Quantum squeezing induced nonreciprocal phonon laser

Phonon lasers or coherent amplifications of mechanical oscillations are powerful tools for fundamental studies on coherent acoustics and hold potential for diverse applications,ranging from ultrasensitive force sensing to phononic information processing.Here,we propose the use of an optomechanical resonator coupled to a nonlinear optical resonator for directional phonon lasing.We find that by pumping the nonlinear optical resonator,directional optical squeezing can occur along the pump direction.As a result,we can achieve the directional mechanical gain using directional optical squeezing,thereby leading to nonreciprocal phonon lasing with a well-tunable directional power threshold.Our work proposes a feasible way to build nonreciprocal phonon lasers with various nonlinear optical media,which are important for a wide range of applications,such as directional acoustic amplifiers,invisible sound sensing or imaging,and one-way phononic networks.

Anti-parity-time symmetry hidden in a damping linear resonator

Phase transition from the over-damping to under-damping states is a ubiquitous phenomenon in physical systems. However, what kind of symmetry is broken associated with this phase transition remains unclear. Here, we discover that this phase transition is determined by an anti-parity-time(anti-PT) symmetry hidden in a single damping linear resonator, which is significantly different from the conventional anti-PT-symmetric systems with two or more modes. We show that the breaking of the anti-PT symmetry yields the phase transition from the over-damping to under-damping states, with an exceptional point(EP) corresponding to the critical-damping state. Moreover, we propose an optomechanical scheme to show this anti-PT symmetry breaking by using the optical spring effect in a quadratic optomechanical system. We also suggest an optomechanical sensor with the sensitivity enhanced significantly around the EPs for the anti-PT symmetry breaking. Our work unveils the anti-PT symmetry hidden in damping oscillations and hence opens up new possibilities for exploiting wide anti-PT symmetry applications in single damping linear resonators.

Quantum parametric amplification of phonon-mediated magnon-spin interaction

The recently developed hybrid magnonics provides new opportunities for advances in both the study of magnetism and the development of quantum information processing.However,engineering coherent quantum state transfer between magnons and specific information carriers,in particular,mechanical oscillators and solid-state spins,remains challenging due to the intrinsically weak interactions and the frequency mismatch between different components.Here,we show how to strongly couple the magnon modes in a nanomagnet to the quantized mechanical motion(phonons)of a micromechanical cantilever in a hybrid tripartite system.The coherent and enhanced magnon-phonon coupling is engineered by introducing the quantum parametric amplification of the mechanical motion.With experimentally feasible parameters,we show that the mechanical parametric drive can be adjusted to drive the system into the strong-coupling regime and even the ultrastrong-coupling regime.Furthermore,we show the coherent state transfer between the nanomagnet and a nitrogen-vacancy center in the dispersive-coupling regime,with the magnon-spin interaction mediated by the virtually-excited squeezed phonons.The amplified mechanical noise can hardly interrupt the coherent dynamics of the system even for low mechanical quality factors,which removes the requirement of applying additional engineered-reservoir techniques.Our work opens up prospects for developing novel quantum transducers,quantum memories and high-precision measurements.

Optomechanical Schrödinger cat states in a cavity Bose-Einstein condensate

Schrödinger cat states,consisting of superpositions of macroscopically distinct states,provide key resources for a large number of emerging quantum technologies in quantum information processing.Here we propose how to generate and manipulate mechanical and optical Schrödinger cat states with distinguishable superposition components by exploiting the unique properties of cavity optomechanical systems based on Bose-Einstein condensate.Specifically,we show that in comparison with its solid-state counterparts,almost a 3 order of magnitude enhancement in the size of the mechanical Schrödinger cat state could be achieved,characterizing a much smaller overlap between its two superposed coherent-state components.By exploiting this generated cat state,we further show how to engineer the quadrature squeezing of the mechanical mode.Besides,we also provide an efficient method to create multicomponent optical Schrödinger cat states in our proposed scheme.Our work opens up a new way to achieve nonclassical states of massive objects,facilitating the development of fault-tolerant quantum processors and sensors.

Complementarity between micro-micro and micro-macro entanglement in a Bose–Einstein condensate with two Rydberg impurities

We theoretically study complementarity between micro-micro and micro-macro entanglement in a Bose–Einstein condensate with two Rydberg impurities.We investigate quantum dynamics of micro-micro and micro-macro entanglement in the micro-macro system.It is found that strong micro-macro entanglement between Rydberg impurities and the BEC can be generated by the use of initial micro-micro entanglement between two Rydberg impurities,which acts as the seed entanglement to create micro-macro entanglement.We demonstrate a curious complementarity relation between micro-micro and micro-macro entanglement,and find that the complementarity property can be sustained to some extent even though in the presence of the BEC decoherence.

Quantum dynamics of an impurity-doped Bose–Einstein condensate system

We study the quantum dynamics of an impurity-doped Bose–Einstein condensate(BEC) system.We show how to generate the macroscopic quantum superposition states(MQSSs) of the BEC by the use of projective measurements on impurity atoms. It is found that the nonclassicality of MQSSs can be manipulated by changing the number of the impurities and their interaction with the BEC. It is shown that the BEC matter-wave field exhibits a collapse and revival phenomenon which reveals the quantum nature of the BEC matter-wave field. We investigate the micro-macro entanglement between the impurities and the BEC, and find enhancement of the micro-macro entanglement induced by the initial quantum coherence of the impurity atoms.