. The resulting circuit exhibits a symmetric and an antisymmetric oscillation which we use as a transmon and ancilla qubit, respectively. The Josephson nonlinearity leads to a cross-Kerr-like coupling of the two oscillations [2] and consequently to a V-shape artificial atom.
We have predicted that such V-shape artificial atom allows to read out the transmon qubit state by using the ancilla qubit frequency [3]. In comparison with the most widely employed readout scheme for superconducting qubits, the dispersive readout in a circuit quantum electrodynamics architecture, this approach promises a quantum non-demolition measurement with a significantly stronger measurement signal and without suffering from Purcell effect. In a measurement chain based on a state-of-the-art Josephson parametric amplifier, we predict a QND fidelity of up to 99.9% for a measurement time down to 60 ns [3]. We will discuss our recent experimental results in comparison with the theoretical prediction.
[1] É. Dumur, et al, “A V-shape superconducting artificial atom based on two inductively coupled transmons”, arXiv1501.04892.
[2] F. Lecocq, I. M. Pop, I. Matei, E. Dumur, et al, "Coherent Frequency Conversion in a Superconducting Artificial Atom with Two Internal Degrees of Freedom", Physical Review Letters 108, 107001 (2012).
[3] I. Diniz, E. Dumur, et al, "Ultrafast quantum nondemolition measurements based on a diamond shaped artificial atom", Physical Review A 87, 033837 (2013).

This work is supported by the ANR-NSFC project.

Non-local entanglement on a chip is a key element for solid state based quantum information and can be implemented from superconducting hybrid multi-terminal devices. The underlying mechanism is referred to crossed Andreev reflection in which a Cooper pair in a superconductor is converted into two spin-entangled electrons located in separate conductors. Here, we have investigated three-terminal superconductor (S) - normal metal (N) - superconductor (S) Josephson junctions. In a geometry where a T-shaped normal metal (Cu) is connected to three superconducting reservoirs (Al), a new sub-gap structure appears in the differential resistance when the bias voltages on two of the three terminals compensate each other exactly [1]. Those correspond to correlated motion of Cooper pairs within the structure and are consistent with the prediction of quartets formed by with the simultaneous splitting of two Cooper pairs in one of the superconducting contacts and the emission of two phase correlated Copper pairs in the two other electrodes [2]. In order to test the quantum coherence of such mechanism, we have studied the effect of radio-frequency irradiation. At two terminal, well-known Shapiro steps are revealed and in particular half integer steps are observed corresponding to the coherent transfer of two Cooper pairs. The quartet resonances show only integer steps as expected. The overall results both at two and three terminals confirms the quartet scenario and demonstrates its quantum nature.
1 - Subgap structure in the conductance of a three-terminal Josephson junction A. H. Pfeffer, J. E. Duvauchelle, H. Courtois, R. Mélin, D. Feinberg, and F. Lefloch, Physical Review B 90, 075401 (2014)
2 - Production of Nonlocal Quartets and Phase-Sensitive Entanglement in a Superconducting Beam Splitter, A. Freyn, B. Douçot, D. Feinberg, and R.Mélin, Phys. Rev. Lett. 106, 257005 (2011).

This work has been partially funded by the French Research National Agency, ANR-NanoQuartet (Grant No. NR12BS1000701). We acknowledge the Nanoscience Foundation for the Ph.D. grant of A. H. Pfeffer, the NanoFab facility at Institut Neel-CNRS for sample fabrication, and fruitful discussions with R. Mélin and D. Feinberg.

Meandered nanometer-wide superconducting strips are commonly used for detection of single photons in the near infrared spectral range. It has recently become clear that current crowding in the bends limits the achievable supercurrent to a value noticeably lower than the depairing current in straight fragments of a meander. Nevertheless, it has been for a long time unclear whether the observed difference between light and dark counts is related to the difference in the mechanism of these events or they originate from different fragments (bended and straight) of the meander. Results of recent experiments with meanders in magnetic field have rather favored the latter explanation. However, unambiguous conclusion was not possible due to the mixed symmetry of the bends in common meander structures with respect to the direction of current and magnetic field.
Our new experimental studies of magnetic field dependencies of the detection efficiency were performed on quadratic spiral structures, where all bends have the same symmetry with respect to the direction of magnetic field and current. Results of these investigations explicitly evidence that dark counts as well as light counts at bias currents noticeably smaller than the critical current come from the bends of the meander. Contrary, at bias currents close to the critical current the light counts are dominated by events in the straight fragments of the meander. We have analyzed experimental data of the last decade, i.e. pulse amplitudes and current dependencies for light and dark counts and show that they all can be understood within the framework of these latest findings.

In 1999 the superconducting spin-valve was proposed theoretically [1,2], comprising a superconducting (S) spacer layer separating two ferromagnetic (F) layers. For ideal operation, the in-plane supercurrent in the S layer can be controlled by the mutual orientation of magnetization in the F layers. Initially the spin-valve effect was observed as a dependence of the critical temperature (Tc) on the magnetic configuration, parallel (P) or antiparallel (AP) in such structures. However, the Tc change between collinear and perpendicular configuration may be much more pronounced than between P and AP alignment in a case of a strongly spin-polarized ferromagnet [3-5] due to appearance of the new channel for drainage of Cooper pairs from the S to the F layers that relates to appearance of the long-range triplet superconducting correlations (LRTC) [6].

We have calculated and observed [5] the angular dependence of the Tc for FSF as well as SFF Co-Nb spin-valves. For both sample types (SFF and FSF) a large suppression of Tc is found when the magnetizations are orthogonal, consistent with the theoretical expectations for the drainage of singlet into the triplet LRTC channel when the magnetization is non-collinear. This suppression may be controlled by the magnitude of the applied field. The effect almost disappears when switching the spin-valve into a collinear state, when LRTC are absent. This shows that LRTC in the ferromagnetic regions are a crucial ingredient contributing to the effect.

Such spin-valves may serve as building blocks of spintronic devices and our investigation provide a mechanism by which such devices might be realized.

[1] A. I. Buzdin, A. V. Vedyayev and N. V. Ryzhanova, Europhys. Lett., 48 686-691 (1999).
[2] L. R. Tagirov, Phys. Rev. Lett. 83 2058 (1999).
[3] P. V. Leksin, N. N. Garif'yanov, I. A. Garifullin, et al., Phys. Rev. Lett. 109 057005 (2012).
[4] A. Singh, S. Voltan, K. Lahabi, J. Aarts. arXiv:1410.4973 (2014).
[5] M. G. Flokstra, T. C. Cunningham, J. Kim, N. Satchell, G. Burnell, P. J. Curran, S. J. Bending, C. J. Kinane, J. F. K. Cooper, S. Langridge, A. Isidori, N. Pugach, M. Eschrig, S. L. Lee. Phys. Rev. B 91, 060501(R) (2015).
[6] Ya. V. Fominov, A. A. Golubov, and M. Yu. Kupriyanov, JETP Letters 77 510 (2003).

We acknowledge the support of the EPSRC through Grants No. EP/J01060X, No. EP/J010626/1, No. EP/J010650/1, No. EP/J010634/1, and No. EP/J010618/1, support of a studentship supported by JEOL Europe and the ISIS Neutron and Muon Source, and the support of the RFBR via Awards No. 13-02-01452-a, and No. 14-02-90018 BEL-a.

It is required to think about a method how to capture a neutron while it is quite useful in the wide range of material science, physics, chemistry, biology, agriculture and spintronics. However, it is not so easy to detect a neutron due to the absence of the electric charge. Traditionally, it is known to use an ionization gas detector, which requires the application of high voltage in operation. We proposed a superconducting nanowire meander in combination with ¹⁰B conversion layer for detecting neutrons. One of two (⁷Li and ⁴He) ions released in opposite direction to create quasiparticles from the Cooper pairs in the nanowire. This detector senses not a chance in the kinetic inductance L_k but a time derivative of the kinetic inductance dL_k/dt induced by a decrease in local Cooper pair density. Our detector is of advantage to work at 4 K when the wire is biased by a constant DC current, of which the amplitude could be much smaller than the critical current. This is different from a microwave kinetic inductance detector (MKID), and we name it a current-biased kinetic inductance detector (CB-KID) [1,2,3]. We succeeded in measuring a clear signal from a single neutron, and the averaged neutron flux from pulsed neutrons (at BL10 of J-PARC) as a function of time. A histogram of averaged neutron in 0.1 ms is in good agreement with that obtained by a Monte-Carlo simulation. This is convincing evidence that our detector successfully detected pulsed neutrons at J-PARC for the first time.
[1] T. Ishida et al., J. Low Temp. Phys. 167, 447 (2012).
[2] N. Yoshioka et al., IEEE Trans. Appl. Supercond. 23, 2400604 (2013).
[3] Y. Narukami, et al., IEEE. Trans. Appl. Supercond. 25, 2400904 (2015).

This work is partially supported by Grant-in-Aid for Young Scientists (A) (No. 24684027), Grant-in-Aid for Scientific Research (S) (No. 23226019), and Grant-in-Aid for Young Scientists (B) (No. 26820130 and No. 26800192) from JSPS. The CB-KID devices were partially fabricated by using the CRAVITY facilities of AIST.

Random tunneling two-level systems (TLSs) in dielectrics have been of interest recently because they adversely affect the performance of superconducting qubits, but they can also be used as coherent resources. Using a calibrated uniform dc electric field, we modify the energy potential of randomly occurring TLSs in an insulating film and probe them using a superconducting microwave resonator at millikelvin temperatures. This allows measurement of the excitation energies of individual TLSs dependent on the z-component of their electric dipole moment p_z. The hyperbolic energy dependence of TLSs reveals the state of double well degeneracy which allows for a precise measurement of p_z and the tunneling strength for each individual TLS. This method of degenerate well resonator (DWR) spectroscopy resolves multiple dipole types from a thick insulating film for the first time, and in silicon nitride we observe distinct types with maxima at p_z=3.3 and 8.3 Debye. Sixty TLSs are measured with this technique, and the microscopic TLS observations are consistent with ensemble-averaged loss tangent data. The TLS-resonator ac coupling is also measured and, with p_z, provides a quantum electrodynamical measurement of the vacuum fluctuation field. The same fluctuation field calculated from the electric field volume is only 16% smaller.

Propagating quantum microwaves are a promising building block for quantum communication and quantum simulation. In particular, such itinerant quantum microwaves can be generated by Josephson parametric amplifiers (JPA) in a form of squeezed photon states. Next, a displacement operation is performed by means of a directional coupler after the squeezing. Finally, we employ a specific "dual-path" setup for quantum state reconstruction. We compare our results are with theory predictions and relate them to quantum simulation algorithms. We discuss our experiments in the context of remote state preparation and quantum teleportation with itinerant continuous microwaves. This work is supported by the DFG via SFB 631 and EU projects CCQED and PROMISCE.

We acknowledge collaboration with Dr. Kunihiro Inomata, Dr. Tsuyoshi Yamamoto and Prof. Dr. Yasunobu Nakamura (NEC / RIKEN, Japan) and with the theory group of Prof. Dr. Enrique Solano (University of the Basque Country UPV/EHU, Bilbao, Spain).