We theoretically investigate a quantum heat diode based on two interacting flux qubits coupled to two heat baths. Rectification of heat currents is achieved by asymmetrically coupling the qubits to the reservoirs, which are modeled as dissipative $RLC$ resonators. We find that the coherent interaction between the qubits can be exploited to enhance the rectification factor, which otherwise would be constrained by the temperatures and couplings of the baths. Remarkably high values of the rectification ratio, up to $\mathcal{R}\sim 3.5$, can be obtained for realistic system parameters, with an enhancement up to approximately $230\mathrm{\%}$ compared to the noninteracting case. The system features the possibility of manipulating both the rectification amplitude and direction, allowing for an enhancement or suppression of the heat flow to a chosen bath. For the parameter regime in which rectification is maximized, we find a significant increase of the rectification above a critical interaction value, which corresponds to the onset of a nonvanishing entanglement in the system. Finally, we discuss the dependence of the rectification factor on the bath temperatures and couplings.

We report on the demonstration of an effective, nonlinearity-induced nonreciprocal behavior in a single nonmagnetic multi-mode Taiji resonator. Nonreciprocity is achieved by a combination of an intensity-dependent refractive index and of a broken spatial reflection symmetry. Continuous wave power-dependent transmission experiments show nonreciprocity and a direction-dependent optical bistability loop. These can be explained in terms of the unidirectional mode coupling that causes an asymmetric power enhancement in the resonator. The observations are quantitatively reproduced by a numerical finite-element theory and physically explained by an analytical coupled-mode theory. This nonlinear Taiji resonator has the potential of being the building block of large arrays for the study of topological and/or non-Hermitian physics. This represents an important step towards the miniaturization of nonreciprocal elements for photonic integrated networks.

Hybrid integrated quantum photonic circuits have the potential to scale up the number of quantum nodes and to build a network with distributed quantum-information-processing units at affordable resources. A central need of such systems is to have near-unity success-rate state transfer between the quantum nodes. This turns out to be a grand challenge since stringent conditions of spatial mode matching and time-reversal symmetry between the sending and receiving nodes have to be simultaneously satisfied. Here we devise a type of hybrid quantum node consisting of a single solid-state quantum system and a coupled-cavity structure toward achieving perfect state transfer between two distant nodes connected via waveguides. The hybrid node possesses flexibility in tailoring the temporal profile of the emitted single-photon wave packet without any dynamic modulation. In particular, we show it could emit time-reversal symmetric single-photon wave packets and fully couple the emission to the waveguide, fulfilling the conditions for perfect quantum state transfer. We develop a complete theoretical framework and provide a clear physical picture of the protocol for transferring a superposition state. We show ideally the simple node structures with one, two, and three chain-coupled microring resonators could lead to total success rates of 90.5%, 97.5%, and 99.3%, respectively. We then further discuss the influence of imperfections and experimental realizations of our scheme with integrated quantum photonics.

Unlike the electrical conductance that can be widely modulated within the same material even in deep-subwavelength devices, tuning the thermal conductance within a single material system or nanostructure is extremely challenging and requires a large-scale device. This prohibits the realization of robust on/off states in switching the flow of thermal currents. Here, we present the theory of a thermal switch based on resonant coupling of three photonic resonators, in analogy to the field-effect electronic transistor composed of a source, a gate, and a drain. As a material platform, we capitalize on the extreme tunability and low-loss resonances observed in the dielectric function of monolayer hexagonal boron nitride ($h$-BN) under controlled strain. We derive the dielectric function of $h$-BN from first principles, including the phonon-polariton line widths computed by considering phonon-isotope and anharmonic phonon-phonon scattering. Subsequently, we propose a strain-controlled $h$-BN–based thermal switch that modulates the thermal conductance by more than an order of magnitude, corresponding to a contrast ratio in the thermal conductance of $98\mathrm{\%}$, in a deep-subwavelength nanostructure.

Random number generators underpin the security of current and future cryptographic systems and are therefore a likely target for attackers. Quantum random number generators have been hailed as the ultimate sources of randomness. However, as shown in this work, the susceptibility of the sensitive electronics required to implement such devices poses a serious threat to their security. We present an out-of-band electromagnetic injection attack on a photonic quantum random number generator through which an adversary can gain full control of the output. In our first experiment, the adversary forces the binary output of the generator to become an alternating string of 1s and 0s, with near 100% success. This attack may be spotted by a vigilant user performing statistical tests on their output strings. We therefore envisage a second more subtle attack in which the adversary forces the output to be a random pattern known to them, thus rendering any protection based on statistical tests ineffective.

Topological matter and topological optics have been studied in many systems, with promising applications in materials science and photonics technology. These advances motivate the study of the interaction between topological matter and light, as well as topological protection in light-matter interactions. In this work, we study a waveguide-interfaced topological atom array. The light-matter interaction is nontrivially modified by topology, yielding optical phenomena. We find topology-enhanced photon absorption from the waveguide for a large Purcell factor, i.e., $\mathrm{\Gamma }/{\mathrm{\Gamma }}_0\gg 1$, where $\mathrm{\Gamma }$ and ${\mathrm{\Gamma }}_0$ are the atomic decays to the waveguide and environment, respectively. To understand this unconventional photon absorption, we propose a multichannel scattering approach and study the interaction spectra for edge- and bulk-state channels. We find that, by breaking inversion and time-reversal symmetries, optical anisotropy is enabled for the reflection process, but the transmission is isotropic. Through a perturbation analysis of the edge-state channel, we show that the anisotropy in the reflection process originates from the waveguide-mediated non-Hermitian interaction. However, the inversion symmetry in the non-Hermitian interaction makes the transmission isotropic. At a topology-protected atomic spacing, the subradiant edge state exhibits huge anisotropy. Because of the interplay between edge- and bulk-state channels, a large topological bandgap enhances nonreciprocal reflection of photons in the waveguide for weakly broken time-reversal symmetry, i.e., ${\mathrm{\Gamma }}_0/\mathrm{\Gamma }\ll 1$, producing complete photon absorption. We show that our proposal can be implemented in superconducting quantum circuits. The topology-enhanced photon absorption is useful for quantum detection. This work shows the potential to manipulate light with topological quantum matter.

Cavity resonators are promising resources for quantum technology, while native nonlinear interactions for cavities are typically too weak to provide the level of quantum control required to deliver complex targeted operations. Here we investigate a scheme to engineer a target Hamiltonian for photonic cavities using ancilla qubits. By off resonantly driving dispersively coupled ancilla qubits, we develop an optimized approach to engineering an arbitrary photon-number-dependent Hamiltonian for the cavities while minimizing the operation errors. The engineered Hamiltonian admits various applications including canceling unwanted cavity self-Kerr interactions, creating higher-order nonlinearities for quantum simulations, and designing quantum gates resilient to noise. Our scheme can be implemented with coupled microwave cavities and transmon qubits in superconducting circuit systems.

Strong coupling of electromagnetic cavity fields with antiferromagnetic spin waves in hematite ($\alpha$-${\mathrm{Fe}}_2{\mathrm{O}}_3$) is achieved above room temperature. A cube of hematite is placed in a metallic tube and the transmission is measured, using a continuous-wave terahertz spectrometer. The spectra, collected as a function of the temperature, reveal the formation of magnetic polaritons.

Quantum key distribution (QKD) allows the establishment of common cryptographic keys among distant parties. Many of the QKD protocols that were introduced in the past, involve the challenge of monitoring the signal disturbance over the communication line, in order to evaluate the information leakage to a potential eavesdropper. Recently, a QKD protocol that circumvents the need for monitoring signal disturbance, has been proposed and demonstrated in initial experiments. Here, we propose an improved version of this so-called round-robin differential phase-shifting (RRDPS) protocol, in which both time and phase degrees of freedom are utilized to enlarge the Hilbert-space dimensionality, without increasing experimental complexity or relaxing security assumptions. We derive the security proofs of the round-robin differential phase-time-shifting (RRDPTS) protocol in the collective attack scenario, and benchmark it against RRDPS for different experimental parameters. Furthermore, a proof-of-concept experiment of the RRDPTS protocol, using weak coherent pulses and decoy state method, is demonstrated over 80 km of fiber link. Our results show that the RRDPTS protocol can achieve higher secret key rate in comparison with the RRDPS, in the condition of high quantum bit error rate.

Electrodes in close proximity to an active area of a device are required for sufficient electrical control. The integration of such electrodes into optical devices can be challenging since low optical losses must be retained to realize high-quality operation. Here, we demonstrate that it is possible to place a metallic shallow phosphorus doped layer in a silicon microring cavity that can function at cryogenic temperatures. We verify that the shallow doping layer affects the local refractive index while inducing minimal losses with quality factors up to ${10}^5$. This demonstration opens up a pathway to the integration of an electronic device, such as a single-electron transistor, into an optical circuit on the same material platform.

Calibration of sensors is a fundamental step in validating their operation. This can be a demanding task, as it relies on acquiring detailed modeling of the device, which can be aggravated by its possible dependence upon multiple parameters. Machine learning provides a handy solution to this issue, operating a mapping between the parameters and the device response, without needing additional specific information on its functioning. Here, we demonstrate the application of a neural-network-based algorithm for the calibration of integrated photonic devices depending on two parameters. We show that a reliable characterization is achievable by carefully selecting an appropriate network training strategy. These results show the viability of this approach as an effective tool for the multiparameter calibration of sensors characterized by complex transduction functions. Furthermore, the approach is proven to be versatile and promising for mass production, as the same neural network is able to calibrate different devices that have the same structure.

In this work, we fabricate a nitride-based photonic crystal (PhC) nanocavity with an embedded active layer for unexplored red regions using simplified selective wet etching of an ${\mathrm{Al}}_{0.82}{\mathrm{In}}_{0.18}\mathrm{N}$ sacrificial layer. Eu,O-codoped $\mathrm{Ga}\mathrm{N}$ ($\mathrm{Ga}\mathrm{N}$:$\mathrm{Eu}$,$\mathrm{O}$) is embedded in L7-type two-dimensional-PhC (2D-PhC) nanocavities with hexagonal holes. Room-temperature photoluminescence (PL) shows $\mathrm{Eu}$ emission coupled to the cavity modes at wavelengths around 620 nm. The quality factor of the fundamental mode in an L7 nanocavity is 5400, which is comparable to those of state-of-the-art 2D-PhC nanocavities with embedded nitride-based active layers for the blue and ultraviolet regions. We show that the Purcell effect due to the small mode volumes of the 2D-PhC nanocavities and enhanced light extraction lead to a 54-fold increase in the integrated PL intensity per unit area with respect to the typical $\mathrm{Ga}\mathrm{N}$:$\mathrm{Eu}$,$\mathrm{O}$ emission.

We propose an approach for wavelength-selective excitation of perpendicular standing spin waves by optical guided modes in a magnetic dielectric film. TM-polarized guided waves with elliptical polarization induce an effective magnetic field due to the inverse transverse magneto-optical Kerr effect (ITMOKE). The ITMOKE is similar to the inverse Faraday effect, except for the induced magnetic field direction that is oriented in the film plane perpendicular to guided-mode propagation. Switching between different guided modes by tuning the wavelength or angle of the laser pulse significantly modifies the spatial distribution of the induced ITMOKE field and, therefore, selectively launches the standing spin-wave modes of different orders. Consequently, one may perform a tunable optical excitation of a certain spin-wave mode via a femtosecond laser pulse of the corresponding wavelength. The proposed approach broadens the spin-wave manipulation capabilities via optical means.

Traditional electronic frequency demodulation of a microwave frequency voltage is challenging because it requires complicated phase-locked loops, narrowband filters with fixed passbands, or large footprint local oscillators and mixers. Herein, a different frequency demodulation concept is proposed based on refractive index modulation of two coupled microcavities excited by an optical wave. A frequency-modulated microwave frequency voltage is applied to two photonic crystal microcavities in a spatially odd configuration. The spatially odd perturbation causes coupling between the even and odd supermodes of the coupled-cavity system. It is shown theoretically and verified by finite-difference time-domain simulations how careful choice of the modulation amplitude and frequency can switch the optical output from on to off. As the modulating frequency is detuned from its off value, the optical output switches from off to on. Ultimately, the optical output amplitude is proportional to the frequency deviation of the applied voltage making this device a frequency-modulated-voltage to amplitude-modulated-optical-wave converter. The optical output can be immediately detected and converted to a voltage that would result in a frequency-demodulated voltage signal. Or the optical output can be fed into a larger radio-over-fiber optical network. In this case the device presents a compact, low power, and tunable route for multiplexing frequency-modulated voltages with amplitude-modulated optical communication systems. The resulting system requires modest modulation amplitudes and operates at frequencies relevant for modern communication systems. The cavity designs have realistic quality factors that are well within the range of experimental implementation. The role of modulation sidebands in reducing switching contrast is explored, and two methods for mitigating these effects are demonstrated.

Second-order photonic topological insulators that host highly localized corner states resilient to defects, are opening alternative routes towards developing fascinating photonic devices. However, the existing works on second-order photonic topological insulators have mainly focused on either transverse-magnetic or transverse-electric modes. In this paper, we propose a dual-polarization topological photonic crystal structure for both transverse magnetic and transverse electric modes through topology optimization. Simple tight-binding lattice models are constructed to reveal the topological features of the optimized photonic crystal structure in a transparent way. The optimized dual-polarization second-order photonic topological insulator hosts four groups of corner states with different profiles and eigenfrequencies for both the transverse-magnetic and transverse-electric modes. Moreover, the robustness of these corner states against defects is explicitly demonstrated. Our results offer opportunities for developing polarization-independent topological photonic devices.

Being a general wave phenomenon, bound states in the continuum (BICs) appear in acoustic, hydrodynamic, and photonic systems of various dimensionalities. Here, we report the first experimental observation of an accidental electromagnetic BIC in a one-dimensional periodic chain of coaxial ceramic disks. We show that the accidental BIC manifests itself as a narrow peak in the transmission spectra of the chain placed between two loop antennas. We demonstrate a linear growth of the radiative quality factor of the BICs with the number of disks that is well described with the developed tight-binding model. We estimate the number of disks when the radiation losses become negligible in comparison to material absorption and, therefore, the chain can be considered as practically infinite. The presented analysis is supported by near-field measurements of the BIC profile. The obtained results provide useful guidelines for practical implementations of structures with BICs opening up horizons for the development of radio-frequency and optical metadevices.

How can a target’s differential image of multiple components at a certain degree of freedom (DOF) of light (e.g., wavelength, polarization, and position) be obtained? Previous schemes often need two steps, i.e., first collecting each component of the target simultaneously with multiple detectors or sequentially with a single detector, then performing the difference operation in the digital postprocessing. Based on the principle of computational ghost imaging (GI), we here take advantage of the natural complementarity in space to design an illumination mode, which is no longer the spatiotemporal fluctuation of photon number (light intensity) but the photons with different components under the same DOF. By applying this engineered illumination to computational GI, termed ghost difference imaging (GDI), we can achieve differential imaging of multiple components of a certain DOF in a single-round acquisition using only one single-pixel detector with no extra digital-subtraction operations but higher SNR. Both multiwavelength-difference GDI and position-difference GDI are well demonstrated in simulations and experiments. Further, the GDI scheme reduces the number of sampling times for differential imaging while also increasing photons’ utilization. Our work, therefore, provides an insight for GI modality, along with a straightforward and low-cost update of the light path, which is suitable for all computational GI systems using the digital micromirror device.

Solid-state atomlike systems have great promise for linear optic quantum computing and quantum communication but are burdened by phonon sidebands and broadening due to surface charges. Nevertheless, coupling to a small-mode-volume cavity would allow high rates of extraction from even highly dephased emitters. We consider the nitrogen-vacancy center in diamond, a system understood to have a poor quantum optics interface with highly distinguishable photons, and design a silicon nitride cavity that allows 99% efficient extraction of photons at 200 K with an indistinguishability of $>$ 50%, improvable by external filtering. We analyze our design using Lumerical fdtd simulations and treat optical emission using a cavity-QED master equation valid at and beyond strong coupling and that includes both zero-phonon line broadening and sideband emission. The simulated design is compact ($<10\mu \mathrm{m}$) and, owing to its planar geometry, can be fabricated using standard silicon processes. Our work therefore points toward scalable fabrication of noncryogenic atomlike efficient sources of indistinguishable photons.