Understanding and suppressing sources of decoherence is a leading challenge in building practical quantum computers. In superconducting qubits, low-frequency charge noise is a well-known decoherence mechanism that is effectively suppressed in the transmon qubit. Devices with multiple charge-sensitive modes can exhibit more complex behaviors, which can be exploited to study charge fluctuations in superconducting qubits. Here we characterize charge sensitivity in a superconducting qubit with two transmonlike modes, each of which is sensitive to multiple charge-parity configurations and charge-offset biases. Using Ramsey interferometry, we observe sensitivity to four charge-parity configurations and track two independent charge-offset drifts over hour time-scales. We provide a predictive theory for charge sensitivity in such multimode qubits which agrees with our results. Finally, we demonstrate the utility of a multimode qubit as a charge detector by spatially tracking local-charge drift.

Symmetry considerations are key to our understanding of the fundamental laws of nature. The presence of a symmetry implies that a physical system is invariant under specific transformations, and this invariance may have deep consequences. For instance, symmetry arguments state that a system will remain in its initial state if incentives to actions are equally balanced. Here, we apply this principle to a chain of qubits and show that it is possible to engineer the symmetries of its Hamiltonian in order to keep quantum information intrinsically protected from both relaxation and decoherence. We show that the coherence properties of this system are strongly enhanced relative to those of its individual components. Such a qubit chain can be realized using a simple architecture consisting of a relatively small number of superconducting Josephson junctions.

Information processing and storage by the same physical system is emerging as a promising alternative to traditional computing platforms. In turn, this requires the realization of elementary units the memory content of which can be easily tuned and controlled. Here, we introduce a polariton-based quantum memristor where the memristive nature arises from the intercavity polariton exchange and is controlled by a time-varying atom-cavity detuning. A dynamical hysteresis is characterized by the fluctuations in the instantaneous polariton number, where the history information is encoded into a dynamical phase. Using a Lindblad master-equation approach, we find that features of the quantum memristor dynamics, such as the area and circulation of the hysteresis loop, showcase a kind of “plasticity” controlled by quantum state initialization. This makes this quantum memristor very versatile for a wide range of applications.

High-energy photons can be generated via inverse Compton scattering (ICS) in the collision between energetic electrons and an intense laser pulse. The development of laser plasma accelerators promises compact and all-optical gamma-ray sources by colliding the electrons from laser wakefield accelerators with high-power driving pulses reflected by a plasma mirror. However, the law of optical focusing hinders realization of both high photon yield and monochromatic spectra in this scenario. We propose an azimuthal spatial-temporal convertor that decouples the focal field strength from laser spot size using helical parabolic geometry. It decomposes the driving laser beam into a pulse train of almost identical divergence angle and focal depth, creating synchronized ICS in the optimized linear regime. The scheme resolves the dilemma between high efficiency and narrow energy spread, facilitating the generation of monochromatic gamma rays using high power lasers beyond relativistic field strengths.

Weyl-like magnon excitations in ordered magnets have attracted significant recent attention. Despite the tantalizing physics and application prospects, the experimental observation of Weyl magnons is still challenging owing to their extraordinarily high frequency that is not accessible to the microstrip antenna technique. Here we predict gigahertz Weyl excitations in the collective dynamics of dipolar-coupled magnetic vortices arranged in a three-dimensional stacked honeycomb lattice. It is found that inversion symmetry breaking leads to the emergence of the type-II Weyl semimetal (WSM) state with tilted dispersion. We derive the full phase diagram of the vortex arrays that support WSMs with both single and double pairs of Weyl nodes, and the topological insulator phase. We observe robust arc surface states in a dual-segment fashion due to the tilted nature of type-II WSMs. Our findings uncover the low-frequency WSM phase in magnetic-texture-based crystals that are indispensable for future Weyltronic applications.

As we are approaching actual application of quantum technology, it is essential to exploit the current quantum resources in the best possible way. With this in mind, it might not be beneficial to use the usual standard gate sets, inspired by classical logic gates, while compiling quantum algorithms when other less standardized gates currently perform better. We, therefore, consider a promising native gate, which occurs naturally in superconducting circuits, known as the diamond gate. We show how the diamond gate can be decomposed into standard gates and, using single-qubit gates, can work as a controlled-not swap (cns) gate. We then show how this cns gate can create a controlled-phase gate. Controlled-phase gates are the backbone of the quantum Fourier-transform algorithm and we, therefore, show how to use the diamond gate to perform this algorithm. We also show how to use the diamond gate in quantum machine learning; namely, we use it to approximate nonlinear functions and classify two-dimensional data.

The search for hardware-compatible strategies for solving nondeterministic polynomial time (NP)-hard combinatorial optimization problems (COPs) is an important challenge of today’s computing research because of their wide range of applications in real-world optimization problems. Here, we introduce an unconventional scalable approach to face maximum-satisfiability (MAX-SAT) problems that combines probabilistic computing with p-bits, parallel tempering, and the concept of invertible logic gates. We theoretically show the spintronic implementation of this approach based on a coupled set of Landau-Lifshitz-Gilbert equations, showing a potential path for energy efficient and very fast (p-bits exhibiting nanosecond timescale switching) architecture for the solution of COPs. The algorithm is benchmarked with hard MAX-SAT instances from the 2016 MAX-SAT competition (e.g., “HG-4SAT-V150-C1350-1.cnf,” which can be described with 2851 p-bits), including weighted MAX-SAT and maximum-cut problems.

Multicolor attosecond electron bunches with tens of megaelectronvolts energy are extremely useful for wide applications, such as multicolor x-ray generation. Here we propose a scheme to generate such electron bunches with a laser wakefield accelerator driven by a millijoule few-cycle laser pulse via carrier-envelope-phase (CEP)-controlled ionization injection. It is found that the rapid CEP shift of the driving pulse in plasma due to plasma dispersion and Gouy phase shift enables the inner shell electrons of background high-$Z$ ions to be ionized periodically. These electrons are then subsequently injected into the wakefield, where each injection lasts for a distance of a few micrometers only, leading to the generation of multicolor electron bunches with narrow peaks in the energy spectrum and a subfemtosecond duration. With typical laser parameters available currently, simulations show that electron bunches with central energy of about $30\mathrm{MeV}$, energy spread of about $2\mathrm{\%}$, and duration of $460$ attoseconds can be obtained. Such electron sources can be driven by current laser systems operating at kilohertz repetition rates, making them attractive for wide applications.

In optical sensing, it is a fundamental principle that retrieving a certain quantity of spatial information requires a commensurate number of photons to be detected. However, on the issue of imaging moving objects under low photon flux, when the detectable photon flow is insufficient to recover the information flow formed by both motion and structural images of the object, photon crisis is induced and a more photon-efficient sensing method is urgently expected. Here we implement a simultaneous tracking and imaging method. The motion information and spatial information of a moving object are encoded by sequentially projecting the laterally shifting patterns and recording the results via a photomultiplier. Then both the trajectory and the image of the moving object are reconstructed gradually along with its evolution. As a photon-efficient encoding-decoding method is achieved, our method realizes capturing moving object with angular velocity larger than 45 mrad/s, and works well with the average number of detected photons even down to about 2.4 photons/speckle under single-laser-pulse illumination.

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.