We have investigated the dynamics of underdamped Josephson junctions with a relatively small Josephson coupling ${E}_{J}$ . 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 ( $T$ , ${E}_{J}$ ) phase diagram with various regimes and show that for a proper description of it, dissipation and level quantization in a metastable well are crucial.We have investigated the dynamics of underdamped Josephson junctions with a relatively small Josephson coupling ${E}_{J}$. 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 ($T$, ${E}_{J}$) phase diagram with various regimes and show that for a proper description of it, dissipation and level quantization in a metastable well are crucial.

arXiv:cond-mat/0501383Physical Review LettersJun 22, 200590 citations

Resonant switching current detector based on underdamped Josephson junctions

V. M. Krasnov

Current-biased Josephson junctions can act as detectors of electromagnetic radiation. At optimal conditions, their sensitivity is limited by fluctuations causing stochastic switching from the superconducting to the resistive state. This work provides a quantitative description of a stochastic switching current detector, based on an underdamped Josephson junction. It is shown that activation of a Josephson plasma resonance can greatly enhance the detector responsivity in proportion to the quality factor of the junction. The ways of tuning the detector for achieving optimal operation are discussed. For realistic parameters of Nb / Al O x / Nb tunnel junctions, the sensitivity and noise-equivalent power (NEP) can reach values of S ≃ 5 × 10 12 (V/W) and NEP ≃ 2 × 10 − 23 ( WHz − 1 / 2 ), respectively. These outstanding characteristics facilitate both bolometric and single-photon detection in microwave and terahertz ranges. Published by the American Physical Society 2024Current-biased Josephson junctions can act as detectors of electromagnetic radiation. At optimal conditions, their sensitivity is limited by fluctuations causing stochastic switching from the superconducting to the resistive state. This work provides a quantitative description of a stochastic switching current detector, based on an underdamped Josephson junction. It is shown that activation of a Josephson plasma resonance can greatly enhance the detector responsivity in proportion to the quality factor of the junction. The ways of tuning the detector for achieving optimal operation are discussed. For realistic parameters of Nb / Al O x / Nb tunnel junctions, the sensitivity and noise-equivalent power (NEP) can reach values of S ≃ 5 × 10 12 (V/W) and NEP ≃ 2 × 10 − 23 ( WHz − 1 / 2 ), respectively. These outstanding characteristics facilitate both bolometric and single-photon detection in microwave and terahertz ranges. Published by the American Physical Society 2024

arXiv:2403.03803Physical Review AppliedAug 6, 20249 citations

Study of Phase Dynamics in Moderately Damped Josephson Junctions

D. Massarotti, Luigi Longobardi, D. Stornaiuolo +5 more

Journal of Superconductivity and Novel MagnetismDec 29, 20122 citations

Experimental tests for the quantum behavior of a macroscopic degree of freedom: The phase difference across a Josephson junction

John M. Martinis, Michel Devoret, John L. Clarke

Experiments are described that demonstrate the quantum behavior of a macroscopic degree of freedom, namely the phase difference \ensuremath{\delta} across a current-biased Josephson tunnel junction. The behavior of \ensuremath{\delta} was deduced from measurements of the escape rate \ensuremath{\Gamma} of the junction from its zero-voltage state. The relevant parameters of the junction, that is, its critical current and shunting admittance, were determined in situ in the thermal regime from the dependence of \ensuremath{\Gamma} on bias current and from resonant activation in the presence of microwaves. It was found that the shunting capacitance was dominated by the self-capacitance of the junction while the shunting conductance was dominated by the bias circuitry. For an underdamped junction in the quantum regime, \ensuremath{\Gamma} became independent of temperature at low temperatures with a value that, with no adjustable parameters, was in excellent agreement with predictions for macroscopic quantum tunneling at T=0. When the critical current was reduced with a magnetic field so that the junction remained in the thermal regime at low temperatures, \ensuremath{\Gamma} followed the predictions of the thermal model, thereby showing the influence of extraneous noise to be negligible. In a further series of experiments, the existence of quantized energy levels in the potential well of the junction was demonstrated spectroscopically. The positions of the energy levels agreed quantitatively with quantum-mechanical predictions involving junction parameters measured in the thermal regime. The relative heights and widths of the resonances are in reasonable agreement with the predictions of a simple model.Experiments are described that demonstrate the quantum behavior of a macroscopic degree of freedom, namely the phase difference \ensuremath{\delta} across a current-biased Josephson tunnel junction. The behavior of \ensuremath{\delta} was deduced from measurements of the escape rate \ensuremath{\Gamma} of the junction from its zero-voltage state. The relevant parameters of the junction, that is, its critical current and shunting admittance, were determined in situ in the thermal regime from the dependence of \ensuremath{\Gamma} on bias current and from resonant activation in the presence of microwaves. It was found that the shunting capacitance was dominated by the self-capacitance of the junction while the shunting conductance was dominated by the bias circuitry. For an underdamped junction in the quantum regime, \ensuremath{\Gamma} became independent of temperature at low temperatures with a value that, with no adjustable parameters, was in excellent agreement with predictions for macroscopic quantum tunneling at T=0. When the critical current was reduced with a magnetic field so that the junction remained in the thermal regime at low temperatures, \ensuremath{\Gamma} followed the predictions of the thermal model, thereby showing the influence of extraneous noise to be negligible. In a further series of experiments, the existence of quantized energy levels in the potential well of the junction was demonstrated spectroscopically. The positions of the energy levels agreed quantitatively with quantum-mechanical predictions involving junction parameters measured in the thermal regime. The relative heights and widths of the resonances are in reasonable agreement with the predictions of a simple model.

Physical review. B, Condensed matterApr 1, 1987572 citations

We have investigated the dynamics of underdamped Josephson junctions with a relatively small Josephson coupling

{E}_{J}

. 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 (

T

{E}_{J}

) phase diagram with various regimes and show that for a proper description of it, dissipation and level quantization in a metastable well are crucial.