Sensing microwave (MW) fields through a quantum protocol has attracted increasing interest, and the phase of MW fields is an essential parameter in communication and quantum technologies. This work proposes a quantum-effect-based method for measuring the phase difference of MW magnetic fields under near-field conditions based on the selective transitions of nitrogen-vacancy- ($\mathrm{N}$-V) center spins in diamond. First, the relationship between the evolutions of the $\mathrm{N}$-V-center spin states and the phase difference of the MW magnetic fields, which are resonant and able to drive the transitions of the $\mathrm{N}$-V-center spins between ground-state energy levels, is deduced. Then, using a double-microstrip-line MW antenna, the variations in the Rabi frequency and maximum contrast as a function of the phase difference are experimentally revealed. Finally, the measurement of the phase difference is validated, and the sensitivity of the proof-of-principle method is estimated to be ${0.0915}^{\circ }/\sqrt{\text{Hz}}$. This method paves the way for the comprehensive quantum sensing of MW fields and has potential applications in the communication and quantum information fields.

In the superconducting state, the presence of a finite gap in the excitation spectrum implies that the number of excitations (quasiparticles) is exponentially small at temperatures well below the critical temperature. Conversely, minute perturbations can significantly impact both the distribution in energy and the number of quasiparticles. Typically, the interaction with the electromagnetic environment is the main perturbation source driving quasiparticles out of thermal equilibrium, while a phonon bath is responsible for restoration of equilibrium. Here we derive approximate analytical solutions for the quasiparticle distribution function in superconducting resonators and explore the impact of nonequilibrium on two measurable quantities: the resonator’s quality factor and its resonance frequency. Applying our results to experimental data, we conclude that while at intermediate temperatures there is clear evidence for the nonequilibrium effects due to heating of the quasiparticles by photons, the low-temperature measurements are not explained by this mechanism.

Active manipulation of the terahertz (THz) beam, especially beam deflection, is urgently required for wide THz applications. Although Pancharatnam-Berry (PB) metasurfaces can flexibly control the spatial phase distribution of circularly polarized waves, active power distribution and modulation between two conjugate spin beams are still challenging. Here, we demonstrate an all-dielectric metadevice consisting of an anisotropic metasurface layer and a PB metasurface layer etched on both sides of a silicon wafer. This structure develops a photonic spin-decoupling mechanism for spin-beam manipulation, which actively controls the power distribution between the two deflected spin beams by rotating the incident linearly polarized direction. The power-distribution ratio reaches 14.9:1 in the experiment. Moreover, the deflected beam can also be modulated by active switching via a laser pump, achieving an intensity-modulation depth of over 90%. The experimental modulation depth of the metadevice is 52% higher than that of the bare silicon wafer at the same pump power, which originates from the THz localized field enhancement and the strong dispersion effect of the PB structures. Therefore, this work provides a scheme for actively manipulating THz-beam deflection, realizing controllable wave-division multiplexing, power distribution, polarization control, and modulation.

Terahertz pulses with controlled spectral-temporal waveform have great potential in expanding the scope of state-of-the-art terahertz researches, such as terahertz-driven resonant and nonresonant control over matter and light, ultrafast terahertz spectroscopy, and terahertz-driven acceleration. Here, we propose and demonstrate a method to generate a prescribed terahertz chirped waveform with a well-designed tapered corrugated wakefield structure. We develop a deterministic procedure to derive the parameters of the corrugated structure that generates the desired spectral-temporal waveform with certain shape of chirp, central frequency, bandwidth, and pulse duration. The experimental results from ultrashort electron bunches passing through a well-designed tapered corrugated structure are demonstrated. Time-frequency analysis of the terahertz electric field measured by electro-optical sampling shows clear chirp from 0.38 to 0.6 THz. Calibrated measurements of the radiated energy indicate 10 µJ per pulse for an incident beam charge of 300 pC.

Non-Gaussian modulation can improve the performance of continuous-variable quantum key distribution (CV QKD). For Gaussian-modulated coherent-state CV QKD, photon subtraction can realize non-Gaussian modulation, which can be equivalently implemented by non-Gaussian postselection. However, non-Gaussian reconciliation has not been deeply researched, which is one of the key technologies in CV QKD. In this paper, we propose a non-Gaussian reconciliation method to obtain identical keys from non-Gaussian data. Multidimensional reconciliation and multiedge-type low-density parity-check codes (MET LDPC) are used in a non-Gaussian reconciliation scheme, where the layered belief propagation decoding algorithm of MET LDPC codes is used to reduce the decoding complexity. Furthermore, we compare the error-correction performance of Gaussian data and non-Gaussian data. The results show that the error-correction performance of non-Gaussian data is better than Gaussian data, where the frame error rate can be reduced by 50 % for code rate 0.1 at SNR of 0.1554 and the average iteration number can be reduced by 25 %.

Using the Landau-Ginzburg-Devonshire approach, we study stress-induced transformations of polarization switching in ferrielectric ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanoparticles for three different shapes: a disk, a sphere, and a needle. Semiconducting properties of a nanoparticle are modeled by a surface-charge layer, whose effective screening length can be rather small due to the field effect. We reveal a very strong and unusual influence of hydrostatic pressure on the appearance of polarization switching in ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanoparticles, hysteresis loop shape, magnitude of the remanent polarization, and coercive fields, and explain the effects by the anomalous temperature dependence and “inverted” signs of ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ linear and nonlinear electrostriction coupling coefficients. In particular, by varying the sign of the applied pressure (from tension to compression) and its magnitude (from zero to several hundreds of MPa), quasistatic hysteresisless paraelectric curves can transform into double, triple, pinched, or single hysteresis loops. Unexpectedly, we predict that stressed ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanospheres and nanoneedles reveal sizeable temperature and pressure ranges of triple loop stability, which are very rare in ferroelectrics and antiferroelectrics. A physical origin of triple loops is the coexistence of “small” and “larger” polarizations in the four-well ferrielectric state. Due to the sufficiently wide temperature and pressure ranges of double, triple, and pinched hysteresis loop stability (at least in comparison with many other ferroelectrics), ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanodisks can be of particular interest for applications in energy storage (in the region of double loops), ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanospheres maybe suitable for dynamic random-access multibit memory, and ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanoneedles are promising for nonvolatile multibit memory cells (in the regions of triple and pinched loops). The stress control of the polarization-switching scenario allows the creation of advanced piezosensors based on ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ nanocomposites.

Event synchronization is a ubiquitous task, with applications ranging from 5G technology to industrial automation and smart power grids. The emergence of quantum communication networks will further increase the demand for precise synchronization in the optical and electronic domains, which implies significant resource overhead, such as the requirement for ultrastable clocks or additional synchronization lasers. Here, we show how temporal correlations of photons may be harnessed for synchronization in quantum networks. We achieve stable synchronization jitter $<68$ ps with as few as 44 correlated detection events per 100-ms data package and demonstrate feasibility in realistic emulated high-loss link scenarios, including atmospheric turbulence. In contrast to previous work, this is accomplished without any external timing reference and only simple crystal oscillators. Our approach replaces the optical and electronic transmission of timing signals with classical communication and computer-aided postprocessing. It can be easily integrated into a wide range of quantum communication networks and could pave the way to future applications in entanglement-based secure time transmission.

Artificial magnetism can be realized with metamaterials in high frequencies, which are important for many applications. However, such metamaterials need to be constructed with metal or high-relative-permittivity dielectric and thus may exhibit absorption loss. Here, we theoretically propose and experimentally demonstrate a pair of magnetic metamaterials that have inversion symmetry and are generated from a parent photonic crystal with low-relative-permittivity dielectric (${\epsilon }_r<4$). Interestingly, one metamaterial is paramagnetic, the other is diamagnetic, and the product of their effective relative permeabilities is close to 1 (${\mu }_{e1}>1$, ${\mu }_{e2}<1$, ${\mu }_{e1}{\mu }_{e2}\approx 1$) in a broad frequency range. As a result, fascinating effects, such as wide-angle and wide-frequency near-perfect transmission and interface states, are observed in the systems.

Polarization-preserving fibers maintain the two polarization states of an orthogonal basis. Quantum communication, however, requires sending at least two nonorthogonal states and these cannot both be preserved. We present an alternative scheme that allows for using polarization encoding in a fiber not only in the discrete, but also in the continuous-variable regime. For the example of a helically twisted photonic crystal fiber, we experimentally demonstrate that using appropriate nonorthogonal modes, the polarization-preserving fiber does not fully scramble these modes over the full Poincaré sphere, but that the output polarization will stay on a great circle; that is, within a one-dimensional protected subspace, which can be parametrized by a single variable. This allows for more efficient measurements of quantum excitations in nonorthogonal modes.

Spontaneous parametric down-conversion (SPDC) is a reliable platform for entanglement generation. Routinely, a coherent laser beam is an essential prerequisite for pumping the nonlinear crystal. Here we break this barrier to generate polarization-entangled photon pairs by using a commercial light-emitting diode (LED) source to serve as the pump beam. This effect is counterintuitive, as the LED source is of extremely low spatial coherence, which is transferred during the down-conversion process to the biphoton wave function. However, the type-II phase matching condition naturally filters the specific frequency and wavelength of LED light exclusively to participate in SPDC such that localized polarization Bell states can be generated, regardless of the global incoherence over the full transverse plane. In our experiment, we characterize the degree of LED light-induced polarization entanglement in the standard framework of the violation of Bell's inequality. We achieve the Bell value $S=2.33\pm 0.097$, obviously surpassing the classical bound $S=2$ and thus witnessing the quantum entanglement. Our work can be extended to prepare polarization entanglement by using other natural light sources, such as sunlight and biolight, which holds promise for electricity-free quantum communications in outer space.

The possibility to control friction through surface microtexturing can offer invaluable advantages in many fields, from wear and pollution reduction in the transportation industry to improved adhesion and grip. Unfortunately, the texture optimization problem is very hard to solve using traditional experimental and numerical methods, due to the complexity of the texture configuration space. Here, we apply machine learning techniques to perform the texture optimization, by training a deep neural network to predict, with extremely high accuracy and speed, the Stribeck curve of a textured surface in lubricated contact. The deep neural network is used to completely resolve the mapping between textures and Stribeck curves, enabling a simple method to solve the texture optimization problem. This work demonstrates the potential of machine learning techniques in texture optimization for friction control in lubricated contacts.

Optically detected magnetic resonance (ODMR) spectroscopy of defect-rich semiconductors is being increasingly exploited for realizing a variety of practical quantum sensing devices. A prime example is the on-going development of compact magnetometers based on the nitrogen-vacancy (N-$V$) defect in diamond for the remote sensing of magnetic signals with high accuracy and sensitivity. In these applications, the ODMR-active material is integrated into the overall apparatus to form a self-contained sensor. However, some emerging applications require the sensing material to be in physical contact with an external object of interest, thus requiring an independent readout device. Here we present an ODMR meter, a compact device specially designed to allow convenient, contactless monitoring of ODMR in a target object, and demonstrate its application to temperature monitoring with N-$V$ defects. Our prototype is composed of a handheld readout head (integrating all the necessary optical components and a microwave antenna) and a control box connected to a laptop computer, all made primarily from commercial off-the-shelf components. We test our device using a N-$V$-rich bulk diamond as the object, demonstrate a temperature sensitivity of $10\mathrm{mK}/\sqrt{\mathrm{Hz}}$ in static conditions, and demonstrate the feasibility of handheld operation. The limitations to measurement speed, sensitivity, and accuracy are discussed. The presented device may find immediate use in medical and industrial applications where accurate thermometry is required, and can be extended to magnetic field measurements.

Impulsive stimulated Raman scattering (ISRS) using a single short femtosecond pump pulse to excite molecular vibrations offers an elegant pump-probe approach to perform vibrational imaging below $200{\mathrm{cm}}^{-1}$. One shortcoming of ISRS is its inability to offer vibrational selectivity as all the vibrational bonds whose frequencies lie within the short pump-pulse bandwidth are excited. To date, several coherent control techniques have been explored to address this issue and selectively excite a specific molecular vibration by shaping the pump pulse. There has not been any systematic work that reports an analogous shaping of the probe pulse to implement preferential detection. In this work, we focus on vibrational imaging and report vibrational selective detection by shaping the probe pulse in time. We demonstrate numerically and experimentally two pulse-shaping strategies with one functioning as a vibrational notch filter and the other functioning as a vibrational low-pass filter. This enables fast ($25\text{μ}\mathrm{s}/$pixel) and selective hyperspectral imaging in the low-frequency regime ($<200{\mathrm{cm}}^{-1}$).

Resonant sensors hold great promise in measuring small masses, to enable future mass spectrometers, and small forces in applications like atomic and magnetic force microscopy. During the last decades, scaling down the size of resonators has led to huge enhancements in sensing resolution, but has also raised the question of what the ultimate limit is. Current knowledge suggests that this limit is reached when a resonator oscillates at the maximum amplitude for which its response is predominantly linear. We present experimental evidence that it is possible to obtain better resolutions by oscillation amplitudes beyond the onset of nonlinearities. An analytical model is developed that explains the observations and unravels the relation between ultimate sensing resolution and speed. In the high-speed limit, we find that the ultimate resolution of a resonator is improved when decreasing its damping. This conclusion contrasts with previous works, which proposed that lowering the damping does not affect or even harms the ultimate sensing resolution.

The detection of prompt photofission neutrons during active interrogation is a strong indication of the presence of special nuclear materials. However, the high-energy photons used for interrogation create a very challenging radiation environment for the detection of prompt fission signatures. These challenges include detector saturation and pulse pile-up. Additionally, there is an elevated neutron background that further challenges the detection of prompt fission neutrons. This background is produced because of (γ, Xn) photonuclear reactions in the surrounding high-Z materials. Here, we demonstrate the detection of prompt photofission neutrons in these challenging environments. Depleted uranium (DU) and lead targets are interrogated with bremsstrahlung photons produced by a 9-MV electron linear accelerator. Fast neutrons are detected with trans-stilbene organic detectors, and scintillation pulses are analyzed using a previously developed and demonstrated artificial neural network system. We observe a 5 times higher photoneutron count rate when the lead target is replaced with the DU target. Additionally, we observe a difference in the photoneutron light-output distributions of lead and DU. This difference in the measured distributions is due to the difference in the photoneutron-energy spectra; DU photoneutrons are emitted with (γ, n) and watt-energy spectra, whereas lead photoneutrons are emitted with only the (γ, n) spectrum.

The optoelectronic reciprocity theorem has been proposed as a theorem relating electroluminescence (EL) and photovoltaic external quantum efficiency in solar cells. The theorem is vital for a fundamental understanding of solar cell operation and its application to device evaluation. Furthermore, with this theorem, it is also possible to estimate the open-circuit voltage ($V_{\mathrm{OC}}$) of solar cells using the external radiative efficiency (${\eta }_{\mathrm{ext}}$) obtained from the absolute value of EL emission intensity, even for solar cells for which we cannot directly measure $V_{\mathrm{OC}}$, such as subcells of multijunction solar cells or individual $\mathrm{Si}$ cells in modules and arrays. However, it is not a priori obvious that the optoelectronic reciprocity theorem holds for the various solar cells that exist. In this study, we report the results of qualitative and quantitative confirmation of the validity of the optoelectronic reciprocity theorem for high-efficiency ${\mathrm{Cu}\mathrm{In}}_{1-x}{\mathrm{Ga}}_x{\mathrm{Se}}_2$ (CIGS) solar cells fabricated by us. The results confirm that our CIGS solar cells qualitatively and quantitatively satisfy the optoelectronic reciprocity theorem within the limits of measurement uncertainty. Also, we experimentally confirm that the value of the diode ideality factor for the applied voltage dependence of the EL emission intensity is exactly unity when the emission mechanism is the band-edge emission due to the direct recombination of electron-hole pairs. Finally, the importance of the ${\eta }_{\mathrm{ext}}$ for improving solar cell efficiency is explained.

Phononic crystal (PnC) enables strong confinement of phonons in a wavelength-scale tiny area due to a band gap, which can enhance the interaction with different physical systems. Here, we utilize the PnC to demonstrate local manipulations of magnons by using ultrahigh frequency phonons confined in a phononic cavity, where the tiny and low-loss vibrations excite magnons in a micromagnet via magnetostriction. Moreover, we find that the magnetoelastic interaction is modulated by selectively exciting the cavity resonant modes. The results open up a realm of PnC cavity magnomechanics and will provide universal controls of ultrahigh frequency magnons and phonons with PnC circuits.

The alternative variant of measurement-device-independent quantum key distribution (MDI QKD), called asynchronous MDI QKD or mode-pairing MDI QKD, offers similar repeaterlike rate-loss scaling but has the advantage of simple technology implementation by exploiting an innovative postmeasurement pairing technique. We herein present an evaluation of the practical aspects of decoy-state asynchronous MDI QKD. To determine its effectiveness, we analyze the optimal method of decoy-state calculation and examine the impact of asymmetrical channels and multiuser networks. Our simulations show that, under realistic conditions, aynchronous MDI QKD can furnish the highest key rate with MDI security as compared to other QKD protocols over distances ranging from 50 to 480 km. At fiber distances of 50 and 100 km, the key rates attain 6.02 and 2.29 Mbps, respectively, which are sufficient to facilitate real-time one-time-pad video encryption. Our findings indicate that experimental implementation of asynchronous MDI QKD in intercity networks can be both practical and efficient.