Based on a two-qubit one-dimensional isotropic spin-$1∕2$ Heisenberg spin chain in a constant external magnetic field, we construct a four-level entangled quantum heat engine (QHE) and investigate the influence of entanglement between two qubits on basic thermodynamic quantities, i.e., the heat transferred and the work done in a cycle, and the efficiency of the QHE. The validity of the second law of thermodynamics is confirmed in the entangled system. We also find several interesting features of the variation of the efficiency with the entanglement of two different thermal equilibrium states in a work cycle in zero and finite magnetic field. An abrupt transition of efficiency is found in zero field.

An ancient and counterintuitive phenomenon known as the Mpemba effect (water can cool faster when initially heated up) showcases the critical role of initial conditions in relaxation processes. How to realize and utilize this effect for speeding up relaxation is an important but challenging task in purely quantum system till now. Here, we experimentally study the strong Mpemba effect in a single trapped ion system in which an exponentially accelerated relaxation in time is observed by preparing an optimal quantum initial state with no excitation of the slowest decaying mode. Also, we demonstrate that the condition of realizing such effect coincides with the Liouvillian exceptional point, featuring the coalescence of both the eigenvalues and the eigenmodes of the systems. Our work provides an efficient strategy to engineer the dynamics of open quantum system, and suggests a link unexplored yet between the Mpemba effect and the non-Hermitian physics.

Parity-time ($\mathcal{PT}$)-symmetric systems, featuring real eigenvalues despite their non-Hermitian nature, undergo spontaneous $\mathcal{PT}$ symmetry breaking at the exceptional point (EP). The phase transition from $\mathcal{PT}$ symmetry to $\mathcal{PT}$ symmetry breaking has shown exotic functionalities in the classical realm, such as loss-induced transparency or lasing revival. This transition is also expected to produce a more unconventional effect in pure quantum $\mathcal{PT}$ devices. Here, we report experimental evidences of spontaneous $\mathcal{PT}$ symmetry breaking in a single cold ${}^{40}\mathrm{Ca}^{+}$ ion and, more importantly, a counterintuitive effect of perfect quantum coherence occurring at the EP. Excellent agreement between experimental results and theoretical predictions is identified. In view of the versatile role of cold ions in building a quantum memory or processor, our experiment provides an alternative platform to explore $\mathcal{PT}$-symmetric physics and utilize pure quantum EP effects.

We investigate speeding up of relaxation of Markovian open quantum systems with the Liouvillian exceptional point (LEP), where the slowest decay mode degenerates with a faster decay mode. The degeneracy significantly increases the gap of the Liouvillian operator, which determines the time scale of such systems in converging to stationarity and thus accelerates the relaxation process. We explore an experimentally relevant three-level atomic system whose eigenmatrices and eigenspectra are obtained completely analytically. This allows us to gain insights into the LEP and examine the respective dynamics with details. We illustrate that the gap can be further widened by Floquet engineering, which further accelerates the relaxation process. Finally, we extend this approach to analyze laser cooling of trapped ions, where vibrations (phonons) couple to the electronic states. An optimal cooling condition is obtained analytically, which agrees with both existing experiments and numerical simulations. In this paper, we provide analytical insights into understanding LEP as well as controlling and optimizing the dissipative dynamics of atoms and trapped ions.

Cavity-enhanced single quantum dots (QDs) are the main approach towards ultra-high-performance solid-state quantum light sources for scalable photonic quantum technologies. Nevertheless, harnessing the Purcell effect requires precise spectral and spatial alignment of the QDs’ emission with the cavity mode, which is challenging for most cavities. Here we have successfully integrated miniaturized Fabry-Perot microcavities with a piezoelectric actuator, and demonstrated a bright single-photon source derived from a deterministically coupled QD within this microcavity. Leveraging the cavity-membrane structures, we have achieved large spectral tunability via strain tuning. On resonance, a high Purcell factor of ~9 is attained. The source delivers single photons with simultaneous high extraction efficiency of 0.58, high purity of 0.956(2) and high indistinguishability of 0.922(4). Together with its compact footprint, our scheme facilitates the scalable integration of indistinguishable quantum light sources on-chip, therefore removing a major barrier to the development of solid-state quantum information platforms based on QDs.

Nonlocality without entanglement is an interesting field. A manifestation of quantum nonlocality without entanglement is the possible local indistinguishability of orthogonal product states. In this paper we analyze the character of operators to distinguish the elements of a full product basis set in a multipartite system, and show that distinguishing perfectly these product bases needs only local projective measurements and classical communication, and these measurements cannot damage each product basis. Employing these conclusions one can discuss local distinguishability of the elements of any full product basis set easily. Finally we discuss the generalization of these results to the locally distinguishability of the elements of incomplete product basis set.