We demonstrate the ultimate sensitivity allowed by quantum physics in the estimation of the time delay between two photons by measuring their interference at a beam splitter through frequency-resolving sampling measurements. This sensitivity can be increased quadratically by decreasing the photonic temporal bandwidth even at values smaller than the time delay when standard two-photon interferometers become inoperable and without adapting the path of the reference photon, nor the need of time-resolving detectors with an unfeasible high resolution. Applications can range from the more feasible imaging of nanostructures, including biological samples, and nanomaterial surfaces to quantum enhanced estimation based on frequency-resolved boson sampling in optical networks.

We address the scaling-up problem for superconducting quantum circuits by using lumped-element resonators based on an alternative fabrication method of aluminum-aluminum oxide-aluminum (Al-${\mathrm{AlO}}_x$-Al) parallel-plate capacitors. The size of the resonators is only $0.04{\mathrm{mm}}^2$, which is more than one order smaller than the typical size of coplanar resonators ($1{\mathrm{mm}}^2$). The fabrication method we develop easily fits into the standard superconducting qubit fabrication process. We obtain capacitance per area $14\mathrm{fF}/\text{μ}{\mathrm{m}}^2$ and the internal quality factor $1\times {10}^3$–$8\times {10}^3$ at the single-photon level. Our results show that such devices based on Al-${\mathrm{AlO}}_x$-Al capacitors could be further applied to the qubit readout scheme, including resonators, filters, amplifiers, as well as microwave metamaterials and innovative types of qubits, such as $0-\pi$ qubit.

Restricted by pulse dispersion, dephasing, and bunch deflection, it has been a challenging task to realize high-gain particle acceleration and bunch focusing by using an ultrashort-pulse laser to drive an integrable acceleration structure, which is necessary to develop an on-chip particle accelerator for practical applications. Here we propose a laser-driven traveling-wave linac (linear accelerator), which uses the cascade reflection and refraction of an ultrashort laser pulse on a microscale dielectric structure to achieve long-range laser-particle interaction, avoiding waveguide dispersion, and enhancing sustainable acceleration gradient. It is scalable and extensible, and can realize full-course particle acceleration by using a single laser source via the inverse Cherenkov effect. With this accelerator scheme, we further propose a dynamic synchronization and focusing method, which not only counteracts dephasing but also restrains the bunch spread and deflection caused by space-charge and emittance effects, realizing stable bunch transport and acceleration in a tiny-size bunch channel without resorting to the need for external focusing equipment. This accelerator scheme paves the way toward a high-efficiency laser-driven all-dielectric on-chip particle accelerator for practical applications.

Alternatively to the full reconstruction of an unknown quantum process, the so-called selective and efficient quantum process tomography (SEQPT) allows estimating, individually and up to the required accuracy, a given element of the matrix that describes such an operation with a polynomial amount of resources. The implementation of this protocol has been carried out with success to characterize the evolution of a quantum system that is well described by a trace-preserving quantum map. Here, we deal with a more general type of quantum process that does not preserve the trace of the input quantum state, which naturally arises in the presence of imperfect devices and system-environment interactions, in the context of quantum information science or quantum dynamics control. In that case, we show with the aid of a priori information on the losses structure of the quantum channel that the SEQPT reconstruction can be adapted to reconstruct the non-trace-preserving map. We explicitly describe how to implement the reconstruction in an arbitrary Hilbert space of finite dimension $d$. The method is experimentally verified on a superconducting quantum processor provided by IBM Quantum services, by estimating several non-trace-preserving quantum processes in dimensions up to $d=6$. Our results show that it is possible to efficiently reconstruct non-trace-preserving processes, with high precision, and with significantly higher fidelity than when the process is assumed to be trace preserving.

The rotational Doppler effect (RDE) is of fundamental importance in the detection of rotating systems, ranging from molecules to macroscopic bodies. Metasurfaces, which are potential substitutes for traditional optical devices, have been reported to induce a RDE. However, the mechanism of the interaction between an electromagnetic wave and a spinning artificial meta-atom is not yet fully understood. Here, we report a multiple RDE based on a time-varying generalized Pancharatnam-Berry phase, obtained from a single spinning meta-atom placed in a rectangular waveguide. Twofold and sixfold rotational Doppler shifts with respect to the rotational velocity are obtained from meta-atoms with C2 and C3 rotational symmetry, respectively. In contrast to full-size devices, a single meta-atom has minimal rotational inertia and size, enabling its application in integrated systems. This work represents the interaction of an electromagnetic wave with a single spinning meta-atom, which may have potential applications in rotation-detection systems, frequency-gradient metasurfaces, and frequency modulators.

Spoof-surface-plasmon polaritons (SSPPs) supported by artificial microwave metawaveguides can mimic other properties of optical-surface-plasmon polaritons (SPPs) and have the potential to build the next generation of circuits and systems. However, the traditional model of SSPP metawaveguides take only physical characters of electrical length and characteristic impedance into consideration and thus can hardly guide the design of integrated SSPP systems efficiently. In this work, a modified model of SSPP metawaveguide is proposed to realize multicharacteristic optimization design of integrated SSPP systems. To validate the modified model, a SSPP-based frequency-shifting keying (FSK) modulation and demodulation system is realized. In comparison to microstrips and SSPP metawaveguides based on the traditional model, the SSPP metawaveguides based on the modified model bring lower discontinuity, lower coupling, and lower system complexity to the FSK modulation and demodulation system, which improves the quality of demodulated signals significantly. Hence, exploring physical characteristics of SSPP metawaveguides by introducing modified models is an effective route to realize outstanding integrated SSPP systems.

This paper presents a compact low-temperature atomic vector magnetometer for weak-field measurements, using an atomic cell containing two orthogonal multipass cavities. At the working temperature of $75{}^{\circ }\mathrm{C}$, the magnetic field sensitivities at all three axes are better than $45\mathrm{fT}{\mathrm{Hz}}^{-1/2}$ at 10 Hz limited by photon noise, and $85\mathrm{fT}{\mathrm{Hz}}^{-1/2}$ at 0.1 Hz. This sensor also shows measurement stabilities better than 1.5 pT at three axes for an integration time of ${10}^4\mathrm{s}$, even with the laser frequency unlocked. The sensor response to a rotation is demonstrated, which is also developed to measure the effective gyromagnetic ratio of atoms in this sensor when the bias field is nulled. This magnetometer makes an important step towards long-term stable measurements and calibrations of ultralow fields.

We design, simulate, and experimentally characterize a reconfigurable elastic metamaterial with beyond-nearest-neighbor (BNN) coupling. The structure is composed from the popular British model-construction system Meccano and supports backward waves with opposite directions of phase and group velocities. We experimentally verify three distinct configurations and acoustically infer their spatial vibration spectra.

Organic field-effect transistors (FETs) are witnessing a revolution in their performance, in part, due to a reduction in contact resistance, and through controlled morphology of the organic semiconducting film. Another major consideration is the role of the dielectric layer in dictating transport properties. The transient electric-field-induced second-harmonic generation (EFISHG) technique allows visualization of carrier dynamics in the channel region of a FET by capturing the movement of the induced dielectric polarization. A microscopic imaging system using a broadband femtosecond laser and a pulse compensation arrangement is constructed for imaging transient EFISHG from organic FETs by synchronized laser and voltage pulses. By varying the dielectric layer in organic FETs comprising pentacene and a donor-acceptor conjugated polymer as semiconducting layers, we show that EFISHG images provide an accurate estimate of the carrier mobility along with providing insights into channel formation and the electric field distribution within the channel region. We further correlate the interface charging effects observed in EFISHG to the interface trap density obtained by capacitance-voltage and conductance-voltage from metal-insulator-semiconductor diodes.

Conventionally, band gaps of water waves can be obtained with periodic arrays of bottom-mounted obstacles or vertical cylinders. However, such structures need to possess a large ratio of volume in water, and may not be easy to build and move in practical ocean engineering. Here, we theoretically propose and experimentally demonstrate that when the water surface is pierced by a fixed, rigid cylinder array, a complete band gap of water waves can be achieved, in which the propagation of water waves is forbidden in all the directions. Such a band gap can exist even when the cylinders have a draft much smaller than the water depth ($d\ll h$). Based on the complete band gap, a semicircular shape of the fixed-surface disk array is further constructed, which can block omnidirectional water waves generated by a point source.

Multiplexing techniques combine multiple signals into one channel, in order to increase the channel capacity and speed. Here we propose a correlation-based imaging method to efficiently measure the quantum states of orbital-angular-momentum (OAM)-multiplexed Gaussian light. The covariance matrices for all OAM components of the multiplexed Gaussian light can be efficiently reconstructed by one homodyne measurement on each beam. We also show that the proposed scheme can be simplified and implemented with only two charge coupled device detectors.

We extend the theory for a Josephson-junction traveling-wave parametric amplifier (TWPA) operating in the three-wave-mixing regime and we propose a scheme for achieving high gain. The continuous three-mode model [P.K. Tien, J. Appl. Phys. 29, 1347 (1958)] is, on one hand, extended to describe a discrete chain of Josephson junctions at high frequencies close to the spectral cutoff where there is no up-conversion. On the other hand, we also develop a continuous multimode theory for the small frequency limit where the frequency dispersion is close to linear. We find that in both cases the gain is significantly reduced compared to the prediction by the continuous three-mode model as the result of increasingly strong dispersion at the high frequencies and generation of up-converted modes at the small frequencies. The developed theory is in quantitative agreement with simulations of the full solution of the dynamical equations. To recover the high gain, we propose to engineer a chain with dispersive features to form a two-band frequency spectrum and to place the pump frequency within the upper band close to the spectral cutoff. We prove that there exists a sweet spot, where the signal and the pump are phase matched, while the up-conversion is inhibited. This results in a high gain, which grows exponentially with the length of the TWPA.

Higher-order topological insulators (HOTIs) in classical systems, featuring robust multidimensional boundary states protected by various crystalline symmetries, have become a fast-growing research branch in the vast topological family. Therein, valley pseudospins and layer pseudospins are separately introduced as extra degrees of freedom (DOFs) to manipulate topological phases. Here, we experimentally demonstrate that, by combining valley and layer DOFs, intriguing HOTIs can be realized. They host topological edge and corner states that are simultaneously valley dependent and layer polarized. We implement such HOTIs in bilayer sonic crystals (SCs) consisting of carefully stacked triangular scatterers. By rotating the scatterers, the valley and layer DOFs are interplayed, producing combined valley-layer polarizations. Correspondingly, the SCs exhibit versatile sound transport and localization. This inspires us to design an acoustic multiplexer, where sound waves can be guided along specific directions or trapped at specific locations, depending on the excitation frequencies. Our study of higher-order topology based on the interplay between valley and layer DOFs enriches the topological physics. With combined valley and layer polarizations, the resultant HOTIs also offer versatile ways to guide and/or trap sound waves, which have potential applications in integrated and multiplexing phononic devices.

In recent years, twisted bilayer graphene has become a hot topic and inspired the upsurge of research on the photonic moiré lattice. Here, we design a photonic moiré superlattice with two synthetic twist angles and construct a synthetic moiré sphere based on these two angles. Thus, we have more degrees of freedom to design the band structure flexibly. A type of transition photonic band (TPB) is obtained in such a moiré superlattice. We investigate the influence of two twist angles on TPBs and find a series of magic angle pairs with optimal band compression of the TPB. Interesting optical properties of TPBs are experimentally demonstrated, including pulse delay, nonlinear optical enhancement, and pulse-width compression. Our work introduces an alternative path to explore multitwist-angle moiré superlattices and reveals that the designed photonic moiré superlattice based on moiré spheres has broad application prospects, including optical signal processing, nonlinear optic processes, and other light-matter interactions.

Tunability of an artificial quantum system is crucial to its capability to process quantum information. However, tunability usually poses significant demand on the design and fabrication of a device. In this work, we demonstrate that Floquet engineering based on longitudinal driving provides distinct possibilities in enhancing the tunability of a quantum system without needing additional resources. In particular, we study a multilevel model based on gate-defined double quantum dots, where coherent interference occurs when the system is driven longitudinally. We develop an effective model to describe the driven dynamics of this multilevel system, and show that it is highly tunable via the driving field. We then illustrate the versatility and rich physics of a driven multilevel system by exploring phenomena such as driving modulation of resonances, adiabatic state transfer, and dark state. In the context of qubit control, we propose noise-resistant quantum gates based on adiabatic passage. The theoretical consideration we present here is rather general, and is in principle valid for other multilevel quantum systems.

Motivated by the synthesized ${\mathrm{Cu}}_4{\mathrm{Ti}\mathrm{Se}}_4$ with ultralow room-temperature thermal conductivity [Angew. Chem., Int. Ed. 60, 9106 (2021)], we systematically investigate its thermoelectric (TE) properties via combining Boltzmann transport equations and first-principles calculations. The results show that the thermal conductivity of ${\mathrm{Cu}}_4{\mathrm{Ti}\mathrm{Se}}_4$ determined considering only three-phonon scattering is reduced by about 40% after including four-phonon scattering at room temperature, indicating the importance of four-phonon scattering in phonon transport. On the other hand, the coexistence of high dispersion and valley degeneracy at the top valence band in the electronic structure causes a high power factor. Consequently, the isotropic figure of merit (ZT) value of 1.5 at 500 K is captured in the p-type doped ${\mathrm{Cu}}_4{\mathrm{Ti}\mathrm{Se}}_4$. In addition, the value of ZT can be further enhanced to 2.2 by applying 2.25% triaxial tensile strain, which is ascribed to the remarkably enhanced four-phonon scattering processes induced by the tensile strain. Meanwhile, the significant suppression of thermal conductivity allows the optimal carrier concentration for the ZT peak to be reduced, which is of important practical significance for the experimental preparation of ${\mathrm{Cu}}_4{\mathrm{Ti}\mathrm{Se}}_4$-based TE devices. Our results pave a way for the design of adjustable medium-temperature TE devices.

Twisted bilayer two-dimensional electronic systems give rise to many exotic phenomena and unveil a new frontier for the study of quantum materials. In photonics, twisted two-dimensional systems coupled via near-field interactions offer a platform to study localization and lasing. Here, we theoretically propose that twisting can be an unprecedented tool to tune the performance of near-field thermophotovoltaic systems. Remarkably, through twisting-induced photonic topological transitions, we achieve significant tuning of the thermophotovoltaic energy efficiency and power. The underlying mechanism is related to the change of the photonic isofrequency contours from elliptical to hyperbolic geometries in a setup where the hexagonal-boron-nitride metasurface serves as the heat source and the indium-antimonide $p$-$n$ junction serves as the cell. We find that a notably high energy efficiency, nearly 53 of the Carnot efficiency, can be achieved in our thermophotovoltaic system, while the output power can reach up to $1.1\times {10}^4\mathrm{W}/{\mathrm{m}}^2$ without requiring a large temperature difference between the source and the cell. Our results indicate the promising future of twisted near-field thermophotovoltaics and paves the way towards tunable, high-performance thermophotovoltaics and infrared detection.

On-resonance Rydberg atom–based radio-frequency- (rf) electric-field sensing methods remain limited by the narrow frequency-signal detection bands available from resonant transitions. An additional rf tuner field can be used to dress or shift a target Rydberg state to return a detuned signal field to resonance and thus dramatically extend the frequency range available for resonant sensing. Here we investigate three distinct tuning-level schemes based on adjacent Rydberg transitions, which are shown to have distinct characteristics and can be controlled with the frequency or the strength of the tuning field. We further show that a two-photon Raman peak can be used as an effective tuning feature separate from conventional Autler-Townes splitting. We compare our tuning schemes with ac Stark effect–based broadband rf-field sensing and show that although the sensitivity is diminished with tuning away from a resonant state, it nevertheless can be used in configurations where there is a low density of Rydberg states, which would result in a weak ac Stark effect.

Relativistic laser-wakefield acceleration is characterized by an unsurpassed accelerating gradient, which is very suitable for electron acceleration over short distances and could be a promising candidate for next-generation compact accelerators. However, using this technique for positron acceleration remains challenging because positively charged particles are commonly defocused in the accelerating structure of a standard nonlinear wakefield driven by an ultrashort laser pulse. Here we propose and numerically demonstrate a scheme to accelerate an externally injected positron beam in a nonlinear laser wakefield in a regime where a tail wave is formed behind density cusps of the wakefield. This tail wave can provide a focusing force in addition to longitudinal acceleration for the positrons. Three-dimensional particle-in-cell simulations demonstrate that a trapping efficiency of positrons of nearly 100% in the nonlinear wakefield is possible. This scheme may open a simple way to achieve compact positron acceleration of hundreds of MeV at high repetition rates with terawatt-class laser systems without the need for special laser modes and plasma structures.