Josephson junctions with nonlinear damping for rapid single-flux-quantum - qubit circuits

Abstract

We demonstrate that shunting of Superconductor-Insulator-Superconductor (S-I-S) Josephson junctions by Superconductor-Insulator-Normal metal (S-I-N) structures having pronounced nonlinear I-V characteristics can remarkably modify the Josephson dynamics. In the regime of Josephson generation the phase behaves as an overdamped coordinate, while in the superconducting state the damping and current noise are strikingly small, that is vitally important for application of such junctions for readout and control of Josephson qubits. Superconducting Nb/AlOx/Nb junction shunted by Nb/AlOx/AuPd junction of S-I-N type was fabricated and, in agreement with our model, exhibited non-hysteretic I-V characteristics at temperatures down to at least 1.4 K.

Dynamics of Josephson junctions and single-flux-quantum networks with superconductor–insulator–normal-metal junction shunts

“…The most challenging condition to be met is the realization of high-quality SIN junctions with a high transparency of the tunnel barrier, having a specific resistance Շ 30 ⍀ m 2 . 11 The recent experiments with Nb-AlO x -Al ͑at T Ն 1.4 K͒ and Al-AlO x -Cu ͑at T Յ 1 K͒ junctions have shown that the ratio of the zero-bias resistance to the asymptotic tunnel resistance can achieve sufficiently large values, i.e., Ͼ30-50. 26,27 ͓This behavior of the SIN junctions can be roughly described by the two-term approximation of the memory kernel equation ͑18͒.͔ Using such SIN junctions as shunts for Nb SIS junctions can make it possible to significantly reduce the noise of the circuits in the quiescent state in the frequency range up to ⌬ Al / h Ϸ 50 GHz, i.e., within the working frequency range of the Josephson qubits of different types.…”

Double SQUID tunable flux qubit manipulated by fast pulses: operation requirements, dissipation and decoherence

“…These problems can be solved by developing an unconventional RSFQ logic [26], together with an appropriate filtering of the transmitted signal and an optimization of the qubit parameters. Different efforts are starting in this direction, in particular in the frame of the UE project "RSFQubit", and first prototypes of chips with an RSFQ flip-flop coupled to a tunable flux qubit are under fabrication.…”

Subgap conductivity in SIN-junctions of high barrier transparency

“…Due to the superconducting energy gap ∆ in the quasiparticle density of states, the current through such an interface is generally suppressed at low temperatures, k B T ≪ ∆, and subgap bias voltages, V < ∼ V g ≡ ∆/e, giving rise to a low-voltage nonlinearity in the IV -curve [1]. Recently this, the most common property of SIN-contacts has been found to be rather useful, e.g., for reducing generation of quasiparticles by building the current shunts into the Josephson-junction qubits [2] or for avoiding an extra decoherence [3] in all-Josephson RSFQ-qubit integrated systems [4].…”