Anti-PT-symmetric Kerr gyroscope

Non-Hermitian systems can exhibit unconventional spectral singularities called exceptional points(EPs).Various EP sensors have been fabricated in recent years,showing strong spectral responses to external signals.Here we propose how to achieve a nonlinear anti-parity-time(PT)gyroscope by spinning an optical resonator.We show that,in the absence of any nonlinearity,the sensitivity or optical mode splitting of the linear device can be magnified up to 3 orders compared to that of the conventional device without EPs.Remarkably,the PT symmetry can be broken when including the Kerr nonlinearity of the materials and,as a result,the detection threshold can be significantly lowered,i.e.,much weaker rotations which are well beyond the ability of a linear gyroscope can now be detected with the nonlinear device.Our work shows the powerful ability of PT gyroscopes in practice to achieve ultrasensitive rotation measurement.

Nonreciprocal transition between two indirectly coupled energy levels

We propose a theoretical scheme to realize nonreciprocal transition between two energy levels that can not coupled directly.Suppose they are coupled indirectly by two auxiliary levels with a cyclic four-level configuration,and the four transitions in the cyclic configuration are controlled by external fields.The indirectly transition become nonreciprocal when the time reversal symmetry of the system is broken by the synthetic magnetic flux,i.e.,the total phase of the external driving fields through the cyclic four-level configuration.The nonreciprocal transition can be identified by the elimination of a spectral line in the spontaneous emission spectrum.Our work introduces a feasible way to observe nonreciprocal transition in a wide range of multi-level systems,including natural atoms or ions with parity symmetry.

Phase-controlled asymmetric optomechanical entanglement against optical backscattering

Quantum entanglement plays a key role in both understanding the fundamental aspects of quantum physics and realizing various quantum devices for practical applications. Here we propose how to achieve a coherent switch of optomechanical entanglement in an optical whispering-gallery-mode resonator, by tuning the phase difference of the driving lasers. We find that the optomechanical entanglement and the associated two-mode quantum squeezing can be well tuned in a highly asymmetric way,providing an efficient way to protect and enhance quantum entanglement against optical backscattering, in comparison with conventional symmetric devices. Our findings shed a new light on improving the performance of various quantum devices in the practical noisy environment, which is crucial in such a wide range of applications as noise-tolerant quantum processing and the backscattering-immune quantum metrology.

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.

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.