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北京理工大学学报：英文版

Zero-Point Energy and Fluctuation in a Field-Josephson Junction-Coupled System

Bin Shao

Published Jan 1, 2000• 0 citations
OpenAlex

Abstract

To investigate the interaction of a mesoscopic Josephson junction with a single mode quantized electromagnetic field, the diagnonalizing process of the coupled system Hamiltonian was performed by using an unitary transformation. Zero? point energy and zero? point fluctuation of the coupled system were shown. For the ground state of the coupled system, the quantum fluctuations of the flux and charge operators in the field and junction subsystems can exhibit smaller quantum noise than the zero point fluctuations of the individual subsystems. Thus, when the coupled system is in its lowest energy state, which means that no radiation appear, the two subsystems exhibit nonclassical behavior.

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To investigate the interaction of a mesoscopic Josephson junction with a single mode quantized electromagnetic field, the diagnonalizing process of the coupled system Hamiltonian was performed by using an unitary transformation. Zero? point energy and zero? point fluctuation of the coupled system were shown. For the ground state of the coupled system, the quantum fluctuations of the flux and charge operators in the field and junction subsystems can exhibit smaller quantum noise than the zero point fluctuations of the individual subsystems. Thus, when the coupled system is in its lowest energy state, which means that no radiation appear, the two subsystems exhibit nonclassical behavior.
Quantum fluctuations of the charge at a current-biased Josephson junction and of the Josephson current couple to the quasiparticle tunneling when the system is at equilibrium in the zero-voltage state. The effect of reducing the capacitance of the junction is studied by means of a model Hamiltonian. When this becomes smaller, charge fluctuations are reduced, but the effective dissipation is also lowered so that the junction displays an underdamped behavior. The Josephson branch is shifted at the voltage e/2C because of charging due to the quasiparticles. A path-integral formulation of the model leads to a high dynamical subgap resistance at I\ensuremath{\rightarrow}0, as recently found experimentally.
Dynamic evolution of a mesoscopic Josephson junction (MJJ) interacting with quantized electromagnetic fields (QEMF) is investigated. It is shown that the current-voltage steps of the MJJ can reflect quantum coherences of the external QEMF; these characteristics may be used to distinguish different types of quantum electromagnetic fields. It is found that supercurrent fluctuations exhibit periodic oscillations in the time evolution. Supercurrent correlation functions are also studied in some detail. It is shown that the thermal noise in the external electromagnetic field seriously affects the second-order correlation function of the supercurrent. \textcopyright{} 1996 The American Physical Society.