Optical nonreciprocity is of paramount importance to optical signal processing and one-way optical communication. Here, we theoretically and experimentally demonstrate nonreciprocal amplification based on four-level hot atoms by exploiting atomic Doppler shifts. Our approach is simple and easy to implement. In fair agreement with our theoretical modeling, forward power amplification of 26 dB and backward isolation of 30 dB are observed. Our results will open up a new avenue towards realistic devices based on nonreciprocal amplification.

We propose a scheme for obtaining an electromagnetically induced grating in an asymmetric semiconductor quantum well (QW) structure via Fano interference. In our structure, owing to Fano interference, the diffraction intensity of the grating, especially the first-order diffraction, can be significantly enhanced. The diffraction efficiency of the grating can be controlled efficiently by tuning the control field intensity, the interaction length, the coupling strength of tunneling, etc. This investigation may be used to develop novel photonic devices in semiconductor QW systems.

By using three mutually perpendicular standing-wave fields, we propose a scheme for three-dimensional (3D) subwavelength atom localization in a five-level M-type atomic system. Based on the electromagnetically induced transparency, position probability distribution of the atom in 3D space could be determined via measuring the probe absorption which is proportional to the filter function. It is shown that patterns of 3D atom localization depends sensitively on the coupling schemes of the standing-wave fields. When the standing-wave fields couple three different transitions, the same atom localization patterns are formed in the eight subspaces of the 3D space. While all the standing-wave fields are applied on one transition, we can realize different atom localization patterns in the eight subspaces. From the view of the xy plane, various symmetric or asymmetric atom localization patterns could be formed at different z positions by adjusting the parameters of the laser fields.

Realization of magnetic-free nonreciprocal transmission for two or more wavelengths simultaneously is in great demand for multiwavelength optical communication and signal processing. Here, making use of optical pumping, we propose and experimentally demonstrate a magnetic-free multiwavelength isolator based on warm V-type atoms with the assistance of the Doppler effect. Applying ${}^{85}\mathrm{Rb}$ atoms, isolation ratios of 25 and 28 dB are obtained simultaneously for signal fields at 795 and 780 nm with transmissions higher than 0.85. This work provides an effective way to realize a multiwavelength optical isolator for optical information processing applications.

We consider the dynamics of intracavity electromagnetically induced transparency (EIT) in an ensemble of strongly interacting Rydberg atoms. By combining the advantage of variable cavity lifetimes with intracavity EIT and strongly interacting Rydberg dark-state polaritons, we show that such an intracavity EIT system could exhibit a very strong photon blockade effect.

Magnetic-free optical isolators are critical components for the realization of integrated optical systems. The underlying physics of passive nonlinear optical isolators is not solely about breaking the Lorentz reciprocity without requiring any external bias. Indeed, one major obstacle to the operation of self-Kerr-type nonlinear optical isolators is the so-called dynamic reciprocity, of which a serious outcome is that a backward signal cannot be isolated in the presence of a forward strong signal. In this work, we advocate the concept of velocity-selective nonlinearity to bypass such dynamic reciprocity. Using a proof-of-principle platform with warm rubidium atoms in an asymmetric cavity, we experimentally achieve the isolation of a backward signal in the presence of a strong forward signal, with experimental observations in agreement with our theoretical simulations. This work has thus made one essential step towards functioning Kerr-type passive optical isolators.