A Sagnac-loop structure can help overcome the major difficulty in the practical implementation of a twin-field quantum key distribution (TFQKD) network, namely, the need to stabilize the phase of a quantum state over many kilometers of fiber. Unfortunately, Rayleigh backscattering noise limits the signal-to-noise ratio for Sagnac systems containing long fibers and lossy photonic devices. Here, we solve this problem by sending optical pulses in long on-off bursts and using time postselection on measurements taken with free-run single-photon avalanche detectors. We also investigate the impact of the residual phase noise uncompensated by the Sagnac structure and find that the variance of the phase noise scales as loop length to the third power, verifying an existing calculation in the literature. We measure the interference visibility in Sagnac loops of varying length without active phase stabilization and achieve $>97\mathrm{\%}$ visibility in 200-km ultralow-loss fiber, which is, to our knowledge, the longest fiber Sagnac interferometer demonstrated. Our results suggest that a Sagnac system is feasible for long-distance TFQKD networks, a key step towards the practical implementation of metropolitan quantum networks.

High-dimensional quantum entanglement is an essential resource in quantum technology since it provides benefits for increasing the information capacity and processing speed. Thus, the controlled harnessing of high-dimensional entanglement has long been hailed as a necessary prerequisite towards practical quantum applications. By using a deterministic quantum state filter implemented through quantum interference, we present a generalized formulation for the complete high-dimensional symmetric and antisymmetric Bell basis and experimentally prepare four-dimensional orbital angular momentum Bell states that provide the well-behaved symmetric or antisymmetric properties. Additionally, we use a concise yet efficient scan of temporal delay to directly observe high-dimensional two-photon interference effects in spatial modes. These results provide an alternative way to harness high-dimensional entanglement and may facilitate the use of quantum interference for more complex quantum information processing tasks beyond qubits.

Octave-spanning soliton frequency combs generated by microresonators present a compact and low-noise tool for high-resolution time and distance measurements through self-referencing. While these soliton combs have been observed on various photonic platforms, their practical deployment is often hindered by complex control mechanisms, resulting in a low yield of useful devices. To address these challenges, we have optimized a dual-mode soliton scheme by incorporating heaters directly onto ${\mathrm{Si}}_3{\mathrm{N}}_4$ microresonators. The thermal control, achieved by adjusting the electrical power supplied to the heaters, effectively tunes the mode separation to a favorable level (∼100 pm for a 1 THz resonator), facilitating efficient thermal compensation and improving the yield of functional dual-mode resonators. Kerr solitons can be deterministically generated by simply reducing the heater voltage via either slow sweeping or fast pulse tuning with the pump wavelength and power fixed. Additionally, bidirectional voltage scanning enables a smooth transition from multisoliton to single-soliton states, optimizing both the bandwidth and comb power. The integrated heater allows for fine-tuning of both the repetition rate and carrier-envelope offset frequency of the soliton comb with slopes of −21.18 MHz/mW and +0.35 GHz/mW, respectively. We have successfully generated octave-spanning solitons across multiple resonators with different repetition rates (∼1 THz and ∼390 GHz). Additionally, a systematic dual-mode design strategy is explored for future optimization. These improvements mark a significant advancement toward cost-effective and practical Kerr frequency combs for applications such as optical clocks and frequency synthesizers.

Integrated and fiber-packaged magnetic field sensors with a sensitivity sufficient to sense electric pulses propagating along nerves and a spatial resolution fine enough to resolve their propagation directions will trigger tremendous steps ahead in medical diagnostics and research. Nitrogen-vacancy centers in diamond are best suitable for such sensing tasks under ambient conditions. Current research on uniting a good sensitivity and high spatial resolution is facilitated by scanning or imaging techniques. However, these techniques employ moving parts or bulky microscopes. Both approaches cannot be miniaturized to build robust, adjustment-free, hand-held devices. In this work, we introduce concepts for spatially resolved magnetic field sensing and two-dimensional gradiometry with an integrated magnetic field camera. The camera utilizes infrared absorption optically detected magnetic resonance (IRA ODMR) mediated by perpendicularly intersecting infrared and pump laser beams forming a pixel matrix. We demonstrate our scalable $3\times 3$ pixel sensor’s capability to reconstruct the position of an electromagnet. In a reference measurement, we show an IRA ODMR sensitivity of $44\mathrm{nT}{\mathrm{Hz}}^{-1/2}$.

Entanglement-based quantum key distribution (QKD) protocols require robust and stable photon-pair sources in terms of high heralding efficiencies or photon-pair generation rates even under harsh environmental conditions, e.g., when operated in the field. Here we report on a type-0 spontaneous parametric down-conversion photon source in periodically poled crystals based on simultaneous operation of second harmonic generation and spontaneous parametric down-conversion (SPDC) in a double-pass configuration within the same nonlinear crystal, further simplifying the hardware requirements of SPDC photon sources. Our source is flexible, tunable, alignment-free, relatively broadband, and completely fiber coupled. Its rack-compatible and modular setup allows a straightforward plug-and-play integration of coding modules, e.g., interferometers to enable various QKD protocols such as phase or phase-time coding. We compare the double-pass configuration with our single-pass type-II and type-0 SPDC source configurations. To evaluate the conversion efficiencies of our modules, we use data postprocessing to remove artifacts from detector after-pulsing and dead times of the detectors.

Rayleigh’s criterion states that resolving point sources below the width of the point spread function is impossible, with the error increasing at shorter distances. This is known as Rayleigh’s curse. While detection mode shaping solves this for equal sources, it fails for unbalanced sources with unknown brightness ratios. We propose BLESS, a technique using Beam moduLation and Examination of Shot Statistics, breaking Rayleigh’s curse for unbalanced sources. Classical and quantum Cramér-Rao bound calculations show the strong potential of BLESS for real imaging experiments.

Multicolored optical switching phenomena are crucial for a broad range of applications, such as telecommunications and optical computing. However, most materials typically exhibit single-colored optical nonlinearity under intense laser illumination. Herein, our femtosecond time-resolved transient transmission measurements reveal the coexistence of the transient Pauli blocking effect in both the $\mathrm{\Gamma }$ and $L$ valleys following intense optical excitation in $\mathrm{Ge}$ films, demonstrating their feasibility as multicolored optical switching materials based on multivalley scattering. Additionally, a split-off energy of 240 meV at the $L$ point, attributed to the spin-orbit coupling effect, is directly observed from time-resolved transient transmission spectra. Based on an accurately calculated band structure, a scheme has been developed to interpret pump-probe transient transmission traces, effectively avoiding overinterpretation of overlapping transient signals caused by multiple-transition pathways. Furthermore, analysis of transmission transients using a three-level model indicates that the intervalley scattering time between the $\mathrm{\Gamma }$ and $L$ valleys is within 196 fs, and the intravalley scattering time is within 1.10 ps in the $\mathrm{\Gamma }$ valley, highlighting the potential for achieving an ultrafast multicolored optical switching device for key applications in multiband communications and optical computing.

A cylindrical ${\mathrm{TM}}_{0,1,0}$ mode microwave cavity resonator was excited using a balanced interferometric configuration that allowed manipulation of the electric field and potential within the resonator by adjusting the phase and amplitude of the interferometer arms driving the resonator. With precise tuning of the phase and amplitude, 25-dB suppression of the electric field at the resonance frequency was achieved while simultaneously resonantly enhancing the time-varying electric-scalar potential. Under these conditions, the system demonstrated electromagnetically induced absorption in the cavity response due to the annulment of the electric field at the resonance frequency. This phenomena can be regarded as a form of extreme dispersion, and led to a sharp increase in the cavity phase versus frequency response by an order of magnitude when compared to the cavity $Q$ factor. This work presents an experimental setup that will allow the electric-scalar Aharonov-Bohm effect to be tested under conditions involving a time-varying electric-scalar potential, without the presence of an electric field or magnetic vector potential, an experiment that has not yet been realized.

Both orbital angular momentum and radial momentum describe the transverse momentum of a light field. Efficient discrimination between and sorting of the two kinds of momentum lie at the heart of further applications. Here, we propose a parabolalike lens that can transform the orbital angular momentum and radial momentum into different positions in the parabolas. We experimentally characterize the performance of our implementation by separating individual angular and radial momenta as well as multiple superposition states. The reported scheme can achieve two kinds of transverse momentum identification and thus can provide a possible way to complete the characterization of the full transverse momentum of an optical field. The proposed device can readily be used in multiplexing and demultiplexing of optical information, and, in principle, achieve unit efficiency, and thus may be suitable for applications that involve quantum states of light.

Quantum frequency conversion is a widely used technique to interface atomic systems with the telecom band in order to facilitate propagation over longer distances in fiber. Here, we demonstrate the difference-frequency conversion from 606 nm to 1552 nm of microsecond-long weak coherent pulses at the single-photon level compatible with $\mathrm{Pr}$${}^{3+}$:$\mathrm{Y}$${}_2$$\mathrm{Si}\mathrm{O}$${}_5$ quantum memories, with a high signal-to-noise ratio (SNR). We use a single-step difference-frequency-generation process with a continuous-wave pump at 994 nm in a periodically poled lithium niobate ($\mathrm{Mg}\mathrm{O}$:ppLN) waveguide and ultranarrow spectral filtering down to a bandwidth of 12.5 MHz. With this setup, we achieve the conversion of weak coherent pulses of duration up to 13.6 µs with a device efficiency of about 25 % and an SNR >460 for 10-$\text{μ}\mathrm{s}$-long pulses containing one photon on average. This SNR is large enough to enable a high-fidelity conversion of qubits emitted from an emissive quantum memory based on $\mathrm{Pr}$${}^{3+}$:$\mathrm{Y}$${}_2$$\mathrm{Si}\mathrm{O}$${}_5$ and to realize an interface with quantum processing nodes based on narrow-linewidth cavity-enhanced trapped ions.

Backscattering in microring cavities induces mode mixing and limits device performance. Existing methods to mitigate backscattering often involve complex fabrication processes or are insufficient for complete suppression. In this work, we introduce a method to eliminate backscattering by operating the cavity at an exceptional point (EP). By engineering nonconservative coupling between degenerate clockwise and counterclockwise modes, we achieve chiral transmission that prevents degeneracy lifting and suppresses unwanted mode coupling. Unlike previous approaches that rely on precise gain-loss balance or complex structures, our method utilizes nonconservative coupling between the counterpropgating cavity modes. Using this method, we further show significant enhancement in the cavity performance in Floquet mode conversion efficiency at the EP. Our highly adaptable approach enables seamless integration into various photonic platforms with electro-optic modulators. This advancement mitigates backscattering and improves the precision of light-matter interactions, offering promising applications in quantum communication and information processing.

We present the implementation and performance analysis of a portable rack-mounted stand-alone warm-vapor quantum memory system that also includes the laser package, control electronics, and data-processing hardware. The optical memory is based on long-lived hyperfine ground states of cesium, which are connected to an excited state via the $D_1$ line at 895 nm in a $\mathrm{\Lambda }$-configuration. The memory is operated with weak coherent pulses containing on average $<1$ photons per pulse. The long-term stability of the memory efficiency and storage fidelity is demonstrated at the single-photon level together with operation in a nonlaboratory environment.

In the past decade, there has been renewed interest in studying materials with time-dependent constitutive parameters. These structures offer solutions to the major limitations of their spatial counterparts, of which metamaterials are a subclass; however, physically realizing such structures is extremely challenging, as controlling the bulk properties of a material periodically and in a controlled manner is a nontrivial task. In contrast, the temporal modulation of a thin impedance layer is more realistic and achievable. In this work, we demonstrate that one of the most significant effects of active materials—the amplified emission of radiation—can be achieved by enclosing a static material within a time-modulated impedance layer. This effect is shown for both one- and two-dimensional cavities. For the latter, we demonstrate that different multipolar modes can be radiated. Additionally, we incorporate angular dependence into the time-modulated impedance, creating the circular analogue of a spatiotemporally modulated material. This enables the radiation of chiral eigenmodes from a circular cavity due to the coupling of time and cylindrical harmonics of the wave field. Our findings indicate that engineered active surfaces can be utilized to develop a new type of lasing system for various classical waves.

Certain quantum sensing protocols rely on qubits that are initialized, coherently driven in the presence of a stimulus to be measured, then read out. The most widely employed driving sequences act locally in the frequency domain. We introduce and experimentally demonstrate a generalized set of sequences that effect a measurement in any fractional Fourier domain, i.e., along a linear trajectory of arbitrary angle through the time-frequency plane. Combined with linear frequency modulation, this scheme could help separate signal from background in nanoscale spectroscopy and other applications.

Hyperentanglement is a promising resource in the realm of quantum communication, owing to its exploitation of multiple degrees of freedom. In this paper, we report the generation of a two-mode hyperentangled state using one nanophotonic periodically poled lithium niobate waveguide. Our approach encompasses the manipulation of polarization, energy-time, and quadrature amplitude-phase degrees of freedom. Theoretical calculations reveal that the magnitude of the Clauser-Horne-Shimony-Holt parameter $|S|$ can reach $2.375$ even considering the multi-photon-pair generation effect, which is indicative of the entanglement in the polarization degree of freedom. Further analysis involved computing the joint spectral amplitude of the two-mode state, followed by Schmidt decomposition and Fourier transformation. The resulting joint uncertainty product $\mathrm{\Delta }({\omega }_s+{\omega }_i)\mathrm{\Delta }(t_s-t_i)=0.4113$, implying that high-quality energy-time entanglement is achieved. Notably, the von Neumann entropy $S_r$ of our source reaches 0.192, confirming the presence of quadrature amplitude-phase entanglement within the generated state. Moreover, we theoretically apply our source in a free-space quantum key distribution system to demonstrate its potential to increase secret key rates. Our proposed scheme is an efficient and practical approach for facilitating high-capacity quantum communication systems.

Integrated photonics plays a pivotal role in quantum communication devices, offering exceptional robustness, scalability, and performance. The growing maturity of silicon photonics technology offers significant potential for miniaturization and cost-effective implementation of quantum key distribution (QKD). While the practical security of lithium niobate modulators in QKD systems has been extensively examined, the adoption of a new modulation mechanism by silicon photonic modulators necessitates a thorough investigation into their impact on the security of QKD systems. For the first time, we reveal that the two-photon absorption effect in silicon photonic intensity modulators will introduce vulnerabilities, enabling an eavesdropper (Eve) to execute malicious attacks on QKD systems. Through experimental demonstration, we show that by externally injecting femtosecond pulses, Eve can manipulate the intensities of decoy states sent by legitimate users. Based on the experimental results, we find that in the presence of the attack, legitimate users consistently overestimate the secret key rate predicted by the security proof. Our work represents an initial exploration into a new security aspect for the design and standardization of integrated QKD systems.

The polarization degree of freedom is an inherent feature of plane waves propagating in an isotropic homogeneous medium. The miniaturization of optical systems leads to the high localization of electromagnetic waves, but also to the loss of polarization control, namely, breaking TE-TM polarization degeneracy. In this work, we discover the near-field polarization degree of freedom for highly localized guided waves propagating along a dielectric metasurface. We demonstrate the opportunity to create a metasurface with the degenerate TE-TM polarization spectrum for the required operating wavelength and different constitutive materials. In particular, we analyze several possible implementations including silicon nitride and ceramic metasurfaces consisting of disk-shaped resonators, and evaluate the impact of substrates. Finally, we experimentally implement one of the metasurface designs and verify its broadband degenerate TE-TM polarization spectrum. The obtained results form an alternative platform for the planar polarization devices utilizing the polarization degree of freedom of localized light.