Ghost imaging can be implemented with illumination light with pronounced quantum characters. In this work, we address the inverse problem of ghost imaging, i.e., the tomography of the quantum states of illumination light based on ghost imaging. We showcase the characterization of multimode Gaussian state light represented by its covariance matrix. With the information of spatial modes, we reconstruct the quantum states of light using the variation of SNR of the ghost images formed by the input light under a given set of optical operations. We demonstrate that ghost imaging can also be used to characterize the non-Gaussian light by its deviation from Gaussian states, which can serve as a criterion for the non-Gaussianity of the input light. The spatial resolving power of the proposed method can be advantageous for quantum information applications involving structured light, such as multiplexing with spatial modes.

We present the results of studying the parameters of the magnetic $M_X$ resonance in an all-optical sensor built according to the two-beam Bell-Bloom scheme in nonzero ultraweak magnetic fields in which the effects of spin-exchange broadening suppression are partially manifested. We report on the features of the resonance under these conditions. We also optimize the resonance parameters to achieve maximum sensitivity in magnetoencephalographic sensors. We demonstrate an improvement in the ultimate achievable sensitivity of an all-optical $M_X$ sensor by a factor of 4 or more, which in our experiment corresponds to a decrease from 13 to $3{\text{fT/Hz}}^{1/2}$ in a volume of $0.13{\mathrm{cm}}^3$. We also report the effect of incomplete suppression of spin-exchange broadening under conditions of strong transverse modulated optical pumping, and propose a semiempirical model to describe it.

Antineutrinos from a nuclear reactor comprise an unshieldable signal, which carries information about the core. A gadolinium-doped, water-based Cherenkov detector has been investigated for detection of reactor antineutrinos for midfield remote reactor monitoring for nonproliferation applications. Two independent reconstruction and analysis pathways have been developed and applied to a number of representative reactor signals to evaluate the sensitivity of a kiloton-scale, gadolinium-doped Cherenkov detector as a remote monitor prototype. The sensitivity of four detector configurations to nine reactor signal combinations was evaluated for a detector situated in Boulby Mine, close to the Boulby Underground Laboratory in the UK. It was found that a 22-m detector with a gadolinium-doped, water-based liquid scintillator fill is sensitive to an approximately 3-${\mathrm{GW}}_{\mathrm{th}}$ reactor at a standoff of approximately $150\mathrm{km}$ within 2 years in the current reactor landscape. A larger detector would be required to achieve a more timely detection or to monitor smaller or more distant reactors.

Controlling the flow of quantum information is a fundamental task for quantum computers, which is unfeasible to realize on classical devices. Coherent devices, which can process quantum states are thus required to route the quantum states that encode information. In this paper we demonstrate experimentally the smallest quantum transistor with a superconducting quantum processor, which is composed of a collector qubit, an emitter qubit, and a coupler (transistor gate). The interaction strength between the collector and emitter qubits is controlled by the frequency and state of the coupler, effectively implementing a quantum switch. Through the coupler-state-dependent Heisenberg (inherent) interaction between the qubits, a single-step (native) conditional $i$swap operation can be applied. To this end, we find that it is useful to take into consideration the higher-energy level for achieving a native and high-fidelity transistor operation. By reconstructing the quantum process tomography, we obtain an operation fidelity of $92.36\mathrm{\%}$ when the transistor gate is open ($i$swap implementation) and $95.23\mathrm{\%}$ in the case of closed gate (identity gate implementation). The architecture has strong potential in quantum information processing applications with superconducting qubits.

A discrete vortex, formed by a one-dimensional (1D) ring array of lasers, has high output power as compared with a conventional continuous vortex, and therefore has attracted considerable interest due to widespread applications in various fields. We present a method for probing the magnitude and sign of the topological charge (l) of an unknown discrete vortex, by analyzing the interference pattern of a 1D ring array of lasers. The interference pattern of an unknown discrete vortex with $l\ne 0$ is averaged with the interference pattern corresponding to $l=0$, which gives rise to a variation in the fringe visibility as a function of the laser number ($j$) in a 1D ring array. The number of dips observed in the fringe-visibility curve is found to be proportional to the magnitude of the topological charge of a discrete vortex. After determination of the magnitude, the sign of $l\ne 0$ is determined by averaging its interference pattern with the interference pattern corresponding to $l=1$. The number of dips in the fringe-visibility curve decreases by 1 for positive l values and increases by 1 for negative l values. Further, we verify our method against the phase disorder, and it is found that the phase disorder does not influence accurate determination of the topological charge of a discrete vortex. The working principle as well as numerical and experimental results are presented for discrete vortices with topological charges from small to large values. Excellent agreement between the experimental results and the numerical simulations is found. Our method can be useful in applications of discrete vortices especially where conventional continuous vortices have power limitations.

The switching of a magnetic tunnel junction (MTJ) using femtosecond laser pulses enables a possible path for nonvolatile, ultrafast, and low-dissipative memories. In this work, we demonstrate successful field-free 50-fs single-laser-pulse-driven magnetization reversal of a [$\mathrm{Tb}$/$\mathrm{Co}$]-based storage layer in a perpendicular MTJ with a estimated absorbed energy of 68.6 fJ/bit. The nanofabricated MTJ devices have an optimized bottom reference electrode and show tunnel-magnetoresistance-ratio (TMR) values up to 74% after patterning down to sub-100-nm lateral dimensions. Experiments on continuous films reveal peculiar reversal patterns of concentric rings with opposite magnetic directions, above a certain threshold fluence. These rings have been correlated with the patterned-device switching probability as a function of the applied laser fluence. Moreover, the magnetization reversal is independent of the duration of the laser pulse. According to our macrospin model, the underlying magnetization-reversal mechanism can be attributed to an in-plane reorientation of the magnetization due to a fast reduction of the out-of-plane uniaxial anisotropy and a magnetization precession around the local effective-anisotropy axis. These aspects are of great interest both for the physical understanding of the switching phenomenon and their consequences for all-optical-switching memory devices, since they allow for a large fluence operation window with high resilience to pulse-length variability.

Measurement-device-independent (MDI) quantum key distribution (QKD) closes all the loopholes in the detection side, which greatly improves the practical security of QKD. Spontaneous parametric down-conversion (SPDC) sources play a significant role in MDI QKD by using the heralding strategies to achieve unique features, such as the passive-decoy method, which reduces side-channel information leakage from the source. However, photon pairs from realistic SPDC sources are often spectrally entangled. This lowers the Hong-Ou-Mandel (HOM) interference visibility, and thus increases the QKD error rate. Unfortunately, research efforts so far have not considered this effect quantitatively. This leads to an overestimation in the secure key rate; therefore their results cannot be applied to practical sources. In this article, we close this gap by decomposing the spectral entangled state into Schmidt eigenstates, then find their contributions to the secure key rate. In addition, our work allows an accurate optimization of pumping and filtering conditions to maximize the final key rate. We note that our model is general and can be applied to other quantum information processes involving HOM interference, such as quantum teleportation, entanglement swapping, and quantum computation.

Interference with a nonlinear cavity mode undergoing a self-sustained limit cycle can lead to superradiant amplification of incident waves. This effect, which results from nonlinear wave-mode coupling, is theoretically predicted and experimentally confirmed in this work. On the theoretical side, we present a nonlinear coupled-mode framework to derive an analytical expression for the scattering matrix. These predictions are then validated against experiments on a superradiant acoustic meta-atom, which uses deep-cavity whistling to create a limit cycle. The proposed all-acoustic meta-atom requires no active elements to manipulate the sound field, but achieves superradiant amplification by nonreciprocal coupling between the incident wave and the synchronized whistling tone (limit cycle). We demonstrate with this setup that the nonlinear phenomena we present can be exploited to quasipassively tune the scattering matrix of a resonant cavity. As a proof of concept, we convert a lossy, weakly reflective scatterer into a reflection amplifier. We explain how the synchronization-based realization of superradiance reported in this work may be exploited in the future to design lossless, nonacoustic scatterers for metamaterial applications.

Currently available quantum computing hardware based on superconducting transmon architectures realizes networks of hundreds of qubits with the possibility of controlled nearest-neighbor interactions. However, the inherent noise and decoherence effects of such quantum chips considerably alter basic gate operations and lead to imperfect outputs of the targeted quantum computations. In this work, we focus on the characterization of crosstalk effects, which manifest themselves in correlations between simultaneously executed quantum gates on neighboring qubits. After a short explanation of the physical origin of such correlations, we show how to efficiently and systematically characterize the magnitude of such crosstalk effects on an entire quantum chip using the randomized benchmarking protocol. We demonstrate the introduced protocol by running it on real quantum hardware provided by IBM and observe significant alterations in gate fidelities due to crosstalk. Lastly, based on the gained information, we propose more accurate means to simulate noisy quantum hardware by devising a crosstalk-aware noise model.

Motivated by recent advances in the inverse design of electromagnetic materials, we develop two methods for manipulating flexural waves on thin elastic plates. First, we derive a technique for determining plate pinning or mass loading of a thin plate, designing structures that focus elastic energy or isolate a region of space from vibrations. Taking inspiration from the adjoint method in electromagnetism, we show how to design graded plates to act as lenses or perform mode shaping. Both of the methods presented are simple, versatile, and straightforward to implement, making them useful for designing a wide range of devices for sensing, energy harvesting, and vibration isolation.

We propose a rotation sensing scheme based on the Sagnac interferometer for a single trapped particle system. In this scheme, the spin and vibrational states of the particle, prepared in a coherent state, are translated into a Schrödinger cat state by applying a strong magnetic field gradient to create the spin-dependent force. By reflecting the phase related to the rotational rate accumulated by the two arms of the interferometer to the spin state, we can obtain the rotational rate of the apparatus by performing spin measurement. The scheme can also be applied to the situation with the vibration state in the thermal state. The sensing sensitivity can reach $S=4.47\times {10}^{-7}\mathrm{rad}/\mathrm{s}\sqrt{\mathrm{Hz}}$ for a suspended diamond nanoparticle and $S=5.65\times {10}^{-6}\mathrm{rad}/\mathrm{s}\sqrt{\mathrm{Hz}}$ for a trapped ${}^{40}{\mathrm{Ca}}^{+}$. Combining with the experimental parameters, our scheme is applied to realize the gyroscope in a trapped single nanoparticle and atomic ion.

We present a method that integrates any quantum algorithm capable of finding solutions to integer linear programs into the branch-and-price algorithm, which is regularly used to solve large-scale integer linear programs with a specific structure. The role of the quantum algorithm is to find integer solutions to subproblems appearing in branch-and-price. Obtaining optimal or near-optimal integer solutions to these subproblems can increase the quality of solutions and reduce the depth and branching factor of the branch-and-price algorithm and hence reduce the overall running time. We investigate the viability of the approach by considering the tail assignment problem and the quantum approximate optimization algorithm (QAOA). Here, the master problem is the optimization problem set partitioning or its decision version exact cover and can be expressed as finding the ground state of an Ising spin glass Hamiltonian. For exact cover, our numerical results indicate that the required algorithm depth decreases with the number of feasible solutions for a given success probability of finding a feasible solution. For set partitioning, on the other hand, we find that for a given success probability of finding the optimal solution, the required algorithm depth can increase with the number of feasible solutions if the Hamiltonian is balanced poorly, which in the worst case is exponential in the problem size. We therefore address the significance of properly balancing the objective and constraint parts of the Hamiltonian. We empirically find that the approach is viable with QAOA if polynomial algorithm depth can be realized on quantum devices.

Although the drift-diffusion model (DDM) is fundamental in semiconductor physics and related fields, the accurate solution of the DDM is unavailable over a long period of time. The DDM is accurately and analytically solved based on the Fermi-Dirac statistics and general mobility model. The differences in current values, carrier density, and electric field functions between the accurate solution, other solutions, and the standard Scharfetter-Gummel (SG) scheme are evident. The results for mobility extracted from the accurate solution are larger and evidently different from those of the SG scheme, Mott-Gurney equation, and the analytic solution under the uniform-electric-field approximation.

Antiferromagnets (AFMs) are promising materials for future high-frequency field-free spintronic applications. Self-localized spin structures can enhance their capabilities and introduce alternative functionalities to AFM-based devices. Here we consider a domain wall (DW), a topological soliton that bridges a connection between two ground states, similar to a Josephson junction link between two superconductors. We demonstrate the similarities between DWs in biaxial AFM with easy-axis primary anisotropy, driven by a spin current, and long Josephson junctions (LJJs). We found that the Bloch line (BL) in DWs resembles the fluxon state of Josephson junctions, creating a close analogy between the two systems. We propose a scheme that allows us to create, move, read, and delete such BLs. This transmission line operates at room temperature and can be dynamically reconfigured in contrast to superconductors. Results of a developed model were confirmed by micromagnetic simulations for ${\mathrm{Cr}}_2{\mathrm{O}}_3$ and ${\mathrm{Dy}\mathrm{Fe}\mathrm{O}}_3$, i.e., correspondingly with weak and strong in-plane anisotropy. Overall, the proposed scheme has significant potential for use in magnetic memory and logic devices.

In this study, we present the fabrication and characterization of $\mathrm{Ni}$/${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$/$\mathrm{Ni}$ trilayers using rf sputtering. These trilayers exhibit thick $\mathrm{Ni}$ layers (10 $\text{μ}\mathrm{m}$) and excellent adherence to the substrate, enabling high magnetoelectric coefficients. By engineering the magnetic anisotropy of nickel through anisotropic thermal residual stress induced during fabrication, and by selecting a carefully chosen cut angle for the ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3$ substrate, we achieved self-biased behavior. We demonstrate that these trilayers can power medical implant devices remotely using excitation by a small ac magnetic field, thereby eliminating the need for a dc magnetic field and bulky magnetic field sources. The results highlight the potential of these trilayers for the wireless and noninvasive powering of medical implants. This work contributes to the advancement of magnetoelectric materials and their applications in healthcare technology.

Ultrafast light-matter interaction has emerged as a powerful tool to control and probe the macroscopic properties of functional materials, especially two-dimensional transition metal dichalcogenides that can form different structural phases with distinct physical properties. However, it is often difficult to accurately determine the transient optical constants. In this work, we developed a near-infrared pump—a terahertz to midinfrared (12–22 THz) probe system in transmission geometry to measure the transient optical conductivity in $2H{\text{−}\mathrm{Mo}\mathrm{Te}}_2$ layered material. By performing separate measurements on bulk and thin-film samples, we are able to overcome issues related to nonuniform substrate thickness and penetration depth mismatch and to extract the transient optical constants reliably. Our results show that photoexcitation at 690 nm induces a transient insulator-metal transition, while photoexcitation at $2\text{μ}\mathrm{m}$ has a much smaller effect due to the photon energy being smaller than the band gap of the material. Combining this with a single-color pump-probe measurement, we show that the transient response evolves towards the $1T^{{}^{\prime }}$ phase at higher fluence. Our work provides a comprehensive understanding of the photoinduced phase transition in the $2H{\text{−}\mathrm{Mo}\mathrm{Te}}_2$ system.

Using analytical and numerical means, we document a geometry-enabled phenomenon, termed herein dispersion morphing, in which lattice stretching in rotator structures modifies the real and imaginary dispersion characteristics of the system. We then demonstrate diverse functionality derived from dispersion morphing under adiabatic (static) and nonadiabatic (dynamic) lattice deformation, to include dramatic changes in group velocity, refractive index, directivity, and amplification. The proposed rotator lattices consist of in-plane rotators coupled by angled elastic linkages the location and spacing of which can be easily reconfigured, allowing significant changes in the dispersion characteristics of the lattices. Under adiabatic lattice deformation, we reconfigure the directivity and refractive index of the periodic structure and present a closed-form solution to achieve flat bands across the entire wave-number domain. We also incorporate chirality in the unit-cell design to counteract pass-band shifting in the process of dispersion morphing, such that a real-time wave manipulation becomes possible. For dynamic lattice deformation, we model the lattice constant as (i) a step function of time and (ii) a harmonic function of time. In the former scenario, we employ the concept of temporal interfaces and achieve on-demand time delay of the propagation. In the latter scenario, we demonstrate a parametric amplification effect with stretching-informed amplification parameters. We report strong agreement between our theoretical analysis and numerical simulations, verifying the aforementioned findings. We believe that the versatile adaptations of such rotator lattices and their rich dynamics may inspire next-generation reconfigurable and multifunctional metamaterial devices.

We propose superconducting-semiconducting (super-semi) qubit and coupler designs based on high-quality compact through-silicon vias (TSVs) to overcome challenges introduced by materials and processing required to fabricate high-quality super-semi Josephson junctions. In our designs, an interposer “probe” wafer containing TSVs is used to contact a sample wafer with, e.g., a superconductor-proximitized epitaxially grown germanium quantum well. By utilizing the capacitance of the probe-wafer TSVs, the majority of the electric field in the qubits is pulled away from lossy regions in the semiconducting wafer, such as the graded buffer layers sometimes required for epitaxial growth. Through simulations, we find that the probe wafer can reduce the electric field participation of the qubit in the sample wafer by an order of magnitude for thin substrates and remains small even when the epitaxial-layer thickness approaches $100\mu \mathrm{m}$. We also show how the qubit-coherence improvements achieved via this scheme are extensible to multiqubit systems that have tunable qubit-qubit couplings without magnetic fields. This approach additionally shrinks the on-chip footprint of voltage-tunable superconducting qubits while homogenizing critical wiring structures independent of semiconducting materials, which could aid in accelerating the understanding of super-semi heterostructures in a variety of systems.