Observation of multiple charge density wave phases in epitaxial monolayer 1T-VSe_(2) film

As a special order of electronic correlation induced by spatial modulation, the charge density wave(CDW) phenomena in condensed matters attract enormous research interests. Here, using scanning-tunneling microscopy in various temperatures, we discover a hidden incommensurate stripe-like CDW order besides the(■) CDW phase at low-temperature of 4 K in the epitaxial monolayer 1T-VSe_(2) film. Combining the variable-temperature angle-resolved photoemission spectroscopic(ARPES) measurements, we discover a two-step transition of an anisotropic CDW gap structure that consists of two parts △_(1) and△_(2). The gap part ?1 that closes around ~ 150 K is accompanied with the vanish of the(√7×√3) CDW phase. While another momentum-dependent gap part △_(2) can survive up to ~ 340 K, and is suggested to the result of the incommensurate CDW phase. This two-step transition with anisotropic gap opening and the resulted evolution in ARPES spectra are corroborated by our theoretical calculation based on a phenomenological form for the self-energy containing a two-gap structure △_(1) +△_(2), which suggests different forming mechanisms between the(√7×√3) and the incommensurate CDW phases. Our findings provide significant information and deep understandings on the CDW phases in monolayer 1T-VSe_(2) film as a two-dimensional(2D) material.

High-efficiency asymmetric diffraction based on PT-antisymmetry in quantum dot molecules

We present preparation of asymmetric grating with higher diffraction efficiency in quantum dot molecules by combining the tunneling effect and parity-time antisymmetry.In the presence of tunneling between two quantum dots,the system exhibits the striking PT antisymmetry via spatially modulating the driving field and the detuning with respect to the driven transition.For this reason,the asymmetric grating could be achieved.The results show that the diffraction efficiency can be adjustable via changing the driving intensity,detuning,tunneling strength,and interaction length,and then the high-order diffraction can be reached.The scheme provides a feasible way to obtain the direction-controlled diffraction grating,which can be helpful for optical information processing and realization of controllable optical self-image.

Quantum nonreciprocality in quadratic optomechanics

We propose to achieve nonreciprocal quantum control of photons in a quadratic optomechanical(QOM)system based on directional nonlinear interactions.We show that by optically pumping the QOM system in one side,the effective QOM coupling can be enhanced significantly in that side,but not for the other side.This,contrary to the intuitive picture,allows the emergence of a nonreciprocal photon blockade in such optomechanical devices with weak single-photon QOM coupling.Our proposal opens up the prospect of exploring and utilizing quantum nonreciprocal optomechanics,with applications ranging from single-photon nonreciprocal devices to on-chip chiral quantum engineering.

Nonreciprocal transition between two nondegenerate energy levels

Stimulated emission and absorption are two fundamental processes of light–matter interaction, and the coefficients of the two processes should be equal. However, we will describe a generic method to realize the significant difference between the stimulated emission and absorption coefficients of two nondegenerate energy levels, which we refer to as a nonreciprocal transition. As a simple implementation, a cyclic three-level atom system, comprising two nondegenerate energy levels and one auxiliary energy level, is employed to show a nonreciprocal transition via a combination of synthetic magnetism and reservoir engineering. Moreover, a single-photon nonreciprocal transporter is proposed using two one-dimensional semi-infinite coupled-resonator waveguides connected by an atom with nonreciprocal transition effect. Our work opens up a route to design atom-mediated nonreciprocal devices in a wide range of physical systems.

Quantum coherence transfer between an optical cavity and mechanical resonators

This study highlights the theoretical investigation of quantum coherence in mechanical oscillators and its transfer between the cavity and mechanical modes of an optomechanical system comprising an optical cavity and two mechanical oscillators that,in this study,were simultaneously coupled to the optical cavity at different optomechanical coupling strengths.The quantum coherence transfer between the optical and mechanical modes is found to depend strongly on the relative magnitude of the two optomechanical couplings.The laser power,decay rates of the cavity and mechanical oscillators,environmental temperature,and frequency of the mechanical oscillator are observed to significantly influence the investigated quantum coherences.Moreover,quantum coherence generation in the optomechanical system is restricted by the system's stability condition,which helps sustain high and stable quantum coherence in the optomechanical system.

Optomechanical ratchet resonators

We describe an optomechanical ratchet scheme to realize nonreciprocal transmission of a light field, which is based on the bias of the optical cavity’s frequency spectrum caused by mechanical ratchet interactions. This approach to break the time-reversal symmetry of light propagation is universally valid in various optomechanical systems with ratchet-oscillator structures. We discuss specifically the implementation of an on-chip Casimir-ratchet optomechanical protocol and demonstrate the optical nonreciprocity with an extremely high isolation ratio and flexible controllability, which does not require external additional optical engineering. Our study opens a door for manipulating flexibly light propagation by using mechanical ratchet resonators, and has potential applications in the on-chip integration of nonreciprocal devices and harness of lateral Casimir forces.

Quantum information-holding single-photon router based on spontaneous emission

In this paper, we propose a single-photon router via the use of a four-level atom system coupled with two one-dimensional coupled-resonator waveguides. A single photon can be directed from one quantum channel into another by atomic spontaneous emission. The coherent resonance and the photonic bound states lead to the perfect reflection appearing in the incident channel.The fidelity of the atom is related to the magnitude of the coupling strength and can reach unit when the coupling strength matches g_a = g_b. This shows that the transfer of a single photon into another quantum channel has no influence on the fidelity at special points.