Eur. Phys. J. D (2021) 75: 167
https://doi.org/10.1140/epjd/s10053-021-00180-x

Regular Article - Quantum Optics

Violations of the Leggett–Garg inequality for coherent and cat states

¹ Nisshin-scientia Co., Ltd., 8F Omori Belport B, 6-26-2 MinamiOhi, Shinagawa-ku, 140-0013, Tokyo, Japan
² Graduate School of Humanities and Sciences, Ochanomizu University, 2-1-1 Ohtsuka, Bunkyo-ku, 112-8610, Tokyo, Japan

^a hiroo.azuma@m3.dion.ne.jp

Received: 17 January 2021
Accepted: 15 May 2021
Published online: 1 June 2021

Abstract

We show that in some cases the coherent state can have a larger violation of the Leggett–Garg inequality (LGI) than the cat state by numerical calculations. To achieve this result, we consider the LGI of the cavity mode weakly coupled to a zero-temperature environment as a practical instance of the physical system. We assume that the bosonic mode undergoes dissipation because of an interaction with the environment but is not affected by dephasing. Solving the master equation exactly, we derive an explicit form of the violation of the inequality for both systems prepared initially in the coherent state $|\alpha ⟩$ and the cat state $(|\alpha ⟩+|-\alpha ⟩)$. For the evaluation of the inequality, we choose the displaced parity operators characterized by a complex number $\beta$. We look for the optimum parameter $\beta$ that lets the upper bound of the inequality be maximum numerically. Contrary to our expectations, the coherent state occasionally exhibits quantum quality more strongly than the cat state for the upper bound of the violation of the LGI in a specific range of three equally spaced measurement times (spacing $\tau$). Moreover, as we let $\tau$ approach zero, the optimized parameter $\beta$ diverges and the LGI reveals intense singularity.