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

TL;DR

This paper demonstrates that in certain conditions, coherent states can violate the Leggett-Garg inequality more strongly than cat states, challenging previous assumptions about quantum state quantumness.

Contribution

It provides a numerical analysis showing coherent states can surpass cat states in LGI violation, with explicit solutions for dissipative cavity modes.

Findings

Coherent states can have larger LGI violations than cat states.

LGI violation intensity diverges as measurement interval approaches zero.

Optimal measurement parameters can lead to intense LGI violations.

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 $|\alpha\rangle$ and the cat state $(|\alpha\rangle+|-\alpha\rangle)$. 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.