Optical lattice clocks demonstrate advantages in metrology and frontier physics because of their high stability. Here, we present approaches to enhancing the stability by decoupling the noise related to the short-term and long-term stability. For the short-term stability, we optimize the clock laser by decoupling the frequency noise, and optimize each noise contribution individually until it is below the thermal noise limit. For the long-term stability, we introduce a method to decouple the instability caused by systematic effects. Having identified that the collision frequency shift was the main limiting factor in our systems, we thus optimized the atom number fluctuations in optical lattices. Through targeted optimization, we achieve a synchronous comparison of two clocks with an average stability of 3.2×10-16/τ and a long-term stability of 2.4 × 10-18 at 8000 s. This work provides an analytical framework for enhancing optical clock stability.

Single-photon detection (SPD) technologies have been applied to underwater optical imaging to overcome the strong attenuation of seawater. However, external photon noise, resulting from the natural light, hinders their further applications due to the extreme sensitivity of SPD and a weakly received optical signal. In this work, we performed noise-resistant underwater correlated biphoton imaging (CPI) to partly solve the influence of the external noise, through a home-built super-bunching laser generated by the stochastic nonlinear interaction between a picosecond laser and a photonic crystal fiber. Compared with a coherent laser, the probabilities of generated bundle N-photons (N ≥ 2) of the super-bunching laser have been enhanced by at least one order of magnitude, enabling CPI under weak light intensity. We experimentally demonstrated CPI with reasonable imaging contrast under the noise-to-signal ratio (NSR) up to 103, and the noise-resistant performance has been improved by at least two orders of magnitude compared to that of the single-photon imaging technology. We further achieved underwater CPI with good imaging contrast under NSR of 150, in a glass tank with a length of 10 m with Jerlov type III water (an attenuation coefficient of 0.176 m-1). These results break the limits of underwater imaging through classical coherent lasers and may offer many enhanced imaging applications through our super-bunching laser, such as long-range target tracking and deep-sea optical exploration under noisy environments.

In this review, we address the emerging field of quantum photonic sensing leveraging the polarization degree of freedom. We briefly discuss the main aspects of treating polarization in quantum optics, and provide an overview of the main trends in the development of the field and the strategies to realize quantum-enhanced polarization-based sensing as well as a comprehensive survey of the main advancements in the field. We aim at promoting quantum approaches to the researchers in classical optical polarimetry as well as underscoring the sustainability and resourcefulness of the field for prospective applications and attracting the researchers in quantum optics to this new emerging field.

Large Purcell enhancement, requiring high-quality factors and small mode volumes, is essential to single-photon sources. Whispering gallery microcavities possessing a high-quality factor are limited by a large mode volume, while dielectric nanoantennas with an ultra-small mode volume suffer from significant scattering loss. Here, by combining the advantages of the microtoroids and the nanoantennas, we achieve large Purcell enhancement with a narrow linewidth in all-dielectric nanoantenna-microtoroid hybrid structures. The scattering loss of the nanoantenna is suppressed by the high-Q microtoroids; meanwhile, its ultra-small mode volume remains almost unchanged. As a result, the Purcell factor of the emitter located at the gap of the nanoantenna reaches as high as 1000–1700, while its linewidth is kept at the order of hundreds of picometers. The proposed mechanism holds promise for applications in on-chip single-photon sources and low-threshold nanolasers.

Quantum correlation imaging plays an important role in quantum information processing. The existing quantum correlation imaging schemes mostly use the Gaussian beam as the pump source, resulting in the entangled two photons exhibiting a Gaussian distribution. In this Letter, we report the experimental demonstration of quantum correlation imaging using a flat-top beam as the pump source, which can effectively solve the problem of imaging distortion. The sampling time for each point is 5 s, and the imaging similarity is 93.4%. The principle of this scheme is reliable, the device is simple, and it can achieve high-similarity quantum correlation imaging at room temperature.

We report a broadband energy-time entangled photon-pair source based on a fiber-pigtailed periodically poled lithium niobate (PPLN) waveguide, designed for applications in the quantum secure network. Utilizing the spontaneous parametric down-conversion nonlinear optical process, the source generates entangled photon pairs within a wavelength range of 64 nm in the telecom band at a pump wavelength of 770.3 nm. Photon pairs from eight paired International Telecommunication Union (ITU) channels are selected, and their correlation and entanglement properties are characterized. The measured coincidence counts of photon pairs from eight paired ITU channels are larger than 152.9 kHz when the coincidence-to-accidental ratios are greater than 260. Entanglement properties are measured through two-photon interference in the Franson interferometer, with all visibilities of interference curves exceeding 98.13%. Our demonstration provides a broadband energy-time entangled photon-pair source, contributing to the development of a large-scale quantum secure network.

Polarimeter is a vital precision tool used for measuring optical parameters through polarization variations. Among the wide range of application fields, the precise measurement of photosensitive materials is an unavoidable task but faces immense obstacles due to the excessive input photons. Facing this situation, introducing a quantum source into the classical precision measurement system is a feasible way to enhance the detection accuracy under the low illumination regime. In this work, we employ polarization-entangled photon pairs in the classical polarimeter to precisely detect the relative phase retardance of uniform anisotropic media. The experimental results show that the accuracy can reach the nanometer scale at extremely low input intensity, and the stabilities are within 0.4% for all samples. Our work paves the way for polarization measurement at low incident light intensity, with potential applications in measuring photosensitive materials and remote monitoring scenarios.

In quantum information processing, unitary transformations are oftentimes used to implement computing tasks. However, unitary transformations are not enough for all situations. Therefore, it is important to explore non-unitary transformations in quantum computing and simulation. Here, we introduce non-unitary transformations by performing singular value decomposition (SVD) on two-photon interference. Through simulation, we show that losses modeled by non-unitary transformation can be perceived as variables to control two-photon interference continuously, and the coincidence statistics can be changed by an appropriate choice of observation basis. The results are promising in the design of integrated optical circuits, providing a way toward fabricating large-scale programmable circuits.

The dissipative Kerr soliton (DKS) frequency comb exhibits broad and narrow-linewidth frequency modes, which make it suitable for quantum communication. However, a scalable quantum network based on multiple independent combs is still a challenge due to fabrication-induced frequency mismatches. This limitation becomes critical in measurement-device-independent quantum key distribution, which requires high visibility of Hong–Ou–Mandel interference between multiple frequency channels. Here, we experimentally demonstrate two independent DKS combs with 10 spectrally aligned lines without any frequency locking system. The visibility for individual comb-line pairs reaches up to 46.72% ± 0.63% via precision frequency translation, establishing a foundation for deploying DKS combs in multi-user quantum networks.

Since the working conditions of classical and quantum signals are very different, how to effectively integrate classical and quantum communication networks without affecting their respective performance has become a great challenge. In this paper, we proposed a scheme to realize classical communication and continuous-variable quantum key distribution (CV-QKD) based on frequency-division multiplexing (FDM), and we verified the feasibility of simultaneously realizing CV-QKD and classical optical communication data synchronous transmission scheme under the same infrastructure. We achieved a 0 bit error rate in 50 frames and a 20 Mb/s bit rate for the classical signal and an average secret key rate of around 5.86 × 105 bit/s for the quantum signal through a 4 dB fiber channel. This work provides a scheme to establish a QKD channel by only reserving a small passband in the entire optical communication instead of an entire wavelength, increasing efficiency and simplifying the integration of QKD and classical communication.

The quantum eraser effect exemplifies the distinctive properties of quantum mechanics that challenge classical intuition and reveal the wave-particle duality of light. Whether the photon exhibits particle-like or wave-like behavior depends on whether the path information is discernible. In this paper, we propose a novel quantum eraser scheme that utilizes photonic phase structures as the which-way indicator. This scheme is implemented using a Mach–Zehnder interferometer (MZI), where one arm is configured with orbital angular momentum (OAM) to establish predetermined which-way information. Consequently, at the output ports of the MZI, the photon displays particle-like characteristics when the which-way information is retained. However, the introduction of an additional spiral phase plate (SPP) to eliminate the phase structure from the output photon of the MZI unveils distinct interference patterns. This result enhances our understanding of the quantum erasure effect.

We demonstrate a resolution enhancement scheme of radio-frequency signals by tailoring a phase-squeezed state. The echo radio-frequency signals collected by photonic radar give rise to displacements in the phase quadrature of a probe laser and are estimated by the balanced homodyne detector. In contrast to the conventional coherent state, the noise variances for radio-frequency estimation with a squeezed state are reduced by approximately 6.9 dB. According to the Rayleigh criterion that defines the resolution limit, the minimum resolvable displacement Δa with a squeezed state is reduced to 45% compared to that with a coherent state, demonstrating the quantum advantage. The squeezing-enhanced technique has extensive applications for multitarget recognition and tracking in contemporary photonic radar systems.

We report the generation of polarization-entangled photon pairs in the 1550 nm band by pumping an uneven nonlinear interferometer loop with two orthogonally polarized counterpropagating pump pulses. The uneven nonlinear interferometer, providing a more ideal interference pattern due to the elimination of secondary maxima, consists of four pieces of dispersion-shifted fibers sandwiched with three pieces of standard single-mode fibers, and the lengths of the nonlinear fibers follow the binomial distribution. The mode number of the photon pairs deduced from the measured joint spectrum is ∼1.03. The collection efficiency of the photon pairs is found to be ∼94% (after background noise correction). The directly measured visibility of two-photon interference of the polarization-entangled photon pairs is ∼92%, while no interference is observed in the direct detection of either the signal or idler photons.

We demonstrate a high-performance acousto-optic modulator-based bi-frequency interferometer, which can realize either beating or beating free interference for a single-photon level quantum state. Visibility and optical efficiency of the interferometer are (99.5±0.2)% and (95±1)%, respectively. The phase of the interferometer is actively stabilized by using a dithering phase-locking scheme, where the phase dithering is realized by directly driving the acousto-optic modulators with a specially designed electronic signal. We further demonstrate applications of the interferometer in quantum technology, including bi-frequency coherent combination, frequency tuning, and optical switching. These results show the interferometer is a versatile device for multiple quantum technologies.

Phase-coherent multi-tone lasers play a critical role in atomic, molecular, and optical physics. Among them, the Raman opeartion laser for manipulating atomic hyperfine qubits requires gigahertz bandwidth and low phase noise to retain long-term coherence. Raman operation lasers generated by directly modulated and frequency-multipled infrared lasers are compact and stable but lack feedback control to actively suppress the phase noise, which limits their performance in practical applications. In this work, we employ a fiber electro-optical modulator driven by a voltage-controlled oscillator (VCO) to modulate a monochromatic laser and employ a second-harmonic generation process to convert it to the visible domain, where the beat note of the Raman operation laser is stabilized by controlling the output frequency of VCO with a digital phase-locked loop (PLL). The low-frequency phase noise is effectively suppressed compared to the scheme without active feedback and it reaches -80 dBc/Hz@5 kHz with a 20 kHz loop bandwidth. Furthermore, this compact and robust scheme effectively reduces the system’s complexity and cost, which is promising for extensive application in atomic, molecular, and optical physics.

Recently, there has been increased attention toward 3D imaging using single-pixel single-photon detection (also known as temporal data) due to its potential advantages in terms of cost and power efficiency. However, to eliminate the symmetry blur in the reconstructed images, a fixed background is required. This paper proposes a fusion-data-based 3D imaging method that utilizes a single-pixel single-photon detector and millimeter-wave radar to capture temporal histograms of a scene from multiple perspectives. Subsequently, the 3D information can be reconstructed from the one-dimensional fusion temporal data by using an artificial neural network. Both the simulation and experimental results demonstrate that our fusion method effectively eliminates symmetry blur and improves the quality of the reconstructed images.

Ghost imaging (GI) is a novel imaging technique that has garnered widespread attention and discussion since its inception three decades ago. To this day, ghost imaging has become an effective bridge between the advantages of quantum light sources and the field of imaging. This article begins by tracing the origin of ghost imaging and reviewing its development journey. Subsequently, we introduce some recent and important achievements and research interests of the field, which mainly include two aspects. First, we review recent works that extend GI from the intensity-only target to the complex field domain, that is, ghost holography. Using quantum correlation, traditional holographic techniques have been reproduced at the single-photon level. Second, we review the recent development of GI with the implementation of the intensified charge-coupled device (ICCD). As detection efficiency improves, ghost imaging will gradually become an important platform for studying physical mechanisms and achieving quantum advantage in imaging.

The cavity quantum electrodynamics (QED) system is a promising platform for quantum optics and quantum information experiments. Its core is the strong coupling between atoms and optical cavity, which causes difficulty in the overlap between the atoms and the antinode of optical cavity mode. Here, we use a programmable movable optical dipole trap to load a cold atomic ensemble into an optical fiber microcavity and realize the strong coupling between the atoms and the optical cavity in which the coupling strength can be improved by polarization gradient cooling and adiabatic loading. By the measurement of vacuum Rabi splitting, the coupling strength can be as high as gN=2π×400 MHz, which means the effective atom number is Neff=16 and the collective cooperativity is CN=1466. These results show that this experimental system can be used for cold atomic ensemble and cold molecule based cavity QED research.

One of the major difficulties in realizing a high-dimensional frequency converter for conventional optical vortex (COV) modes stems from the difference in ring diameter of the COV modes with different topological charge numbers l. Here, we implement a high-dimensional frequency converter for perfect optical vortex (POV) modes with invariant sizes by way of the four-wave mixing (FWM) process using Bessel–Gaussian beams instead of Laguerre–Gaussian beams. The measured conversion efficiency from 1530 to 795 nm is independent of l at least in subspace l∈{-6,…,6}, and the achieved conversion fidelities for two-dimensional (2D) superposed POV states exceed 97%. We further realize the frequency conversion of 3D, 5D, and 7D superposition states with fidelities as high as 96.70%, 89.16%, and 88.68%, respectively. The proposed scheme is implemented in hot atomic vapor. It is also compatible with the cold atomic system and may find applications in high-capacity and long-distance quantum communication.

The sensitivity of optical measurement is ultimately constrained by the shot noise to the standard quantum limit. It has become a common concept that beating this limit requires quantum resources. A deep-learning neural network free of quantum principle has the capability of removing classical noise from images, but it is unclear in reducing quantum noise. In a coincidence-imaging experiment, we show that quantum-resource-free deep learning can be exploited to surpass the standard quantum limit via the photon-number-dependent nonlinear feedback during training. Using an effective classical light with photon flux of about 9×104 photons per second, our deep-learning-based scheme achieves a 14 dB improvement in signal-to-noise ratio with respect to the standard quantum limit.

Multimode photonic quantum memory could enhance the information processing speed in a quantum repeater-based quantum network. A large obstacle that impedes the storage of the spatial multimode in a hot atomic ensemble is atomic diffusion, which severely disturbs the structure of the retrieved light field. In this paper, we demonstrate that the elegant Ince-Gaussian (eIG) mode possesses the ability to resist such diffusion. Our experimental results show that the overall structure of the eIG modes under different parameters maintains well after microseconds of storage. In contrast, the standard IG modes under the same circumstance are disrupted and become unrecognizable. Our findings could promote the construction of quantum networks based on room-temperature atoms.

Remarkable progress has been made in satellite-based quantum key distribution (QKD), which can effectively provide QKD service even at the intercontinental scale and construct an ultralong-distance global quantum network. But there are still some places where terrestrial fiber and ground stations cannot be constructed, like harsh mountainous areas and air space above the sea. So the airborne platform is expected to replace the ground station and provide flexible and relay links for the large-scale integrated communication network. However, the photon transmission rate would be randomly reduced, owing to the randomly distributed boundary layer that surrounds the surface of the aircraft when the flight speed is larger than 0.3 Ma. Previous research of airborne QKD with boundary layer effects is mainly under the air-to-ground scenario in which the aircraft is a transmitter, while the satellite-to-aircraft scenario is rarely reported. In this article, we propose a performance evaluation scheme of satellite-to-aircraft QKD with boundary layer effects in which the aircraft is the receiver. With common experimental settings, the boundary layer would introduce a ∼31 dB loss to the transmitted photons, decrease ∼47% of the quantum communication time, and decrease ∼51% of the secure key rate, which shows that the aero-optical effects caused by the boundary layer cannot be ignored. Our study can be performed in future airborne quantum communication designs.

Reliable generation of single photons is of key importance for fundamental physical experiments and quantum protocols. The periodically poled lithium niobate (LN) waveguide has shown promise for an integrated quantum source due to its large spectral tunability and high efficiency, benefiting from the quasi-phase-matching. Here we demonstrate photon-pair sources based on an LN waveguide periodically poled by a tightly focused femtosecond laser beam. The pair coincidence rate reaches ∼8000 counts per second for average pump power of 3.2 mW (peak power is 2.9 kW). Our results prove the possibility of application of the nonlinear photonics structure fabricated by femtosecond laser to the integrated quantum source. This method can be extended to three-dimensional domain structures, which provide a potential platform for steering the spatial degree of freedom of the entangled two-photon states.

We report an all-fiber telecom-band energy-time entangled biphoton source with all physical elements integrated into a compact cabinet. At a pump power of 800 µW, the photon pairs generation rate reaches 6.9 MHz with the coincidence-to-accidental ratio (CAR) better than 1150. The long-term stability of the biphoton source is characterized by measuring the Hong–Ou–Mandel interference visibility and CAR within a continuous operation period of more than 10 h. Benefiting from the advantages of compact size, light weight, and high stability, this device provides a convenient resource for various field turnkey quantum communication and metrology applications.

Quantum parameter estimation is a crucial tool for inferring unknown parameters in physical models from experimental data. The Jaynes–Cummings model is a widely used model in quantum optics that describes the interaction between an atom and a single-mode quantum optical field. In this Letter, we systematically investigate the problem of estimating the atom-light coupling strength in this model and optimize the initial state in the full Hilbert space. We compare the precision limits achievable for different optical field quantum states, including coherent states, amplitude- and phase-squeezed states, and provide experimental suggestions with an easily prepared substitute for the optimal state. Our results provide valuable insights into optimizing quantum parameter estimation in the Jaynes–Cummings model and can have practical implications for quantum metrology with hybrid quantum systems.

Reference frame independent and measurement device independent quantum key distribution (RFI-MDI-QKD) has the advantages of being immune to detector side loopholes and misalignment of the reference frame. However, several former related research works are based on the unrealistic assumption of perfect source preparation. In this paper, we merge a loss-tolerant method into RFI-MDI-QKD to consider source flaws into key rate estimation and compare it with quantum coin method. Based on a reliable experimental scheme, the joint influence of both source flaws and reference frame misalignment is discussed with consideration of the finite-key effect. The results show that the loss-tolerant RFI-MDI-QKD protocol can reach longer key rate performance while considering the existence of source flaws in a real-world implementation.

We investigate the single-photon transport problem in the system of a whispering-gallery mode microresonator chirally coupled with a two-level quantum emitter (QE). Conventionally, this chiral QE-microresonator coupling system can be studied by the master equation and the single-photon transport methods. Here, we provide a new approach, based on the transfer matrix, to assess the single-photon transmission of such a system. Furthermore, we prove that these three methods are equivalent. The corresponding relations of parameters among these approaches are precisely deduced. The transfer matrix can be extended to a multiple-resonator system interacting with two-level QEs in a chiral way. Therefore, our work may provide a convenient and intuitive form for exploring more complex chiral cavity quantum electrodynamics systems.

Quantum information technology requires bright and stable single-photon emitters (SPEs). As a promising single-photon source, SPEs in layered hexagonal boron nitride (hBN) have attracted much attention recently for their high brightness and excellent optical stability at room temperature. In this review, the physical mechanisms and the recent progress of the quantum emission of hBN are reviewed, and the various techniques to fabricate high-quality SPEs in hBN are summarized. The latest development and applications based on SPEs in hBN in emerging areas are discussed. This review focuses on the modulation of SPEs in hBN and discusses possible research directions for future device applications.

The ultracold molecule is a promising candidate for versatile quantum tasks due to its long-range interaction and rich internal rovibrational states. With the help of the cavity quantum electrodynamics (QED) effects, an optical cavity can be employed to increase the efficiency of the formation of the photoassociated molecules and offers a non-demolition detection of the internal states of molecules. Here, we demonstrate the production of the high-finesse optical fiber microcavity for the Rb2 molecule cavity QED experiment, which includes the fabrication of fiber-based cavity mirrors, testing, and the assembly of ultra-high vacuum-compatible optical fiber microcavity. The optical fiber microcavity offers high cooperativity between cavity mode and ultracold molecule and paves the way for the study of molecule cavity QED experimental research.

The quantum key distribution (QKD) network is a promising solution for secure communications. In this paper, we proposed a polarization-independent phase-modulated polarization encoding module, and it can be combined with a dense wavelength division multiplexer (DWDM) to achieve multi-user QKD. We experimentally test the encoding module with a repetition rate of 62.5 MHz, and its average quantum bit error rate (QBER) is as low as 0.4%. Finally, we implement a principle verification test for simultaneous QKD for 1 to 2 users in 100 min, and the average QBER of two users under the transmission distance of 1 km and 5 km is kept below 0.8%. Due to the use of polarization encoding, the module can also realize scalable network architecture in free-space QKD systems in the future.

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.

Heterodyne detectors as phase-insensitive (PI) devices have found important applications in precision measurements such as space-based gravitational-wave (GW) observation. However, the output signal of a PI heterodyne detector is supposed to suffer from signal-to-noise ratio (SNR) degradation due to image band vacuum and imperfect quantum efficiency. Here, we show that the SNR degradation can be overcome when the image band vacuum is quantum correlated with the input signal. We calculate the noise figure of the detector and prove the feasibility of heterodyne detection with enhanced noise performance through quantum correlation. This work should be of great interest to ongoing space-borne GW signal searching experiments.

Our previous work had proved pump field noise coupling in the seed field injected optical parametric amplifier (OPA) at a certain analysis frequency. Inspired by this noise coupling mechanism, the frequency dependent squeezing factor due to excess pump noise was experimentally demonstrated. Apart from a reduced squeezing level with an increased noise, the results also prove that a broadband squeezing noise spectrum is not frequency dependent on the amplitude modulated pump field, but limited by the bandwidth of the amplitude modulator and OPA resonator, and the effective measurement is carried out in the frequency range of 2–10 MHz. It provides a guidance to design a broader-bandwidth, higher-level bright squeezed light.

Using the quantum interference of photon pairs in N-stage nonlinear interferometers (NLIs), the contour of the joint spectral function can be modified into an islands pattern. We perform two series of experiments. One is that all of the nonlinear fibers in pulse pumped NLIs are identical; the other is that the lengths of N pieces of nonlinear fibers are different. We not only demonstrate how the pattern of spectral function changes with the stage number N, but also characterize how the relative intensity of island peaks varies with N. The results well agree with theoretical predictions, revealing that the NLI with lengths of N pieces of nonlinear fibers following binomial distribution can provide a better active filtering function. Our investigation shows that the active filtering effect of multi-stage NLI is a useful tool for efficiently engineering the factorable two-photon state—a desirable resource for quantum information processing.

The first photon bias of photon detection results in distortion of the photon waveform, which seriously affects the accurate acquisition of target information. A rapid universal recursive correction method is proposed, which is suitable for multi-trigger and single-trigger modes of photon detection. The calculation time is 2 to 3 orders of magnitude faster than that of Xu et al.’s method. In the experiment, we have obtained good correction results for area targets and targets with varying depths. When the average number of echo photons is 0.89, the correlation distance of the correction waveform is reduced by 85%.

Remarkable achievements have been witnessed in free-space quantum key distribution (QKD), which acts as an available approach to extend the transmission range of quantum communications. The feasibility of transmitting qubits through the free-space channel with the aid of moving platforms like satellites, aircraft, unmanned aerial vehicles (UAVs) has been verified. In view of the limited working time and resource consumption of the satellite-based QKD and the last-mile challenges of connecting satellite nodes with terrestrial networks, the airborne QKD is expected to provide flexible and relay links for the large-scale integrated network. This paper reviews the recent significant progress of QKD based on aircraft or UAVs, highlights their critical techniques, and prospects the future of airborne quantum communications.

We investigate the dynamics of a system that consists of ultra-cold three-level atoms interacting with radiation fields. We derive the analytical expressions for the population dynamics of the system, particularly, in the presence and absence of nonlinear collisions by considering the rotating wave approximation (RWA). We also reanalyze the dynamics of the system beyond RWA and obtain the state vector of the system to study and compare the time behavior of population inversion. Our results show that the system undergoes two pure quantum phenomena, i.e., the collapse–revival and macroscopic quantum self-trapping due to nonlinear collisional interactions. The occurrence of such phenomena strongly depends on the number of atoms in the system and also the ratio of interaction strengths in the considered system. Finally, we show that the result of the perturbed time evolution operator up to the second-order is in agreement with the numerical solution of the Schrödinger equation. The results presented in the paper may be useful for the design of devices that produce a coherent beam of bosonic atoms known as an atom laser.

Quantum random access codes (QRACs) are important communication tasks that are usually implemented in prepare-and-measure scenarios. The receiver tries to retrieve one arbitrarily chosen bit of the original bit-string from the code qubit sent by the sender. In this Letter, we analyze in detail the sequential version of the 3→1 QRAC with two receivers. The average successful probability for the strategy of unsharp measurement is derived. The prepare-and-measure strategy within projective measurement is also discussed. It is found that sequential 3→1 QRAC with weak measurement cannot be always superior to the one with projective measurement, as the 2→1 version can be.

Recently, the nested Mach–Zehnder interferometer [Phys. Rev. Lett. 111, 240402 (2013)] was modified by adding Dove prisms in a paper [Quantum Stud.: Math. Found. 2, 255 (2015)], and an interesting result is that, after the Dove prisms were inserted, a signal at the first mirror of the nested interferometer was obtained. But, according to the former original paper, the photons have never been present near that mirror. In this work, we interpret this result naturally by resorting to the three-path interference method. Moreover, we find that even though the photons have been somewhere, they can hide the trace of being there.

We employ quantum state and process tomography with time-bin qubits to benchmark a city-wide metropolitan quantum communication system. Over this network, we implement real-time feedback control systems for stabilizing the phase of the time-bin qubits and obtain a 99.3% quantum process fidelity to the ideal channel, indicating the high quality of the whole quantum communication system. This allows us to implement a field trial of high-performance quantum key distribution using coherent one way protocol with an average quantum bit error rate and visibility of 0.25% and 99.2% during 12 h over 61 km. Our results pave the way for the high-performance quantum network with metropolitan fibers.

Quantum walks, a counterpart of classical random walks, have many applications due to their neoteric features. Since they were first proposed, quantum walks have been explored in many fields theoretically and have also been demonstrated experimentally in various physical systems. In this paper, we review the experimental realizations of discrete-time quantum walks in photonic systems with different physical structures, such as bulk optics and time-multiplexed framework. Then, some typical applications using quantum walks are introduced. Finally, the advantages and disadvantages of these physical systems are discussed.

Squeezed vacuum, as a nonclassical field, has many interesting properties and results in many potential applications for quantum measurement and information processing. Here, we investigate a single atom–cavity quantum electrodynamics (QED) system driven by a broadband squeezed vacuum. In the presence of the atom, we show that both the mean photon number and the quantum fluctuations of photons in the cavity undergo a significant depletion due to the additional transition pathways generated by the atom–cavity interaction. By measuring these features, one can detect the existence of atoms in the cavity. We also show that two-photon excitation can be significantly suppressed by the quantum destructive interference when the squeezing parameter is very small. These results presented here are helpful in understanding the quantum nature of the broadband squeezed vacuum.

Nonlocal correlations observed from entangled quantum particles imply the existence of intrinsic randomness. Normally, locally projective measurements performed on a two-qubit entangled state can only certify one-bit randomness at most, while non-projective measurement can certify more randomness with the same quantum resources. In this Letter, we carry out an experimental investigation on quantum randomness certification through a symmetric informationally complete positive operator-valued measurement, which in principle can certify the maximum randomness through an entangled qubit. We observe the quantum nonlocal correlations that are close to the theoretical values. In the future, this work can provide a valuable reference for the research on the limit of randomness certification.