Owing to the potential applications in all-optical quantum information processing and quantum optical networks, magnet-free optical non-reciprocity transmission has attracted great interest and has been studied in many fields, such as parity-time-symmetry enhanced nonlinearity, optomechanical systems, photonic crystal, cold atomic Bragg lattices, chiral quantum optics, and hot atoms. In particular, the random thermal motion of hot atoms can be a useful resource to realize optical non-reciprocity. Here in this work, based on the susceptibility-momentum-locking of atomic thermal motion and the strong coupling characteristics of cavities, a magnetic-free optical reciprocity-nonreciprocity transmission conversion scheme is designed and realized through the atom-cavity compound system. Theoretical and experimental analysis show that the coupling field conditions determine the nonreciprocity of the system. Under the action of single traveling-wave field, the nonreciprocity in hot atoms depends on the propagation direction of the coupling field due to the Doppler effect. Therefore, by changing the opening and closing of the opposite coupling field, the two-way single channel optical nonreciprocal transmission based on intracavity electromagnetically induced transparency can be controlled. When the two coupling fields propagate simultaneously in the opposite directions, however, the cavity transmission changes from single-dark-state to double-dark-state peaks, in which the reciprocity outputs depend on the frequency difference between the two coupling fields. By tuning the frequency difference, the two-way multi-channel reciprocal-nonreciprocal transmission regulation based on double dark polar peaks can be realized. The study can be applied to all-optical quantum devices and quantum information processing, such as optical transistors, optical switching and routing, and quantum gate manipulation.
Owing to the potential applications in all-optical quantum information processing and quantum optical networks, magnet-free optical non-reciprocity transmission has attracted great interest and has been studied in many fields, such as parity-time-symmetry enhanced nonlinearity, optomechanical systems, photonic crystal, cold atomic Bragg lattices, chiral quantum optics, and hot atoms. In particular, the random thermal motion of hot atoms can be a useful resource to realize optical non-reciprocity. Here in this work, based on the susceptibility-momentum-locking of atomic thermal motion and the strong coupling characteristics of cavities, a magnetic-free optical reciprocity-nonreciprocity transmission conversion scheme is designed and realized through the atom-cavity compound system. Theoretical and experimental analysis show that the coupling field conditions determine the nonreciprocity of the system. Under the action of single traveling-wave field, the nonreciprocity in hot atoms depends on the propagation direction of the coupling field due to the Doppler effect. Therefore, by changing the opening and closing of the opposite coupling field, the two-way single channel optical nonreciprocal transmission based on intracavity electromagnetically induced transparency can be controlled. When the two coupling fields propagate simultaneously in the opposite directions, however, the cavity transmission changes from single-dark-state to double-dark-state peaks, in which the reciprocity outputs depend on the frequency difference between the two coupling fields. By tuning the frequency difference, the two-way multi-channel reciprocal-nonreciprocal transmission regulation based on double dark polar peaks can be realized. The study can be applied to all-optical quantum devices and quantum information processing, such as optical transistors, optical switching and routing, and quantum gate manipulation.
Abstract Hot atoms can exhibit non-magnetic optical non-reciprocal transmission due to their chiral properties, which are characteristic of most alkali metal atoms. In fact, the nonreciprocity in hot atoms depends on the propagation direction of the coupling field due to the Doppler effect. Herein, the reciprocal to non-reciprocal conversion based on the single- and double-dark states is realized by controlling the bidirectional coupling fields in a three-level electromagnetically induced transparent medium coupled with a ring cavity. Tuning the frequency difference between the two coupling fields causes the multi-frequency-channel reciprocity and nonreciprocity manipulation to occur. The experimental proof can be applied to quantum communications and quantum networks, such as optical transistors, all-optical switching or routing and logic gate operation.
Multi-band signal propagation and processing play an important role in quantum communications and quantum computing.In recent years, optical nonreciprocal devices such as an optical isolator and circulator are proposed via various configurations of atoms, metamaterials, nonlinear waveguides, etc.In this work, we investigate all-optical controlled nonreciprocity of multi-band optical signals in thermal atomic systems.Via introducing multiple strong coupling fields, nonreciprocal propagation of the probe field can happen at some separated frequency bands, which results from combination of the electromagnetically induced transparency (EIT) effect and atomic thermal motion.In the proposed configuration, the frequency shift resulting from atomic thermal motion takes converse effect on the probe field in the two opposite directions.In this way, the probe field can propagate almost transparently within some frequency bands of EIT windows in the opposite direction of the coupling fields.However, it is well blocked within the considered frequency region in the same direction of the coupling fields because of destruction of the EIT.Such selectable optical nonreciprocity and isolation for discrete signals may be greatly useful in controlling signal transmission and realizing selective optical isolation functions.
Optical nonreciprocity plays an important role in optical communication and quantum networks. In this research, we propose and demonstrate a conversion scheme of magnetic-free dual-channel optical reciprocal amplification (RA) and nonreciprocal amplification (NRA) based on the multiple degenerate four-wave mixing (FWM) process in hot atoms. In our experiment, the dual-channel NRA works with the action of a single pump field based on the establishment of FWM in the same direction and breaking in the opposite direction. Based on stable ground-state Zeeman coherence, by introducing a counter-propagating pump field again, NRA can be changed to RA in the opposite direction of the two original amplified conjugate signals. Moreover, the frequencies of RA signals are very dependent on those of co-propagating pump fields. The experimental realization of NRA-RA conversion may have applications for multichannel angular momentum spatial multiplexing and quantum gate manipulation.
Magnet-free optical nonreciprocity has significant applications in quantum communication, quantum networks, and optical information processing. In this research, considering a degenerate two-level thermal atomic system with the Doppler effect of thermal atoms, the nonreciprocal amplification (NRA) of dual-path degenerate four-wave mixing (FWM) signals is achieved under the action of a co-propagating pumping field. On this basis, spatially multiplexed multiple FWM processes are formed by introducing another counter-propagating pumping field, thereby achieving the reciprocal amplification (RA) of the dual-channel FWM signals. Furthermore, by using multiple sets of spiral phase plates to load spiral phases on the signal light and the pumping light respectively, higher-order Laguerre-Gaussian vortex beams carrying different optical orbital angular momentum (OAM) are generated and participate in the FWM process, achieving the transfer of the OAM of the pumping light to the amplified FWM fields. Simultaneously, using the Mach-Zehnder interferometer, the conservation characteristics of the OAM of each FWM signal in the NRA-RA conversion are further analyzed. Furthermore, experimental results demonstrate that in the multiple FWM process induced by a pair of counter-propagating pump fields, the OAM of the amplified FWM signal in each channel varies with that of the pump field. However, the overall process maintains the OAM conservation. This study provides a feasible solution for expanding the channel capacity using OAM based on NRA-RA system, showing that the OAM has potential application prospects in achieving high-capacity optical communication and multi-channel signal processing.
Optical nonreciprocity plays an essential role in optical information communication and information processing. Based on electromagnetically induced transparency, a nonreciprocal cavity dark-state polariton (DSP) could be achieved using spin-biased cold atoms. The DSP induces a nonreciprocal window with high transmission and low insertion loss around the cavity resonant frequency. By decreasing the strength of the control field, the contribution of the atom excitation dominates the behavior of the DSP, which exhibits a narrow cavity linewidth and thus a long lifetime of cavity photons in the form of the DSP. Further, we investigate the nonreciprocity for the statistical properties of the system in the single-atom (N=1) and multiatom (N>1) cases. Due to the existence of DSP, the photon statistics leads to totally different profiles for the light propagating in both directions in the two cases. In the single-atom (N=1) case, the light in the form of DSP shows apparent sub-Poissonian distribution simultaneously with relatively high transmission and a narrow bandwidth corresponding to the one-photon excitation due to quantum interference. Such a quasiparticle may provide a platform for novel applications in nonreciprocal quantum devices and quantum simulation.
Because the nonreciprocity has both theoretical and applicable prospects, it is desired to break through the restriction of the reciprocity principle for nonreciprocity optical transmission. Theoretically, the reciprocity principle is equivalent to the limited time reversal symmetry. To break the symmetry, the system should include nonlinearity, magneto-optical effects, or modulation in time. Some key components in the optical circuits, such as isolators, circulators, and phase shifters, are typical nonreciprocal devices. They are realized based on the Faraday effect of magneto-optical materials in traditional free-space optics. However, the magneto-optical materials are not naturally compatible with the integrated optics. Therefore, while pursuing the magneto-optical integration on a chip, people also devote to developing nonreciprocal theories and devices without magnetic materials. Whispering-gallery-modes (WGMs) optical microcavity has the advantages of small mode volume and high-quality factor. Therefore, many nonreciprocal strategies in photonic integration are based on optical microcavities. Similar to the nonreciprocal principle of the magneto-optical effect, the external bias fields (such as magnetic field, angular momentum, linear momentum, etc.) are necessary to break the time reversal symmetry. In this paper, we review the research on nonreciprocal optics in WGMs optical microcavities, and summarize recent research achievements in the following interaction categories: Magneto-optical effect, chiral coupling of atoms with energy level distribution bias, macroscopic Doppler effect with angular momentum bias, optomechanical driving, acousto-optic oscillation and nonlinear driving. Firstly, even on the chip, the magneto-optical effect is an important way to realize nonreciprocity. Nonreciprocity can be achieved by depositing/bonding magneto-optical materials on a silicon chip or using the spin-orbit interaction of WGMs in the YIG microsphere with the help of an external magnetic field. Secondly, preparing the atom to a specific Zeeman energy level can also lead to the polarization concerned interaction between the atoms and the photons, so that the nonreciprocity of the photon can be realized based on the control of the atomic ground state population. Thirdly, the Doppler effect can make the moving dielectric medium produce an additional refractive index related to the speed. Then the effective refractive index is different between the light propagating in the opposite directions. The reciprocity of the system is destroyed, thereby achieving nonreciprocity. Finally, the nonreciprocity of optomechanical systems, acousto-optic modulation, and nonlinear effects can be explained by the conservation of angular momentum of photon-phonon interactions. As a summary, some representative parameters of isolators obtained in related experiments are listed in the table. The existing solutions for nonreciprocity have also be classified in terms of angular momentum and interactions, which is beneficial to deeply understanding the existing solutions and to developing potential solutions. The research of nonreciprocity is also discussed by listing problems to be studied.