*Article* **Coherence and Anticoherence Induced by Thermal Fields**

**Lihui Sun 1, Ya Liu 1, Chen Li 1, Kaikai Zhang 1, Wenxing Yang 1,2,\* and Zbigniew Ficek 3,\***


**Abstract:** Interesting coherence and correlations appear between superpositions of two bosonic modes when the modes are parametrically coupled to a third intermediate mode and are also coupled to external modes which are in thermal states of unequal mean photon numbers. Under such conditions, it is found that one of linear superpositions of the modes, which is effectively decoupled from the other modes, can be perfectly coherent with the other orthogonal superposition of the modes and can simultaneously exhibit anticoherence with the intermediate mode, which can give rise to entanglement between the modes. It is shown that the coherence effects have a substantial effect on the population distribution between the modes, which may result in lowering the population of the intermediate mode. This shows that the system can be employed to cool modes to lower temperatures. Furthermore, for appropriate thermal photon numbers and coupling strengths between the modes, it is found that entanglement between the directly coupled superposition and the intermediate modes may occur in a less restricted range of the number of the thermal photons such that the modes could be strongly entangled, even at large numbers of the thermal photons.

**Keywords:** coherence; anticoherence; entanglement; nonlinear systems

**Citation:** Sun, L.; Liu, Y.; Li, C.; Zhang, K.; Yang, W.; Ficek, Z. Coherence and Anticoherence Induced by Thermal Fields. *Entropy* **2022**, *24*, 692. https://doi.org/ 10.3390/e24050692

Academic Editor: Adam Gadomski

Received: 25 April 2022 Accepted: 12 May 2022 Published: 13 May 2022

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