The supercurrent of a Josephson junction is reduced by phase diffusion. For ultrasmall capacitance junctions the current may be further decreased by Coulomb blockade effects. We calculate the Cooper pair current by means of time-dependent perturbation theory to all orders in the Josephson coupling energy and obtain the current-voltage characteristic in closed form in a range of parameters of experimental interest. The results comprehend phase diffusion of the coherent Josephson current in the classical regime as well as the supercurrent peak due to incoherent Cooper pair tunneling in the Coulomb blockade regime.
. In circuits of underdamped, small-capacitance Josephson junctions, the charge and phase variables behave quantum mechanically. Superpositions of charge states and flux states have been demonstrated, as well as microwave induced quantum dynamics (see e.g. refs [3, 4]). These e#ects have their origin in the conjugation of the phase and charge variables of Josephson circuits. The qubit design that we present here also contains small-capacitance Josephson junctions. However, even though charge-phase conjugation underlies the quantum behavior of the qubit, it was explicitly designed to be very insensitive to charge e#ects. In practice, random background charges and 1/f charge noise are always present in Josephson circuits, but these charge e#ects have negligible influence on the qubit. The qubit loop contains three Josephson junctions. Two junctions have Josephson coupling E J , the third junction has a coupling of #E J .Weaim
Josephson junctions constructed from superconductor-semiconductor-superconductor heterostructures have been used to realize a variety of voltage-tunable superconducting quantum devices, including qubits and parametric amplifiers. To date, theoretical descriptions of these systems have been restricted to small quantum fluctuations of the junction phase, making them inapplicable to many experiments. In this paper, we relax this, employing a path-integral formulation where the phase quantum dynamics is obtained self-consistently from an underlying many-body formalism. Our method recovers previously known results for small phase fluctuations, and predicts effects outside of that limit: (i) system capacitances undergo a gate-voltage-dependent renormalization; and (ii) an additional charge offset appears for asymmetric junctions. Our main results can be summarized in terms of a single-particle Hamiltonian, which can be directly compared to that of an ordinary Josephson junction. This more general theory could be a first step towards designing new quantum devices that go qualitatively beyond voltage-tunable variants of previously known circuits.
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. \\
A quantum mesoscopic electrical LC-circuit with charge discreteness including a Josephson junction is considered and a nonlinear Hamiltonian that describing the dynamic of such circuit is introduced. The quantum dynamical behavior (persistent current probability) is studied in the charge and phase regimes by numerical solution approaches. The time evolution of charge and current, number-difference and the bosonic phase and also the energy spectrum of a quantum mesoscopic electric LC-circuit with charge discreteness that coupled with a Josephson junction device are investigated. We show the role of the coupling energy and the electrostatic Coulomb energy of the Josephson junction in description of the quantum behavior and the spectral properties of a quantum mesoscopic electrical LC-circuits with charge discreteness.
Here we present the direct observation of macroscopic quantum properties in an all high-critical-temperature superconductor d-wave Josephson junction. Although dissipation caused by low-energy excitations is expected to strongly suppress macroscopic quantum effects, we demonstrate energy level quantization in our d-wave Josephson junction. The result indicates that the role of dissipation mechanisms in high-temperature superconductors has to be revised, and it may also have consequences for the class of solid-state "quiet" quantum bits with superior coherence time.