Dense high-energy monoenergetic proton beams are vital for wide applications, thus modern laser-plasma-based ion-acceleration methods are aiming to obtain high-energy proton beams with energy spread as low as possible. In this work, we put forward a quantum radiative compression method to postcompress a highly accelerated proton beam and convert it to a dense quasimonoenergetic one. We find that when the relativistic plasma produced by radiation-pressure acceleration collides head on with an ultraintense laser beam, large-amplitude plasma oscillations are excited due to quantum radiation reaction and the ponderomotive force, which induce compression of the phase space of protons located in its acceleration phase with negative gradient. Our three-dimensional spin-resolved quantum electrodynamics (QED) particle-in-cell simulations show that hollow-structure proton beams with a peak energy approximately $\mathrm{GeV}$, relative energy spread of few percents, and number $N_p\sim {10}^{10}$ (or $N_p\sim {10}^9$ with a $1\mathrm{\%}$ energy spread) can be produced in near-future laser facilities, which may fulfill the requirements of alternative applications, such as, for radiography of ultrathick dense materials, or as injectors of hadron colliders.

The bulk photovoltaic (BPVE) effect in crystals lacking inversion symmetry offers great potential for optoelectronic applications due to its unique properties, such as above-band-gap photovoltage and switchable photocurrent. Because of their large spontaneous polarizations, ferroelectric materials are ideal platforms for studying the BPVE. However, identifying the origin of an experimentally observed photovoltaic response is often challenging due to the entanglement between bulk and interface effects, leading to much debate in the field. This issue is particularly pronounced in vertical heterostructures, where the two effects are comparable. Here, we report crossover between bulk- and interface-dominant responses in vertical ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$ heterostructures when changing the photon energy. We show that well-above-band-gap excitation leads to a bulk photovoltaic response, but band-edge excitation requires interface band bending to separate the photocarriers. Our findings not only help to clarify contradicting reports in the literature, but also lay the foundation for a deeper understanding of the ferroelectric photovoltaic effect and its applications in various devices.

With the advent of quantum technology, nitrogen vacancy ($\mathrm{N}$-$V$) centers in diamond turn out to be a frontier that provides an efficient platform for quantum computation, communication, and sensing applications. Due to the coupled spin-charge dynamics of the $\mathrm{N}$-$V$ system, knowledge of $\mathrm{N}$-$V$ charge-state dynamics can help to formulate efficient spin-control sequences strategically. Here, we report two spectroscopy-based deconvolution methods to create charge-state mapping images of ensembles of $\mathrm{N}$-$V$ centers in diamond. First, relying on the fact that an off-axis external magnetic field mixes the electronic spins and selectively modifies the photoluminescence (PL) of $\mathrm{N}$-$V^{-}$, we perform decomposition of the optical spectrum for an ensemble of $\mathrm{N}$-$V$ and extract the spectra for $\mathrm{N}$-$V^{-}$ and $\mathrm{N}$-$V^0$ states. Next, we introduce an optical-filter-based decomposition protocol and perform PL imaging for $\mathrm{N}$-$V^{-}$ and $\mathrm{N}$-$V^0$. Previously obtained spectra for $\mathrm{N}$-$V^{-}$ and $\mathrm{N}$-$V^0$ states are used to calculate their transmissivities through a long-pass optical filter. These results help us to determine the spatial distribution of the $\mathrm{N}$-$V$ charge states in a diamond sample.

In measurement-device-independent quantum key distribution (MDI QKD), the setup of Bell state measurement (BSM) usually requires that the photons sent by different users should be frequency degenerate. It increases the complexity to implement MDI QKD since a complicated feedback system is usually required to calibrate photons’ frequencies in different users. It also limits the development of MDI QKD networks since wavelength division multiplexing (WDM) technology is not compatible with this requirement. In this work, we propose a MDI QKD scheme of frequency-nondegenerate photons, in which the users send photons with different frequencies. It is based on a BSM setup with a frequency-domain beam splitter (FBS), which is realized by a phase modulator (PM). A proof-of-principle experiment is realized, showing that the BSM of frequency-nondegenerate photons could select specific users to realize MDI QKD by adjusting the modulation frequency of the PM to match the frequency difference of photons sent by the users. The results manifest that our scheme has great potential for simplifying the implementation and realizing MDI QKD networks combined with WDM.

Thermal noise from the suspension fibers used in the mirror pendulums in current gravitational-wave detectors is a critical noise source. Future detectors will require improved suspension performance with the specific ability to suspend much heavier masses to reduce radiation pressure noise, while retaining good thermal noise performance. In this paper we propose and experimentally demonstrate a design for a large-scale fused-silica suspension, demonstrating its suitability for holding an increased mass of 160 kg. We demonstrate the concepts for improving thermal noise via longer suspension fibers supporting a higher static stress. We present a full thermal noise analysis of our prototype, meeting requirements for conceptual third-generation detector designs such as the high frequency interferometer of the Einstein Telescope, and closely approaching that required for Cosmic Explorer.

The ability to overcome the adverse effect induced by the obstacles within the transmission link is a central challenge in long-distance optical image transmission and is significantly crucial in free-space optical communication. In this work, we introduce an efficient protocol to realize the robust far-field optical image-signal transmission by modulating the spatial-coherence structure of a partially coherent random light source. The image information encoded in the spatial-coherence structure can be stably transmitted to the far field and can resist the influence of obstructions within the communication link. This is due to the self-reconstruction property of the spatial-coherence structure embedded with the cross phase in the far field. We demonstrate experimentally that the image information can be recovered well by measuring the second-order spatial-coherence structure of the obstructed random light in the far field. Our findings open a door for robust optical signal transmission through the complex environment and may find application in optical communication through a turbulent atmosphere.

The combination of finite semiconductor band gap and ultrasmall thickness makes few-layer transition-metal dichalcogenides (TMDCs) promising materials for use in optoelectronics devices. Here, we use density-functional theory to investigate the photon-energy-dependent optical properties of two-layer (2L), three-layer, four-layer, and five-layer ${\mathrm{Mo}\mathrm{S}}_2$, ${\mathrm{Mo}\mathrm{Se}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{W}\mathrm{Se}}_2$ TMDCs. We find that the values of the dielectric constant and the refractive index always increase with the number of layers as well as with external electric field in the static region. However, in the visible region (400–600 nm), the values of the dielectric constant and the refractive index decrease with increasing external electric field. Among these TMDCs, ${\mathrm{Mo}\mathrm{Se}}_2$ shows the strongest response under electric field. We find that in contrast to a small electro-optic response in the weak-field limit, a large change in the refractive index is applied under large applied fields, with the increasing response obtained for thinner TMDC films. The magnitude and even the sign of the electro-optic response changes with the photon frequency and reaches the peak values in the 1.5–2.5 eV range. The position of the peak electro-optic response can be tuned by both changing the number of layers and the TMDC composition. 2L ${\mathrm{Mo}\mathrm{Se}}_2$ exhibits the highest response of 8.8 pm/V at 2.3 eV that is comparable to the response of typical oxide electro-optic ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$. Analysis of the sensitivity trends of these TMDC systems shows that compared to the changes in the band gap, the changes of the onset of strong absorption under electric field and change in the number of layers show better correlation with the changes in the electro-optic response. Our results provide guidance for the possible use of these materials in optical micro- or nanocavities for devices such as tunable optical filters and fast optical switches.

Crystal-strain variation imposes significant limitations on many quantum sensing and information applications for solid-state defect qubits in diamond. Thus, the precision measurement and control of diamond crystal strain is a key challenge. Here, we report diamond strain measurements with a unique set of capabilities, including micron-scale spatial resolution, a millimeter-scale field of view, and a 2-order-of-magnitude improvement in volume-normalized sensitivity over previous work, reaching $5(2)\times {10}^{-8}/\sqrt{\mathrm{Hz}\mu {\mathrm{m}}^{-3}}$ (with spin-strain coupling coefficients representing the dominant systematic uncertainty). We use strain-sensitive spin-state interferometry on ensembles of nitrogen-vacancy ($\mathrm{N}$-$V$) color centers in single-crystal bulk diamond with low strain gradients. This quantum interferometry technique provides insensitivity to magnetic-field inhomogeneity from the electronic and nuclear spin bath, thereby enabling long $\mathrm{N}$-$V$–ensemble electronic spin dephasing times and enhanced strain sensitivity, as well as broadening the potential applications of the technique beyond isotopically enriched or high-purity diamond. We demonstrate the strain-sensitive measurement protocol first on a confocal scanning laser microscope, providing quantitative measurement of sensitivity as well as three-dimensional strain mapping; and second on a wide-field-imaging quantum diamond microscope. Our strain-microscopy technique enables fast, sensitive characterization for diamond material engineering and nanofabrication; as well as diamond-based sensing of strains applied externally, as in diamond anvil cells or embedded diamond stress sensors, or internally, as by crystal damage due to particle-induced nuclear recoils.

Disorder in condensed matter and atomic physics is responsible for a great variety of fascinating quantum phenomena, which are still challenging for understanding, not to mention the relevant dynamical control. Here we introduce proof of the concept and analyze a neural-network-based machine-learning algorithm for achieving feasible high-fidelity quantum control of a particle in random environment. To explicitly demonstrate its capabilities, we show that convolutional neural networks are able to solve this problem as they can recognize the disorder and, by supervised learning, further produce the policy for the efficient low-energy cost control of a quantum particle in a time-dependent random potential. We show that the accuracy of the proposed algorithm is enhanced by a higher-dimensional mapping of the disorder pattern and using two neural networks, each properly trained for the given task. The designed method, being computationally more efficient than the gradient-descent optimization, can be applicable to identify and control various noisy quantum systems on a heuristic basis.

In this work, we use $\mathrm{N}$-phenylglycine (NPG) molecules as a universal passivant for all typical surface defects of perovskite to improve the photovoltaic performance and stability of perovskite. Our results show that NPG molecules can passivate the organic cation vacancy, $\mathrm{I}$ vacancy, $\mathrm{Pb}$ vacancy, and $\mathrm{Pb}$-$\mathrm{I}$ antisite surface defects of perovskite, through either coordinate and/or hydrogen bonding, or the van der Waals interaction. More interestingly, after the $\mathrm{Pb}$ vacancy on the perovskite surface is passivated by NPG, a transition from metallic to semiconducting properties occurs. The perovskite solar cells (PSCs) passivated by NPG have a high power conversion efficiency (PCE) of 21.16% and retain 91% of the initial efficiency after 700 h in humid air, compared with PSCs without passivation (PCE of 18.29% and maintained efficiency of 75%). Our study proposes one general surface-defect passivant for PSCs that significantly boosts photovoltaic performance and stability.

Optical parametric oscillation in a Kerr nonlinear microresonator can generate coherent laser light with frequencies that are widely separated from the pump frequency, allowing, for example, visible light to be generated using a near-infrared pump. To be practically useful, the pump-to-signal conversion efficiency must be far higher than what has been demonstrated in microresonator-based oscillators with widely separated output frequencies. To address this challenge, here we theoretically and numerically study parametric oscillations in Kerr nonlinear microresonators, revealing an intricate solution space that arises from an interplay of nonlinear processes. As a start, we use a three-mode approximation to derive an efficiency-maximizing relation between pump power and frequency mismatch. However, realistic devices, such as integrated microring resonators, support far more than three modes. Hence, a more accurate model that includes the entire modal landscape is necessary to determine potential inefficiencies arising from unwanted competing nonlinear processes. To this end, we numerically simulate the Lugiato-Lefever equation that accounts for the full spectrum of nonlinearly coupled resonator modes. We observe and characterize two nonlinear phenomena linked to parametric oscillations in multimode resonators: mode competition and cross-phase modulation-induced modulation instability. Both processes may impact conversion efficiency. Finally, we show how to increase the conversion efficiency to approximately 25% by tuning the microresonator loss rates. Our analysis will guide microresonator designs that aim for high conversion efficiency and output power.

We propose an approach for the excitation of polariton Tamm states with a controllable nontrivial topology. The Tamm polaritons emerge at the interface of two binary one-dimensional photonic crystals belonging to a $C_{3v}$ point group, with an exciton resonance within their matching band gaps. The external magnetic field applied in the Faraday geometry endows the dispersion of the Tamm polaritons with the nonreciprocity: it lifts the degeneracy between opposite propagation directions in the interface plane. The phenomenology of Tamm polariton currents closely resembles one of a $Z_2$ topological insulator. The proposed structure acts as an optical spin splitter controlled by the magnetic field magnitude.

Strain engineering is a promising approach for suppressing the off-state conductance in graphene-based devices that arises from Klein tunneling. In this work, we derive a comprehensive tight-binding Hamiltonian for strained graphene that incorporates strain-induced effects that have been neglected hitherto, such as the distortion of the unit cell under strain, the effect of strain on the next-nearest-neighbor coupling, and the second-order contributions of the strain tensor. We derive the corresponding low-energy effective Hamiltonian about the Dirac points and reformulate the boundary conditions at the interfaces between strained and unstrained graphene in light of additional terms in the Hamiltonian. By applying these boundary conditions, we evaluate the transmission across a strained graphene heterostructure consisting of a central segment sandwiched between two unstrained leads. Modulation of the transmitted current can be effected by varying the magnitude and direction of the applied strain, as well as by the application of a gate voltage. Based on realistic parameter values, we predict that high on:off ratios of up to ${10}^{12}$, as well as high current-valley polarization, can be achieved in the strain-modulated device.

We present a portable optically pumped magnetometer instrument for ultrasensitive measurements within the Earth’s magnetic field. The central part of the system is a sensor head operating a MEMS-based Cs vapor cell in the light-shift dispersed $M_z$ mode. It is connected to a compact, battery-driven electronics module by a flexible cable. We briefly review the working principles of the device and detail the realization of both the sensor head and electronics. We show shielded and unshielded measurements within a static magnetic field amplitude of $50\mu \mathrm{T}$, demonstrating a noise level of the sensor system down to $140\mathrm{fT}/\sqrt{\mathrm{Hz}}$ and a sensor bandwidth of several 100 Hz. In a detailed analysis of sensor noise, we reveal the system to be limited by technical sources with straightforward strategies for further improvement toward its fundamental noise limit of $12\mathrm{fT}/\sqrt{\mathrm{Hz}}$. We assess the parameters defining the sensor bandwidth by theoretical modeling based on the Bloch equations. Finally, we compare our sensors’ performance to a commercial superconducting quantum interference device system in a measurement environment typical for geomagnetic observatory practice and geomagnetic prospection.

Accurately measuring the three-dimensional position of an atomic cloud is essential for improving the precision of atom interferometers. In the work presented here, the three-dimensional position of an atomic cloud in an atom interferometer is measured with only one laser beam, generated by a phase modulator. The vertical position is determined from the change in the additional-sideband effect with the time interval $T$ between adjacent Raman pulses in the atom interferometer. Based on this method, the vertical position of the atomic cloud is measured with an uncertainty of 0.2 mm; therefore, the uncertainty in a gravity measurement contributed by the gravity gradient is not limited by the uncertainty in the measurement of the vertical position at the level of $0.1\mu \mathrm{Gal}$. Furthermore, a method of measuring Rabi oscillations at different horizontal positions to determine the horizontal position of the atomic cloud is proposed and is demonstrated experimentally. With this method, the horizontal position can be determined with an uncertainty of less than 0.1 mm, and the horizontal velocity of the atomic cloud can also be deduced. The methods demonstrated in this paper are helpful for evaluating systematic errors in atom interferometers, and also useful in precision measurements based on an atomic fountain.