Quantum key distribution (QKD) is one of the most important aspects of quantum cryptography. Using laws of quantum mechanics as the basis for security, the key-distribution process makes information theoretically secure in QKD. With the advancement and commercialization of QKD, an end-to-end QKD simulation software is required that can include experimental imperfections. Software of this kind will ensure that resources are invested only after prior performance analysis, and is faithful to experimental capacities and limitations. In this work, we introduce our QKD simulation toolkit qkdSim, which is ultimately aimed at being developed into such a software package that can precisely model and analyze any generic QKD protocol. We present the design, implementation, and testing of a prototype of qkdSim that can accurately simulate our own experimental demonstration of the B92 protocol. The simulation results match well with experiment; a representative key rate and the quantum bit error rate from experiment is $51\pm 0.5$ kbit/sec and $4.79\mathrm{\%}\pm 0.01\mathrm{\%}$ respectively, wherein the simulation yields $52.83\pm 0.36$ kbit/sec and $4.79\mathrm{\%}\pm 0.01\mathrm{\%}$, respectively.

Metasurfaces that enable wave-front manipulation within the subwavelength range exhibit fascinating capabilities and application potentials in ultrathin functional devices. Therein, various metasurfaces to realize delicate transverse and even three-dimensional structured light fields have been proposed for applications such as holographic displays, imaging, optical manipulation, etc. However, a metasurface with the capability of tailoring the axial structure of a light field has not been reported so far. Here, we propose and experimentally demonstrate a dielectric metalens to tailor the axial intensity distribution of a light field, based on the independent control of amplitude and phase. The metalens is designed according to an optimized Fourier spectrum encoding method, which allows construction of an ultrasmall nondiffractive light field with any arbitrary preestablished axial structure. This axial modulation scenario enriches the three-dimensional wave-front modulation functionality of the metasurface, and can be implemented for other waves beyond optics, from acoustic and elastic waves to matter waves.

Nonreciprocal optical devices, such as isolators and circulators, provide highly desirable functionalities in routing signals. This motivates the active research on high-efficiency nonreciprocal devices, especially for miniaturized and integrable magnetic-free schemes. All the present experimental demonstrations of circulators in optical domains are based on the high-$Q$ factor cavity. In this work, we demonstrate a cavity-free optical circulator with low insertion loss by making use of a far-detuned nonlinear Raman process with the assistance of the atomic Doppler effect. Due to the large detuning, the excited state can be adiabatically eliminated, resulting in nearly vanishing absorption. An optical circulator with average transmission of more than $85\mathrm{\%}$ and fidelity above $0.98$ is well achieved in an experiment with hot atoms. Our scheme shows the possibility of cavity-free nonreciprocal optical devices with low insertion loss.

A simple and flexible scheme for high-dimensional linear quantum operations is demonstrated on optical discrete spatial modes in the transverse plane. Quantum-state tomography via symmetric informationally complete positive-operator-valued measures and quantum Fourier transformation are implemented with dimensionality of 15. The statistical fidelity of symmetric informationally complete positive-operator-valued measures and the fidelity of quantum-state tomography are approximately 0.97 and up to 0.853, respectively, while the matrix fidelity of quantum Fourier transformation is 0.85. We believe that our approach has the potential for further exploration of high-dimensional spatial entanglement provided by spontaneous parametric down-conversion in nonlinear crystals.

We present two configurations for analog 0 to 2π optical phase modulation using liquid crystals (LCs), each of which achieve switching times that are 1 ms or less. One configuration is based on the switching behavior of a so-called nematic pi cell, and the other is based on the flexoelectro-optic effect in chiral nematic LCs when operated in the uniform lying helix geometry. Both configurations exploit a multipass optical arrangement to enhance the available optical phase range, while maintaining a fast switching speed. Moreover, these devices can be operated at or close to room temperature. Experimental data are found to be in good agreement with results predicted from theory for these multipass phase-modulation configurations.

Gradual loss of the entanglement of a twin beam containing around 25 photon pairs with the increasing external noise is experimentally investigated. The entanglement is quantified by the nonclassicality depths and the nonclassicality counting parameters related to several nonclassicality criteria. The reduction of intensity moments of the analyzed multimode twin beams to single-mode ones allows determination of the negativity as another quantifier of the entanglement. Both the raw photocount histograms and the reconstructed photon-number distributions are analyzed in parallel.

Vortex $\gamma$-ray beams with large orbital angular momentum (OAM) have significant potential applications in material science, nuclear detection, and astrophysics. An all-optical scheme for production of intense vortex $\gamma$-ray beams using relativistic laser-plasma interactions is proposed, where the OAM of $\gamma$-photons is converted from the spin angular momentum of circularly polarized (CP) lasers. The spin-to-orbital angular momentum conversion is achieved by two processes: resonant acceleration of electrons in CP laser-plasma interactions and nonlinear Compton scattering (NCS) of another linearly polarized laser from the former. Three-dimensional particle-in-cell simulations show that vortex $\gamma$-ray beams with a large and controllable OAM of $2.5\times {10}^{18}\hslash$ and energy up to hundreds of $\mathrm{MeV}$ can be obtained by using two lasers at intensities of approximately ${10}^{22}{\mathrm{W}/\mathrm{cm}}^2$, which is much more practical and more efficient than previously proposed NCS of a Laguerre-Gaussian beam.

We study, both experimentally and theoretically, the modification of ${\mathrm{Er}}^{3+}$ photoluminescence properties in $\mathrm{Si}$ dielectric nanoslots. The ultrathin nanoslot (down to 5-nm thickness), filled with $\mathrm{Er}$ in ${\mathrm{Si}\mathrm{O}}_2$, boosts the electric and magnetic local density of states via coherent near-field interaction. We report an experimental 20-fold enhancement of the radiative decay rate with negligible losses. Moreover, via modifying the geometry of the all-dielectric nanoslot, the outcoupling of the emitted radiation to the far field can be strongly improved, without affecting the strong decay-rate enhancement given by the nanoslot structure. Indeed, for a periodic square array of slotted nanopillars an almost one-order-of-magnitude-higher ${\mathrm{Er}}^{3+}$ PL intensity is measured with respect to the unpatterned structures. This has a direct impact on the design of more efficient CMOS-compatible light sources operating at telecom wavelengths.

A promising method to leverage near-field power densities without the use of nanoscale vacuum gaps is through hyperbolic metamaterial (HMM) waveguides. When placed between a hot and cold reservoir, an ideal HMM can transmit surface waves across several microns, enabling an extension of near-field enhancements. However, when accounting for transmission loss due to realistic levels of absorption within the waveguide, previous studies have shown that the enhancements are significantly curtailed at wide separations. In our study, we investigate the role of internal sources within nonisothermal HMMs. We demonstrate that, in some cases, the emission from the HMM accounts for over 90% of the total radiative heat transfer to the receiver, and that these additional sources can largely compensate for optical losses associated with decreased transmission from the emitter to the receiver. Lastly, we investigate the spectral transport in a realistic three-body system that has mismatching optical properties between the boundaries (emitter and receiver) and the waveguide (HMM). Our model shows that the near-field thermal transport remains spectrally selective to the boundaries, even as major radiative contributions come from the waveguide. This work may enable the design of nonisothermal emitter-waveguide-receiver systems that transmit near-field power levels over wider separations.

We demonstrate a method to generate multiparameter controllable chiral optical patterns. In the method, we introduce a phase calculation method of the rotation matrix and the transformation matrix. The controllable characteristics of the chiral optical patterns, which are generated by an annular subzone vortex phase plate, are demonstrated. The direction and spatial distribution of the eccentricity of the generated optical patterns can be freely regulated. The restriction on the direction of a chiral optical pattern by the single phase gradient (±Δl) of an annular subzone vortex is solved. A chiral optical pattern is modulated by using the light-field superposition and phase shift, which is experimentally achieved by a spatial light modulator. The numerical and experimental results indicate that the controllable degrees of freedom are as many as five, namely, shape, size, direction, eccentricity, and number of arrays. Furthermore, we analyze the spatial propagation of the controllable chiral optical patterns in turbulence. The generation of chiral optical patterns has great potential application in optical tweezers, optical micromechanics, chiral microstructure fabrication, rotating microscopic illumination, and so on.

The topologically protected one-way edge modes for light show a way to manipulate the interaction between a single photon and a quantum emitter. We systematically study single-photon quantum optics in a topological waveguide supporting the frequency-dependent one-way edge mode at the boundary of a dynamically modulated photonic system, and compare the results of the single-photon interacting dynamics with the quantum emitter in a unidirectional waveguide. As a major difference, a photon in a topological waveguide propagates around the resonant site coupled to the quantum emitter due to the topological protection. Our results show that, despite different subtleties, photon-emitter interactions for one photon at the edge mode and for one photon propagating in a unidirectional waveguide hold similar features. In addition, we consider the interaction between the photon and a $\mathrm{\Lambda }$-type three-level quantum emitter in the topological waveguide, where we show the nonlocal correlated photon-state generation as well as the single-photon reflection with the frequency conversion. Our work provides insight for understanding the interaction between the topologically protected unidirectional-propagating photon and the quantum emitter, which holds promise for potential applications in quantum-information processing.

We study high-order harmonic generation (HHG) with an annularly shaped high-power, high-repetition-rate laser. We investigate how the spatial separation (or divergence) of the incident laser and the generated high-order harmonics depends on the gas pressure when the laser intensity is in the overdriven regime. It is shown that, when the cone size of the incident annular beam is relatively large, the resulting harmonics are highly collimated along the propagation axis and are spatially separated from the incident beam. Under the overdriven condition, dynamic phase matching favors HHG emissions from short-trajectory electrons that are efficiently built up in the second half of the gas medium. We verify that using an annular beam is a simple and effective method for separating the intense incident infrared laser from the generated high harmonics. Such high-flux and low-divergence light pulses are expected to be powerful sources in the laboratory for ultrafast probing of electron dynamics.

We demonstrate multiphoton interference using a resource-efficient frequency multiplexing scheme, suitable for quantum information applications that demand multiple indistinguishable and pure single photons. In our source, frequency-correlated photon pairs are generated over a wide range of frequencies by pulsed parametric down conversion. Indistinguishable single photons of a predetermined frequency are prepared using frequency-resolved detection of one photon to control an electro-optic frequency shift applied to its partner. Measured photon statistics show multiplexing increases the probability of delivering a single photon, without a corresponding increase to multiphoton events. Interference of consecutive outputs is used to bound the modal purity and demonstrate the nonclassical nature of the emitted light.

High-dimensional quantum key distribution (QKD) allows achievement of information-theoretic secure communications, providing high key-generation rates, which cannot, in principle, be obtained by QKD protocols with binary encoding. Nonetheless, the amount of experimental resources needed increases as the quantum states to be detected belong to a larger Hilbert space, thus raising the costs of practical high-dimensional systems. Here, we present an alternative scheme for fiber-based four-dimensional QKD, with time and phase encoding and one-decoy state technique. Quantum state transmission is tested over different channel lengths up to 145 km of standard single-mode fiber, evaluating the enhancement of the secret key rate in comparison to the three-state two-dimensional BB84 protocol, which is tested with the same experimental setup. Our scheme allows measurement of the four-dimensional states with a simplified and compact receiver, where only two single-photon detectors are necessary, thus making it a cost-effective solution for practical and fiber-based QKD.

Self-injection locking is a dynamic phenomenon representing stabilization of the emission frequency of an oscillator with a passive cavity enabling frequency-filtered coherent feedback to the oscillator cavity. For instance, self-injection locking of a semiconductor laser to a high-quality-factor (high-$Q$) whispering-gallery-mode microresonator can result in reduction of the laser linewidth by multiple orders of magnitude. The phenomenon was broadly studied in experiments, but there was no detailed theoretical model allowing improvement of the stabilization performance. In this paper we develop such a theory. We introduce five parameters identifying the efficiency of the self-injection locking in an experiment, comprising backscattering efficiency, phase delay between the laser and the high-$Q$ cavities, frequency detuning between the laser and the high-$Q$ cavities, the pump coupling efficiency, and the optical path length between the laser and the microresonator. Our calculations show that the laser linewidth can be reduced by 2 orders of magnitude compared with the case of nonoptimal self-injection locking. We present recommendations for experimental realization of the optimal self-injection-locking regime. The theoretical model provides deeper understanding of self-injection locking and benefits multiple practical applications of self-injection-locked oscillators.

Interference between coherent and dissipative coupling results in nonreciprocity in cavity magnonic devices, in which the isolation ratio can theoretically be infinite when the magnon-mode frequency exactly matches a zero-damping condition (ZDC). In order to obtain a nonreciprocal device possessing both a high isolation ratio and a broad operating bandwidth, we tune the side ZDCs by integrating a movable loop antenna, which can effectively control the radiation damping of the magnon mode. As a consequence, the effective operating bandwidth of the giant nonreciprocity can be broadened to a few hundred times the magnon linewidth. Our study offers a feasible method to design a broadband nonreciprocal device with a high isolation ratio by engineering photon-magnon coupling, which may be of benefit for future quantum and coherent information processing.

Controlling and guiding optical spins and pseudospins over an interface is of great importance in the emerging fields of spintronics and valleytronics, holding the promise for new degrees of freedom in the quest for efficient information transport. Here we explore the general possibilities offered by metasurfaces to route optical momentum, spin and helicity through surface waves. We show how anisotropy and bianisotropy can be engineered to impart extreme directionality to local spin excitations. While most previous works have focused on out-of-plane spins, in-plane excitations have become particularly important for valleytronic applications relying on two-dimensional materials, and they offer specific challenges addressed here with suitably engineered nonlocality in metasurfaces.

We introduce an optimal metagrating design for transverse-position metrology in presence of photon shot noise. The proposed working principle is closely related to the formation of a phase vortex in the diffraction orders in the parameter space. Using the topological robustness, we optimize the design and compress all the transverse-position information around a certain point into a small number of detected photons, saturating the shot-noise limit had all the photons used for probing the position been detected. The current scheme avoids the problem of detector saturation in the presence of high probe power while maintaining all the information detected, allowing one to make full use of the high power that is available. Besides, the direct link between the resonant property in the unit cell and the conditions to achieve the bound is given: one with the zeroth dipole resonance and the other one with the anapole condition of the first dipole. The connection between the metagrating design and the optimization using topological robustness along with the fundamental precision limit using classical light gives new insights in all of these fields.