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Applied Physics Letters

Extremely underdamped Josephson junctions for low noise applications

B. Ruggiero, C. Granata, E. Esposito, M. Russo, P. Silvestrini

Published Jul 5, 1999• 16 citations
Publisher

Abstract

We present a design of extremely underdamped Josephson junctions either for use in experiments in the quantum limit or for devices using the switching dynamics of the junction, when a low noise level is mandatory. High resistance molybdenum meander lines integrated with the junction allow one to increase the insulation of the sample from its electromagnetic environment. We report an experimental study of the effective dissipation relevant in the thermally activated supercurrent decay presenting evidence of a substantial reduction of the dissipation level at low temperature.

DOI
10.1063/1.124295

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We present a design of extremely underdamped Josephson junctions either for use in experiments in the quantum limit or for devices using the switching dynamics of the junction, when a low noise level is mandatory. High resistance molybdenum meander lines integrated with the junction allow one to increase the insulation of the sample from its electromagnetic environment. We report an experimental study of the effective dissipation relevant in the thermally activated supercurrent decay presenting evidence of a substantial reduction of the dissipation level at low temperature.
We present an experimental study of the effective dissipation relevant in the thermally activated supercurrent decay of extremely underdamped Josephson junctions. Data referring to the supercurrent decay of Nb/AlOx/Nb{\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}\mathrm{O}}_{x}/\mathrm{Nb}Nb/AlOx​/Nb Josephson junctions are compared with the Kramers theory. Our measurements allow us to obtain the ``effective'' resistance to be used in the resistively shunted junction model that results to be the subgap resistance due to the presence of thermally activated quasiparticles. The extremely low dissipation level obtained at low temperatures renders our result quite interesting in view of experiments in the quantum limit.
We have studied the temperature dependence of escape phenomena in various underdamped Josephson junctions (JJs). The junctions had different Josephson coupling energies E J which were relatively small, but larger than the charging energy E C . Upon increasing the temperature T , we first observe the usual cross-over between macroscopic quantum tunnelling and thermally activated (TA) behaviour at temperatures k B T ∼ ℏω p , where ω p is the plasma frequency of the junction. Increasing T further, the width of the switching current distribution has, counterintuitively, a non-monotonic temperature dependence. This can be explained by the novel cross-over from TA behaviour to underdamped phase diffusion. We show that this cross-over is expected to occur at temperatures such that k B T ∼ E J (1 − 4/π Q ) 3 /2, where Q is the quality factor of the junction at the plasma frequency, in agreement with experiment. Our findings can be compared with detailed model calculations which take into account dissipation and level quantization in a metastable well. Particular attention is paid to the sample with the smallest E J , which shows extensive phase diffusion even at the lowest temperatures. This sample consists of a dc-SQUID and a single JJ close to each other, such that the SQUID acts as a tunable inductive protection for the single junction from fluctuations of a dissipative environment. By varying the flux through the dc-SQUID, we present, for the first time, experimental evidence of the escape of a JJ from the phase diffusion regime to the free running state in a tunable environment. We also show that in the zero voltage state the losses mainly occur at frequencies near the plasma resonance.
Macroscopic quantum effects in Josephson systems have recently attracted interest in the scientific community both for the physics involved and in view of applications. Here we present experiments on the presence of energy level quantization at temperatures above the classical-quantum regime crossover temperature in different Josephson junctions. This has been possible by extending the measurements of the escape rate out of the zero-voltage state at higher sweeping bias rate (dI/dt up to 100 A/sec) in order to induce nonstationary conditions in the energy potential describing the junction dynamics. Our measurements allow us to obtain as fitting parameter the "effective" resistance to be used in the RSJ model which is compatible with the subgap resistance at very low voltage. The very low dissipation level obtained at low temperatures renders our results quite interesting in view of applications in the quantum limit.
We present an experimental characterization of Nb∕AlOx∕Nb\mathrm{Nb}∕\mathrm{Al}{\mathrm{O}}_{x}∕\mathrm{Nb}Nb∕AlOx​∕Nb Josephson devices in the quantum regime. Josephson junctions and fully integrated Josephson devices consisting of an rf SQUID coupled to a read-out system based on a dc SQUID have been measured. Data on the decay rates from the metastable states show evidence of macroscopic quantum tunneling with a very low dissipation level and a high isolation of the devices from the external environment.
We have investigated the dynamics of underdamped Josephson junctions with a relatively small Josephson coupling EJ{E}_{J}EJ​. In addition to the usual crossover between macroscopic quantum tunneling and thermally activated (TA) behavior we observe, for the first time, the transition from TA behavior to underdamped phase diffusion. Above the crossover temperature, the threshold for switching into the finite-voltage state becomes extremely sharp. We propose a (TTT, EJ{E}_{J}EJ​) phase diagram with various regimes and show that for a proper description of it, dissipation and level quantization in a metastable well are crucial.
A new phase diffusive regime in a current biased Josephson junction is theoretically explored which originates from embedding the junction in a circuit environment with anomalous dissipation. This is realized by placing parallel to the junction a resistor in series with a capacitor such that electromagnetic fluctuations effectively couple also to the charge of the junction. This leads to rich Josephson dynamics, in particular for the switching of the junction out of a zero voltage state. Modelled as the escape process of a fictitious phase-particle out of a metastable well, a detailed study reveals that anomalous dissipation has a strong impact at low temperatures when quantum tunneling dominates against thermal activation. As a manifestation, a regime is found, where for realistic circuit parameters the quantum escape process is substantially enhanced, followed by a short voltage pulse and re-trapping with high probability. This class of circuits may be leveraged for detecting microwave photons or dissipative quantum annealing processes. In addition, the analysis provides a general framework for engineering dissipative dynamics in nonlinear systems using anomalous environments.