We study Kerr frequency combs and dissipative Kerr solitons in silicon photonic crystal coupled-cavity waveguides (CCWs) with globally optimized dispersion at telecom wavelengths. The corresponding threshold for comb generation is found to explicitly depend on the main CCW figures of merit, namely, the mode volume, the normal-mode quality factor, and the slow-light group index. Our analysis is carried out by solving the nonlinear dynamics of the CCW Bloch modes in the presence of Kerr nonlinearity and two-photon absorption. Our results open the way to CCW comb generation via dispersion engineering and slow-light enhancement.

A coherent interface between temporal and spatial degrees of freedom (DOFs) allows one to use photons to take advantage of both types of DOF in quantum information processing. Previous interfaces either suffer from a typical loss of 1–3 dB or disturb the polarization DOF in the conversion process. Here we realize a low-loss polarization-independent coherent interface between temporal and spatial DOFs, specifically the time-bin and path modes. The experimental loss is 0.2 dB, and the fidelity of the conversion process is found by tomography to be 0.970 (0.947) without (with) considering the polarization DOF. As an application, our interface transforms a time-bin qubit output from an entanglement distribution channel to a path qubit, which allows postselection-loophole-free measurement of the Bell inequality. With its low loss and polarization independence, our interface promises applications in coherent conversion from hybrid temporal-polarization states to spatial-polarization states and could be put into a cavity to boost the generation rate of multiphoton polarization-entangled sources or enhance the scalability of sequential quantum metrology.

Developing a high-efficiency coherent nonlinear light source is a critical step for photonic quantum technologies in signal processing, imaging, and switching. A promising approach is second-harmonic generation (SHG) via intersub-band transitions in a semiconductor quantum-well structure with a specific plasmon resonant mode of metal structures. There are, however, two significant challenges with this approach. First, limited by the conduction-band offset, the SHG wavelength based on intersub-band transitions is unlikely to extend into the visible region. Second, the high dissipative losses of plasmonic metal nanostructures could severely limit their applicability. Here we demonstrate an alternative configuration using interband excitonic transitions in an all-dielectric multiple-quantum-well metasurface capable of achieving high SHG conversion efficiency in the visible region. Taking advantage of the magnetic resonance of the multiple-quantum-well metasurface, the tightly concentrated optical field that is induced in the metasurface is responsible for boosting the conversion efficiency to about $2\times {10}^{-7}$. This demonstration opens a viable path toward a coherent light source using SHG that is extended to the visible region and beyond with high conversion efficiency, enabling nanophotonic quantum-information applications.

We suggest and discuss a concept of a deterministic integrated source of nonclassical light based on the coherent diffusive photonics, a coherent light flow in a system of dissipatively coupled waveguides. We show how this practical quantum device can be realized with a system of single-mode waveguides laser inscribed in nonlinear glass. We describe a hierarchy of models, from the complete multimode model of the waveguide network to the single mode coupled to a bath, analyze the conditions for validity of the simplest single-mode model and demonstrate feasibility of the generation of bright sub-Poissonian light states merely from a coherent input. Notably, the generation of nonclassical states occurs at the initial stages of the dynamics, and can be accounted for in the linear model that allows us to circumvent the prohibiting computational complexity of the exact full quantum representation.

Alignment error is an important element, which affects the secure key rate of quantum key distribution (QKD). With the large-scale application of weak coherent states in practical QKD systems, the model of misalignment error reflects, however, the behavior of a single photon. Here, based on the recent afterpulse model, we develop a realistic model to include the propagation of multiphoton pulses in the misalignment-error analysis. Moreover, double-click events are also included. The former narrow the gap between the model and real systems and the latter secure the model. We present that the double-click events can be depicted precisely in our model, which is underestimated in previous works. Also, the compatibility of multiphoton pulses in our model almost does not amplify the influence of afterpulse. Besides, the results show that other factors of real-life QKD systems such as the modulation accuracy and the system’s misalignment have a similar impact on the secure key rate.

Quantum key distribution (QKD) based on the laws of quantum physics allows the secure distribution of secret keys over an insecure channel. Unfortunately, imperfect implementations of QKD compromise its information-theoretical security. Measurement-device-independent quantum key distribution (MDI QKD) is a promising approach to remove all side channels from the measurement unit, which is regarded as the “Achilles’ heel” of QKD. An essential assumption in MDI QKD is, however, that the sources are trusted. Here we experimentally demonstrate that a practical source based on a semiconductor laser diode is vulnerable to a laser-seeding attack, in which light injected from the communication line into the laser results in an increase of the intensities of the prepared states. The unnoticed increase of intensity may compromise the security of QKD, as we show theoretically for the prepare-and-measure decoy-state BB84 and MDI QKD protocols. Our theoretical security analysis is general and can be applied to any vulnerability that increases the intensity of the emitted pulses. Moreover, a laser-seeding attack might be launched as well against decoy-state-based quantum cryptographic protocols beyond QKD.

Dispersive coupling based on the Rabi model with large detuning is widely used for qubit quantum nondemolition (QND) readout in quantum computation. However, the measurement speed and fidelity are usually significantly limited by the Purcell effects (i.e., Purcell decay, critical photon numbers) and qubit-dependent Kerr nonlinearity. To avoid these effects, we propose how to realize an ideal QND readout of a gradiometric flux qubit with a tunable gap via its nonperturbative dispersive coupling (NPDC) to a frequency-tunable measurement resonator. We show that this NPDC-based readout mechanism is free of dipole-field interactions and that the qubit QND measurement is not deteriorated by intracavity photons. Both qubit-readout speed and qubit-readout fidelity can avoid the Purcell limitations. Moreover, NPDC can be conveniently turned on and off via an external control flux. We show how to extend this proposal to a multiqubit architecture for a joint qubit readout.

We propose a squeezed-light source meeting the stringent requirements of continuous-variable quantum sampling. Using the time-dependent effective second-order nonlinear interaction induced by a strong driving beam in the presence of the ${\chi }_3$ response in a microresonator, our approach is compatible with established nanophotonic platforms. With typical realistic parameters, squeezed states with a mean photon number of 10 or higher can be generated in a single consistent temporal mode at repetition rates of over 100 MHz. Over 15 dB of squeezing is expected in existing ultralow-loss platforms.

Transparent conducting oxides (TCOs) and graphene have been extensively investigated as promising materials for electro-optic modulation applications due to their dynamically configurable near-infrared properties. In this study, a detailed comparison between modulators comprising either a TCO film or a graphene-insulator-graphene structure is presented, considering a conventional silicon photonic waveguide as the underlying physical system. Both in-line and resonant configurations are investigated, integrating an electro-optic switching mechanism that changes the free-carrier concentration in the configurable material by means of electrical gating. The carrier-concentration change effect is rigorously modeled using solid-state physics principles, providing a physically consistent and straightforward comparison between the investigated material platforms. The performance of the designed modulators is thoroughly evaluated in terms of the extinction ratio, insertion loss, energy consumption, and bandwidth in both TE and TM operation. On the whole, both technologies support high-performance optical modulation, with TCO designs exhibiting an up to $40\mathrm{\%}$ smaller footprint, with their graphene counterparts standing out for their subpicojoule-per-bit performance on a physical system as simple as the employed silicon-wire waveguide. The low insertion losses as well as the impressive over-100-GHz switching speeds suggest both TCOs and graphene as very promising material candidates for future on-chip modulation applications.

Topological one-way edge states promise robust information transport through the notion of photonic crystals. For further applications of edge-state circuits and devices, a high gap Chern number, ${\mathcal{C}}_g$, is required. Contrary to previous realizations using gyrotropic photonic crystals, we show how $|{\mathcal{C}}_g|>1$ can be obtained by gapping multiple Dirac cones, which can surprisingly occur in a reciprocal continuous medium with limited degrees of freedom. A maximum of four one-way edge states is allowed on a single interface between two reciprocal continuous media. One-way signal combiners and splitters are designed with exact control on the branching and transmission efficiencies as immediate applications. Our theoretical investigations will be useful for constructing edge-state devices with multiplexing and demultiplexing capability.

The orbital angular momentum (OAM) of light has attracted a great amount of interest in recent times, due to the wide variety of applications it has made possible. Generally, the OAM spectrum is produced by coaxial superposition of several vortex beams, which might increase the complexity of the system. Here, we propose and experimentally demonstrate a simple way to produce a state of light with a controllable OAM spectrum using a binary array of pinholes. More specifically, we show that a spiral structure can convert a plane wave into a beam with a wide OAM spectrum, which can be easily tuned to pure or multiple OAM modes by adapting the structure of the pinhole plate. Furthermore, we show that a simple pinhole plate can produce structured beams with particular OAM states, such as photonic gears (superposition of OAM modes with opposite topological charges ±ℓ) and OAM combs (an optical mode formed by a series of discrete and equally spaced OAM modes, akin to an optical frequency comb). It is worth noting that we demonstrate the OAM comb experimentally. This study provides an avenue for the flexible generation of OAM spectra and the simplicity of the setup could make this approach convenient for many applications, such as optical communications and quantum information.

We consider theoretically exciton-polaritons in a strip of honeycomb lattice with zigzag edges and it is shown that the interplay among the spin-orbit coupling, Zeeman splitting, and an on-site detuning between sublattices can give rise to a band structure where all the edge states of the system split in energy. Within an energy interval, one of the spin-polarized edge states resides with the gapless bulk that has opposite spin. Being surrounded by opposite spin, and the absence of the backward-propagating edge state, ensures both reflection-free and feedback-suppressed one-way flow of polaritons with one particular spin in the system. The edge states in this system are more localized than those in the standard topological-polariton systems and are fully spin polarized. This paves the way for feedback-free spin-selective polariton channels for transferring information in polariton networks.

We demonstrate the tunable quantum beat of single photons through the co-development of core nonlinear nanophotonic technologies for frequency-domain manipulation of quantum states in a common physical platform. Spontaneous four-wave mixing in a nonlinear resonator is used to produce nondegenerate, quantum-correlated photon pairs. One photon from each pair is then frequency shifted, without degradation of photon statistics, using four-wave-mixing Bragg scattering in a second nonlinear resonator. Fine tuning of the applied frequency shift enables tunable quantum interference of the two photons as they are impinged on a beam splitter, with an oscillating signature that depends on their frequency difference. Our work showcases the potential of nonlinear nanophotonic devices as a valuable resource for photonic quantum-information science.

Off-axis vortex beams are generated by superposing a Gaussian beam onto a symmetric optical vortex beam of unit topological charge in a single-path interferometer with a control of their relative intensities and phases. The radial displacement of the point vortex from the center of the beam is controlled by varying the relative intensity of the superposed beams, while the azimuthal displacement of the vortex is controlled by the phase difference between the superposed beams. This phase difference is employed through the Pancharatnam-Berry geometric phase by different cyclic evolutions of the polarization states of the superposed beams on the Poincaré sphere. Interferometric field reconstruction of the resultant beams from experiment, simulation, and numerical calculations are used to obtain the transverse linear momentum density. The net transverse linear momentum vector and the resulting extrinsic orbital angular momentum in an off-axis vortex beam is demonstrated to be related to the radial and azimuthal position of the vortex across the beam. Controlling the Pancharatnam-Berry geometric phase and intensity ratio of the component beams is thus proposed as an effective and robust technique to tune the extrinsic orbital angular momentum of off-axis vortex beams. The presented results can be useful in applications ranging from optical manipulation of trapped microparticles, controlling micromachines using light with orbital angular momentum to enabling more flexibility in superresolution microscopy and controlled asymmetric interaction of light with atom, molecule, and Bose-Einstein condensate.

Nonlinear processes in the quantum regime are essential for many applications, such as quantum-limited amplification, measurement, and control of quantum systems. In particular, the field of quantum error correction relies heavily on high-order nonlinear interactions between various modes of a quantum system. However, the required order of nonlinearity is often not directly available or weak compared to dissipation present in the system. Here, we experimentally demonstrate a route to obtain higher-order nonlinearity by combining more easily available lower-order nonlinear processes, using a generalization of the Raman transition. In particular, we show a transformation of four photons of a high-$Q$ superconducting resonator into two excitations of a superconducting transmon mode and two pump photons, and vice versa. The resulting eight-wave mixing process is obtained by cascading two fourth-order nonlinear processes through a virtual state. We expect this type of process to become a key component of hardware-efficient quantum error correction using continuous-variable error-correction codes.

We introduce a conceptual design of a free-space thin-sheet isolator with an unlimited aperture and the possibility of a broadband omnidirectional rejection of the backward-propagating light. The proposed design involves a multilayered resonant cavity incorporating subwavelength magnetic layers, dichroic nanolayers, and an optional metallic nanolayer. The cavity resonance enhances the Faraday rotation produced by the subwavelength magnetic layers, while providing nearly total absorption of the backward-propagating light by the dichroic nanolayers. The latter is a necessary and the most challenging condition for a thin-sheet isolator with an unlimited aperture to function. The (optional) metallic nanolayer provides rejection of the obliquely incident light, which otherwise would be partially transmitted in either direction. Our numerical simulations and quasioptical measurements at millimeter-wave frequencies illustrate how the key elements of the layered-sheet isolator work. Our approach can be scaled down to long- and midinfrared wavelengths.

This Paper introduces analyzing and modeling dual silicon-oxy-nitride [${\mathrm{Si}\mathrm{O}}_x{\mathrm{N}}_y$] racetrack resonators (RRs) for biosensing applications. The geometric configuration of the resonators and design of alternative small sensing windows result in the realization of optical biosensors with a highly improved intrinsic limit of detection (ILOD). A small sensing window, located above the coupling region of dual RRs, can be used to maximize the total quality factor (minimized ILOD). The best characteristics for each optical component in the parallel-coupled RR-based biosensors are single-mode-channel waveguides with dimensions of $35\times 1\mu {\mathrm{m}}^2$; the radius of curvature $90\mu \mathrm{m}$ and quality factors up to approximately $5.53\times {10}^6$ for RRs. The biosensing performance of our proposed configuration is compared with other experimental and simulated biosensor devices. Our structure shows an ILOD of approximately $1.81\times {10}^{-6}$ refractive index units (r.i.u.), which significantly improved 100 times compared with the traditional single ${\mathrm{Si}\mathrm{O}}_x{\mathrm{N}}_y$ RR biosensor. This ultra-improved ILOD parameter is found in the dynamic refractive index range of 1.332–1.350 r.i.u. It shows that the proposed biosensor could be a promising diagnostic candidate for analyzing different components of blood samples, which is especially important in the early detection of some critical diseases such as prostate cancer.

Coupling-enabled frequency synchronization is essential for an array of light sources operating in a photonic system. Using a two-dimensional nonlinear oscillator model of a laser, we analyze the role of two distinct types of coupling, dispersive and dissipative, in promoting frequency locking between two nonidentical lasers. In both scenarios the two oscillators synchronize into a frequency-locked state when the coupling level exceeds a critical value. We show that the onset of dispersive and dissipative synchronization processes is associated with hard and soft frequency transitions, respectively. Through analysis and numerics, we demonstrate that the dispersive coupling yields bistable synchronization modes, accompanied by asymmetric intensities, and the frequency controlled by the coupling strength. In contrast, dissipative coupling induces monostable synchronization with symmetric intensities and a coupling-independent frequency. Our results are expected to provide a basis for understanding the coupling mechanisms of frequency locking toward controlling synchronization in laser arrays.