In this study we engineer the $z$ component of the Dzyaloshinskii-Moriya interaction mediated by photons to emulate ground-state chiral excitation based on three-level atoms driven by quantum and classical fields. We employ adiabatic elimination techniques to derive an effective Dzyaloshinskii-Moriya interaction Hamiltonian of two-level systems in the ground-state manifold, which can ensure the desired dynamics is achieved through the implementation of periodic modulation. Meanwhile, three-state and multistate chiral excitation can be obtained by choosing appropriate driving frequencies and phases. The numerical simulation results clearly indicate that our proposal can generate the comparatively perfect three-state chiral excitation and relatively reliable multistate chiral excitation. Moreover, the influence of unfavorable factors on the chiral current is discussed in detail, and the potential experimental feasibility further shows that our results provide possibilities for quantum state transfer and future quantum networks.

The characterization of multi-time correlations in open quantum systems is of fundamental importance. In this work, we investigate multi-time processes using the process matrix formalism and show that the presence of a quantum-memory environment acts as a resource in enhancing the communication capacity in sequential prepare-transform-measure quantum random access codes (QRAC). The correlated environment enables a quantum advantage to multiple parties, even with projective measurements. In particular, we show that Markovian and classical-memory processes, i.e., non-Markovian quantum processes with classical feedback from the environment, do not yield a sequential quantum advantage. In contrast, it is possible to achieve an advantage in the presence of a quantum-memory environment. Therefore, this approach allows a semi-device-independent certification of quantum non-Markovianity. As opposed to entanglement-detection criteria which require knowledge of the complete process, this method allows to certify the presence of a quantum-memory environment from the observed measurement statistics. Moreover, quantum memory ameliorates the unambiguous certifiable region of unsharp instruments in a semi-device-independent manner.

We propose a theoretically designed laser resonance chromatography (LRC) experiment on ${\mathrm{Rf}}^{+}$ $(Z=104)$ drifting in He buffer gas. To this end, we first developed a four-level rate equation model that simulates the optical pumping of ${\mathrm{Rf}}^{+}$ from its ground state, ${}^{2}D_{3/2}$ $\left(7s^{2}6d^1\right)$, to the metastable ${}^{4}F_{3/2}$ $\left(7s^{1}6d^2\right)$ state via laser resonant excitation of the intermediate ${}^{4}F_{3/2}$ $\left(7s^{1}6d^{1}7p^1\right)$ state prior to electronic state chromatography. This model predicts a 93% pumping efficiency that suffices to enable efficient laser resonance chromatography of this ion. We then performed accurate relativistic multireference configuration-interaction calculations to model the interaction of ${\mathrm{Rf}}^{+}$ with He in the ground ${}^{2}D_{3/2}$ $\left(7s^{2}6d^1\right)$, low-lying ${}^{2}D_{5/2}$ $\left(7s^{2}6d^1\right)$, and metastable ${}^{4}F_{3/2}$ $\left(7s^{1}6d^2\right)$ states. These ion-atom interaction potentials were used to calculate the state-specific ion mobilities. For gas temperatures above 100 K and small applied electric fields, the reduced ion mobilities of the ground and metastable states differ significantly. In particular, at room temperature the difference between the reduced ion mobilities of these states is larger than 11% and as such sufficiently large to ensure LRC of this ion.

Double-core-hole (DCH) states in isolated-water and heavy-water molecules, resulting from the sequential absorption of two x-ray photons, have been investigated. A comparison of the subsequent Auger emission spectra from the two isotopes provides direct evidence of ultrafast nuclear motion during the 1.5-fs lifetime of these DCH states. Our numerical results align well with the experimental data, providing for various DCH states an in-depth study of the dynamics responsible of the observed isotope effect.

We propose to extend coherent laser cooling from narrowband to broadband transitions by using trains of ultrashort broadband pulses. We analytically study two possible methods to reduce the momentum spread of a distribution by several units of photon momentum in a single spontaneous emission lifetime. We report on numerical simulations of one-dimensional laser cooling of a two-level system with realistic parameters. With this scheme, coherent laser cooling can in particular be implemented in the case of fast species with a short lifetime such as positronium.

The nonlinear Thouless pumping is an exciting frontier of topological physics. While recent works have revealed the quantized motion of solitons in Thouless pumps, the interplay between the topology, nonlinearity, and disorder remains largely unexplored. Here, we investigate the nonlinear Thouless pumping of solitons in the presence of an impurity in the context of a Bose–Einstein condensate. Using both the Gross-Pitaevskii equation and Lagrange variational approach, we analyze the interaction between a moving soliton and an impurity. Without the pump, the soliton can pass through when the impurity strength is weak; however, it can get trapped when the impurity strength increases. In contrast, we find Thouless pump soliton in Thouless pumps can transit through also for strong impurity strength, and its motion is topologically quantized. Our result explicitly showcases the robustness of topological soliton pumping against microscopic imperfections, and opens a new perspective in the information processing with solitons.

We present a phase-compensation rule for achieving coherent control of frequency combs via high-order harmonic generation (HHG). The phase-compensation rule is illustrated by performing the generation of frequency combs via HHG from a hydrogen atom subjected to an intense laser pulse train. The numerical technique involves accurate and efficient solution of the time-dependent Schrödinger equation by means of the time-dependent generalized pseudospectral method. The results indicate that the frequency combs are fully optimized by employing the proposed phase-compensation rule, allowing direct control and generation of frequency combs with either the regular structure or the denser repetition combs. The performance of phase compensation is evaluated by analyzing the dynamical evolution of the harmonic spectra driven by several phase-compensation schemes. We believe that our findings serve as a guide for the experimental realization of phase coherent control of frequency combs via HHG at extremely high repetition frequencies.

For harnessing the full potential of quantum phenomena, light-matter interfaces and complexly connected quantum networks are required, relying on the joint quantum operation of different physical platforms. In this work, we analyze the quantum properties of multipartite quantum systems, consisting of an arbitrarily large collection of optical cavities with two-level atoms. In particular, we explore quantum walks in such systems and determine the resulting entanglement. Realistic imperfections are included in the model as optical losses and spontaneous decays of atoms. The topology of torus and the nonorientable Möbius strip serve as examples of complex networks that we consider, demonstrating the versatility of our approach and resulting in interesting quantum dynamics and interference effects for quantum simulation applications.

Continuous variable quantum teleportation provides a path to the long-distance transmission of quantum states. Photon-varying non-Gaussian operations have been shown to improve the fidelity of quantum teleportation when integrated into the protocol. However, for a given type of non-Gaussian operation, the achievable fidelity varies with the parameters associated with the operation. Previous work only focused on particular settings of the parameters, over which an optimization was missing. The potential of such operations is not fully uncovered. Given a fixed non-Gaussian operation, the achievable fidelity also varies with input states. An operation that increases the fidelity for teleporting one class of states might do the contrary for other classes of states. A performance metric, upon which an operation is optimized, suitable for different input states, is also missing. In this work, we build a framework for photon-varying non-Gaussian operations for multimode states, upon which we propose a performance metric suitable for arbitrary teleportation input states. We then apply the new metric to evaluate different types of non-Gaussian operations. Starting from simple multiphoton photon subtraction and photon addition, we find that increasing the number of ancillary photons involved in the operation does not guarantee performance improvement. We then investigate a generalization of the operations mentioned above, finding that operations that approximate a particular form provide the best improvement. The results provided here will be valuable for real-world implementations of quantum teleportation networks and applications that harness the non-Gaussianity of quantum states.

Nonadiabatic holonomic operations are based on nonadiabatic non-Abelian geometric phases, hence possessing the inherent geometric features for robustness against control errors. However, nonadiabatic holonomic operations are still sensitive to the systematic amplitude error induced by imperfect control of pulse timing or laser intensity. In this work, we present a scheme of nonadiabatic holonomic operations in order to mitigate the said systematic amplitude error. This is achieved by introducing a monitor qubit along with a conditional measurement on the monitor qubit that serves as an error correction device. We shall show how to filter out the undesired effect of the systematic amplitude error, thereby improving the performance of nonadiabatic holonomic operations.

This paper presents a computational analysis of a superconducting transmon qubit design, in which the superconductor-insulator-superconductor (SIS) Josephson junction is replaced by a coplanar, superconductor-constriction-superconductor (ScS) nanobridge junction. Within the scope of Ginzburg-Landau theory, we find that the nanobridge ScS transmon has an improved charge dispersion compared to the SIS transmon, with a tradeoff of smaller anharmonicity. These calculations provide a framework for estimating the superconductor material properties and junction dimensions compatible with gigahertz frequency ScS transmon operation.

Variational quantum algorithms have become the de facto model for current quantum computations. A prominent example of such algorithms—the quantum approximate optimization algorithm (QAOA)—was originally designed for combinatorial optimization tasks, but has been shown to be successful for a variety of other problems. However, for most of these problems the optimal circuit depth remains unknown. One such problem is unstructured search, which consists of finding a particular bit string or, equivalently, preparing a state of high overlap with a target state. To bound the optimal QAOA depth for such a problem we build on its known solution in a continuous time quantum walk (CTQW). We Trotterize a CTQW to recover a QAOA sequence, and employ recent advances on the theory of Trotter formulas to bound the query complexity (circuit depth) needed to prepare a state that approaches perfect overlap with the target state. The obtained complexity exceeds Grover's algorithm complexity $O\left(N^{\frac{1}{2}}\right)$, but remains smaller than $O\left(N^{\frac{1}{2}+c}\right)$ for any $c>0$, which shows quantum advantage of QAOA over classical solutions. We verify our analytical predictions by numerical simulations of up to 68 qubits.

The entropic uncertainty relation imposes a limit on the accuracy of measurement outcomes of two conjugate observables, which can be reduced in the presence of quantum memory. We theoretically study the dynamical behaviors of the quantum-memory-assisted entropic uncertainty relation for a bipartite system under local nonequilibrium dephasing environments and a common nonequilibrium dephasing environment. Particularly, we study the influence of the competition between nonequilibrium and non-Markovianity in controlling the entropic uncertainty bound (EUB) in weak- and strong-coupling regimes. We find that the nonequilibrium nature renders a promising protocol to efficiently harness the EUB in both regimes without requiring any operations on the main system. Further, we reveal that the EUB can be significantly reduced in a common bath case compared to the case of local baths. Moreover, we elucidate the primary factor responsible for governing the dynamical behaviors of EUB by examining the interplay between quantum discord and the minimal missing information about the particle to be measured. In addition, we investigate the state preparation and measurement choice (SPMC) condition in both environmental setups with weak- and strong-coupling regimes. Remarkably, we show that the SPMC condition does not hold in the common environment scenario.

A quantum algorithm for solving the advection equation by embedding the discrete time-marching operator into Hamiltonian simulations is presented. One-dimensional advection can be simulated directly since the central finite-difference operator for first-order derivatives is anti-Hermitian. Here this is extended to industrially relevant multidimensional flows with realistic boundary conditions and arbitrary finite-difference stencils. A single copy of the initial quantum state is required and the circuit depth grows linearly with the required number of time steps, the sparsity of the time-marching operator, and the inverse of the allowable error. State-vector simulations of a scalar transported in a two-dimensional channel flow and lid-driven cavity configuration are presented as a proof of concept of the proposed approach.

The exploration of dimensional crossover carries profound fundamental significance, serving as a crucial bridge in comprehending the remarkable disparities observed in transitional phenomena across the two distinct dimensions of a physical system. The prevalent strategy for manipulating the dimensionality involves meticulously controlling the external trapping geometry, thereby restricting the degrees of freedom of the kinetic energy from three-dimensional (3D) to lower-dimensional spaces, while maintaining the 3D nature of the interaction energy degrees of freedom. The aim of this work is to introduce an innovative scenario to achieve dimensional crossover, characterized by lower-dimensional nature of both the kinetic and the interaction energy degrees of freedom. To accomplish this objective, we delve deeply into the realm of a two-dimensional optically trapped Bose gas, focusing specifically on its finite-range interaction. Our emphasis lies in exploring the lattice-induced dimensional crossover from full 3D to one-dimensional (1D) in both kinetic and interaction terms. Utilizing the functional path integral method, we derive the equation of states of the model system, encompassing crucial quantities, such as the ground-state energy and quantum depletion. These equations enable us to analyze the combined effects of finite-range interaction and an optical lattice on quantum fluctuations of the BEC system. Notably, our analytical findings reconcile the Lee-Huang-Yang (LHY) correction to the ground-state energy in the 3D and Lieb-Liniger (LL) ones in the 1D limit, thereby providing fresh insights into the intriguing disparities between LHY and LL corrections.

Focusing on two-level atoms, we apply the positive $P$ representation to a full-wave mixed bosonic and fermionic system of Jaynes-Cummings type and identify an advantageous degree of freedom in the choice of the involved nonorthogonal fermionic basis states. On this basis, we propose a stochastic correction to the Maxwell-Bloch equations by relating them to a stochastic differential equation on a nonclassical phase space, which captures the full second quantization dynamics of the system. This approach explores the connection between semiclassical and field-quantized treatments of light-matter interaction and can potentially be used for the simulation of nonclassical light sources while retaining the main advantages of a semiclassical model.

Superoscillations occur when a globally band-limited function locally oscillates faster than its highest Fourier component. We generalize this effect to arbitrary quantum-mechanical operators as a weak value, where the preselected state is a superposition of eigenstates of the operator with eigenvalues bounded to a range, and the postselection state is a local position. Superbehavior of this operator occurs whenever the operator's weak value exceeds its eigenvalue bound. We give illustrative examples of this effect for total angular momentum and energy. In the latter case, we demonstrate a sequence of harmonic oscillator potentials where a finite-energy state converges everywhere on the real line, using only bounded superpositions of states whose asymptotic energy vanishes—“energy out of nothing.” This limit requires postselecting the particle in a region whose size diverges in the considered limit. We further show in this example that the superenergy is associated with superoscillations in time with a rate given by the local superenergy divided by the reduced Planck's constant. This example demonstrates the possibility of mimicking a high-energy state with coherent superpositions of nearly zero-energy states for as wide a spatial region as desired. We provide numerical evidence of these features to further bolster and elucidate our claims.

The cost of classical simulations of quantum many-body dynamics is often determined by the amount of entanglement in the system. In this paper, we study entanglement in stochastic quantum trajectory approaches that solve master equations describing open quantum system dynamics. First, we introduce and compare adaptive trajectory unravelings of master equations. Specifically, building on [Phys. Rev. Lett. 128, 243601 (2022)], we study several greedy algorithms that generate trajectories with a low average entanglement entropy. Second, we consider various conventional unravelings of a one-dimensional open random Brownian circuit and locate the transition points from area- to volume-law-entangled trajectories. Third, we compare various trajectory unravelings using matrix product states with a direct integration of the master equation using matrix product operators. We provide concrete examples of dynamics, for which the simulation cost of stochastic trajectories is exponentially smaller than the one of matrix product operators.