Contemporary quantum computers with tens to hundreds of physical quantum bits (qubits) are susceptible to quantum noise. As a result, running quantum circuits on these computers is error prone. In particular, deep quantum circuits with a large number of quantum gates and qubits are more likely to fail on quantum computers. In this work, we propose a design for the quantum arithmetic division operation which runs successfully on contemporary quantum computers. A quantum division circuit is needed for realization of quantum algorithms in scientific and image processing applications. While there have been a limited number of prior works on quantum division, none of them can be implemented on quantum computers due to excessive circuit complexity. We propose a different design that exploits dynamic circuits and approximate computing to deploy a quantum division circuit in quantum computers. The dynamic circuit is a new feature in recent quantum computers for midcircuit measurement in hardware. We exploit this feature to reduce the number of qubits and increase the fidelity of quantum division. We also exploit approximate computing to overcome noise in quantum hardware. We carefully tune the scope of approximation in our circuits to offer an acceptable level of accuracy. We run our division circuit on an IBM quantum computer and show that our circuit overcomes quantum noise and generates meaningful results.

The Schwinger model describes quantum electrodynamics in $1+1$ dimensions, it is a prototype for quantum chromodynamics, and its lattice version allows for a quantum link model description that can be simulated using modern quantum devices. In this work, we devise quantum simulations to investigate the dynamics of this model in its low-dimensional form, where the gauge field degrees of freedom are described by spin-$\frac{1}{2}$ operators. We apply Trotterization to write quantum circuits that effectively generate the evolution under the Schwinger model Hamiltonian. We consider both sequential circuits, with a fixed-gate sequence, and randomized ones. Utilizing the correspondence between the Schwinger model and the PXP model, known for its quantum scars, we investigate the presence of quantum scar states in the Schwinger model by identifying states exhibiting extended thermalization times in our circuit evolutions. Our comparison of sequential and randomized circuit dynamics shows that the nonthermal sector of the Hilbert space, including the scars, is more sensitive to randomization.

Dissipation and decoherence of quantum systems in thermal environments is important to various spectroscopies. It is generally believed that dissipation can broaden the line shape of spectroscopies, and thus stronger system-bath interaction can result in more significant homogeneous broadening of two-dimensional electronic spectroscopy (2DES). Here we show that the case can be the opposite in the regime of electromagnetically induced transparency (EIT). We predict that assisted by EIT, the homogeneous broadening of the 2DES at a higher temperature can be significantly reduced due to the detailed balance. This anomalous effect is due to the long-lasting off-diagonal peaks in 2DES.

We consider massive vortices in binary condensates, where the immiscibility condition entails the trapping of the minority component in the vortex cores of the majority component. We study such vortices by means of a two-dimensional pointlike model, and show how the relevant dynamical equations exhibit vortex-pair solutions characterized by different vortex masses and circular orbits of different radii $a$ and $b$. These solutions are validated by the simulations of the Gross-Pitaevskii equations for binary condensates. After examining the properties of the vortex-pair rotational frequency $\mathrm{\Omega }$ as a function of the vortex masses for a given pair geometry, we define the rotational-state diagram $\mathcal{D}$, describing all the possible vortex-pair solutions in terms of the orbit radii at given $\mathrm{\Omega }$. This includes solutions with equal-mass pairs but $a\ne b$ or with one of the two masses (or both) equal to zero. Also, we analytically find the minimum value of $\mathrm{\Omega }$ for the existence of such solutions, and obtain numerically the critical frequency ${\mathrm{\Omega }}_c$ below which $\mathcal{D}$ changes its structure and the transition to an unstable vortex-pair regime takes place. Our paper highlights an indirect measurement scheme to infer the vortex masses from the orbits' radii $a$ and $b$, and a link between the vortex masses and the vortex-pair small-oscillation properties.

Vibrational resonance (VR) is a nonlinear phenomenon in which the system response to a weak signal can be resonantly enhanced by applying a high-frequency modulation signal with an appropriate amplitude. The majority of VR research has focused on amplifying the amplitude or intensity of the system response to a weak signal, whereas the study of the phase information of system responses in VR remains limited. Here, we investigate the VR phenomena in both amplitude and phase quadratures of an optical field in a Kerr nonlinear cavity driven by a near-resonant weak signal and a far-detuned modulation signal. Analytical and numerical results demonstrated that the resonant enhancement in the amplitude and phase quadratures of the system response to a weak signal simultaneously occurs as the amplitude of the modulation signal is varied. There is a linear relation between the amplitude and frequency of the modulation signal for achieving an optimal VR effect. Furthermore, we generalized our study to investigate the quadrature at an arbitrary phase and determined that the VR enhancement sensitively depends on the phase. Our findings not only broaden the scope of VR research by incorporating phase information but also introduce an approach for amplifying an optical field by manipulating another optical field.

We study the semiclassical limit of the two-photon Dicke model with both cavity decay and qubit dissipation, and extend a recent analysis of its stationary points [Garbe et al., Sci. Rep. 10, 13408 (2020)], where a large unstable region was found. By considering the explicit dynamical evolution, we show that the unstable region actually hosts a rich nonlinear behavior. At variance with other types of Dicke models (without qubit dissipation or with one-photon interactions), the occurrence of chaos does not rely on a large counterrotating interaction. Furthermore, new routes to chaos appear in addition to period-doubling bifurcations, i.e., intermittent chaos and quasi-periodic oscillations. The transition mechanisms under these three distinct routes are investigated in detail through the system's long-time evolution, the optical field power spectrum, Lyapunov exponents, and bifurcation diagrams. Additionally, we provide a comprehensive phase diagram detailing the existence of stable fixed points, limit cycles, and the aforementioned chaos-related dynamics.

Entanglement is beneficial to enhancing a phase sensitivity beyond the classical limit that is given by a coherent state. The best performance is achieved with symmetric entangled states, which is enhanced further by sending more photons to the signal mode. We delve into which entanglement structure is valuable for quantum target sensing, such as a reflectivity parameter. We show that an asymmetric entangled state can approach the local joint measurement bound of a two-mode squeezed vacuum state which is a nearly optimal input state for quantum target sensing, whereas it cannot beat the performance of the coherent state for quantum phase sensing. The result is demonstrated with an asymmetric entangled coherent state whose performance is evaluated with quantum Fisher information and confirmed by the signal-to-noise ratio with an optimal observable. The best quantum advantage is achieved in the case of sending fewer photons to the signal mode and many more photons to the idler mode.

We propose a chirped-pulse driving protocol and reveal its exceptional property for quantum coherent control. The nonadiabatic passage generated by the driving protocol, which includes the population inversion and the nonadiabaticity-induced transition as its ingredients, is shown to be robust against pulse truncation. We further demonstrate that the protocol allows for universal manipulation on the qubit system through designing pulse sequences with either a properly adjusted sweeping frequency or pulsing intensity.

Photoemission spectroscopy is used to quantitatively determine the partial cross section of the $1t_2$ and $2a_1$ subshells of methane. Photoelectron spectra are recorded using a hemispherical electron analyzer and monochromatized synchrotron radiation. The corresponding emission peaks are analyzed to calculate the partial cross sections with the associated uncertainties in a photon energy range from 16.5 to 70 eV. The presented results cover this broad energy range with corresponding uncertainties and belong to the framework of photoemission orbital tomography, aiming to bridge the experimental gap between simple single atoms and complex molecular monolayers.

The plasmon-enhanced photoemission process significantly improves the quality of electron emission current, making it applicable for next-generation free-electron devices. However, classical descriptions are less suitable for understanding the interactions between plasmonic structures and other materials at the nanoscale. In this study, we investigate the underlying mechanisms and dynamics of a plasmonic emitter composed of a Au nanoparticle and a sodium atom using real-time time-dependent density functional theory. Calculation results reveal three interaction mechanisms, namely, a near-field enhancement effect, orbital hybridization, and charge-transfer plasmon, that strongly affect the emission behavior. Specifically, the presence of the charge-transfer plasmon leads to a substantial deviation of the excitation laser frequency and significantly enhances photoemission currents. The maximum emission current increases by more than twice compared to the excitation at the original resonance frequency of Au nanoparticles. This work provides a guidance for the practical construction and experimentation of plasmon-enhanced photoemission electron sources.

In this paper we generalize the adjoint method of inverse design to nonreciprocal media. As a test case, we use three-dimensional topology optimization via the level-set method to optimize one-way energy transfer for pointlike source and observation points. To achieve this we introduce a suite of tools, chiefly what we term the “Faraday-adjoint” method which allows for efficient shape optimization in the presence of magneto-optical media. We carry out an optimization based on a very general equation that we derive for energy transfer in a nonreciprocal medium, and link finite-different time-domain numerics to analytics via a modified Born series generalized to a tensor permittivity. This paper represents a stepping stone towards practical nanophotonic optical isolation, often regarded as the “holy grail” of integrated photonics.

In a fiber-based quantum network, the utilization of the telecom band is crucial for long-distance quantum information (QI) transmission between quantum nodes. However, the near-infrared wavelength is identified as optimal for storing and processing QI through alkaline atoms. Recognizing the challenge of efficiently bridging the frequency gap between atomic quantum devices and telecom fibers while maintaining the QI carried by photons, quantum frequency conversion (QFC) serves as a pivotal quantum interface. In this study, we explore an efficient telecom-band QFC mechanism based on diamond-type four-wave mixing (FWM) with rubidium energy levels. The mechanism enables the conversion of photons between the near-infrared wavelength of 795 nm and the telecom band of 1367 or 1529 nm. Using the Heisenberg-Langevin approach, we optimize conversion efficiency (CE) across varying optical depths while considering quantum noises and present corresponding experimental parameters. Unlike previous works neglecting the applied field absorption loss, our results are more relevant to practical scenarios. Moreover, by employing the reduced-density-operator theory to construct a theoretical framework, we demonstrate that this diamond-type FWM scheme can maintain the quantum characteristics of input photons with high fidelity, such as quadrature variances and photon statistics. Importantly, these properties remain unaffected by vacuum field noise, enabling the system to achieve high-purity QFC. Another significant contribution lies in examining how this scheme impacts QI encoded in photon-number, path, and polarization degrees of freedom. These encoded qubits exhibit remarkable entanglement retention under sufficiently high CE. In the case of perfect CE, the scheme can achieve unity fidelity. This comprehensive exploration establishes a theoretical foundation for the application of the diamond-type QFC scheme based on atomic ensembles in quantum networks, laying essential groundwork for advancing the scheme in distributed quantum computing and long-distance quantum communication.

Recent years have seen great interest in extending causal inference concepts developed in the context of classical statistical models to quantum theory. So far, this program has only barely addressed causal identification, a type of causal inference problem concerned with recovering from observational data and qualitative assumptions the causal relationships generating the data, and hence the effects of hypothetical interventions. A major obstacle to a theory of causal identification in the quantum setting is the question of what should play the role of “observational data,” as any means of extracting data at a locus will almost certainly disturb the system. One might think a priori that quantum measurements are already too much like interventions, so that the problem of causal identification is trivialized. This is not the case: when we fix a limited class of quantum measurement instruments (namely, the class of all projective measurements) to play the role of “observations,” there exist scenarios for which causal identification is impossible. In this paper, we present a framework, based on process theories (also known as strict symmetric monoidal categories), for studying quantum causal identification scenarios on the same footing as their classical counterparts. Within this framework, we present sufficient conditions for quantum causal identification in networks with unobserved confounding systems, including quantum analogs of the well-known “back-door” and “front-door” criteria. These results arise from a type of causal model designed to facilitate the transfer of inference techniques from the classical to the quantum setting.

In recent years, substantial research effort has been devoted to quantum algorithms for ground-state-energy estimation (GSEE) in chemistry and materials. Given the many heuristic and nonheuristic methods being developed, it is challenging to assess what combination of these methods will ultimately be used in practice. One important metric for assessing utility is the runtime, which depends on the ground-state preparation (GSP) for most GSEE algorithms. Towards assessing the utility of various combinations of GSEE and GSP methods, we asked under which conditions a GSP method should be accepted over a reference method, such as the Hartree-Fock method. We introduce a criterion for accepting or rejecting a GSP method for the purposes of GSEE. We consider different GSP methods ranging from heuristics to algorithms with provable performance guarantees and perform numerical simulations to benchmark their performance on different chemical systems, starting from small molecules like the hydrogen molecule to larger systems like jellium. In the future, this approach may be used to abandon certain variational quantum eigensolver (VQE) ansatzes and other heuristics. Yet our findings do not provide evidence against using VQE and more expensive heuristic methods, like the low-depth booster. This work sets a foundation from which to further explore the requirements to achieve quantum advantage in quantum chemistry.

Variational quantum algorithms are tailored to perform within the constraints of current quantum devices, yet they are limited by performance-degrading errors. In this study we consider a noise model that reflects realistic gate errors inherent to variational quantum algorithms. We investigate the decoherence of a variationally prepared quantum state due to this noise model, which causes a deviation from the energy estimation in the variational approach. By performing a perturbative analysis of optimized circuits, we determine the noise threshold at which the criterion set by the stability lemma is met. We assess our findings against the variational quantum eigensolver and quantum approximate optimization algorithm for various problems with up to 14 qubits. Moreover, we show that certain gate errors have a significantly smaller impact on the coherence of the state, allowing us to reduce the execution time without compromising performance.

Achieving precise preparation of quantum many-body states is crucial for the practical implementation of quantum computation and quantum simulation. However, the inherent challenges posed by unavoidable excitations at critical points during quench processes necessitate careful design of control fields. In this work, we introduce a promising and versatile dynamic control neural network tailored to optimize control fields. We address the problem of suppressing defect density and enhancing cat-state fidelity during the passage across the critical point in the quantum Ising model. Our method facilitates seamless transitions between different objective functions by adjusting the optimization strategy. In comparison to gradient-based power-law quench methods, our approach demonstrates significant advantages for both small system sizes and long-term evolutions. We provide a detailed analysis of the specific forms of control fields and summarize common features for experimental implementation. Furthermore, numerical simulations demonstrate the robustness of our proposal against random noise and spin number fluctuations. The optimized defect density and cat-state fidelity exhibit a transition at a critical ratio of the quench duration to the system size, coinciding with the quantum speed limit for quantum evolution.