Dispersive Non-reciprocity between a Qubit and a Cavity

TL;DR

This paper demonstrates a non-reciprocal dispersive interaction between a qubit and a cavity, controlled via a ferrite component, revealing asymmetric dynamics and photon dephasing, modeled effectively by a simple master equation.

Contribution

It experimentally shows non-reciprocal dispersive coupling in a qubit-cavity system using a ferrite, and introduces a simple model to describe this complex interaction.

Findings

Observation of asymmetric frequency shifts and dephasing.

Tuning non-reciprocity via magnetic bias of ferrite.

Effective description with a non-reciprocal master equation.

Abstract

The dispersive interaction between a qubit and a cavity is ubiquitous in circuit and cavity quantum electrodynamics. It describes the frequency shift of one quantum mode in response to excitations in the other, and in closed systems is necessarily bidirectional, i.e.~reciprocal. Here, we present an experimental study of a non-reciprocal dispersive-type interaction between a transmon qubit and a superconducting cavity, arising from a common coupling to dissipative intermediary modes with broken time reversal symmetry. We characterize the qubit-cavity dynamics, including asymmetric frequency pulls and photon shot-noise dephasing, under varying degrees of non-reciprocity by tuning the magnetic field bias of a ferrite component in situ. Furthermore, we show that the qubit-cavity dynamics is well-described in a wide parameter regime by a simple non-reciprocal master-equation model, which provides a compact description of the non-reciprocal interaction without requiring a full understanding of the complex dynamics of the intermediary system. Our result provides an example of quantum non-reciprocal phenomena beyond the typical paradigms of non-Hermitian Hamiltonians and cascaded systems.