The ability to create phase-controlled pulses of light with wavelength tunability has applications spanning quantum and classical communications networks. Traditionally, optical transmitters are able to either produce phase-controlled pulses at a fixed wavelength or require a chain of bulky and expensive external modulators to convert wavelength tunable continuous-wave light into optical pulses. One technology of great interest is quantum key distribution (QKD), a technology for generating perfectly random keys at remote nodes to ensure secure communications. Environments such as data centers, where the user needs change regularly, will require adaptability in the deployment of QKD to integrate into classical optical networks. Here we propose and demonstrate an alternative quantum transmitter design consisting of a multimodal Fabry-Perot laser optically injection locked by a wavelength tunable laser. The transmitter is able to produce phase-controlled optical pulses at GHz speeds with a tunable wavelength range of $>65\mathrm{nm}$ centered at 1550 nm. With this transmitter, we perform proof-of-principle QKD with secure bit rates of order Mb/s.

We report on multiphoton statistics of correlated twin beams produced in a periodic poled microring resonator on thin-film lithium niobate. Owing to high-cavity confinement and near-perfect quasi-phase-matching, the photon pairs are produced efficiently in single modes at rates reaching 27 MHz per $\text{μ}\mathrm{W}$ pump power. By using a pump laser whose pulse-width impedance matches with the cavity, those photons are further created in single longitudinal modes with purity reaching 99%, without relying on later-on filtering. With a dual-channel photon-number-resolving detection system, we obtain directly the joint detection probabilities of multiphoton states up to three photons, with high coincidence to accidental contrast for each. Used as a single-photon source, it gives heralded $g_H^{(2)}(0)$ around 0.04 at a single-photon rate of 650 kHz on chip. The findings of our research highlight the potential of this nanophotonic platform as a promising platform for generating nonclassical, few-photon states with ideal indistinguishability, for fundamental quantum optics studies and information applications.

Sophisticated positive-operator-valued measures (POVMs) are fundamental to obtain a quantum advantage in many informational problems but are difficult to implement. We design a universal measurement device composed of cascaded quantum-walk modules able to realize arbitrary entangled POVMs on two qubits. Only parameters in the modules need reprogramming for different POVMs. We apply our device to the task of direction guessing, realizing nine different five-outcome entangled measurements (fidelities above 0.9850) via nine-step photonic quantum walks, recovering optimal direction guessing from nonideal input states. Our work marks an important step toward building a universal versatile quantum measurement device able to implement optimal measurements for various quantum information tasks.

We propose an on-chip fiber-based quantum transducer to convert microwave photons into optical-telecommunication-band photons. The device consists of a coplanar-waveguide microwave resonator and a pair of coupled optical cavities integrated into a rare-earth-doped fiber. The microwave resonator is formed by two aluminum strips deposited onto the cladding of the fiber, whose diameter has been etched down to approximately $8\text{μ}\mathrm{m}$. We study the conversion process using the cavity electro-optic formalism for a system operating at liquid-helium temperature (around 4 K), as well as using a three-wave mixing approach for operation at millikelvin temperatures achievable in a dilution refrigerator. Both methods predict that unity conversion efficiency could be achievable with realistically achievable experimental parameters. The proposed device acts as a quantum transducer for coherent microwave-to-optical conversion, which has great promise for application in all-fiber integrated quantum devices for quantum information processing.

We present a scheme of quantum repeater that uses entangled multimode coherent states that are obtained by electro-optic modulation of symmetric and antisymmetric Schrödinger cat states. Part of the generated entangled frequency modes is sent to a symmetric beam splitter at the central node, while the remaining modes are stored locally in quantum memories. The entangled coherent states between remote quantum memories are conditionally prepared by photon-counting measurements at the output channels of the beam splitter. We study how the effects of decoherence in the quantum channel affect statistics of photocounts and, for the heralding outcomes determined by the parity of photocounts, evaluate the probability of success and the fidelity of the prepared entanglement depending on the symmetry of input cat states. It is demonstrated that the generated entanglement can be employed for teleportation of the phase information from the modulated states that are utilized in quantum key distributions with subcarrier wave encoding.

Recent advances in laser technology demand more-effective measurement of ultrashort optical pulses in fiber optics and nanophotonics devices. Here we propose an in-fiber strategy to characterize ultrashort pulses by incorporating a gallium selenide–assisted microfiber, as a frequency-doubling block, in an all-fiber interferometer. Pulses to be retrieved were first split into the two arms of the interferometer and then overlapped, exciting the interferometric autocorrelated second harmonic in the microfiber due to high second-order nonlinearity of gallium selenide. In the experiment, the autocorrelator shows high accuracy in measuring 7.13-ps, 1239-fs, and 683-fs pulses. In comparison with a commercial frequency-resolved optical gating, the relative errors are only 1.53%, 3.74%, and 6.86%, respectively. Moreover, by attenuation of the incident laser of one arm of the interferometer, the measured peak-to-background ratio was less than the conventional theoretical value. The proposed autocorrelator could be very compatible with current systems for future applications of in-fiber or on-chip ultrafast optical technology.

Non-Hermiticity has emerged as a promising tool to induce topological phase transitions, allowing for the design of tunable and functional topological devices such as topological lasers and switches on on-chip integrated optical platforms. In this work, we experimentally demonstrate loss-modulated photonic topological edge states in anti-parity-time-symmetric waveguide arrays. Loss is introduced via carefully designed subwavelength grating (SWG) waveguides, allowing for on-chip control of topological responses. Our results show that loss not only induces topological phase transition but also enhances topological responses, leading to the increased localization of topological edge states and the anomaly reduced intrinsic loss of the topological edge states. Our work provides a platform based on SWG waveguides for exploring non-Hermiticity-induced photonic topological responses and corresponding applications.

We apply inverse-design methods to produce two-dimensional triangular-lattice plasma-metamaterial (PMM) devices, which are then constructed and demonstrated experimentally. Finite-difference frequency-domain simulations are used along with forward-mode automatic differentiation to optimize the plasma densities of each of the plasma elements in the PMM to perform beam steering and demultiplexing under transverse magnetic polarization. The optimal device parameters are then used to assign plasma-density values to elements that make up an experimental version of the device. Device performance is evaluated against both the simulated results and human-designed alternatives, showing the benefits and disadvantages of in silico inverse design and paving the way for future fully in situ optimization.

Bound states in the continuum (BICs) in open cavities have attracted considerable attention in wave physics due to their ability to confine light and produce high-quality-factor resonances with promising applications for filtering and sensing. One of the most interesting types of BICs is Friedrich-Wintgen (FW) BICs, which result from destructive interference of two interacting modes belonging to the same radiation channel. Here, we investigate theoretically and experimentally FW BICs in a photonic and plasmonic T-shaped cavity made of two horizontal guides of lengths $d_2$ and $d_3$ coupled to a vertical stub of length $d_1$. We demonstrate that the necessary condition for obtaining BICs consists in taking the lengths of the two horizontal guides $d_2$ and $d_3$ commensurate. This BIC is a common mode of the guides of lengths $d_2$ and $d_3$, such as the electric field vanishes at their connection point with the stub of length $d_1$; this BIC is independent of $d_1$ and the infinite waveguide to which the whole cavity will be attached. We show that, depending on $d_1$, the FW BIC appears as the consequence of the interaction between two eigenmodes of the originally isolated cavity where the width of one mode vanishes giving rise to FW BIC, while the width of the second mode becomes broad. In addition, we show that by slightly deviating from the BIC condition, the latter transforms to either electromagnetically induced transparency (EIT) or reflection or Autler-Townes splitting (ATS) resonances. Both EIT and ATS effects are qualified as a transparency window between two transmission zeros, but with different physical origins. We exploit the Akaike’s information criterion test to discern EIT from ATS and distinguish the regime where the EIT or ATS effect dominates. The theoretical results, obtained by means of the Green’s function method, are validated both by experimental measurements using coaxial cables in the radiofrequency domain and numerical simulations using metal-insulator-metal plasmonic waveguides operating in the infrared domain. The sensitivity of the PIT (the plasmonic analogue of EIT) resonances to the dielectric inside the waveguides can be used to design a highly sensitive sensor, which makes it suitable for an on-chip optical sensing platform.

Quantum digital signatures (QDSs), generating correlated bit strings among three remote parties for signatures through quantum law, can guarantee nonrepudiation, authenticity, and integrity of messages. Recently, a one-time universal-hashing QDS framework, exploiting the quantum asymmetric encryption and universal hash functions, has been proposed to significantly improve the signature rate and ensure unconditional security by directly signing the hash value of long messages. However, similar to quantum key distribution, this framework utilizes keys with perfect secrecy by performing privacy amplification that introduces cumbersome matrix operations, thereby consuming large computational resources, causing delays, and increasing failure probability. Here, we prove that, different from private communication, imperfect quantum keys with partial information leakage can be used for digital signatures and authentication without compromising the security while having eight orders of magnitude improvement on signature rate for signing a megabit message compared with conventional single-bit schemes. This study significantly reduces the delay for data postprocessing and is compatible with any quantum key generation protocols. In our simulation, taking two-photon twin-field key generation protocol as an example, QDS can be practically implemented over a fiber distance of 650 km between the signer and receiver. For the first time, this study offers a cryptographic application of quantum keys with imperfect secrecy and paves a way for the practical and agile implementation of digital signatures in a future quantum network.

Recently, three-dimensional (3D) metamaterials have undergone a revolution driven mainly by the popularization of 3D-printing techniques, which has enabled the implementation of modern microwave and photonic devices with advanced functionalities. However, the analysis of 3D metamaterials is complex and computationally costly in comparison with the analysis of their one-dimensional and two-dimensional counterparts due to the intricate geometries involved. In this paper, we present a fully analytical framework based on Floquet-Bloch modal expansions of the electromagnetic fields and integral-equation methods for the analysis of 3D metamaterials and metagratings. Concretely, we focus on 3D configurations formed by periodic arrangements of rectangular waveguides with longitudinal slot insertions. The analytical framework is computationally efficient compared with full-wave solutions and also works under oblique-incidence conditions. Furthermore, it comes associated with an equivalent circuit that allows one to gain physical insight into the scattering and diffraction phenomena. The analytical equivalent circuit is tested against full-wave simulations in cst. Simulation results show that the proposed 3D structures provide independent polarization control of the two orthogonal polarization states. This key property is of potential interest for the production of full-metal polarizers, such as the one illustrated here.

Tuning $\mathrm{Ga}\mathrm{As}$-based quantum emitters to telecom wavelengths makes it possible to use existing mature technology for applications in, e.g., long-haul ultrasecure communication in fiber networks. A promising method redeveloped recently is to use a metamorphic $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ buffer that redshifts the emission by reducing strain. However, the impact of such a buffer also causes the simultaneous modification of other quantum dot (QD) properties. Knowledge of these effects is crucial for actual implementations of QD-based nonclassical light sources for quantum communication schemes. Here, we thoroughly study single $\mathrm{Ga}\mathrm{As}$-based quantum dots grown by molecular-beam epitaxy on specially designed, digital-alloy $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ metamorphic buffers. With a set of structures varying in the buffer indium content and providing quantum dot emission through the telecom spectral range up to 1.6 µm, we analyze the impact of the buffer and its composition on QD structural and optical properties. We identify the mechanisms of quantum dot emission shifts with varying buffer compositions. We also look at charge-trapping processes and compare excitonic properties for different growth conditions with single-dot emission successfully shifted to both the second and third telecom windows.

We present a field-deployed quantum key distribution (QKD) link based on energy-time entanglement. Three nodes are connected, bridging the cities of Nice and Sophia Antipolis, by means of commercial grade optical fibers, spanning over a total distance of 50 km. We have built a packaged and resilient source of energy-time entangled photon pairs and implemented remote interferometer stabilization. Moreover, the clocks of the two end users are synchronized exploiting photon-counting techniques only, therefore requiring no dedicated channel. The system runs over the ITU 100-GHz standard telecom grid, through which a secure key rate of 7.0 kbit/s is extracted for one pair of channels. A homemade software performs all the necessary postprocessing procedures enabling to establish secret keys in real time. We present the first fully operational entanglement-based metropolitan QKD system being totally self-sufficient, i.e., requiring no additional overheads to be operated on a conventional metropolitan fiber network.

Ghost imaging can be implemented with illumination light with pronounced quantum characters. In this work, we address the inverse problem of ghost imaging, i.e., the tomography of the quantum states of illumination light based on ghost imaging. We showcase the characterization of multimode Gaussian state light represented by its covariance matrix. With the information of spatial modes, we reconstruct the quantum states of light using the variation of SNR of the ghost images formed by the input light under a given set of optical operations. We demonstrate that ghost imaging can also be used to characterize the non-Gaussian light by its deviation from Gaussian states, which can serve as a criterion for the non-Gaussianity of the input light. The spatial resolving power of the proposed method can be advantageous for quantum information applications involving structured light, such as multiplexing with spatial modes.

Topological phases of matter feature exotic edge states. However, the fractional topological numbers at edges, although predicted long ago by Jackiw and Rebbi, remain elusive in topological photonic systems. Here we report on the observation of fractional topological numbers at the topological edges and corners in one- and two-dimensional photonic crystals. The fractional topological numbers are determined via measurements of the photonic local density of states. In one-dimensional photonic crystals, we witness a rapid change of the fractional topological number at the edges rising from 0 to 1/2 when the photonic band gap experiences a topological transition, confirming the well-known prediction of Jackiw and Rebbi. In two-dimensional systems, we discover that the fractional topological number in the corner region varies from 0 to 1/2 and 1/4 in phases with different photonic band gaps. Our study paves the way toward topological manipulation of fractional quantum numbers in photonics.

Imaging systems measuring intensity in the far field succumb to Rayleigh’s curse, a resolution limitation dictated by the finite aperture of the optical system. Many proof-of-principle and some two-dimensional imaging experiments have shown that, by using spatial mode demultiplexing (SPADE), the field information collected is maximal, and, thus, the resolution increases beyond the Rayleigh criterion. Hitherto, the SPADE approaches are based on resolving the lateral splitting of a Gaussian wave function. Here, we consider the case in which the light field originates from a biphoton source, i.e., spontaneous parametric down-conversion, and a horizontal separation is introduced in one of the two photons. We show that a separation induced in the signal photon arm can be superresolved using coincidence measurements after projecting both photons on Hermite-Gauss modes. Remarkably, the Fisher information associated with the measurement is enhanced compared to the ordinary SPADE techniques by $\sqrt{K}$, where $K$ is the Schmidt number of the two-photon state that quantifies the amount of spatial entanglement between the two photons.

When the energy of a quantum emitter aligns with the photonic bandgap of the waveguide, variety of distinct quantum optical phenomena emerges. The paradigmatic example of such nontrivial interaction results in the formation of a qubit-photon bound state (QPBS) where a single photon is exponentially localized around the qubit position. Superconducting quantum circuits has been extensively used for studying QPBS where the photonic band gaps are generated in two-dimensional periodic structures such as photonic crystals and metamaterials. Nonetheless, in all the experiments reported so far, the exponential nature of the bound state wave function is justified by making two assumptions (1) that the waveguide is infinite in length and (2) the coupling to the external ports are absent. These assumptions are difficult to achieve for a physical device and the transition from an experimentally realizable finite-size system to the theoretically assumed infinite device size has never been thoroughly understood. Here, we have realized an alternative platform based on a three-dimensional (3D) rectangular waveguide that has a naturally occurring low-frequency cutoff for generating QPBS as proposed in Shahmoon and Kurizki [Phys. Rev. A 87, 033831 (2013)]. In its theoretical description, we explicitly take into consideration the finite nature of the 3D waveguide arising due to its metallic boundaries and coupling to microwave ports and demonstrate how the infinite waveguide limit arises in such a finite system.