Coherence in a dissipative two-level system

Jiří Blažek

Weichen Wang, Mao-Fa Fang

We have constructed a non-Hermitian two-level system (a PT -symmetric system) in dissipative environments, and investigated the quantum coherence in the non-Hermitian two-level system. Our results show that, quantum coherence can be created by PT -symmetric systems, even if the initial state of the twolevel system is incoherent state. Even though two-level system is interacted with dissipative environments, the quantum coherence exhibits a long-lived revival, and can be protected. We find that the two-level system can obtain more coherence with the coupling strength Ω increases. And we should point out that the PT -symmetric system can be regarded as a good candidate system for creation of the long-lived quantum coherence in dissipative environments.

Katja Kustura, Oriol Romero‐Isart, Carlos Gonzalez-Ballestero

We derive a Born-Markov master equation describing the dissipation induced by a bath of lossy but coherently driven two-level systems (TLSs) coupled to a bosonic system via Jaynes-Cummings interaction. We analytically derive all the master-equation rates. We characterize these rates for the particular case of a single-mode system coupled to identical TLSs. We study the steady state of the system and its exotic properties stemming from the nonthermal stationary state of the driven TLS bath. These properties include dissipative amplification, bath-induced linear instability, and both coherent and dissipative squeezing. The master equation is valid for arbitrarily strong TLS driving and it can be generalized to include multilevel systems or other system-bath interaction terms, among others. Our work provides a tool to study and characterize TLS-induced decoherence, a key limiting factor in quantum technological devices based on, for instance, superconducting circuits, magnonic systems, or quantum acoustics.

Zhen-huan Yang, Dan-Bo Zhang

Spontaneous macroscopic quantum synchronization is an emergent phenomenon where an ensemble of quantum oscillators achieves global phase coherence through the interplay of interaction and dissipation. To illuminate this phenomenon, we study an ensemble of two-level systems (TLS) and establish its associated nonlinear quantum master equation, for which self-consistent analytical solutions of quantum synchronization can be obtained. The trajectories on the Bloch sphere vividly illustrate how dissipation and interaction drive the system toward a synchronized state. We present a phase diagram for macroscopic synchronization as a function of interaction strength and the gain-to-damping ratio. Furthermore, we demonstrate full synchronization and partial synchronization between two groups of TLS with different natural frequencies. This work establishes ensemble of TLS as a remarkable system for understanding spontaneous quantum synchronization.

Hao Wang, V. N. Freire, Xian-Geng Zhao

The time-dependent average population in a two-level system interacting with an Ohmic boson bath and driven by a dc-ac field is obtained. The study is performed within the noninteracting-blip approximation and for the high-frequency driving case. The integro-differential master equation and the exact formal solution for the average transition probability are obtained. The dissipative quantum decay and the transition temperature from the coherent to the incoherent motion are investigated. The effect of the external electric field on the the dissipative two-level system is found to destroy its coherence.