Dynamics of Josephson junctions in a superconducting ring using a “Three-Coils” technique

Joonho Jang, Raffi Budakian, David Ferguson +3 more

John Gilbert Wilson

Vasili K. Semenov, Yuri A. Polyakov, Sergey K. Tolpygo

Existing concepts of reversible superconducting circuits as well as demonstrated adiabatic circuits require three-phase bias/clock signals generated by room temperature sources. A while ago, we suggested that a multi-phase bias/clock could be provided by a local Josephson junction-based generator. The generator looks like a long annular Josephson junction, only composed of discreet elements - junctions and inductors, and closed into a ring via a flux pump to inject any required number of vortices into the ring. A steady motion of the vortices forced by a uniformly distributed dc bias current applied to the ring is accompanied by a nearly harmonic ac currents flowing via the Josephson junctions (JJs) connected in series with small inductors. These ac currents serve as multi-phase bias/clock for nSQUID-based circuitry. To verify this concept and compare the dissipated energy with kBTln2 threshold, we developed a ring composed of 256 unshunted JJs with 20 μA target critical current, Ic. We investigated the behavior of the ring oscillator at each vortex count from 0 to 256. The measured critical current of the ring with vortices was about 0.1 μA per one JJ, which can be explained by unavoidable nonuniformity of the ring components and the influence of fluxes frozen near the ring. The corresponding energy dissipation, about 10kBT per passage of one vortex through one JJ, should be reduced further for prospective experiments with reversible circuits. However, obtained I-V characteristics could be of interest for scientists working with long Josephson junctions. Superiority of the fabrication process used in this work is demonstrated by the obtained about 200 times reduction of Icof the ring with vortices with respect to a single comprising JJ, much larger than in any previously described case.

J. Wilson, Robert I. Gayley

Joseph Weston

Yituo Zhao, Hai Wang, Shijian Wang +4 more

Superconducting detectors, with their advantages of low noise and high energy resolution, have become key components for astronomical observation. Time-division multiplexing (TDM) scheme is a widely used technology for the readout of large sensor arrays, owing to its advantages in scalability, ease of integration, and low-noise performance. A primary limitation, however, is the degradation of the signal-to-noise ratio due to noise aliasing, which constrains the scaling of the multiplexing factor. To address this issue, we developed a front-end superconducting switch based on Josephson junction (JJ). Specifically, we fabricated Josephson junction switch (JJ-switch) cells where each junction is shunted by a 4 Ω resistor to ensure its non-hysteretic current-voltage characteristics. Three independent channels are designed on one switch chip, the inductance of Lin and SQUID are fixed values, but the resistance of Rin has three values: 0.1 Ω, 1 Ω and 2.5 Ω. The output of each switch channel is magnetically coupled to a dedicated input superconducting quantum interference device (SQUID) via an independent input coil, with a coefficient of 4.68 μA/Φ0. Finally, the signal transmission ratio, noise, and system sensitivity of the switch were characterized at an operating temperature of 4.2 K. This work verifies the transmission characteristics of the JJ-switch, which will provide a new device foundation for the TDM readout of large-scale superconducting detector arrays