We demonstrate quantum sensing of dc magnetic fields that exceeds the sensitivity of conventional $T_2^{\ast }$-limited dc magnetometry by more than an order of magnitude. We use nitrogen-vacancy centers in a diamond rotating at periods comparable to the spin coherence time, and characterize the dependence of magnetic sensitivity on measurement time and rotation speed. Our method up-converts only the dc field of interest and preserves the quantum coherence of the sensor. These results definitively improve the sensitivity of a quantum magnetometer to dc fields, demonstrating that sensitivity below the $T_2^{\ast }$ limit is possible and can be applied to any diamond magnetometer where $T_2\gg T_2^{\ast }$ to yield an order-of-magnitude sensitivity improvement.

Plasmon antennas allow subwavelength confinement and enhancement of electromagnetic fields at the “hotspot” where the radiation efficiency of emitters can be substantially enhanced. Such enhancement, however, is often polarization dependent. Consequently, the radiation behaviors (e.g., radiation pattern and polarization states) of the emitter placed at the hotspot are also modified significantly. Enhancing the radiation efficiency without altering the original radiation pattern and polarization state of the emitter is highly desired for many sought-after applications involving chiral emitters but remains a challenging task, especially at low frequencies. To this end, spoof-plasmon antennas with fourfold and sixfold rotational symmetries are designed and realized experimentally. These plasmon antennas support polarization-independent localized plasmon resonances, which can significantly enhance the local density of photonic states at the structural center without changing the polarization state of the emitter. As a typical example, the structure with sixfold rotational symmetry is coupled with a half-wave dipole antenna. The measurement results show that the far-field radiation pattern of the dipole antenna is maintained with the radiation efficiency enhanced by more than 2 orders of magnitude, irrespective of the dipole orientation.

Charge detection is a crucial component used in semiconductor quantum circuits. It allows the noninvasive readout of charge and spin qubits in quantum information protocols and can also be leveraged for quantum sensing applications. In this work, we demonstrate that van der Waals heterostructure devices with gated nanoconstrictions in monolayer ${\mathrm{WSe}}_2$ can be used as charge detectors for nearby quantum dots. This is a demonstration of charge detection using transition-metal dichalcogenides, and we find that the detection sensitivity is comparable to that of other two-dimensional (2D) material charge detectors based on graphene. These results open the possibility of implementing charge detection schemes based on 2D materials in complex quantum circuits.

Mapping the positions of single electron spins is a highly desired capability for applications such as nanoscale magnetic resonance imaging and quantum network characterization. Here, we demonstrate a method based on rotating an external magnetic field to identify the precise location of single electron spins in the vicinity of a quantum spin sensor. We use a nitrogen-vacancy center in diamond as a quantum sensor and modulate the dipolar coupling to a proximate electron spin in the crystal by varying the magnetic field vector. The modulation of the dipolar coupling contains information on the coordinates of the spin, from which we extract its position with an uncertainty of 0.9 Å. We show that the method can be used to locate electron spins with nanometer precision up to 10 nm away from the sensor. We discuss the applicability of the method to mapping hyperfine coupled electron spins and show that it may be applied to locating nitroxide radicals. The magnetic tomography method can be utilized for distance measurements for studying the structure of individual molecules.

Fully coupled photon-electron-positron computational schemes are needed in a broad range of domains, from radiotherapy and radiosurgery treatment planning to nuclear-reactor safety analysis. Industrial deployment of such schemes remains problematic because of the prohibitively time-consuming charged-particle-transport aspects of both Monte Carlo algorithms and Boltzmann-Fokker-Planck solvers. In this paper, a partially coupled scheme based on reactor-physics models for secondary photons (RPSPs) is presented. RPSPs include thick-target-bremsstrahlung, annihilation, and full atomic cascade models, recovering long-range effects of both electrons and positrons. The RPSP-based scheme is compared with state-of-the-art fully coupled and uncoupled schemes in medical, nuclear, and multidisciplinary contexts. Within tissues, the average discrepancies from the fully coupled scheme are about $0.5\mathrm{\%}$, with a maximum error below $1.3\mathrm{\%}$ at high energy. The CPU time of the coupled schemes is reduced by 58 and 78 times for 3- and 6-MeV beams, respectively. The discrepancies are also within the accuracy of the cross sections for ${}_4\mathrm{Be}$, ${}_6\mathrm{C}$, ${}_{13}\mathrm{Al}$, ${}_{26}\mathrm{Fe}$, ${}_{29}\mathrm{Cu}$, ${}_{32}\mathrm{Ge}$, ${}_{47}\mathrm{Ag}$, ${}_{79}\mathrm{Au}$, ${}_{82}\mathrm{Pb}$, and ${}_{92}\mathrm{U}$ slabs. The same discrepancies are about $0.1\mathrm{\%}$, $0.4\mathrm{\%}$ and $0.8\mathrm{\%}$, respectively, in the reactor's shielding, vessel and fuel slabs. A power peaking factor reduction of 4.35 times from the state-of-the-art uncoupled scheme is demonstrated for uranium oxide and mixed plutonium-uranium oxide slabs.

We report $\mathrm{Ga}\mathrm{As}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ nanowires in the one-dimensional (1D) quantum limit. The ultrathin wurtzite $\mathrm{Ga}\mathrm{As}$ cores between 20 and 40 nm induce large confinement energies of several tens of millielectronvolts, allowing us to experimentally resolve up to four well-separated subband excitations in microphotoluminescence spectroscopy. Our detailed experimental and theoretical polarization-resolved study reveals a strong diameter-dependent anisotropy of these transitions: We demonstrate that the polarization of the detected photoluminescence is governed, on the one hand, by the symmetry of the wurtzite 1D quantum wire subbands but also, on the other, by the dielectric mismatch of the wires with the surrounding material. The latter effect leads to a strong attenuation of perpendicularly polarized light in thin-dielectric wires, making the thickness of the $(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ shell an important factor in the observed polarization behavior. Including the dielectric mismatch to our $\mathbf{k}\cdot \mathbf{p}$-based simulated polarization-resolved spectra of purely wurtzite $\mathrm{Ga}\mathrm{As}$ quantum wires, we find excellent agreement between experiment and theory. These findings reveal insights relevant to spectroscopic studies of the 1D quantum regime and the design of photonic quantum wire applications such as lasers and hopefully open up paths for selective subband excitations.

We report on a wave-driven microfluidic ultrasonic shaker that can stably trap and oscillate microparticles in water in a microchannel. The microfluidic ultrasonic shaking system mainly consists of a microfluidic chip and two orthogonally excited piezoelectric transducers: one for creating radiation force potential wells for three-dimensional trapping of microparticles and the other for generating sound destabilization to shake the trapped microparticles. It is shown that two standing waves of different but close frequencies provide a shaking force [approximately $F_0$cos(2πΔft), with $F_0$ and Δf being the force amplitude and the frequency difference, respectively] that can repeatedly perturb a trapped microparticle and cause it to execute spontaneous oscillations. The dynamic behavior of microparticle oscillation driven by two sound waves is found to be equivalent to a forced harmonic oscillator (i.e., a mass on a spring) that has been extensively described in the literature. Specifically, experiments at various ultrasonic excitations show that the oscillating frequency of a stably trapped microparticle is determined by the frequency difference between the two sound waves, and the shaking amplitude scales inversely to the frequency difference and is affected by pressure amplitudes of both sound waves. We foresee that the illustrative microfluidic ultrasonic shaker presented here opens perspectives for dynamic manipulation of single microparticles or clusters in microscale acoustofluidic systems.

When two identical periodic structures are stacked at a series of discrete rotation angles, the resulting moiré pattern brings an extreme flat band with transport enhancement. It provides insights into the fascinating physics of insulating states and unconventional superconductivity with delocalization-localization transitions and commensurable-incommensurable phases. However, the exploration of symmetry- and geometry-independent flat-band physics with moiré patterns is still rare, limited by the stringent requirement in high dimension. Here, we experimentally observe the localized flat-band state by mapping a moiré model into a one-dimensional photonic lattice using a femtosecond laser direct writing technique. By accurately controlling the external periodic field, we construct moiré photonic lattices with different moiré band structures. We successfully observe the photon-walker evolution from splitting with dispersion to nondispersive propagation with larger localization in the input ports as the result of a flat band induced by moiré patterns with larger period. Our approach to engineering moiré model, together with the integrated photonic implementation, establishes a powerful tool for exploring the effects accompanying the transition from commensurate to incommensurate phases.

The emergence of ratchet effects in two-dimensional materials is strongly correlated with the introduction of asymmetry into the system. In general, dual-grating-gate structures forming lateral asymmetric superlattices provide a suitable platform for studying this phenomenon. Here, we report on the observation of ratchet effects in $\mathrm{Hg}\mathrm{Te}$-based dual-grating-gate structures hosting different band-structure properties. Applying polarized terahertz laser radiation we detect linear and polarization-independent ratchets, as well as a radiation-helicity-driven circular ratchet effect. Studying the ratchet effect in devices made of quantum wells (QWs) of different thickness, we observe that the magnitude of the signal substantially increases with decreasing QW width with a maximum value for devices made of QWs of critical thickness hosting Dirac fermions. Furthermore, sweeping the gate voltage amplitude we observe sign-alternating oscillations for gate voltages corresponding to $p$-type conductivity. The amplitude of the oscillations is more than two orders of magnitude larger than the signal for $n$-type conducting QWs. The oscillations and the signal enhancement is shown to be caused by the complex valence band structure of $\mathrm{Hg}\mathrm{Te}$-based QWs. These peculiar features of the ratchet currents make these materials an ideal platform for the development of terahertz applications.

A physics-based model that predicts the emitted current from thermionic cathodes is developed, which accurately spans from the temperature-limited (TL) to full-space-charge-limited (FSCL) regions. Experimental observations of thermionic electron emission demonstrate a smooth transition between TL and FSCL regions of the emitted-current-density-versus-temperature (J-T) (Miram) curve and the emitted-current-density-versus-voltage (J-V) curve. Knowledge of the temperature and shape of the TL-FSCL transition is important in evaluating the thermionic electron-emission performance of cathodes, including predicting the lifetime. However, there are no first-principles physics-based models that predict the smooth TL-FSCL transition region for real thermionic cathodes without applying a priori assumptions or empirical phenomenological equations that are physically difficult to justify. Previous work detailing the nonuniform thermionic emission found that the effects of three-dimensional space charge, patch fields (electrostatic potential nonuniformity on the cathode surface based on local work-function values), and Schottky barrier lowering can lead to a smooth TL-FSCL transition region from a model thermionic cathode surface with a checkerboard spatial distribution of work-function values. In this work, we construct a physics-based nonuniform emission model for commercial dispenser cathodes. This emission model is obtained by incorporating the cathode surface grain orientation via electron-backscatter diffraction and the facet-orientation-specific work-function values from density-functional-theory calculations. The model enables the construction of two-dimensional emitted-current-density maps of the cathode surface and corresponding J-T and J-V curves. The predicted emission curves show excellent agreement with experiment, not only in the TL and FSCL regions but, crucially, also in the TL-FSCL transition region. This model provides a method to predict the thermionic emission from the microstructure of a commercial cathode and improves the understanding of the relationship between thermionic emission and cathode microstructure, which is beneficial for the design of vacuum electronic devices.

The dissemination of atomic clocks with fiber-based techniques finds application in the fields of metrology, fundamental physics, navigation, and spectroscopy but is a challenge in terms of reliability, maintenance, and performance. Here, we describe the realization of a 1023-km-long fiber link between the metrological institutes of Italy and France that shares the infrastructure with the Internet traffic and exploits segmentation in shorter, cascaded spans to fight optical losses exceeding 280 dB. With four months of quasicontinuous operation of this link, we compared the $\mathrm{Cs}$, $\mathrm{Rb}$, and $\mathrm{Yb}$ atomic clocks at our laboratories, highlighting the potential of this tool to assess the clock uncertainty budgets, characterize advanced satellite techniques, and develop optical timescales. The integration of the metrological, fiber-based infrastructures in the two countries, connecting photonics and spectroscopy laboratories as well as telescope facilities, provides the research community with a physical layer over which applications can be built on.

Here we theoretically and experimentally demonstrate a way to control the intensity of optical vortices by a magnetic field directed along the beam axis. We describe the principle of an optical vortex modulator and switch based on the Faraday effect in an iron garnet film. Importantly, the magnetic field influences only the polarization plane of the vortex and keeps its topological charge.

Quantum router as a key device in quantum information processing always attracts researchers’ attention. This work suggests a single-photon (SP) router based on one bus-waveguide side coupled to numerous Jaynes-Cummings emitters (JCEs). Each JCE connects with a semi-infinite branch waveguide as an output channel. Different from the commonly used scheme that the JCEs are set in the tuning regime, we here let them work in the large-detuning regime. This change brings about the following amazing benefits for the SP router. Firstly, the number of channels can reach more than a thousand. Secondly, the router is adjustable, that is, the SP can be routed into any output channel on demand. Thirdly, the router has a remarkable monochromaticity. Fourthly, such a SP router can also depress the influence of cavity losses and interferential levels in atoms. Finally, the routing probability can reach approximately $100\mathrm{\%}$ for each channel by adding a mirror and a phase shifter to the end of the bus waveguide. This work takes the Rydberg atoms ${}^{87}\mathrm{Rb}$ whose levels can be adjusted by an external electrostatic field as an example to demonstrate the feasibility of the router, which can be applied to quantum informatics.

Photons entangled in their position-momentum degrees of freedom serve as an elegant manifestation of the Einstein-Podolsky-Rosen paradox, while also enhancing quantum technologies for communication, imaging, and computation. The multimode nature of photons generated in parametric down-conversion has inspired a generation of experiments on high-dimensional entanglement, ranging from complete quantum state teleportation to exotic multipartite entanglement. However, precise characterization of the underlying position-momentum state is notoriously difficult due to limitations in detector technology, resulting in a slow and inaccurate reconstruction riddled with noise. Furthermore, theoretical models for the generated two-photon state often forgo the importance of the measurement system, resulting in a discrepancy between theory and experiment. Here we formalize a description of the two-photon wave function in the spatial domain, referred to as the collected joint-transverse momentum amplitude (JTMA), which incorporates both the generation and measurement system involved. We go on to propose and demonstrate a practical and efficient method to accurately reconstruct the collected JTMA using a simple phase-step scan known as the $2D\pi$ measurement. Finally, we discuss how precise knowledge of the collected JTMA enables us to generate tailored high-dimensional entangled states that maximize discrete-variable entanglement measures such as entanglement of formation or entanglement dimensionality, and optimize critical experimental parameters such as photon heralding efficiency. By accurately and efficiently characterizing photonic position-momentum entanglement, our results unlock its full potential for discrete-variable quantum information science and lay the groundwork for future quantum technologies based on multimode entanglement.

Entanglement swapping and heralding are at the heart of many protocols for distributed quantum information. For photons, this typically involves Bell-state measurements based on two-photon interference effects. In this context, hybrid systems that combine high rate, ultrastable, and pure quantum sources with long-lived quantum memories are particularly interesting. Here, we develop a theoretical description of pulsed two-photon interference of photons from dissimilar sources to predict the outcomes of second-order cross-correlation measurements. These are directly related to, and hence used to quantify, photon indistinguishability. We study their dependence on critical system parameters such as quantum state lifetime and emission frequency, and quantify the impact of time jitter, pure dephasing, and spectral wandering. We show that for a fixed lifetime of one of the two emitters, for each frequency detuning there is an optimal lifetime of the second emitter that leads to the highest photon indistinguishability. Expectations for different hybrid combinations involving III-V semiconductor quantum dots, color centers in diamond, atom-scale defects in two-dimensional materials and neutral atoms are quantitatively compared for real-world system parameters. Our work provides a theoretical basis for the treatment of dissimilar emitters and enables assessment of which imperfections can be tolerated in hybrid photonic quantum networks.

Atomic heterodyne dressed by a local oscillator resonant with Rydberg transitions allows high sensitivity and robust phase measurement of a microwave electric field, but it is typically limited to detection of discrete frequencies within the narrow bandwidth of Rydberg transitions. Here we demonstrate an atomic heterodyne scheme for continuous-frequency electric field measurement based on multilevel Rydberg atoms in a room-temperature vapor cell. Driven by two off-resonant microwaves acting as a tunable local oscillator field, the heterodyne receiver can retrieve the amplitude, phase, and frequency information of signal microwave in a continuous frequency band. In our experiment, the receiver achieves an electric field sensitivity of up to $1.5\mu \mathrm{V}{\mathrm{cm}}^{-1}{\mathrm{Hz}}^{-1/2}$, 80-dB linear dynamic range, and over 1 GHz of continuous frequency range. We also demonstrate the reliable reception of continuously tunable phase-modulated carriers in the digital communication. This work will facilitate the application of atomic heterodyne in the areas such as radar technique, radio monitoring, and radio astronomy.

A detailed investigation of a superconducting spin-triplet valve is presented. This spin valve consists of a superconducting film covering a metal with an intrinsic spiral magnetic order, which could result from competing isotropic exchanges or, if the crystal lattice breaks central symmetry, from asymmetric Dzyaloshinskii-Moriya exchange. Depending on the anisotropy, such a metal may change its magnetization either from a spiral to uniform order, as seen in $\mathrm{Ho}$ and $\mathrm{Er}$, or in the direction of the spiral itself, as in crystals of the B20-type structure [such as $\mathrm{Mn}\mathrm{Si}$, ($\mathrm{Fe}$,$\mathrm{Co}$)$\mathrm{Si}$, $\mathrm{Fe}\mathrm{Ge}$, etc.]. The nonuniform magnetic order controls the appearance of long-range triplet superconducting correlations at strong exchange fields, affecting the detailed character of the proximity effect. We show that the magnetic control of the spin-valve behavior can also be obtained from moderately low exchange fields (typically associated to negligible long-range triplet correlations), thanks to an orientation-dependent averaging mechanism of the magnetic inhomogeneity on the scale of the Cooper pairs. Our numerical calculations reveal that the spin-valve effect is in fact magnified at moderately low exchange fields, when the exchange splitting in the spiral magnet is comparable to the superconducting gap, and the spiral period is less than or equal to the superconducting coherence length in the magnet multiplied by $2\pi$.

Measurements to estimate parameters of a model are commonplace in the physical sciences, where the traditional approach to automation is to use a sequence of preselected settings followed by least-squares fitting of a model function to the data. This measure-then-fit approach is simple and effective and entirely appropriate for many applications but when measurement resources are limited, efficiency becomes more important. To increase efficiency, Bayesian experiment design allows measurement settings to be chosen adaptively based on accumulated data and utility, the predicted improvement in results as a function of settings. However, the calculation of utility has been judged too impractical for most applications. In this paper, we introduce computational methods and simplified algorithms that accelerate utility calculations by over an order of magnitude, with only slight degradation in measurement efficiency. The methods eliminate utility calculation as a barrier to practical application of efficient adaptive measurement.