Thermoelectrics and thermophotovoltaics are promising solid-state systems for power generation. They convert heat directly into electricity via the Seebeck effect and photovoltaic effect, respectively. Due to their different physical principles and operation conditions, these technologies are almost always discussed separately in the literature. In this work, using the theoretical framework of irreversible thermodynamics that has been commonly applied in the discussions for thermoelectrics, we analyze thermophotovoltaics under small temperature differences and demonstrate the mathematical equivalence of the maximum efficiency expressions for thermoelectrics and thermophotovoltaics. As a result, we define a dimensionless figure of merit of thermophotovoltaics $Z_{\mathrm{TPV}}T$, analogous to the well-known thermoelectric figure of merit $Z_{\mathrm{TE}}T$. We theoretically and numerically discuss the ideal operation under which $Z_{\mathrm{TPV}}T$ approaches infinity and the conversion efficiency reaches the Carnot limit. Our numerical studies on thin-film thermophotovoltaic systems show that $Z_{\mathrm{TPV}}T$ can be significantly greater than one at room temperature in the radiative limit. However, we also demonstrate that nonradiative recombination processes significantly reduce $Z_{\mathrm{TPV}}T$. By accurately incorporating the temperature dependence of short-circuit current and radiative heat transfer, we derive an expression of the maximum efficiency with an engineering figure of merit $(Z_{\mathrm{TPV}}T{)}_{\mathrm{eng}}$, which predicts maximum efficiency at larger temperature differences within the linear current–voltage regime. Finally, we compare the efficiency of thermophotovoltaic systems with linear and nonlinear current–voltage curves and highlight that thermophotovoltaics greatly benefit from the nonlinearity in achieving higher efficiency. Our work achieves a unified treatment and, hence, a direct comparison of the two energy conversion technologies in the linear response regime, while emphasizing distinct differences in the nonlinear regime.

Suitably patterned single crystals made of the cuprate superconductor ${\text{Bi}}_2{\text{Sr}}_2{\text{CaCu}}_2{\text{O}}_{8+x}$ (BSCCO), intrinsically forming a stack of Josephson junctions, can generate electromagnetic radiation in the lower terahertz regime. Because of Joule heating, the emission power of single stacks seems to be limited to values below $100\text{μ}\mathrm{W}$. To increase the radiation power, mutually synchronized arrays situated on the same BSCCO base crystal have been studied. A maximum power of almost 1 mW has been achieved by synchronization of three stacks. Mutual electromagnetic interactions via a connecting BSCCO base crystal have been considered essential for synchronization, but the approach still suffers from Joule heating, preventing the synchronization of more than three stacks. In the present paper we show, on the basis of two emitting stacks, that mutual synchronization can also be achieved by standalone stacks contacted by gold layers and sharing only a common gold layer. Compared with BSCCO base crystals, the gold layers have a much higher thermal conductivity and their patterning is not very problematic. We analyze our results in detail, showing that the two oscillators exhibit phase correlations over a range of $\pm 0.4$ GHz relative to their center frequencies, which we studied mainly between 745 and 765 GHz. However, we also find that strong phase gradients in the beams radiated from both the mutually locked stacks and the unlocked stacks play an important role and, presumably, diminish the detected emission power due to destructive interference. We speculate that the effect arises from higher-order cavity modes that are excited in the individual stacks. Our main message is that the mutual interaction provided by a common gold layer may open new possibilities for relaxing the Joule-heating problem, allowing the synchronization of a higher number of stacks. The approach may also allow one to synchronize several stacks that are comparatively small in size and less prone to the strong phase gradients we observed. Our findings may boost attempts to substantially increase the output power levels of BSCCO terahertz oscillators.

The long-lived and optically addressable high-spin state of the negatively charged silicon vacancy ($V_{\mathrm{Si}}$) in silicon carbide makes it a promising system for applications in quantum technologies. Most studies of its spin dynamics have been performed under external magnetic fields applied along the symmetry axis. Here, we find that the application of weak magnetic fields perpendicular to the symmetry axis leads to nontrivial behavior caused by the dynamical reorientation of the $V_{\mathrm{Si}}$ spin multipole under optical excitation. Particularly, we observe the inversion of the quadrupole-spin polarization in the excited state and the appearance of a dipole-spin polarization in the ground state. The latter is much higher than the thermal polarization and cannot be induced solely by optical excitation. Our theoretical calculations reproduce well all sharp features in the spin-resonance spectra and shine light on the complex dynamics of spin multipoles in these kinds of solid-state systems.

The bistable reflective cholesteric-liquid-crystal display exhibits two stable states with contrasting optical properties at zero voltage. One is the reflecting planar state where the helical axis is perpendicular to the display substrate, and the other is the nonreflecting focal conic state where the orientation of the helical axis is random throughout the display. The display is switched directly from the planar state to the focal conic state by a low voltage. It is switched from the focal conic state back to the planar state in an indirect manner: it is switched to a third state, known as the homeotropic state, by a high voltage, and then relaxes back to the planar state when the applied high voltage is removed. Here we report a new mechanism to switch the bistable reflective display. We make use of liquid-crystal dimers, which have abnormally small bend elastic constants and exhibit a heliconical state. The bistable display is switched from the focal conic state to the heliconical state by an intermediate voltage and then relaxes back to the planar state after the applied voltage is removed. Through this mechanism the driving voltage is significantly reduced.

The measurement of optical vortices constitutes a critical aspect within the photonics domain, with the fast identification of orbital-angular-momentum (OAM) states in vector beams presenting considerable challenges, especially when compared with their counterparts in scalar beams. In this work, we introduce a swift and efficient identification scheme designed specifically for OAM states in vector beams. This innovative scheme enables the simultaneous detection of vortex modes and polarization characteristics through a single-shot measurement. Employing a vector mode decomposition method technique, our approach utilizes advanced polarization conversion and polarization preservation strategies. This methodology has been rigorously developed within a theoretical framework and has been demonstrated in a straightforward, easily implementable experimental setup. The proposed method holds substantial promise for advancing fundamental research in optical topology and quantum state engineering, while also facilitating the applications of OAMs in optical communication.

Quantum secret sharing (QSS) plays a significant role in multiparty quantum communication and is a crucial component of future quantum multiparty computing networks. Therefore, it is highly valuable to develop a QSS protocol that offers both information-theoretic security and validation in real optical systems under a finite-key regime. In this work, we propose a three-user QSS protocol based on phase-encoding technology. By adopting symmetric procedures for the two players, our protocol resolves the security loopholes introduced by asymmetric basis choice without prior knowledge of the identity of the malicious player. Kato’s concentration inequality is exploited to provide security against coherent attacks with the finite-key effect. Moreover, the practicality of our protocol has been validated under a 30-dB channel loss with a transmission distance of 5-km fiber. Our protocol achieves secure key rates ranging from 432 to 192 bps by choosing different pulse intensities and basis selection probabilities. Offering enhanced security and practicality, our protocol stands as an essential element for the realization of quantum multiparty computing networks.

The field of nonlinear metamaterials with exciting intracoupling of multiple physics fields has recently become a topic for vital research. However, most nonlinear dynamic processes are limited to solid-state metamaterials. Here, we propose a strategy for a thermally nonlinear liquid metaresonator by designing a high-$Q$-factor reflective-type electric-$LC$ structure based on the fluid metal, mercury ($\mathrm{Hg}$). We first demonstrate that our proposed metaresonator exhibits thermally tunable properties with high temperature tuning rate. Then, the mutual couplings among the electromagnetic field, thermal field, and mechanical deformation are theoretically and numerically revealed. On this basis, the resonant frequency shift under different power levels of an incident electromagnetic wave is experimentally demonstrated to verify the thermally nonlinear properties. Our strategy for such a thermally nonlinear liquid metaresonator builds a new avenue for dynamically controlling the electromagnetic response of metamaterials.

Nuclear spins in solid-state platforms are promising for building rotation sensors due to their long coherence times. Among these platforms, nitrogen-vacancy ($\mathrm{N}$-$V$) centers have attracted considerable attention with ambient operating conditions. However, the current performance of $\mathrm{N}$-$V$ gyroscopes remains limited by the degraded coherence when they are operating with large spin ensembles. Protecting the coherence of these systems requires a systematic study of the coherence-decay mechanism. Here we present the use of nitrogen-15 nuclear spins of $\mathrm{N}$-$V$ centers in building gyroscopes and compare their performance to the better studied nitroge-14 nuclear spins: while nitrogen-15 spins benefit from their simpler energy structure and lack of a quadrupolar interaction, they suffer from other challenges in coherence protection. We systematically reveal the coherence-decay mechanism of the nuclear spin in different $\mathrm{N}$-$V$ electronic spin manifolds and further develop a robust coherence-protection protocol based on controlling the $\mathrm{N}$-$V$ electronic spin only, achieving a 15-fold increase in dephasing time. With the coherence protection developed, we demonstrate an emulated gyroscope by measuring a designed rotation-rate pattern, showing an order-of-magnitude sensitivity improvement.

Quantum infrared spectroscopy (QIRS), which uses quantum interference of the generation processes of photon pairs, enables infrared spectroscopy using visible light sources and detectors. Until recently, continuous wave (cw) lasers with a narrow linewidth have been used for excitation in most QIRS experiments. Although the use of pulsed laser pumping is attractive, it has been expected that the inherent finite linewidth of pulsed lasers may degrade the spectral resolution of QIRS. Here, we show that this is not always true. The theoretical analysis shows that the resolution of quantum Fourier transform infrared (QFTIR) spectroscopy is not affected by the linewidth of the pump laser. To verify this, we construct an experimental setup for QFTIR spectroscopy using both cw and pulsed laser excitation under the same experimental conditions. High-resolution spectra are successfully obtained for a silicon parallel plate and an acetylene gas cell as samples using a pulsed pump laser with a finite linewidth, and no resolution degradation is observed compared to the case of cw pumping. These results will open up the use of QFTIR spectroscopy using visible light sources and detectors in novel applications including high-speed time-resolved infrared spectroscopy, which will be beneficial for various fields including physics, chemistry, and environmental science.

The spin dynamics in two electron double quantum dots embedded in a two-dimensional electron gas at the interface between ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ and ${\mathrm{La}\mathrm{Al}\mathrm{O}}_3$ is studied by an exact numerical solution of the time-dependent Schrödinger equation in the context of the electric-dipole-spin-resonance experiment. On the basis of the three-band model of $3d$ electrons localized at $\mathrm{Ti}$ ions on the square lattice, we analyze in detail the singlet-triplet transition induced by the ac electric field, in the magnetic field range close to the avoided crossing that appears as a result of the spin-orbit coupling. For symmetric double quantum dots the single-photon spin-flip transition is prohibited due to the parity symmetry, and the transition can occur only by the higher-order two-photon processes. For a weakly asymmetric system, when the first-order singlet-triplet transitions are released due to the parity symmetry breaking, the spin-flip transition has a character of the Rabi oscillations for a low electric field amplitude. As the amplitude is increased, the frequency of the transition is blueshifted (redshifted) for the magnetic field below (above) the single-triplet avoided crossing. For a sufficiently high magnetic field and high ac-field amplitude, the electric field drives the system across the avoided crossing, inducing the spin flip by the Landau-Zener-Stueckelberg-Majorana effect. Finally, the optimization of the geometrical parameters of the system with respect to the time of spin flip of its fidelity is also presented.

We describe microfabricated rubidium vapor cells with integrated temperature-control functionality and demonstrate their suitability for use in miniaturized ultrasensitive magnetometers. These functionalized vapor cells (FVCs) embody a dual-chamber design in low-conductivity silicon with antipermeation coatings and microstructured thin-film platinum surface traces as resistive heaters and temperature sensors. Thermal tests show our ability to control alkali-metal distribution within the FVCs, ensuring a clean sensing chamber for optical measurements. Optical-absorption spectroscopy is used to correlate the temperature readings with vapor density and to measure buffer-gas pressure, of interest for optimizing sensitivity. Finally, we demonstrate zero-field resonance magnetometry with $18\mathrm{fT}/\sqrt{\mathrm{H}}\mathrm{z}$ sensitivity in the 10- to 100-Hz band, limited by laser noise and magnetic shield noise, which indicates that the functionalization does not introduce significant magnetic noise.

Acoustic wave modulation plays a pivotal role in various applications, including sound-field reconstruction, wireless communication, and particle manipulation, among others. However, current acoustic metamaterial and metasurface designs typically focus on controlling either reflection or transmission waves, often overlooking the coupling between the amplitude and phase of acoustic waves. To fill this gap, we propose and experimentally validate a design enabling complete control of reflected and transmitted acoustic waves individually across a frequency range of 4 to 8 kHz, allowing arbitrary combinations of amplitude and phase for reflected and transmitted sound in a broadband manner. Additionally, we demonstrate the significance of our approach for sound manipulation by achieving acoustic diffusion, reflection, and focusing, and generating a two-sided three-dimensional hologram at three distinct frequencies. These findings open an alternative avenue for extensively engineering sound waves, promising applications in acoustics and related fields.

In this work, a compact acoustic coupler based on the Su-Schrieffer-Heeger topological model is presented. The coupler is composed of a cavity array and a series of slits, and the driving pulses in the system are achieved by modulating the width of the slits. The propagation process of acoustic energy is greatly accelerated by selecting and optimizing the driving pulses within a compact configuration. An optimized protocol is adopted to drive the system and the coupler is greatly shortened to 0.3 m. Stable and asymmetric energy transfer is realized from the initial cavity to the end cavity via adiabatic state evolution. Benefitting from the topological protection exhibited by the energy band gap in the proposed multicavity coupler, the energy transfer remains robust after accelerating operation, which is verified experimentally. More uniquely, the coupler has application potential in unidirectional or nonreciprocal energy transfer when a certain loss or spatiotemporal modulation are introduced into the coupler, guaranteeing the security of signal exchange.

Absent of dissipation, wave energy transport occurs with the group velocity, and we exploit this principle to resolve the Dirac cone at the Brillouin-zone edge for ultrasonic surface waves in a hexagonal lattice of resonant cavities. Wave propagation measured along varying real-space lattice angles resolves dispersion along the Dirac cone in reciprocal space where the isofrequency contour gradient is oriented along the angle for which energy propagation is measured. Through the Dirac point, we measure group and phase velocities, and the Dirac-point frequency, which agree with finite-element predictions. The results afford the study of Dirac cone evolution in development of alternative acoustic systems, and demonstrate how a one-dimensional line-scan measurement can resolve targeted reciprocal space dispersion about a specific high-symmetry point for a two-dimensional crystal.

Recently, several single-pixel-imaging schemes have emerged for imaging fast-moving objects and have shown dramatic results. However, fast image reconstruction of a moving object with high quality is still challenging for single-pixel imaging, thereby limiting its practical application. In this paper, we present a simultaneous tracking and imaging method that incorporates position encoding and spatial information encoding through Fourier patterns. The use of Fourier patterns with specific spatial frequencies ensures robust and accurate object localization. By exploiting the properties of the Fourier transforms, our method achieves a remarkable reduction in time complexity while significantly increasing image quality. Furthermore, we introduce an optimized sampling strategy specifically designed for small moving objects, significantly reducing the required dwell time for imaging. The proposed method provides a practical solution for real-time tracking, imaging, and edge detection of moving objects, underscoring its considerable potential for diverse applications.

The transport of microparticles in fluids is of current interest and controlling their trajectories at will is of paramount importance for fundamental research and applications. Here, we demonstrate the effect of an ac electric field and a dc magnetic field on the orientation and electrophoretic propulsion of superparamagnetic microparticles in a nematic liquid crystal. The magnetic field changes the orientation of the elastic dipoles in the plane, and the competing effect of the elastic and magnetic torques is used to measure the induced magnetic moment of a single particle. In the absence of the magnetic field, the particles propel along the far-field director under the application of a low-frequency transverse electric field. The rotating magnetic field changes the direction of motion, facilitating remote control of the particle trajectories. Our study demonstrates the potentialities of orthogonal electric and magnetic fields for maneuvering microparticle trajectories, which are important for practical applications, ranging from targeted delivery and microfluidics to microrobotics.

We propose a strategy to continuously tune the amplitude of acoustic waves based on the interference between two mode-conversion paths in passive acoustic structures. The interference phenomenon is attributed to two conjugate acoustic geometric phases obtained with two mode-conversion processes in a hybrid-type geometric-phase meta-atom (HGPM) pair. Notably, 100% modulation depth of the wave amplitude is achievable by simply varying the local orientation angle of the meta-atom. We utilize an acoustic structure made of two cylindrical resonators to construct a deep-subwavelength secondary source with designated initial phase delay, and the HGPM supporting the desired mode-conversion functionality is accordingly fabricated with four secondary sources. Both theory and experiment consistently verify the continuous amplitude modulation function of the HGPM pair, which showcases a general scheme for reconfigurable amplitude-type acoustic metadevices, i.e., those that require grayscale amplitude modulation for acoustic field engineering.