Microwave frequency mixing is a fundamental functionality in microwave photonics, playing a crucial role in modern radar and wireless communication systems. Classical microwave photonic mixers require both the radio frequency (rf) signal and the local oscillator to be loaded onto the same optical light source, thereby impeding their capability in the context of distributed microwave systems. Using time-energy–entangled biphotons as the optical carrier, the quantum microwave photonic (QMWP) mixer offers a promising solution for attaining nonlocal mixing and surmounting such limitations. In this paper, we explore two types of QMWP mixer based on the configuration of the intensity modulators: cascade type and parallel type. Both QMWP mixers feature the properties of high linearity and dual outputs, with their spurious-free dynamic ranges being identical to that of the equivalent classical microwave photonic mixer. Specifically, the parallel-type QMWP mixer can nonlocally mix two physically separated rf signals, demonstrating its superiority in distributed microwave and millimeter-wave radar systems. Further, by application of multiphoton frequency-entangled sources as optical carriers, the nonlocal microwave frequency conversion capability endowed by the QMWP mixer can be extended to enhance the performance of multiple-path microwave mixing, which is essential for radar net systems.

We demonstrate theoretically and experimentally that linear coupling due to the backscattering between two counterpropagating modes in a Kerr microresonator leads to extreme sensitivity of the intensity asymmetry of nonlinear light states to the relative phase between the bidirectional pumps of equal power. In the absence of linear coupling, the relative pump phase does not affect counterpropagating intraresonator intensities, and two asymmetric states arise due to spontaneous symmetry breaking. Contrariwise, in the presence of weak linear intermode coupling, the asymmetry of the states is deterministically controlled via changes in phase (for all phases except 0 and $\pi$). Spontaneous symmetry breaking at zero phase is suppressed when the linear intermode coupling is increased, while for the $\pi$ phase it can be enhanced, so that the overall threshold for the spontaneous symmetry breaking can be significantly lower for nonzero linear coupling. These results are key for a fundamental understanding of the processes in Kerr resonators and other systems with Kerr-like nonlinearities and linear intermode coupling and have high prospects for the development of photonic devices, such as ultrasensitive sensors.

Recent progress has made high-bandwidth electro-optic modulators (EOMs) available. Here we show the cascaded operation of two such modulators driven by custom-built electronics delivering on-demand picosecond pulses and application to a quantum dot–cavity-QED single-photon source. We implement an EOM-based correlation technique and demonstrate light pulses as short as $24\pm 2$ ps with a single EOM. The EOMs can be synchronized and operated in series, and we then produce optical pulses down to $17$ ps. To optimize the pulse contrast, for two different EOM devices, we analyze the transmission as a function of EOM bias, wavelength, and temperature, and we show that, by temperature tuning and stabilization, a pulse contrast above 25 dB can be obtained at the targeted wavelength. With this high contrast, we demonstrate resonant excitation of an $\mathrm{In}\mathrm{Ga}\mathrm{As}$ quantum dot–microcavity-based single-photon source, demonstrating a crucial technology for scalability and synchronization of large-scale photonic quantum applications.

Multiphoton hyperentangement is fundamental in optical quantum information processing. Existing theory and experiments usually focus on producing two-photon hyperentanglement and multiphoton entangled states encoded in a single degree of freedom. Multiphoton entangled states have until now relied on the outcome postselection, a procedure where only the measurement results corresponding to the desired state are considered. Such an approach severely limits the usefulness of the resulting entangled states. We present the protocols of direct production of three- and four-photon hyperentanglement and extend the approach to an arbitrary number of photons through a straightforward cascade of spontaneous parametric down-conversion (SPDC) sources. The generated multiphoton hyperentangled states are encoded in polarization-spatial modes and polarization-time bin degrees of freedom, respectively. Numerical calculation shows that if the average photon number $\mu$ is 1, the down-conversion efficiency is $7.6\times {10}^{-6}$ and the repetition frequency of the laser is ${10}^9$ Hz, the number of the generation of three-photon and four-photon hyperentanglement after cascading can reach about $5.78\times {10}^{-2}$ and $4.44\times {10}^{-7}$ pairs per second, respectively. By eliminating the constraints of outcome postselection, our protocols may represent progresses for multiphoton hyperentangement generation and provide a pivotal role in future multiparty and high-capacity communication networks.

The emergence of topological edge states is a key feature of various topological phases. This phenomenon, termed the bulk-edge correspondence, is regarded as the major experimental signature of topological materials. Recent theoretical studies reveal that topological defects, such as dislocations, can reveal more information about the topological properties of the hosting materials. Here, we observe such a phenomenon in experiments by engineering dislocations in topological photonic crystals. Instead of the topological edge states, we show that a fractional topological number and a local mode bound to the dislocation can emerge as the experimental manifestation of the underlying topology. The fractional topological number depends on both the photonic band topology in momentum space and the geometry of the dislocation in real space and, thus, it serves as a versatile probe of the band topology. We confirm with experiments that the photonic band topology can indeed be inferred from the fractional topological numbers at various dislocations, uncovering the underlying bulk-defect correspondence. Moreover, the geometry of the dislocation can be engineered, which in turn affects the fractional topological number, indicating the role of the real-space topology of the dislocation. Our work serves as a stepping stone toward probing topological phases with dislocations, especially for crystalline materials and metamaterials with time-reversal symmetry.

Nonlinear quantum photonics serves as a cornerstone in photonic quantum technologies, such as universal quantum computing and quantum communications. The emergence of an integrated photonics platform not only offers the advantage of large-scale manufacturing but also provides a variety of engineering methods. Given the complexity of integrated photonics engineering, a comprehensive simulation framework is essential to fully harness the potential of the platform. In this context, we introduce a nonlinear quantum photonics simulation framework, which can accurately model a variety of features such as adiabatic waveguide, material anisotropy, linear optics components, photon losses, and detectors. Furthermore, utilizing the framework, we have developed a device scheme, chip-scale temporal walk-off compensation, that is useful for various quantum information-processing tasks. We further show that the proposed device scheme can enhance the squeezing parameter of photon-pair sources and the conversion efficiency of quantum frequency converters without relying on higher pump power.

Dielectric nanophotonics enables almost-lossless resonant interaction between light and matter at the nanoscale. We show both theoretically and by electromagnetic simulations that the peculiar nature of Mie resonance–induced effective magnetic fields contrasts sharply with the optomagnetism of smooth bulk materials. Mie resonances produce strongly nonuniform effective magnetic fields generated by the inverse Faraday effect. Different orders of optical resonances are characterized by different types of the effective magnetic field patterns. The number of pointlike inverse-Faraday-effect sources can be controlled by adjustment of the pump wavelength, allowing the selective launch of spin waves with submicron wavelengths. A distinguishing feature of the Mie resonance–induced effective magnetic fields is the vortex structure that can be used for magnetic skyrmion generation. The proposed approach considerably broadens the scope of nanoscale optomagnetism.

Negatively charged nitrogen-vacancy ($\mathrm{N}\text{−}$$V^{-}$) centers in diamonds have rapidly developed due to their exceptional quantum optical and spin properties at room temperature. However, their applicability is limited by broad phonon-sideband emission with feeble coherent emission at the zero-phonon line (ZPL). This calls for the exploration of photonic cavities to enhance ZPL emission. We propose an easy-to-fabricate hybrid metal-dielectric nanocavity with low mode volume to enhance ZPL emission while suppressing phononic emission channels due to different $\mathrm{N}\text{−}$V charge states. We find a 400% enhancement in the emission rate and outcoupling of emitted light at the ZPL with better directivity by deterministic positioning of the $\mathrm{N}\text{−}$$V^{-}$ inside the nanocavity, corroborated by numerical simulations. The pump-dependent measurements confirm better emission saturation intensity for the cavity-coupled $\mathrm{N}\text{−}$$V^{-}$s. The proposed tunable nanocavity structure can be applied to a broad range of quantum emitters to control their emission so that their use in quantum technologies could be improved.

We explore the optical properties of plasmon localization and local field enhancement in metal-plastic metasurface. The flexible metal-dielectric metasurfaces are made from modulated polycarbonate substrate coated by a thin silver film. Localization of an optical excitation is experimentally observed by near-field scanning optical microscopy within subwavelength areas in the regular open-resonator metasurface. The localized modes are seen as giant fluctuations of the local electric field spatially concentrated in hotspots, where the local field is much larger than the amplitude of the incident light. Local near-field spatial spectra consist of regularly distributed strong peaks. The maxima of the electric field are highly dependent on the sample structure. The form of the regularly distributed strong resonance peaks follows the topography obtained by atomic force microscopy. It is shown that strong electromagnetic field is concentrated in recesses where the silver film is typically thinner than on the bumps. This experimental observation is consistent with the results of computer simulations of a double-periodic metal-dielectric metasurface and the predictions of our analytical theory. It is found the silver nanofilm has strong adhesion to polycarbonate substrate.

Reference-frame-independent quantum key distribution (RFI-QKD) has gained widespread attention due to its unique advantage for practical application, as it circumvents the need for active reference-frame alignment within the system. However, in comparison to the standard BB84 protocol, the original six-state RFI protocol requires a greater number of quantum states to be operated by Alice and Bob, which is an aspect that merits optimization. In this work, we propose a four-state RFI protocol and illustrate that Alice and Bob each require only four quantum states to perform channel estimation that remains independent of reference-frame deviation, which can proficiently reduce the system complexity. Furthermore, through numerical simulations taking the finite-key-size effect into consideration, we show that the four-state RFI protocol can achieve a secure key rate and transmission distance on par with those of the original six-state RFI protocol. Finally, an experiment over 200 km is conducted to evaluate the feasibility of our scheme. We believe that our protocol can streamline the implementation of RFI-QKD and thereby contribute to the practical advancement of RFI-QKD.

Lightsails are a promising spacecraft concept that can reach relativistic speeds via propulsion by laser light, allowing travel to nearby stars within a human lifetime. The success of a lightsail mission requires that any motion in the plane transverse to the propagation direction is bounded and damped for the entire acceleration phase. Here we demonstrate that a previously unappreciated relativistic force, which generalizes the Poynting-Robertson effect, can passively damp this transverse motion. We show that this purely optical effect can be enhanced by 2 orders of magnitude compared to the effect on plane-mirror sails by judicious design of the scattering response. We thus demonstrate that exploiting relativistic effects may be a practical means to control the motion of lightsails.

Suitably tailored forms of spatiotemporal modulation in electronic circuit networks have been recently employed to overcome fundamental challenges in modern electronic systems, including breaking reciprocity, squeezing the footprint of high-Q resonators, and overcoming the delay-bandwidth limit. Rotating patterns of temporal modulations have been used to synthesize angular momentum, which replaces magnetic bias to break reciprocity in integrated circuits. However, this approach is limited by trade-offs between modulation speed, footprint, and bandwidth of operation. Rotating switching patterns in commutated capacitor networks also enables compact filters and quasielectrostatic wave propagation, overcoming the delay-bandwidth limit. In this paper, we combine these mechanisms in an integrated-circuit ring that synthetically rotates in two dimensions, realizing an effective helicoidal motion that provides ultrabroadband quasielectrostatic nonreciprocal responses fitting within a theoretically infinitesimal size. We also analyze the impact of modulation signal noise on time-modulated nonreciprocal components and unveil the role of a dynamic noise mechanism based on which the noise level increases in the presence of a strong signal passing through the component, along with methods to mitigate this effect. We experimentally verify these principles in a three-port integrated circulator based on a 65-nm CMOS process that operates from dc to 1 GHz with a miniaturization factor of 2 × ${10}^6$.

The reference-frame-independent quantum key distribution (RFI QKD) protocol enables QKD systems to function effectively despite slowly varying reference frames, offering a distinct advantage in practical scenarios, particularly in mobile platforms. In this study, we successfully distribute secure key bits over a 250-km optical fiber distance by developing an RFI QKD system with a repetition rate of 150 MHz. Benefiting from high repetition rate, we achieve a finite-key secret key rate of 49.65 bit/s at a distance of 200 km, which is more than 3 times higher than state-of-the-art systems. Our work dramatically extends the transmission distance and enhances the secret key rate of RFI QKD, significantly promoting its practical application.

Here, we investigate the effective response of three-dimensional spacetime crystals formed by spherical scatterers under a traveling-wave modulation. We develop an analytical formalism to homogenize the spacetime crystals that extends the renowned Clausius-Mossotti formula to time-varying platforms. Our formalism shows that traveling-wave spacetime crystals can be used to engineer a wide range of classes of nonreciprocal bianisotropic couplings in the long-wavelength limit. In particular, our theory reveals the possibility of realizing a purely isotropic Tellegen response in crystals formed by interlaced sublattices of scatterers subjected to different modulation velocities. Furthermore, we introduce a class of generalized Minkowskian crystals that display invariance under arbitrary Lorentz boosts aligned with a fixed spatial direction. We prove that such systems are formed by pseudouniaxial materials with the principal axis aligned parallel to the modulation velocity. The electromagnetic response of such generalized Minkowskian crystals is indistinguishable from that of moving photonic crystals.

Reinforcement learning is a subfield of machine learning that is having a huge impact in the different conventional disciplines, including physical sciences. Here, we show how reinforcement learning methods can be applied to solve optimization problems in the context of radiative heat transfer. We illustrate their use with the optimization of the near-field radiative heat transfer between multilayer hyperbolic metamaterials. Specifically, we show how this problem can be formulated in the language of reinforcement learning and tackled with a variety of algorithms. We show that these algorithms allow us to find solutions that outperform those obtained using physical intuition. Overall, our work shows the power and potential of reinforcement learning methods for the investigation of a wide variety of problems in the context of radiative heat transfer and related topics.

Scintillation describes the conversion of high-energy particles into light in transparent media and finds diverse applications such as high-energy particle detection and industrial and medical imaging. This process operates on multiple timescales, with the final radiative step consisting of spontaneous emission, which can be modeled within the framework of quasiequilibrium fluctuational electrodynamics. Scintillation can therefore be controlled and enhanced via nanophotonic effects, which has been proposed and experimentally demonstrated. Such designs have thus far obeyed Lorentz reciprocity, meaning there is a direct equivalence between scintillation emission and absorption by the scintillator. However, scintillators that do not obey Lorentz reciprocity have not been explored, even though they represent an alternative platform for probing emission, which is both nonequilibrium and nonreciprocal in nature. In this work, we propose to harness nonreciprocity to achieve directional control of scintillation emission, granting an additional degree of control over scintillation. Such directionality of light output is useful in improving collection efficiencies along the directions where detectors are located. We present the design of a nonreciprocal scintillator using a one-dimensional magnetophotonic crystal in the Voigt configuration. Our work demonstrates the potential of controlling nonequilibrium such as scintillation by breaking reciprocity and expands the space of nanophotonic design for achieving such control.

Optical interferometers are extensively used in fundamental physics studies, gravitational wave detection, quantum metrology, topological photonics, and quantum information processing. Fiber-based interferometers are compact, mechanically robust, and cheap, and thus are ubiquitously deployed. However, the optical phase in fiber interferometers is sensitive to ambient perturbation, resulting in compromised phase-sensing precision. Therefore, phase control, shifting, and stabilization of fiber interferometers are essential. Methods to create stable interference patterns and to lock a fiber interferometer at arbitrary phase have been shown, which however are sophisticated, bulky, and delicate, preventing wider application in harsh environments outside laboratories or in space. Here we demonstrate an innovative method for arbitrary-phase locking of fiber unbalanced Mach-Zehnder interferometers. Compared with existing methods, our method is much simpler. We showcase the preparation and characterization of a narrow-band, energy-time-entangled photon state generated in integrated nonlinear microresonators, where two-photon interference visibility reaches 0.993(6). Our method constitutes a critical building block for photonic quantum networks, and is useful to emerging single-photon interference in curved space-time that facilitates exploration of the interface of quantum mechanics and general relativity.