Non-hysteretic superconducting quantum interference proximity transistor with enhanced responsivity

This Letter presents fabrication and characterization of an optimized superconducting quantum interference proximity transistor. The present device, characterized by reduced tunnel junction area and shortened normal-metal section, demonstrates no hysteresis at low temperatures as we increased the Josephson inductance of the weak link by decreasing its cross section. It has consequently almost an order of magnitude improved magnetic field responsivity as compared to the earlier design. The modulation of both the current and the voltage across the junction have been measured as a function of magnetic flux piercing the superconducting loop.Peer reviewe

Erratum: “Non-hysteretic superconducting quantum interference proximity transistor with enhanced responsivity”

Peer reviewe

Low-temperature characterization of Nb-Cu-Nb weak links with Ar ion-cleaned interfaces

We characterize niobium-based lateral Superconductor (S) - Normal metal (N) - Superconductor weak links through low-temperature switching current measurements and tunnel spectroscopy. We fabricate the SNS devices in two separate lithography and deposition steps, combined with strong argon ion cleaning before the normal metal deposition in the last step. Our SNS weak link consists of high-quality sputtered Nb electrodes that are contacted with evaporated Cu. The two-step fabrication flow enables great flexibility in the choice of materials and pattern design. A comparison of the temperature-dependent equilibrium critical supercurrent with theoretical predictions indicates that the quality of the Nb-Cu interface is similar to that of evaporated Al-Cu weak links. Aiming at increased sensitivity, range of operation temperatures, and thermal isolation, we investigate how these SNS structures can be combined with shadow-evaporated aluminum tunnel junctions for sensor applications that utilize the superconducting proximity effect. To this end, we demonstrate a hybrid magnetic flux sensor based on a Nb-Cu-Nb SNS junction, where the phase-dependent normal metal density of states is probed with an Al tunnel junction.Comment: 5 pages, 3 figure

Spatial and energy resolution of electronic states by shot noise

| openaire: EC/H2020/766025/EU//QuESTechShot-noise measurements are widely used for the characterization of nonequilibrium configurations in electronic conductors. The recently introduced quantum tomography approach was implemented for the studies of electronic wave functions of few-electron excitations created by periodic voltage pulses in phase-coherent ballistic conductors based on the high-quality GaAs two-dimensional electron gas. Still relying on the manifestation of Fermi correlations in noise, we focus on the simpler and more general approach beneficial for local measurements of energy distribution (ED) in electronic systems with arbitrary excitations with well-defined energies and random phases. Using biased diffusive metallic wire as a test bed, we demonstrate the power of this approach and extract the well-known double-step ED from the shot noise of a weakly coupled tunnel junction. Our experiment paves the way for local measurements of generic nonequilibrium configurations applicable to virtually any conductor.Peer reviewe

Driving a low critical current Josephson junction array with a mode-locked laser

We report proof-of-concept experiments on an optically driven Josephson voltage standard based on a mode-locked laser (MLL), a time-division multiplexer, and a cryogenic ultrafast photodiode driving an overdamped Josephson junction array (JJA). Our optical pulse pattern generator (PPG) concept builds on the capability of MLLs to produce trains of picosecond-wide optical pulses with little amplitude and temporal spread. Our present setup enables multiplication of the original 2.3 GHz pulse repetition frequency by a factor of 8. A commercial photodiode converts the optical pulses into about 25 ps wide electrical pulses in liquid helium several cm from the JJA. Using a custom-made MLL, we can drive a JJA with a low critical current of 360 μA at multiple Shapiro steps. We have performed experiments with pulse pairs whose time interval can be set freely without distorting the shapes of individual pulses. Experimental results are in qualitative agreement with theoretical simulations, and they demonstrate, e.g., crossover in the Shapiro step pattern when the time interval between the pulses is approximately equal to the inverse of the characteristic frequency of the JJA. However, there are quantitative discrepancies, which motivate an improved integration of photodiodes and JJAs to improve both the understanding and fidelity of Josephson Arbitrary Waveform Synthesizers. Considering future quantum technologies in a wider perspective, our optical approach is a potential enabler for fast and energy-efficient pulse drive without an expensive high-bandwidth electrical PPG and without high-bandwidth electrical cables that yield too high thermal conductance between cryogenic and room temperatures

Broadband Continuous-Variable Entanglement Generation Using a Kerr-Free Josephson Metamaterial

| openaire: EC/H2020/862644/EU//QUARTET | openaire: EC/H2020/820505/EU//QMiCS | openaire: EC/H2020/820363/EU//OpenSuperQ | openaire: EC/H2020/824109/EU//EMP | openaire: EC/H2020/670743/EU//QuDeT Funding Information: We thank Alpo Ahonen, Paula Holmlund, and Harri Pohjonen for technical assistance and Terra Quantum AG for scientific support. K.V.P. is funded by the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 862644 (FET-Open project: Quantum readout techniques and technologies, QUARTET). The work at VTT is funded from the EU Flagship on Quantum Technology Grant No. H2020-FETFLAG-2018-03 Project Nos. 820363 OpenSuperQ and 820505 QMiCS. The contribution of M.R.P., I.L., M.W., and A.S. is supported by Grant Agreement No. 824109 (European Microkelvin Platform project, EMP), and ERC Grant Agreement No. 670743 (QuDeT). P.J.H. and V.V. acknowledge financial support from the Academy of Finland through Grants No. 314448 and No. 321700, respectively. The work of S.H.R. and P.J.H. is supported by a MATINE research grant. G.S.P. and K.V.P. thank Saab for scientific collaboration under a research agreement with Aalto University. This work is done under the “Finnish Center of Excellence in Quantum Technology QTF” of the Academy of Finland, Projects No. 312059, No. 312294, No. 312295, No. 336810, and No. 312296. Publisher Copyright: © 2022 American Physical Society.Entangled microwave photons form a fundamental resource for quantum information processing and sensing with continuous variables. We use a low-loss Josephson metamaterial comprising superconducting, nonlinear, asymmetric inductive elements to generate frequency-entangled photons from vacuum fluctuations at a rate of 2 giga entangled bits per second spanning over the 4-GHz bandwidth. The device is operated as a traveling-wave parametric amplifier under Kerr-relieving biasing conditions. Furthermore, we demonstrate single-mode squeezing in such devices - 3.1±0.7dB below the zero-point level at half of modulation frequency.Peer reviewe