Nonreciprocal transition between two indirectly coupled energy levels

TL;DR

This paper presents a theoretical scheme for achieving nonreciprocal transitions between two energy levels that are indirectly coupled, using a cyclic four-level system controlled by external fields and synthetic magnetic flux.

Contribution

It introduces a novel method to realize nonreciprocal transitions in multi-level systems via external control and symmetry breaking, applicable to natural atoms and ions.

Findings

Nonreciprocal transition identified by spectral line elimination.

Breaking time reversal symmetry via synthetic magnetic flux.

Feasible observation method in natural multi-level systems.

Abstract

We propose a theoretical scheme to realize nonreciprocal transition between two energy levels that can not coupled directly. Suppose they are coupled indirectly by two auxiliary levels with a cyclic four-level configuration, and the four transitions in the cyclic configuration are controlled by external fields. The indirectly transition become nonreciprocal when the time reversal symmetry of the system is broken by the synthetic magnetic flux, i.e., the total phase of the external driving fields through the cyclic four-level configuration. The nonreciprocal transition can be identified by the elimination of a spectral line in the spontaneous emission spectrum. Our work introduces a feasible way to observe nonreciprocal transition in a wide range of multi-level systems, including natural atoms or ions with parity symmetry.