We propose compact plasmonic-power-splitters with arbitrary phase responses in the millimetre waveband based on spoof surface plasmon polaritons (SSPPs). Benefitting from the strong field confinements of SSPPs, the branches of the proposed plasmonic-power-splitters can be flexibly bent to meet the design requirement of various situations, while their transmission performance is nearly not affected. Of note, the output phase responses of the plasmonic-power-splitters can be flexibly controlled by adjusting the SSPP propagation constant of each branch only via optimizing the unit size, without changing the overall layout of the power splitter. At a proof of concept, two eight-way plasmonic-power-splitters with exactly the same layout are designed, simulated and measured. They can achieve eight-way equal output powers, but their output phase shifts are 0° and 45°, respectively, at the center frequency of 26 GHz. We show that the designed power splitters have good power allocation performance in an ultrawideband range from 5 to 30 GHz. Though the phase responses cannot maintain good consistency in such a wide bandwidth, it can be designed at any center frequency in the band. The proposed power splitters show big advantages of path compatibility, high compactness, and customizable phase response, which is promising in large-scale integrated circuits.

Atoms are ideal quantum sensors and quantum light emitters. Interfacing atoms with nanophotonic devices promises nanoscale sensing and quantum optical functionalities. But precise optical control of atomic states in these devices is challenged by the spatially varying light-atom coupling strength, which is generic to nanophotonic devices. We demonstrate numerically that despite the inhomogeneity, composite picosecond optical pulses with optimally tailored phases are able to evanescently control the atomic electric dipole transitions nearly perfectly, with fidelity $f>99\mathrm{\%}$ across volumes large enough for, e.g., controlling cold atoms confined in near-field optical lattices. Our proposal is followed by a proof-of-principle demonstration with a ${}^{85}$$\mathrm{Rb}$-vapor–optical-nanofiber interface, where the excitation by an $N=3$ sequence of guided picosecond $D_1$ control pulses reduces the absorption of a coguided nanosecond $D_2$ probe pulse by up to approximately $70\mathrm{\%}$. The close-to-ideal performance is corroborated by comparing the absorption data across the parameter space with first-principles modeling of the mesoscopic atomic vapor response. Extension of the composite technique to $N\ge 5$ appears highly feasible to support arbitrary local control of atomic dipoles with exquisite precision. This unprecedented ability would allow error-resilient atomic spectroscopy and open up a variety of research opportunities in nanophotonic quantum optics.

Thermal emissions with both narrow spectral bandwidth and compressed angular distributions suggesting high temporal and spatial coherence are useful for the realization of an alternative family of highly efficient optical devices, and they play an increasingly significant role in numerous applications. Although people have endeavored to realize thermal emitters by harnessing different physics implemented in various artificial metasurfaces mainly made from metals, most of the reported results to date fail to produce emitters possessing the above merits. This work presents a proof-of-principal experimental demonstration on how to make use of the concept of quasiguided modes to realize alternative thermal mid-infrared emitters. Supported by an elaborately designed binary grating structure containing a period-doubling perturbation, these modes possess ultrahigh and perturbation-dependent Q factors. Although an amorphous form of evaporated materials is used, and the measured signal is a superposition of multiangle emission peaks, an emission bandwidth of about 25 nm is still experimentally obtained at the wavelength of 5.8 μm, which is one order of magnitude smaller than conventional thermal emitters based on metallic metamaterials. This bandwidth implies that a similar level of enhancement in the temporal coherence is achieved. Furthermore, our calculations show that the thermal emissions based on the quasiguided modes can achieve a spatial coherence length of 1.7 mm. Both of these two coherence properties are the state of the art to date in the mid-infrared thermal emitters.

The optical spin Hall effect, as an optical analogue of spin-orbit coupling of fermionic electrons, has attracted increasing interest in fundamental physics and nanophotonic applications. In addition to spin, charge of particles, as the other degree of freedom, plays a significant role in spin-orbit interaction as well. However, the optical counterpart of charge conjugation has been absent due to the charge-free nature of photons. Here, in a one-dimensional photonic crystal, we report reversible optical spin currents from the opposite band edges of the photonic band gap, which can provide an optical analogue of charge conjugation in spin-orbit coupling. The effective magnetic field acting on the photons is controlled by the tunable splitting of the transverse electric and transverse magnetic optical modes, which is confirmed by both experiment and simulation. Based on the tunable effective magnetic field, the optical spin current can be controlled by the tunable cavity length in an open cavity. Our results will facilitate the development of optical spintronic devices and guide the understanding of analogies and generalizations involving quantum and classical wave theories.

Since the first proof-of-principle experiments 25 years ago, quantum metrology has matured from fundamental concepts to versatile and powerful tools in a large variety of research branches, such as gravitational-wave detection, atomic clocks, plasmonic sensing, and magnetometry. At the same time, two-photon interferometry, which underpins the possibility of entanglement to probe optical materials with unprecedented levels of precision and accuracy, holds the promise to stand at the heart of innovative functional quantum sensing systems. We report a quantum based method for measuring the frequency dependence of the velocity in a transparent medium, i.e., chromatic dispersion (CD). This technique, using energy-time-entangled photons, allows straightforward access to CD value from the visibility of two-photon fringes recorded in a free-evolution regime. In addition, our quantum approach features all the advantages of classical measurement techniques, i.e., flexibility and accuracy, all in a plug-and-play system.

Understanding the computational overheads imposed by classical control systems on quantum computing platforms becomes critically important as these quantum machines grow in scale and complexity. In this work, we calculate the overheads imposed by the implementation of real-time graph traversal algorithms needed to find computational paths through incomplete cluster states for the implementation of one-qubit gates; a necessary requirement for a realistic implementation of photonic measurement-based quantum computing. By implementing two different algorithms, a global breadth-first search that searches the entire cluster state and an incremental version that traverses a narrow subsection of the cluster state, we analyze the trade-off between the accuracy of finding viable paths and the speed at which this operation can be performed, which constrains the overall photonic clock cycle of the system. We also outline the broader implications of our results for implementing classical control systems for measurement-based photonic quantum computing.

Recently, much attention has been paid to second-order photonic topological insulators (SPTIs), because of their support for highly localized corner states with excellent robustness. SPTIs have been implemented in either transverse-magnetic (TM) or transverse-electric (TE) polarizations in two-dimensional photonic crystals (PCs), and the resultant topological corner states are polarization dependent, which limits their application in polarization-independent optics. However, to achieve polarization-independent corner states is not easy, since they are usually in-gap and the exact location in the topological band gap is not known in advance. Here we report on an SPTI based on a two-dimensional square-lattice PC made of an elliptic metamaterial, and we report that whether the band gap is topological or trivial depends on the choice of the unit cell. It is found that locations of topological band gaps of TM and TE polarizations in the frequency spectrum can be independently controlled by the out-of-plane permittivity ${\epsilon }_{\perp }$ and the in-plane permittivity ${\epsilon }_{\parallel }$, respectively, and more importantly, the location of in-gap corner states can also be separately manipulated by them. From this, we achieve topological corner states for both TM and TE polarizations with the same frequency in the PC by adjusting ${\epsilon }_{\perp }$ and ${\epsilon }_{\parallel }$, and their robustness with regard to disorders and defects is numerically demonstrated. The proposed SPTI provides a potential application scenario for polarization-independent topological photonic devices.

The generation, propagation, and control of twisted light in a photonic chip has developed to meet the demands of modern optics, contributing to different research fields such as classical and quantum information technologies and integrated microfluidics. Here, we show how a single Gaussian illumination can excite different twisted-light modes into a buried annular waveguide, including that carrying orbital angular momentum (OAM) or superpositions of its constituents. We show that is possible to encode one-by-multiple information of independent OAM modes, unveiling the dynamics of the generation of OAM modes in a photonic chip. Owing to the degree of freedom of OAM light, OAM-circuit photonics can be explored to enhance high-capacity data processing for quantum or classical networking.

Many quantum sensing suggestions rely on temporal correlations found in photon pairs generated by parametric down-conversion. In this work, we show that the temporal correlations in light with a thermal photon statistics can be equally useful for such applications. Using a subthreshold laser diode as an ultrabright source of thermal light, we demonstrate optical range finding to a distance of up to 1.8 km.

Quantum reservoir computing (QRC) exploits the information-processing capabilities of quantum systems to solve nontrivial temporal tasks, improving over their classical counterparts. Recent progress has shown the potential of QRC exploiting the enlarged Hilbert space, but real-time processing and the achievement of a quantum advantage with efficient use of resources are prominent challenges towards viable experimental realizations. In this work, we propose a photonic platform suitable for real-time QRC based on a physical ensemble of reservoirs in the form of identical optical pulses recirculating through a closed loop. While ideal operation achieves maximum capacities, statistical noise is shown to undermine any quantum improvement. We propose a strategy to overcome this limitation and sustain the QRC performance when the size of the system is scaled up. The protocol is conceived for experimental implementations to be viable with current technology.

A label-free, low-photodamage, low-cost, and in vivo high-resolution microscopic imaging technique has been challenging to develop. Ghost imaging (GI) is a computational imaging protocol with the spatially nonlocal property, which enables low-light imaging. Considering the advantages of GI, we propose a method, called high-resolution microscopic ghost imaging (HRMGI), to achieve high-resolution imaging by simply demagnifying the signal beam. The analytical formulas for resolution and depth of field (DOF) of HRMGI are provided. Experimental results show that the HRMGI system can have 10.5 times higher resolution than the conventional GI, which can exceed the pixel size limit of multipixel detectors, and 58 times greater DOF than the traditional diffraction-limited optical (TDLO) system. To present the performance of HRMGI, blood vessels within an ovarian section are also imaged. The proposed method is expected to facilitate the development of bioimaging techniques and promote the practical application of GI.

A high-efficiency spin-photon interface is an essential piece of quantum hardware necessary for various quantum technologies. Self-assembled ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{As}$ quantum dots have excellent optical properties: if embedded in an optical microcavity, they can show near-deterministic spin-photon entanglement and spin readout. To address the individual spin states, an external magnetic field is required, and a superconducting magnet is usually used. Here we show a compact cryogenically compatible $\mathrm{Sm}$-$\mathrm{Co}$ magnet design that delivers a 475-mT in-plane (Voigt-geometry) magnetic field at $5\mathrm{K}$, which is suitable for lifting the energy degeneracy of the electron spin states and trion transitions of a single ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{As}$ quantum dot. This quantum dot is embedded in a birefringent high-finesse optical microcavity that enables efficient collection of single photons emitted by the quantum dot. We demonstrate spin-state manipulation by addressing the trion transitions with a single laser field and two laser fields. The experimental data agree well with our model, which covers single- and two-laser cross-polarized resonance fluorescence, Purcell enhancement in a birefringent cavity, and variation of the laser powers.

Interaction of counterpropagating (CP) solitons in a high-$Q$ microcavity can lead to phase locking of the two microcombs, endowing a Vernier-like comb pair with high mutual coherence. Here, we study numerically and analytically the soliton dynamics in this phase-locking state. Our simulations unveil that the relative group velocity of CP solitons will change periodically as opposed to being locked to a fixed value. CP solitons are kicked into the frequency-locking state instead of via continuous interaction when assuming a point backscatterer. The group-velocity modulation and periodic soliton motion induce sidebands around the comb modes. The simulated locking dynamics can be analytically modeled by a generalized Adler equation, which accounts for injection locking among multiple comb-line pairs. Our work elucidates the relevance of considering the frequency-locking dynamics when using CP solitons for dual-comb or time-of-flight-based applications.

Nitrogen-vacancy (N-V) centers are diamond lattice defects that may be manipulated and controlled by visible light and microwave irradiation. They are considered a promising solid-state platform for a broad range of quantum technologies, such as magnetic field sensing. A major limitation in realizing such applications is the weak optical signal attained from the NVs, making the readout inefficient and prone to noise. Here, we report the increased brightness and shortened lifetime of N-V centers coupled to hyperbolic metamaterial photonic cavities with optimized dispersion characteristics. As a result, we demonstrate the enhancement of magnetic field sensitivity and measurement SNR. These results introduce a broadly applicable, robust, and technically accessible platform, promising improved performance relevant for a multitude of solid-state defects and their applications.