Spectroscopy of Quantum Phase Slips: Visualizing Complex Real-Time Instantons

TL;DR

This paper investigates quantum phase slips in parametrically driven oscillators, revealing how weak AC signals influence phase-slip rates and how the spectrum of response can visualize real-time instantons, offering insights for qubit control.

Contribution

It demonstrates that the phase-slip rate is highly sensitive to weak AC perturbations and introduces the use of the logarithmic susceptibility spectrum to visualize real-time instantons.

Findings

Phase-slip rate exponentially depends on weak AC signals.

Logarithmic susceptibility spectrum reveals real-time instanton features.

Potential for improved qubit control through spectral analysis.

Abstract

Parametrically driven oscillators can emerge as a basis for the next generation of qubits. Classically, these systems exhibit two stable oscillatory states with opposite phases. Upon quantization, these states turn into a pair of closely spaced Floquet states, which can serve as the logical basis for a qubit. However, interaction with the environment induces phase-slip events which set a limit on qubit coherence. Such phase slips persist even at zero temperature due to a mechanism known as quantum activation \cite{QuantumActivation}. In contrast to conventional tunneling, the quantum activation is described by a {\em real-time} instanton trajectory in the complexified phase space of the system. In this work, we show that the phase-slip rate is exponentially sensitive to weak AC perturbations. The spectrum of the system's response -- captured by the so-called logarithmic susceptibility (LS) -- enables a direct observation of characteristic features of real-time instantons. Studying this spectrum suggests new means of efficient qubit control.