Optical nonreciprocity is important in photonic information processing to route the optical signal or prevent the reverse flow of noise. By adopting the strong nonlinearity associated with a few atoms in a strongly coupled cavity QED system and an asymmetric cavity configuration, we experimentally demonstrate the nonreciprocal transmission between two counterpropagating light fields with extremely low power. The transmission of 18% is achieved for the forward light field, and the maximum blocking ratio for the reverse light is 30 dB. Though the transmission of the forward light can be maximized by optimizing the impedance matching of the cavity, it is ultimately limited by the inherent loss of the scheme. This nonreciprocity can even occur on a few-photon level due to the high optical nonlinearity of the system. The working power can be flexibly tuned by changing the effective number of atoms strongly coupled to the cavity. The idea and result can be applied to optical chips as optical diodes by using fiber-based cavity QED systems. Our work opens up new perspectives for realizing optical nonreciprocity on a few-photon level based on the nonlinearities of atoms strongly coupled to an optical cavity.

We experimentally demonstrate strong coupling between a one-dimensional (1D) single-atom array and a high-finesse miniature cavity. The atom array is obtained by loading single atoms into a 1D optical tweezer array with dimensions of $1\times 11$. Therefore, a deterministic number of atoms is obtained, and the atom number is determined by imaging the atom array on a CCD camera in real time. By precisely controlling the position and spacing of the atom array in the high finesse Fabry-Perot cavity, all the atoms in the array are strongly coupled to the cavity simultaneously. The vacuum Rabi splitting spectra are discriminated for deterministic atom numbers from 1 to 8, and the $\sqrt{N}$ dependence of the collective enhancement of the coupling strength on atom number $N$ is validated at the single-atom level.

Breaking the time‐reversal symmetry of light is of great importance for fundamental physics and has attracted increasing interest in the study of non‐reciprocal photonic devices. Here, a chiral cavity quantum electrodynamics system with multiple atoms strongly coupled to a Fabry–Pérot cavity is experimentally demonstrated. By polarizing the internal quantum state of the atoms, the time‐reversal symmetry of the atom‐cavity interaction is broken. The strongly coupled atom‐cavity system can be described by non‐reciprocal quasiparticles, that is, the cavity polariton. When it works in the linear regime, the inherent nonreciprocity makes the system work as a single‐photon‐level optical isolator. Benefiting from the collective enhancement of multiple atoms, an isolation ratio exceeding 30 dB on the single‐quanta level (≈ 0.1 photon on average) is achieved. The validity of the non‐reciprocal device under zero magnetic field and the reconfigurability of the isolation direction are also experimentally demonstrated. Moreover, when the cavity polariton works in the nonlinear regime, the quantum interference between polaritons with weak anharmonicity induces non‐reciprocal nonclassical statistics of cavity transmission from coherent probe light.

We propose a scheme to realize nonreciprocal mechanical squeezing in a spinning optomechanical system, where a spinning resonator is driven by a periodically modulated laser field via a tapered optical fiber. The spinning resonator supports two counterpropagating optical modes and a mechanical breathing mode induced by the radiation pressure. According to the optical Sagnac effect, the two counterpropagating optical modes obtain opposite frequency shifts. By utilizing the modulated pump field with a given frequency, mechanical squeezing can only be achieved by driving the resonator from one direction but not from the other. We analyze the impact of backscattering losses on mechanical squeezing and find that this negative impact can be almost completely avoided by increasing the angular velocity of the resonator. We also show that the presented scheme is robust to mechanical thermal noise and can be realized under the current experimental conditions. Therefore, this work may be meaningful for the study of quantum nonreciprocity and quantum precision measurement.

We consider a dissipative cavity optomechanical system which couples to an auxiliary cavity with a high-quality factor and is driven by an amplitude-modulated laser field. Due to the introduction of the auxiliary cavity, we find that the steady-state mechanical squeezing can be enhanced compared to the case without an auxiliary cavity and the so-called 3 dB limit can also be broken in the resolved-sideband regime. Moreover, the squeezing can also be achieved even in the unresolved-sideband regime. We study the optomechanical entanglement in the presented system. It is found that the enhanced entanglement can be obtained by introducing the auxiliary cavity, and the entanglement can also be generated in the unresolved-sideband regime for appropriate system parameters. Numerical results show that the presented schemes have strong robustness against the thermal noise acting on the mechanical mode.

With the help of light-emitting diodes at the center wavelength of 470 nm, we demonstrate that light-induced atom desorption (LIAD) can be used for flexibly controlling the loading of magneto-optical traps (MOT) of cesium atoms. Under an ultralow background pressure in a quartz cell without any wall coating, we show that low intensity blue light can be used to control the loading rates (from 200 to 4000 atoms/s) and the number of cesium atoms (on the order of ${10}^4$ in our experiment) in a MOT without the use of dispensers or any secondary atom source. A theoretical model based on an atom loading rate equation is built which can simulate the magneto-optical trap loading process with LIAD. The theoretical results are in agreement with experimental data. Some important parameters of the vacuum system are determined accordingly. The decay time of the vacuum pressure is about 70 ms, which is much shorter than the usual vacuum system for experiments on atomic ensembles. It is very difficult to measure such short times by the traditional fluorescence detection of cold atoms due to the slow loading process. The change of desorption rate (on the order of ${10}^{13}\text{atoms}/\text{s}$) for the different desorption intensities is also determined based on the background pressure caused by noncesium atoms in the cell. According to the experimental data and theoretical calculations, we obtain a partial pressure of about $1.4\times {10}^{-10}\text{Torr}$ for the untrapped cesium atoms and about $6.4\times {10}^{-10}\text{Torr}$ for the noncesium atoms at the moment the desorption light is turned off. The system is almost an ideal vacuum system for transferring atoms into another region for further experiments.