Quantum information masking, an emerging concept in quantum information processing, refers to the complete quantum information transfer from a single quantum carrier to the correlations of multiple quantum carriers, so that any single carrier no longer contains any information of the pre-masked state. Quantum information masking has applications in the fields such as qubit commitment and secret sharing; however, similar to operations like the cloning, broadcasting, and hiding of quantum states, masking of quantum information cannot be accomplished with a universal bipartite isometry. We will provide a systematical introduction of recent progress about quantum information masking: firstly, we will describe the geometric characteristics of the maskable set, discuss the realization of the masking operation, its information-theoretical implication and its relationship with quantum error correction; secondly, we will present the experimental realizations of quantum information masking, expound the feasibility of quantum information masking in complex systems such as high-dimensional systems, and demonstrate its application in quantum secret sharing and noise-resilient quantum communication.

Photonic transverse spatial mode includes two degrees of freedom, i.e., azimuthal modes and radial modes, either of which can be exploited to construct a high-dimensional Hilbert space. In the past two decades, azimuthal modes such as photonic orbital angular momentum have been deeply studied, and widely used in the fields of classical optics and quantum information. However, the research on the radial modes is still lacking at present, and it is still in the exploratory stage, so the quantum number of the radial mode is once called “a forgotten quantum number”. Recent years have witnessed a growing research interest in photonic radial modes because of their unique characteristics in light field. The preparation, measurement and manipulation methods of high-order radial modes are introducted systematically, especially the quantum entanglement correlation characteristics of photonic radial modes and the research progresses of their applications in the detection of the fundamental issues in quantum mechanics and the processing of high-dimensional quantum information.

Compared with the classical system, quantum information protocol has great advantages in substantially improving the security, fidelity and capacity of information processing. Various quantum information protocols with diverse functionalities have been proposed and implemented. A multifunctional platform compatible with multiple different quantum information protocols based on rubidium atomic four-wave mixing process is introduced. Based on this multifunctional platform, the parallel 9-channel all-optical quantum teleportation, the partially disembodied quantum state transfer and optimal N→M coherent state quantum cloning can be implemented. More importantly, the partially disembodied quantum state transfer protocol can link the all-optical quantum teleportation protocol and the optimal 1→M coherent state quantum cloning protocol. These quantum information protocols have potential applications in all-optical quantum communication.

Optical lattice clocks based on optical frequency transitions of neutral atoms have demonstrated excellent system stability and uncertainty in recent years and are one of the most promising candidates for the next generation of “second” replications. With the improvement of the performance of ground optical lattice clocks, the transportable optical lattice clock, which can operate outside the laboratory, has been realized, and the space optical lattice clock that can operate in space is being developed. In this paper, we review the key factors affecting the stability and accuracy of optical lattice clocks and the main techniques to suppress or eliminate these factors. The technical characteristics and research progress of ground optical lattice clocks, portable optical lattice clocks and space optical lattice clocks are reviewed.

The strongly coupled system between optical cavity and atoms is a basic system in the research of quantum physics, which not only has important physical significance, but also provides an ideal system for the generation of key technologies and the research of key devices of quantum information, quantum computation and quantum precision measurement. The experiment of the strongly coupled interaction between cavity and atom has been developed since 1990s. After years of research, great progress has been made in the strong coupling between single atom and optical cavity and the coupling between atomic ensemble and optical cavity. With the progress of quantum manipulation technology of multi-atom array, the strongly coupled system of controllable multi-atom array and optical microcavity has become an important research direction of cavity quantum electrodynamics in recent years. However, the strong coupling between the deterministic controllable multi-atom array and the cavity is still faced with great technical challenges at present, and the number of controllable atoms remains at two. The main experimental progress and corresponding experimental schemes of the strongly coupled cavity quantum electrodynamics system in the optical frequency region in recent years are briefly reviewed, and the future development is prospected.

Metasurfaces are optical elements that use two-dimensional planar microstructures to control the light field. In recent years, the research and application of metasurfaces in quantum optics have received more and more attention. The metasurface is able to realize the miniaturization and integration of quantum devices. In addition, it can also improve the luminous efficiency and quality of quantum light sources. Based on the combination of quantum optics and metasurfaces, this article introduces the latest research and progress in five aspects, including quantum plasmonics, the use of metasurfaces to optimize quantum light sources, the use of metasurfaces to measure and manipulate quantum states, the application of quantum optics, and the quantum vacuum engineering of quantum emitters. Finally, some potential applications of metasurfaces were prospected.

Quantum communication is the current research frontier and hotspot in quantum field. The gain-switched semiconductor laser generation technology is a relatively mature method to generate pulsed laser, and can meet the demand for pulsed laser in quantum communication when it is combined with the injection-locking technology. This article systematically introduces the working principle of gain-switched semiconductor lasers, injection-locking schemes, and their applications in quantum key distribution, quantum random number generation, etc. The relevant physical principles, experimental schemes, and recent progress are emphasized, and a brief outlook on the development in the future is finally given.

With the rapid development of quantum information science, quantum entangled-photon source has become an important resource for quantum nonlocality test, quantum communication, quantum computing, and quantum metrology. Using spontaneous parametric down-conversion process in a nonlinear medium, polarized two-photon entanglement sources have been rapidly developed in terms of brightness and quality. From the early bulk crystal of β-barium borate to the later periodically poled crystal based on quasi-phase-matching, the brightness of the entanglement source has been increased, providing the possibility of large-scale quantum communication and fundamental test of quantum physics with satellites. Here we systematically introduce the development and latest achievements of quantum entangled-photon sources for space platform application in recent years, especially the spaceborne entangled-photon source represented by the Micius quantum science satellite. In addition, the international progress and future trend of satellite-based quantum entanglement source in recent years are also introduced and analyzed.

Chiral quantum optics has received extensive attention in the field of quantum information technology, which mainly studies the spin-dependent chiral coupling and transmission behavior of light in micro and nano structures. The interaction between chiral light and matter can enhance the coupling between photons and quantum emitters and endow nano-photonic devices with new functions and applications, thus promoting the large-scale application of chiral quantum optics in the field of quantum information. In this paper, the on-chip chiral nano-photon devices based on semiconductor quantum dots are reviewed, with emphasis on the optical properties of semiconductor quantum dots and the physical mechanism of the interaction between chiral light and matter. On this basis, the multi-functional chiral photon devices realized by chiral coupling principle in recent years are summarized, and the future application scenes of chiral quantum optics are prospected.

According to Lorentz reciprocity theorem, we know that in ordinary optical systems, the received optical signal is unchanged even if the positions of the signal source and the detector are exchanged. This brings us convenience for optical design and analysis. However, in all-optical communication and quantum network, it is necessary to effectively control the directional transmission of optical signals to avoid the interference of reflected light and separate the optical signals transmitted in opposite direction. Therefore, nonreciprocal devices that only allow one-way transmission of light are also essential components. This review starts from the background of the magneto-optical isolators and then briefly introduces the research status of the magneto-optical field. The research focus is the development of magnetic-free nonreciprocity, from both the active and passive aspects, which undoubtedly demonstrates the important role of quantum optics in the field of magnetic-free nonreciprocity.

The third-order nonlinear effect in optical fiber can be used to prepare a variety of quantum states, which provides an effective tool for the study of quantum information. Because of the long interacting length and low transmission loss, the waveguide structure of optical fibers can significantly enhance the nonlinear effects. On the other hand, the quantum states originate from the waveguide have a well-defined pure spatial mode, which is helpful for the collection and manipulation of the quantum states. Moreover, the commercially available fiber components are mature, effective, and low-priced. Therefore, there is a prospect of developing all fiber quantum light sources, which are not only miniaturized and low-cost, but also compatible with the optical fiber network. In this paper, we review the generation of quantum correlated photon pairs in different kinds of optical fibers, including the photon pairs in different wavelength bands and with different spectral properties, and entangled photon pairs in different degrees of freedoms.

As an important degree of freedom, orbital angular momentum (OAM) plays a key role in the research of photonic quantum information. Combined OAM with other degrees of freedom of photons such as polarization, multi-degree-of-freedom photonic quantum information processing is possible. In addition, due to the property of natural discrete high dimensions, OAM is one of the optimal degrees of freedom for the research of high dimensional quantum information processing. Based on the spontaneous parametric down-conversion nonlinear optical process, the entangled source with OAM can be easily obtained. In recent years, quantum entanglement based on OAM of photons has attracted wide attention and many significant progresses have been made in many directions, such as multiple degrees of freedom, high dimension and multiple photons. However, there are still many key scientific issues that need to be further studied in this realm, including how to achieve efficient and high-quality OAM sorter, how to achieve higher-dimensional frequency conversion, how to improve the quality of multi-degree-of-freedom entangled sorter, how to obtain high-dimensional entangled states with more dimensions and more photons, and how to construct feasible high-dimensional quantum gates. Starting from the most basic two-dimensional manipulation of OAM of photons, the quantum state regulation of single photon with OAM and the entanglement manipulation of two photons and multiple photons with OAM are reviewed. Based on the characteristics of multiple degrees of freedom, large angular momentum and high dimension, quantum entanglement of OAM of photons is discussed systematically from the perspectives of generation, regulation, measurement and application. Meanwhile, the possible methods to overcome the challenges in this realm are explored.

A high-precision optical interferometer is one of the basic tools for precision measurement, and the ultimate sensitivity of the interferometer is limited by the standard quantum limit (SQL) determined by vacuum fluctuations. By making use of quantum resources, the phase measurement beyond SQL can be realized. In a quantum interferometer based on optical parametric amplifier (OPA), squeezed states produced by OPAs in the two arms are directly used as phase-sensitive quantum states. In addition, the sensitivity of the quantum interferometer can unconditionally surpass SQL by the simultaneous shot noise reduction and amplification of phase-sensitive field intensity. However, quantum states are very sensitive to losses. By analyzing the influence of all kinds of losses on sensitivity, the relationship between sensitivity of the quantum interferometer and losses is obtained. Meanwhile, through analyzing the relationship between sensitivity and other physical parameters such as analysis frequency, the experimental parameters for further optimization of performance including system sensitivity and bandwidth are obtained.

Since quantum coherence was proposed as a resource theory, the research on its quantification and experimental measurement has always attracted widespread attention. At present, studies have proved that collective measurement can improve the measurement accuracy of coherence, but in experiments, the impact of experimental conditions on the performance of collective measurement to estimate quantum coherence is still unclear. In this paper, we numerically simulate the error of collective measurement to estimate coherence under different detection efficiency and resource number, and we compare them with the errors caused by other methods. We also notice that there exists a parameter range where collective measurement method with the detection efficiency of 60% outperforms the tomography method with the detection efficiency of 90%. In addition, with the increase of the number of resources, the performance of collective measurement is almost independent of the quantum state and coherence measure, and the errors are decreasing at the same scale.

A mixed cavity optomechanical system consists of both the first-order and quadratic optomechanical interactions. We study the two-photon scattering problem in the mixed cavity optomechanical system. By solving the scattering process within the Wigner-Weisskopf framework, we obtain the analytical expression of the scattering state and find four physical processes associated with the two-photon scattering process in this system: 1) two photons are directly reflected by the fixed end mirror, without entering the cavity; 2) one photon is reflected directly and the other photon enters the cavity; 3) two photons enter the cavity in sequence, but there is at most one photon in the cavity; 4) two photons enter the cavity. By analyzing the two-photon scattering spectrum, we find that the two-photon frequency anticorrelation can be induced via this scattering process. We also find the relationship between the parameters of the mixed cavity optomechanical system and the characteristics of the two-photon scattering spectrum. This study not only provides a scattering method to create correlated-photon pairs, but also presents a spectrometric way to characterize the optomechanical system.

The quantum entangled state is the key resource of quantum information. A feasible experimental scheme is proposed that a sum-frequency process is cascaded by a non-degenerate optical parametric oscillation with an injection signal in an optical superlattice, which can generate quadripartite continuous variable entangled light fields with different frequencies. First, the pump light and the injected signal light generate idle light through the difference frequency process. Then, the pump light and idle light generate sum-frequency light through a cascaded sum-frequency process in the same optical superlattice. According to the determination method of multi-component continuous variable entanglement, the characteristics of quantum entanglement among pump light, signal light, idle light, and sum-frequency light field are theoretically proved. The quadripartite entanglement characteristic decreases with the increase of pump power, and a better quadripartite entangled light field can be obtained by selecting larger injection signal power, coupling parameters of cascade nonlinear process and pump light attenuation constant. In this experimental scheme, a four-color continuous variable entangled light field can be generated by using only one optical superlattice, and the experimental device is simple.

The measurement-device-independent quantum key distribution (MDI-QKD) can remove all detector side-channel attacks, which greatly boosts the practical application of the quantum key distribution. However, this protocol still has a strict assumption that the states in source side must be perfectly prepared. The imperfections of the modulators in source side would bring side-channels and threaten the practical security of the system. To protect against state-preparation imperfections, the uncharacterized-source MDI-QKD, which can still generate secret keys when the prepared states are uncharacterized, have been put forward. This protocol is a great combination of the theoretical information security and the practical security. Through the three-intensity decoy state method and the self-developed Sagnac-Asymmetric-Mach-Zehnder coding structure, the measurement device-independent quantum key distribution system without characteristic source is successfully built, and the secure key distribution rate of 1.91×10 -6 is achieved under the fiber channel of 50.4 km and the repetition rate of 25 MHz.

The wireless laser communication technology using optical quantum detection has broad application prospects in heaven and earth integrated secure communication networks. Based on the Poisson distribution model of photon number of a laser source, the atmospheric turbulence Gamma-Gamma model, the response model of the single photon detector, and the correlation counting signal processing method, we establish a calculation model of bit error rate of the optical quantum communication system in the turbulent channel. The effects of the distribution of photon number of the laser source, turbulent channel parameter, performance parameters of single photon detectors, and correlation counting method on the system BER are simulated. The results show that the system BER is negatively correlated with the average photon number of laser pulses, pulse frequency, and detector detection efficiency, while it is positively correlated with atmospheric turbulence intensity and detector dark counting. The system adopting the correlation counting method can effectively reduce the system BER. When the number of door openings is determined, the system has an optimal threshold for discrimination. The proposed model can provide references for the design and optimization of the optical quantum communication systems.

Quantum positioning system (QPS) is a high precision and safe positioning system. The change of photon number has a great impact on the positioning error and security. In order to reduce the positioning error and improve the safety performance of the system in rainy weather, based on the decoy state quantum key distribution protocol and the optimal average photon number adaptive (PNA) algorithm, an adaptive adjustment strategy of QPS decoy state against rainfall interference is proposed. The adaptive relationship among rainfall intensity, transmission distance and the optimal average photon number is established, and the positioning error and the security key generation rate before and after adaptive adjustment are compared. The simulation results show that when the rainfall intensity is 10 mm/h and the transmission distance is 10 km, the system positioning error decreases from 13.81 cm to 1.13 cm by using PNA algorithm. When the rainfall intensity is 1.47 mm/h and the transmission distance is 25 km, the security key generation rate of the system is improved from 5.5×10 -4 to 6.3×10 -4 by using PNA algorithm. It can be seen that the reliability of QPS in rainy weather can be improved by adaptively adjusting the average number of photons per pulse of the system.

We propose a new scheme of simulating search algorithm in an optical system via weak value amplification and post-selection. In this scheme, we encode the database on the transverse distribution of the input beam, and then perform pre-selection and post-selection on the polarization state of the input beam. We first discuss a general input beam and obtain some interesting results. Then we analysis a Gaussian input beam. Results show that by choosing the post-selection state of the auxiliary system properly and using the weak value amplification, it is possible that we can achieve a database search in only one iteration.