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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Quantum system dynamics with a weakly nonlinear Josephson junction bath

Here, we investigate the influence of a weakly nonlinear Josephson bath consisting of a chain of Josephson junctions on the dynamics of a small quantum system (LC oscillator). Focusing on the regime where the charging energy is the largest energy scale, we perturbatively calculate the correlation function of the Josephson bath to the leading order in the Josephson energy divided by the charging energy while keeping the cosine potential exactly. When the variation of the charging energy along the chain ensures fast decay of the bath correlation function, the dynamics of the LC oscillator that is weakly and capacitively coupled to the Josephson bath can be solved through the Markovian master equation. We establish a duality relation for the Josephson bath between the regimes of large charging and Josephson energies, respectively. The results can be applied to cases where the charging energy either is nonuniformly engineered or disordered in the chain. Furthermore, we find that the Josephson bath may become non-Markovian when the temperature is increased beyond the zero-temperature limit in that the bath correlation function gets shifted by a constant and does not decay with time.

1-dimensional systems↗
DOE OSTI2024

Correlating aluminum layer deposition rates, Josephson junction microstructure, and superconducting qubits’ performance

An ultra-thin aluminum oxide layer is a key component for Josephson junctions (JJ) in superconducting quantum bits (qubits). This layer serves as a barrier layer for Cooper pairs tunneling between the superconducting electrodes and significantly influences the overall performance of the junction. In this study, we investigate the impact of aluminum deposition rates on the microstructure and chemical variation of the aluminum oxide layer, as well as the device's yields and qubits’ lifetimes. Surprisingly, although the oxide layer roughness and thickness variation improve noticeably with an increasing Al deposition rate from 0.5 Å/s to 5 Å/s, the qubit's coherence time is almost unchanged. We attribute this to the fact that Cooper pairs tunnel only through the thinnest region of the barrier. Additionally, we revealed a stress-induced grain boundary sliding short-circuit failure mechanism in the JJ. Our discovery provides a vital understanding of oxide microstructure and JJ functionality, which are critical for improving the coherence time of superconducting qubits.

Coherence time↗
DOE OSTI2022

Effective field theory of fluctuating wall in open systems: from a kink in Josephson junction to general domain wall

We investigate macroscopic behaviors of fluctuating domain walls in nonequilibrium open systems with the help of the effective field theory based on symmetry. Since the domain wall in open systems breaks the translational symmetry, there appears a gapless excitation identified as the Nambu-Goldstone (NG) mode, which shows the non-propagating diffusive behavior in contrast to those in closed systems. After demonstrating the presence of the diffusive NG mode in the (2+1)-dimensional dissipative Josephson junction, we provide a symmetry-based general analysis for open systems breaking the one-dimensional translational symmetry. A general effective Lagrangian is constructed based on the Schwinger-Keldysh formalism, which supports the presence of the gapless diffusion mode in the fluctuation spectrum in the thin wall regime. Besides, we also identify a term peculiar to the open system, which possibly leads to the instability in the thick-wall regime or the nonlinear Kardar-Parisi-Zhang coupling in the thin-wall regime although it is absent in the Josephson junction.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Superconducting neural networks with disordered Josephson junction array synaptic networks and leaky integrate-and-fire loop neurons

Fully coupled randomly disordered recurrent superconducting networks with additional open-ended channels for inputs and outputs are considered the basis to introduce a new architecture to neuromorphic computing in this work. Various building blocks of such a network are designed around disordered array synaptic networks using superconducting devices and circuits as an example, while emphasizing that a similar architectural approach may be compatible with several other materials and devices. A multiply coupled (interconnected) disordered array of superconducting loops containing Josephson junctions [equivalent to superconducting quantum interference devices (SQUIDs)] forms the aforementioned collective synaptic network that forms a fully recurrent network together with compatible neuron-like elements and feedback loops, enabling unsupervised learning. This approach aims to take advantage of superior power efficiency, propagation speed, and synchronizability of a small world or a random network over an ordered/regular network. Additionally, it offers a significant factor of increase in scalability. Here, a compatible leaky integrate-and-fire neuron made of superconducting loops with Josephson junctions is presented, along with circuit components for feedback loops as needed to complete the recurrent network. Several of these individual disordered array neural networks can further be coupled together in a similarly disordered way to form a hierarchical architecture of recurrent neural networks that is often suggested as similar to a biological brain.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1975

Parametric amplification by unbiased Josephson junctions

The theory of parametric amplification by a series of unbiased Josephson junctions is presented. The various regimes of operation are explored. This device is different in many ways from the usual varactor parametric amplifier. In particular, a very small pump power is required, and a large bandwidth is obtained. Comparison with experimental data is made.

Feldman, M. J.↗
NASA NTRS1975

Perturbation treatment of mixing in Josephson junctions

The mixing properties of a current-biased resistively-shunted Josephson junction irradiated at two frequencies are calculated by a perturbation technique, treating both signals as perturbations. The size of the mixing steps is obtained from second-order calculations. It is shown that subharmonic mixing steps are absent in the case of a single frequency. The amplitude of the voltage oscillations at the difference and sum frequencies is shown to be nonzero at all voltages. The microwave resistance is calculated for one frequency to third order in the perturbation.

Levinsen, M. T.↗
NASA NTRS1985

A closed cycle cascade Joule Thomson refrigerator for cooling Josephson junction magnetometers

A closed cycle cascade Joule Thomson refrigerator designed to cool Josephson Junction magnetometers to liquid helium temperature is being developed. The refrigerator incorporates 4 stages of cooling using the working fluids CF4 and He. The high pressure gases are provided by a small compressor designed for this purpose. The upper stages have been operated and performance will be described.

Tward, E.↗
DOE OSTI2026

Current-Phase Relations of Superconducting PtSi Constriction Josephson Junctions

Here, we study the current-phase relations of superconductor–constriction–superconductor Josephson junctions made from platinum silicide thin films by fabricating dc-superconducting quantum interference devices incorporating pairs of junctions and measuring their magnetic-field-dependent electrical transport. By comparing the supercurrent interference patterns with numerical Ginzburg–Landau simulations, we extract the current-phase relations of individual junctions and quantify the degree of nonlinearity. These measurements show that while the constrictions themselves sustain substantial nonlinearity, the effect is moderated by the kinetic inductance of the device leads, which is an important consideration for practical superconducting circuit applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Quantum critical dynamics of a Josephson junction at the topological transition

We find the admittance Y(ω) of a Josephson junction at or near a topological transition. The dependence of the admittance on frequency and temperature at the critical point is universal and determined by the symmetries of the system. Despite the absence of a spectral gap at the transition, the dissipative response may remain weak at low energies: ReY(ω)∝max(ω,T) 2 . This behavior is strikingly different from the electromagnetic response of a normal metal. Away from the critical point, the scaling functions for the dependence of the admittance on frequency and temperature are controlled by at most two parameters.

36 MATERIALS SCIENCE↗
NASA NTRS1973

An investigation of the SNS Josephson junction as a three-terminal device

A particular phenomenon of the SNS Josephson junction was investigated; i.e., control by a current entering the normal region and leaving through one of the superconducting regions. The effect of the control current on the junction was found to be dependent upon the ration of the resistances of the two halves of the N layer. A low frequency, lumped, nonlinear model was proposed to describe the electrical characteristics of the device, and a method was developed to plot the dynamic junction resistance as a function of junction current. The effective thermal noise temperature of the sample was determined. Small signal linearized analysis of the device suggests its use as an impedance transformer, although geometric limitations must be overcome. Linear approximation indicates that it is reciprocal and no power gain is possible. It is felt that, with suitable metallurgical and geometrical improvements, the device has promise to become a superconducting transistor.

Meissner, H.↗
NASA NTRS1973

Josephson junction mixing.

A theory is presented which, though too simple to explain quantitative details in the Josephson junction mixing response, is sufficient for explaining qualitatively the results observed. Crucial to the theory presented, and that which differentiates it from earlier ones, is the inclusion of harmonic voltages across the ideal Josephson element.

Thompson, E. D.↗
NASA NTRS2003

Fluctuations of the Phase Difference Across an Array of Josephson Junctions in Superfluid He-4

We present a formal thermodynamic treatment of superfluid flow in a Josephson junction. We show that the current and the phase difference are thermodynamic conjugate variables. We derive quantitative expressions for the rms fluctuations of these variables. Also, we discuss the thermodynamic stability and the thermal activation to the phase slip region. We apply the developed formalism to show why an array of apertures in He-4 can exhibit the Josephson effect near the Lambda transition despite strong thermal fluctuations.

Chui, T.↗
DOE OSTI2025

Designing an exciton-condensate Josephson junction in quantum Hall heterostructures

The exciton condensate, a counterpart of superconductors realized in two-dimensional quantum Hall bilayer systems, has yet to demonstrate definitive experimental evidence of phase coherence such as the Josephson effect. This work introduces a gate-defined Josephson junction designed to exhibit the in-plane Josephson effect in van der Waals heterostructures. Unlike superconducting S-I-S junctions, the proposed device leverages a nearly layer-polarized gated region to mediate Josephson coupling via layer pseudospin variation. This coupling mechanism is tunable through gate voltages and magnetic fields, offering a high degree of experimental control. We demonstrate the feasibility of the design using realistic parameters, paving the way for direct observation of excitonic phase coherence in clean quantum Hall systems.

Wang, Tianle↗
NASA NTRS1979

Characteristics of a Josephson junction harmonic mixer with external pumping

Numerical calculations are carried out on harmonic mixing with an externally pumped Josephson junction. Bias conditions for optimum conversion efficiency are studied for a variety of junction parameters. The predicted noise temperature for harmonic mixing is compared with that for fundamental mixing at the same signal frequency.

Taur, Y.↗
DOE OSTI2025

Sn-InAs Nanowire Shadow-Defined Josephson Junctions

Hybrid superconductor–semiconductor platforms are foundational to advancing quantum information technologies, motivating the integration of materials with clean interfaces, robust superconductivity, and scalable architectures. Here, in this work, we report the synthesis and analysis of inclined InAs nanowires, conformally coated with β-Sn shells. These nanowires extend in opposite in-plane directions, forming a self-aligned, criss-cross network. This enables the deterministic formation of nanowire-shadow Josephson junctions through angle-controlled, low-temperature Sn deposition. Structural characterization shows uniform polycrystalline β-Sn shells forming a sharp, diffusion-free interface with InAs. Low-temperature transport measurements reveal a hard induced superconducting gap ≈ 600 μeV, switching currents up to ≈ 500 nA, and parallel magnetic field resilience beyond 1T. These results establish β-Sn/InAs nanowire networks as a promising platform for superconducting qubits, low-noise microwave devices, and the exploration of exotic superconducting phases including triplet pairing and topological superconductivity.

B-Sn↗
NASA NTRS1974

The SNS Josephson junction with a third terminal

Discussion of the operating characteristics of a three-terminal thin-film SNS Josephson junction whose diameter is much greater than the electron pair coherence length in the N metal. It is shown that a junction of this type is essentially a two-terminal device even though the third terminal of the junction supplies the control current. The mechanism underlying this finding is discussed.

Prans, G. P.↗
NASA NTRS1977

Parametric amplification and oscillation at 36 GHz using a point-contact Josephson junction

The paper reports observation of doubly degenerate parametric amplification and oscillation at 36 GHz from a single point-contact Josephson junction. The experimental results agree qualitatively with theoretical calculations based on the resistively shunted junction model. The estimated noise temperature of an amplifier with 11 dB net gain is consistent with zero, but has an upper limit of 50 K. Attempts to observe parametric amplification in the singly degenerate mode with a pump frequency of 72 GHz were not successful.

Taur, Y.↗