The moderately damped regime in Josephson junctions (JJ) is quite common in devices characterized by low critical currents and, therefore, by low Josephson energies. Measurements of the switching current distribution (SCD) are a direct way of determining the phase dynamics in the nontrivial case of moderate damping, which is becoming more common with advances in nanopatterning superconductors and in materials science for fabricating hybrid systems. We report measurements of SCDs, in both the thermal and quantum regimes for moderately damped YBaCuO grain boundary biepitaxial JJs. A direct transition from a phase diffusion regime to macroscopic quantum tunnelling occurs at about 130 mK. The crossover to the quantum regime is determined by the magnetic field and the phase dynamics can be described by a fully consistent set of junction parameters obtained by numerical simulation.
We have measured the switching current versus applied magnetic flux of a highly asymmetric, hysteretic dc superconducting quantum interference device (SQUID) from 30 mK to 400 mK. The SQUID loop had an inductance of 1 nH, and the capacitances of the two Al-Al2O3-Al Josephson junctions comprising the SQUID were 100 fF and 5 fF. As expected, the switching current of the SQUID displayed a small, periodic modulation with applied flux, from which the switching currents of the large and small junctions at 30 mK were determined to be 846.3 ± 0.1 nA and 0.7 ± 0.08 nA, respectively. Measurements of an identically prepared small junction exhibited a highly suppressed switching current (IS < 10 pA), consistent with phase diffusion in this device. The observed modulation in the SQUID thus demonstrates that the quantum mechanical uncertainty in the phase difference across the small junction was significantly reduced by coupling to the large junction through the loop inductance. We discuss the sensitivity of this technique for determining the gauge-invariant phase difference of small Josephson junctions, and the implications for imaging the phase of superconductors at the atomic scale using scanning tunneling microscopy.
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.
The observation of macroscopic quantum tunneling (MQT) manifests the quantumness of macroscopic variables, such as the phase difference across a Josephson junction (JJ). Since then, MQT has become a hallmark experiment to show the quantum nature of JJ-based devices. It has also paved a way to test other macroscopic quantum effects and has led to engineering a variety of artificial atoms, i.e., qubits. However, in the progress of quantum circuits for different applications, it is crucial to understand the influence of thermal and quantum fluctuations, dissipation, and noise to various quantum devices. Thus, the role of the factors mentioned above and the junction parameters
EJ (Josephson coupling energy) and
EC (charging energy) can be scrutinized by understanding the dynamics of the fictitious phase particle. Similarly, in the application perspective, Szilard engine based on SET device are too slow to convert heat into work and are not efficient in-terms of power performance. To overcome the drawbacks, we proposed a high-efficiency fast Szilard engine based on a flux logic device (FLD) and investigated its property numerically and optimize its performance. \\\\ In the first part of the dissertation, we studied the phase dynamics of moderately damped Josephson junctions. The time of flight technique is used to measure the switching current distributions (SCDs). Different dynamical states, thermal activation (TA), MQT, and phase diffusion (PD) are identified from SCDs through well-defined criteria. We observed unambiguous evidence of MQT when temperature is below
∼0.5-0.8 K. The importance of the tilt in the phase dynamics of the junctions, which has been overlooked in previous studies, is clarified. We found that the junction with larger
EJ but the same
ωp0 requires greater tilt and lower temperature to enter MQT regime. \\ \\ In the second part of the dissertation, to overcome the limitation of the SET based Szilard engine, we used superconducting quantum interference device (SQUID) as a one-bit flux logic cell to realize a fast Szilard engine for optimal power. In the past decade, due to technological advancement, the study of energy fluctuations in nano-scale systems such as optical or electrical traps and single-electron tunneling (SET) devices has transformed Szilard's engine to experimental realizations. However, the heat engine realized are slow to convert heat into work limiting their performance in speed and power. The SQUID-based flux logic device (FLD) has a high characteristic frequency,
ωp/2$\pi \sim
10^{10}$ Hz, making it an excellent candidate for realizing a high-efficiency Szilard engine with unprecedented performance. We showed that, on average, the proposed heat engine can extract k
BTln2 of heat per cycle from the thermal reservoir in the quasi-static (QS) limit. In addition, we showed that the FLD-based Szilard engine delivers maximum power when operated about 200 times faster than the threshold speed of quasi-static operation. Our result demonstrates that Szilard's engine's performance based on FLD exceeds other nano-device implementations by orders of magnitude. \\
By measuring the current–voltage characteristics and the switching current distributions as a function of temperature, we have investigated the phase dynamics of Al tunnel ferromagnetic Josephson junctions (JJs), designed to fall in the typical range of parameters of state-of-the-art transmons, providing evidence of phase diffusion processes. The comparison with the experimental outcomes on non-magnetic JJs with nominally the same electrodynamical parameters demonstrates that the introduction of ferromagnetic barriers does not cause any sizeable detrimental effect and supports the notion of including tunnel ferromagnetic JJs in qubit architectures.
We study a 0-$\ensuremath{\pi}$ dc superconducting quantum interference device (SQUID) with asymmetric inductances and critical currents of the two Josephson junctions (JJs). By considering such a dc SQUID as a black box with two terminals, we calculate its effective current-phase relation ${I}_{s}(\ensuremath{\psi})$ and the Josephson energy $U(\ensuremath{\psi})$, where $\ensuremath{\psi}$ is the Josephson phase across the terminals. We show that there is a domain of parameters where the black box has the properties of a $\ensuremath{\varphi}$ JJ with degenerate ground state phases $\ensuremath{\psi}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\varphi}$. The $\ensuremath{\varphi}$ domain is rather large, so one can easily construct a $\ensuremath{\varphi}$ JJ experimentally. We derive the current phase relation and show that it can be tuned in situ by applying an external magnetic flux resulting in a continuous transition between the systems with static solutions $\ensuremath{\psi}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\varphi},\phantom{\rule{0.16em}{0ex}}\ensuremath{\psi}={\ensuremath{\varphi}}_{0} ({\ensuremath{\varphi}}_{0}\ensuremath{\ne}0,\ensuremath{\pi})$ and even $\ensuremath{\psi}={\ensuremath{\varphi}}_{0}\ifmmode\pm\else\textpm\fi{}\ensuremath{\varphi}$. The dependence of ${\ensuremath{\varphi}}_{0}$ on applied magnetic flux is not $2\ensuremath{\pi}$ (one flux quantum) periodic.