Inverse Superconductor-Insulator Transition in Weakly Monitored Josephson Junction Arrays

TL;DR

This paper demonstrates how continuous weak measurements and feedback in Josephson junction arrays can induce an inverse superconductor-insulator transition, revealing new non-equilibrium phase behaviors driven by quantum feedback.

Contribution

It introduces a novel mechanism where monitoring and feedback control induce inverse phase transitions in Josephson junction arrays, distinct from thermal or equilibrium dissipation effects.

Findings

Monitoring leads to a steady state with an effective quantum temperature.

Quantum dissipation from monitoring differs from thermal bath dissipation.

Inverse transition occurs from insulator to superconductor at intermediate temperatures.

Abstract

Control and manipulation of quantum states by measurements and bath engineering in open quantum systems have emerged as new paradigms in many-body physics. Here, taking a prototypical example of Josephson junction arrays (JJAs), we show how repetitive monitoring through continuous weak measurements and feedback can transform an insulating state in these systems to a superconductor and vice versa. We show that, even in the absence of any external thermal bath, the monitoring leads to a long-time steady state characterized by an effective `quantum' temperature in a suitably defined semiclassical limit. However, we show that the quantum dissipation due to monitoring has fundamental differences with equilibrium quantum and/or thermal dissipation in the well-studied case of JJAs in contact with an Ohmic bath. In particular, using a variational approximation, and by considering various limiting cases, we demonstrate that this difference can give rise to re-entrant steady-state phase transitions, resulting in unusual inverse transition from an effective low-temperature insulating normal state to superconducting state at intermediate temperature. Our work emphasizes the role of quantum feedback, that acts as an additional knob to control the effective temperature of non-equilibrium steady state leading to a phase diagram, not explored in earlier works on monitored and open quantum systems.