Inverse superconductor-insulator transition in weakly monitored Josephson junction arrays

Abstract

Control and manipulation of quantum states by measurements and bath engineering in open quantum systems, like in a quantum computer, have emerged as new paradigms in many-body physics. Taking a prototypical example of Josephson junction arrays (JJAs), we show how monitoring through continuous weak measurements and feedback can transform an insulating steady state in JJAs to a superconductor (SC) and vice versa. We show that the quantum feedback, when not too large, is crucial to realize such a superconducting steady state by maintaining an effective temperature in a semiclassical limit. However, we show that large feedback can destroy superconductivity by enhancing quantum phase fluctuations. Thus, dissipation and phase fluctuations in the presence of monitoring have fundamental differences from those in the well-studied case of JJAs in contact with an equilibrium Ohmic bath. In general, a description in terms of only an effective temperature is not adequate for the monitored JJAs. Using a variational approximation, and by considering various limiting cases, we demonstrate that these differences can give rise to reentrant SC-insulator phase transitions leading to an unusual inverse transition from a seemingly low-effective-temperature insulating normal state to a superconducting state at intermediate temperature.

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