Unauthorized light injection has always been a vital threat to the practical security of a quantum-key-distribution (QKD) system. An optical-power limiter (OPL) based on the thermo-optical defocusing effect has been proposed and implemented, limiting the injected hacking light. As a hardware countermeasure, the performance of the OPL under various light-injection attacks will be tested to clarify the security boundary before it is widely deployed. To investigate the security boundary of the OPL in quantum cryptography, we comprehensively test and analyze the behavior of the OPL under continuous-wave (cw) light-injection attacks and pulse-illumination attacks with pulse repetition rates of $0.5$ Hz, $40$ MHz, and $1$ GHz. The test results illuminate the security boundary of the OPL, which allows one to properly employ the OPL in use cases. The methodology of testing and analysis proposed here is applicable to other power-limitation components in a QKD system.

Distributing quantum information between remote systems will necessitate the integration of emerging quantum components with existing communication infrastructure. This requires understanding the channel-induced degradations of the transmitted quantum signals, beyond the typical characterization methods for classical communication systems. Here we report on a comprehensive characterization of a Boston-Area Quantum Network (BARQNET) telecom fiber testbed, measuring the time-of-flight, polarization, and phase noise imparted on transmitted signals. We further design and demonstrate a compensation system that is both resilient to these noise sources and compatible with integration of emerging quantum memory components on the deployed link. These results have utility for future work on the BARQNET as well as other quantum network testbeds in development, enabling near-term quantum networking demonstrations and informing what areas of technology development will be most impactful in advancing future system capabilities.

In quantum illumination, various detection schemes have been proposed for harnessing the remaining quantum correlations of the entanglement-based resource state. To date, the only successful implementation in the microwave domain [R. Assouly, R. Dassonneville, T. Peronnin, A. Bienfait, and B. Huard, Nat. Phys. 19, 1418 (2023)] has relied on a specific mixing operation of the respective return and idler modes, followed by single-photon counting in one of the two mixer outputs. We investigate the performance of this scheme for realistic detection parameters in terms of the detection efficiency, dark-count probability, and photon-number resolution. Furthermore, we take the second mixer output into account and investigate the advantage of correlated photon counting (CPC) for a varying thermal background and optimum postprocessing weighting in CPC. We find that the requirements for photon-number resolution in the two mixer outputs are highly asymmetric due to different associated photon-number expectation values.

Next-generation microwave communications systems face several challenges, particularly from congested communications frequencies and complex propagation environments. We present, and experimentally test, a framework based on the coupled-dipole approximation for designing structures composed of a single simple emitter with a passive disordered scattering structure of rods that is optimized to provide a desired radiation pattern. Our numerical method provides an efficient way to model, and then design and test, otherwise inaccessibly large scattering systems.

Implementation of quantum logic gates with linear optical elements plays a prominent role in quantum computing due to their relatively easy manipulation and realization. We present efficient schemes to implement controlled-not (cnot) gates and controlled-swap (Fredkin) gates by using solely linear optics. We encode the control qubits and target qudits in photonic polarization (two-level) and spatial degrees of freedom ($d$ level), respectively. On the basis of the hybrid encoding, cnot and Fredkin gates are constructed in a deterministic way without any borrowed ancillary photons or measurement-induced nonlinearities. Remarkably, the number of linear optics required to implement a cnot gate has been reduced to one polarizing beam splitter, while only $d$ polarizing beam splitters are necessary to implement a generalized Fredkin gate. The optical depths of all schemes are reduced to 1 and and are dimension independent. Besides, the fidelity of our three-qubit Fredkin gate is higher than 99.7% under realistic conditions, which is higher than in previous schemes.

Integrated microresonator facilitates the realization of quantum frequency comb (QFC), which provides a large number of discrete frequency modes with broadband spectral range and narrow linewidth. However, all previous demonstrations have focused on the generation of energy-time or time-bin entangled photons from QFC. Realizing polarization-entangled quantum frequency comb, which is a useful resource for the fundamental study of quantum mechanics and quantum information applications, remains challenging. Here, we demonstrate a broadband polarization-entangled quantum frequency comb by combining an integrated silicon-nitride microresonator with a Sagnac interferometer. With a free spectral range of about 99 GHz and a narrow linewidth of about 190 MHz, our source provides 22 polarization-entangled photon pairs with frequency covering the whole telecom C-band. The entanglement fidelities for all 22 pairs are above 81%, including 17 pairs with fidelities higher than 90%. Our demonstration paves the way for employing the polarization-entangled quantum frequency comb in quantum network using CMOS technology as well as standard dense wavelength division-multiplexing technology.

Rydberg atoms have shown remarkable performance in sensing microwave fields. The sensitivity of such an electrometer based on optical readout of the atomic ensemble has been demonstrated to approach the photon-shot-noise limit. However, the sensitivity cannot be promoted infinitely by increasing the power of the probe light due to the increased collision rates and power broadening. Compared with classical light, the use of quantum light may lead to a better sensitivity with a lower number of photons. In this paper, we exploit entanglement in a microwave-dressed Rydberg electrometer to suppress the fluctuation of noise. The results show a sensitivity enhancement that beats the shot-noise limit in both cold- and hot-atom schemes. Through optimizing the transmission of the optical readout, our quantum advantage can be maintained with differing absorptive indexes of the atomic vapor, which makes it possible to apply a quantum light source in the absorptive electrometer.

III-V semiconductor quantum dots (QDs) are near-ideal and versatile single-photon sources. Because of the capacity for monolithic integration with photonic structures as well as optoelectronic and optomechanical systems, they are proving useful in an increasingly broad application space. Here, we develop monolithic circular dielectric gratings on bulk substrates—as opposed to suspended or wafer-bonded substrates—for greatly improved photon collection from $\mathrm{In}\mathrm{As}$ quantum dots. The structures utilize a unique two-tiered distributed-Bragg-reflector structure for vertical electric field confinement over a broad angular range. Opposing “openings” in the cavities induce strongly polarized QD luminescence without harming collection efficiencies. We describe how measured enhancements depend on the choice of collection optics. This is important to consider when evaluating the performance of any photonic structure that concentrates far-field emission intensity. Our cavity designs are useful for integrating QDs with other quantum systems that require bulk substrates, such as surface acoustic wave phonons.

The manipulation of topological states in topological systems can be achieved through gain-loss domain walls, offering flexibility and reconfigurability within the bulk. However, this typically necessitates a global non-Hermitian modulation, involving the modulation of all lattice points within each domain, thereby increasing the complexity of system modulation. In this study, we propose a local-non-Hermitian-modulation approach to inducing photonic topological states within topologically trivial systems. By solely modulating the lattice points along the domain boundary, we significantly reduce the complexity of modulation. We theoretically and experimentally demonstrate the generation of photonic topological states induced by local non-Hermitian modulation on a silicon chip in a one-dimensional system. This process is controlled precisely through an effective Hamiltonian. Furthermore, we extend this concept to generate higher-order topological corner states in two-dimensional systems. Our findings showcase the potential of local non-Hermitian modulation in facilitating the manipulation of photonic topological states, thereby offering new possibilities for higher-dimensional systems and other topological systems.

Topological half vortices in momentum space have recently attracted great interest in photonic research. Characterized by winding polarization distributions, topological half vortices carry half-integer topological charges, underlying many exotic optical phenomena. To date, in systems with continuous translational symmetry, the study of momentum-space topological vortices is based mostly on the band degeneracies. Here we theoretically propose that by optical cavities that contain uniaxial materials, two types of momentum-space topological half vortex can emerge. With both a Hamiltonian model and eigenmode simulations, we reveal that paired degenerate and nondegenerate half vortices can be spawned from zero topological charge with different energy detuning of bands. The winding configurations of topological half vortices are closely associated with the distinctive characteristics of Berry curvature. Our work broadens methods to generate and manipulate topological half vortices in momentum space, and also offers a perspective of polarization vortices in the investigation of cavity photons in anisotropic media.

Recently, the spectral manipulation of single photons has been achieved through spatial-temporal modulation of the optical refractive index. Here, we utilized the spatial-temporal modulation of the potential to manipulate the motion of massive particles, i.e., realizing the MeV-magnitude electron acceleration and atom decelerator on a millimeter scale. With the language of spatial-temporal modulation, manipulation of the photons and massive particles are connected and clearly investigated. The spatial-temporal modulation approach is also powerful for the optimization of multistage modulation with a large number of single stages on a photonic integrated chip and can be generalized to a wide range of systems. Thus, this approach may play a significant role in hybrid photonic chips and microscale particle manipulation.

We present a heuristic mathematical model of the relation between the geometry of a photonic crystal waveguide and the Purcell-enhancement factor at a particular wavelength of interest. We use this model to propose approaches to the design of a photonic crystal waveguide maximizing the Purcell enhancement at a target wavelength. Numerical simulations indicate that the proposed model allows controlled variation of Purcell-factor peak width and height, which can be used to achieve increased robustness with regard to fabrication defects.

We investigate the effects of photon loss and spectrally impure sources on Gaussian boson sampling (GBS) when used in dense-subgraph-finding algorithms. We find that the effectiveness of these algorithms is remarkably robust to such errors, to such an extent that there exist classical algorithms that can efficiently simulate the underlying GBS. These results suggest that, unlike the GBS problem itself, the speed-up of GBS-based algorithms over classical approaches when applied to the dense-subgraph problem is not exponential. They do suggest, however, that any advantage offered could be achieved on a quantum device with far less stringent requirements on photon loss and purity than general GBS.

We theoretically and numerically study the dual-pumped optical parametric oscillation in Kerr nonlinear microresonators with and without the thermo-optic effect. Parametric gain at degeneracy points, which exceeds the intrinsic loss of the waveguide, can be achieved with both normal and anomalous dispersion by optimizing the pump power, detuning, and frequency offset. Our theoretical modeling includes the various forms of dynamic behavior of the dual-pumped cavity, such as build-up, bistability, and chaotic behavior. Besides, the conditions for spectral phase transition, including the power threshold and critical point, have also been numerically investigated to achieve degenerate parametric oscillation. Notably, we analyze the phase-sensitive amplification regimes leading to phase bifurcation, which enable various applications in quantum random number generation, all-optical coherent Ising machines, and on-chip photon-pair generation. Our study will hopefully provide guidelines for Kerr-based microresonator designs, which aim for degenerate optical parametric oscillation.

In this work, we develop a homogenization model to determine the effective response of a metallic nanowire array embedded in an electric gyrotropic material. We study the interaction of electromagnetic waves with the metamaterial and demonstrate that the nanowire array can greatly enhance the nonreciprocal response of the gyrotropic substrate. In particular, the metamaterial can either absorb the incoming energy almost entirely or reflect it with little loss, depending on the sign of the incidence angle. We explore the implications of our theory in the context of Kirchhoff's law of thermal radiation. Our results demonstrate that the wire array can boost the difference between the emissivity and absorptivity over a broad spectrum of frequencies and incidence angles compared to an unstructured gyrotropic substrate. These findings suggest potential applications for the nonreciprocal wire medium in thermal management, radiative cooling, and others.

A distributed quantum computing system requires a quantum communication channel between spatially separated processing units. In superconducting circuits, such a channel can be realized by using propagating microwave photons to encode and transfer quantum information between an emitter and a receiver node. Here we experimentally demonstrate a superconducting circuit that deterministically transfers the state of a data qubit into a propagating microwave mode, with a process fidelity of 94.5%. We use a time-varying parametric drive to shape the temporal profile of the propagating mode to be time symmetric and with constant phase, so that reabsorption by the receiving processor can be implemented as a time-reversed version of the emission. We demonstrate a self-calibrating routine to correct for time-dependent shifts of the emitted frequencies due to the modulation of the parametric drive. Our work provides a reliable method to implement high-fidelity quantum state transfer and remote entanglement operations in a distributed quantum computing network.

The measurement of a wave function plays a pivotal role in quantum physics and presents a distinctive challenge in experiment. Recent works have shown that both the real and imaginary components of the wave function can be extracted by employing weak or strong measurements, thereby enabling the determination of its amplitude and phase. Here, we propose a simple approach for reconstructing the wave function utilizing the spin-orbit interaction of light at the air-glass interface. By directly measuring the amplitude and employing spatial differentiation to capture the phase gradient, it becomes possible to successfully reconstruct an unknown wave function. To demonstrate its feasibility, we experimentally measure the pure wave function of photons with a Gaussian state. Furthermore, we conduct measurements on a custom state featuring a targeted phase jump to examine the accuracy of our methodology. The measured results show a distribution with high contrast and considerable accuracy, with a fidelity that can exceed 85%. We believe that this work contributes valuable insights into the practical applications of spin-orbit interaction, including optical image processing, wave-front sensing, and quantitative phase imaging.

Integrated photonic chips have gradually become a significant option for information transmission and processing, in which the integration density will play an increasingly important role similar to that witnessed in integrated circuits. To date, great efforts have been made for silicon-on-insulator wafers to achieve dense integration with low crosstalk, although it remains quite challenging in the emerging lithium-niobate-on-insulator (LNOI) platform. Here, we report a strategy utilizing Floquet-mode-index modulation to realize broadband zero crosstalk with minimal impact on other performance metrics on the LNOI chips. The underlying physics of zero crosstalk is attributed to the collapse of the Floquet quasienergy, which is further experimentally verified by ultrabroadband low-crosstalk transmissions with low excess loss. Moreover, we demonstrate broadband eight-channel light transmissions in a compact LNOI waveguide array, showing an advantage over the conventional ones in comparison. Our work provides an alternative approach for improving the integration density of on-chip photonic circuits, opening up different possibilities for dense waveguide applications in the promising LNOI platform.

Nowadays quantum key distribution (QKD) represents the most mature quantum technology, and multiple countries as well as private institutions are building their quantum network. However, QKD devices are still far from representing a product within everyone’s reach. Indeed, limitations in terms of compatibility with existing telecom infrastructure and limited performances in terms of secret key rate, using noncryogenic detection systems, are still critical. In this work, we implemented a quantum key distribution link between Sicily (Italy) and Malta utilizing two different single-photon avalanche diode (SPAD) detectors. The performances of a standard commercial SPAD have been compared with the results achieved with an alternative prototype of fast-gated system in a package (SIP) SPAD; the SIP detector has shown to be able to accomplish a 14 times higher key rate compared with the commercial device over the channel showing 20 dB of losses.