We study the local efficiency of symmetric informationally complete positive operator-valued measures (SIC-POVMs) from an information geometrical perspective. The local efficiency of a SIC-POVM is defined as the relative magnitude of the density of distinguishable states acquired via the measurement over the intrinsic density of quantum states. The two densities measure the local distinguishability of quantum states and its corresponding quantum resources, respectively. Strikingly, the efficiency is intrinsic in the sense of invariance under reparametrization. Specific to two-level systems, the efficiency is highly anisotropic and has a tetrahedral symmetry. Especially for a shell of the Bloch sphere, states in directions parallel (antiparallel) to the measurement basis have minimal (maximal) efficiency. Furthermore, the optimality of SIC-POVM versus other equal-length measurements is shown.

We demonstrate the existence of genuine tripartite non-Gaussian entanglement in a quantum gravitational system formed by a quantum harmonic oscillator coupled to a single frequency of a quantized gravitational wave. For this purpose, we introduce an entanglement witness, well suited for the graviton-matter interaction Hamiltonian analyzed here. We show that the witness is nonzero for the three-mode states generated by the Hamiltonian when the system starts in the ground state, thus proving the generation of genuine multipartite quantum entanglement at the most fundamental level of theoretical graviton-matter interactions.

We present calculations of the self-energy correction to the $E1$ transition amplitudes in hydrogenlike ions, performed to all orders in the nuclear binding strength parameter. Our results for the $1s\text{−}2p_{1/2}$ transition for the hydrogen isoelectronic sequence show that the perturbed-orbital part of the self-energy correction provides the dominant contribution, accounting for approximately 99% of the total correction for this transition. Detailed calculations were performed for $ns\text{−}{n}^{\prime }p$ and $np\text{−}{n}^{\prime }d$ transitions in H-like cesium. We conclude that the perturbed-orbital part also remains dominant for other $ns\text{−}{n}^{\prime }p$ transitions, whereas for the $np\text{−}{n}^{\prime }d$ matrix elements, this dominance no longer holds. Consequently, the self-energy corrections for the $np\text{−}{n}^{\prime }d$ one-electron matrix elements cannot be well reproduced by means of effective QED operators constructed for energy levels.

The quantum–classical hybrid variational quantum eigensolver (VQE) algorithm is among the most actively studied topics in atomic and molecular calculations on quantum computers, yet few studies address properties other than energies or account for relativistic effects. This work presents high-precision 18-qubit relativistic VQE simulations for calculating the permanent electric dipole moments (PDMs) of alkaline-earth metal monohydride molecules (BeH to RaH) on traditional computers, and 6- and 12-qubit PDM computations for SrH on IonQ quantum devices. To achieve high precision on current noisy intermediate-scale era quantum hardware, we apply various resource-reduction methods, including reinforcement learning-based and causal flow preserving ZX–calculus routines, along with error mitigation and postselection techniques. Our approach reduces the two-qubit gate count in our 12-qubit circuit by $99.71\%$, with only a $2.35\%$ trade-off in precision for PDM when evaluated classically within a suitably chosen active space. On the current generation IonQ Forte-I hardware, the error in PDM is $-1.17\%$ relative to classical calculations and only $1.21\%$ compared with the unoptimized circuit.

Heading errors of atomic magnetometers refer to the dependence of measurement results on the sensor orientation with respect to the external magnetic field. There are three main sources of such errors: the light shift effect, the linear nuclear-spin Zeeman effect, and the nonlinear Zeeman effect. In this work, we suppress the first two effects by using the Bell-Bloom optical pumping method and probe the atomic signals while the pumping beam is off, and focus on the heading error induced by nonlinear Zeeman effect while the sensor operates in the geomagnetic field range. We demonstrate several schemes to suppress this remaining heading error within 1 nT using a single magnetometer or a comagnetometer. In the magnetometer system, two schemes are developed to average out the horizontal atomic polarization in space or in time, respectively. In the comagnetometer system, we combine the simultaneously measured Larmor frequencies of two different alkali atoms to either suppress the heading error or extract the orientation of the pumping beam relative to the bias field.

We report on the observation of Feshbach resonances in ultracold ${}^6\mathrm{Li}\text{−}{}^{164}\mathrm{Dy}$ mixtures, where ${}^6\mathrm{Li}$ atoms are respectively prepared in their three lowest spin states and ${}^{164}\mathrm{Dy}$ atoms are prepared in their lowest-energy state. We observe 21 interspecies scattering resonances over a magnetic-field range from 0 to 702 G using atom loss spectroscopy, three of which exhibit relatively broad widths. These broad resonances provide precise control over the interspecies interaction strength, enabling the study of strongly interacting effects in ${}^6\mathrm{Li}\text{−}{}^{164}\mathrm{Dy}$ mixtures. Additionally, we observe a well-isolated interspecies resonance at 700.1 G, offering a unique platform to explore novel impurity physics, where heavy dipolar ${}^{164}\mathrm{Dy}$ atoms are immersed in a strongly interacting Fermi superfluid of ${}^6\mathrm{Li}$ atoms.

In this work we establish a general scattering theory for two-dimensional quantum systems with periodically modulated interactions. We show that scattering quantities, such as the scattering cross section, effective scattering length, and effective-range parameter, can be defined in a manner analogous to conventional scattering theory, even in the presence of modulation. Unlike systems with static interactions, the incident wave in our setup can be scattered inelastically into different Floquet channels, absorbing or emitting quanta from the modulation field. A family of Floquet resonances is identified, at which universal recurrence relations are derived for the Floquet scattering amplitudes. These relations are equivalent to a discrete Schrödinger equation describing a pointlike source trapped by an effective potential. The resonance mechanism is explained as the coincidence of the point source's energy with the bound states of the effective potential. We discuss the scattering properties both at and near the resonances, finding that the resonance width and the ratio of the inelastic to elastic cross sections can be effectively controlled through the modulation of the interaction. Our results provide a pathway to precisely manipulate many-body interactions in periodically driven quantum systems.

We theoretically propose a cavity magnomechanical system with a levitated yttrium iron garnet sphere to study the entanglement characteristics between subsystems. The system consists of microwave photon, magnon, and mechanical phonon modes, where the photon and magnon modes can be driven by external microwave sources. We find that with the appropriate choice of the suspension position, the effective couplings between the cavity field and the magnon and phonon modes can be established, thus inducing the generation of bipartite entanglement and genuine tripartite entanglement in the system. Further, we also analyze in detail the influences of the radius of the sphere, the detunings of the cavity and magnon modes, the driving power, the frequency of the phonon mode, and the environment temperature on the characteristics of the quantum entanglement. The results obtained here indicate that the bipartite and tripartite entanglements can be manipulated by changing the radius and the oscillation frequency of the sphere in addition to changing the parameters of the cavity field, and therefore, the levitated magnomechanical system has a potential application in the study of macroscopic quantum phenomena.

In optimal quantum-mechanical evolutions, motion can occur along nonpredetermined paths of shortest length in an optimal time. Alternatively, optimal evolutions can happen along predefined paths with no waste of energy resources and $100\%$ speed efficiency. Unfortunately, realistic physical scenarios typically result in less-than-ideal evolutions. In this paper we study different families of suboptimal qubit Hamiltonians, both stationary and time varying, for which the so-called geodesic efficiency and the speed efficiency of the corresponding quantum evolutions are less than one. Furthermore, after proposing an alternative hybrid efficiency measure constructed out of the two previously mentioned efficiency quantifiers, we provide illustrative examples where the average departures from time optimality and $100\%$ speed efficiency are globally captured over a limited time period. In particular, due to this hybrid measure, quantum evolutions are partitioned in four categories: geodesic unwasteful, nongeodesic unwasteful, geodesic wasteful, and nongeodesic wasteful. Finally, we discuss Hamiltonians specified by magnetic-field configurations, both stationary and nonstationary, yielding optimal hybrid efficiency, that it, both time optimality and $100\%$ speed efficiency, over a finite time interval.

Neutron-scattering characterization of materials allows for the study of entanglement and microscopic structure but is inefficient to simulate classically for comparison to theoretical models and predictions. However, quantum processors, notably analog quantum simulators, have the potential to offer an unprecedented, efficient method of Hamiltonian simulation by evolving a state in real time to compute phase transitions, dynamical properties, and entanglement witnesses. To elucidate the response of materials to neutron scattering, we present a method for simulating the spin dynamics in the transverse field Ising chain on QuEra's Aquila processor by calculating the dynamic structure factor (DSF) and propose a method for error mitigation. We provide numerical simulations and experimental results for the performance of the procedure on the hardware, up to a chain of length $L=25$. Additionally, the DSF result is used to compute the quantum Fisher information (QFI) density, where we confirm bipartite entanglement in the system experimentally.

We investigate the dynamics of quantum information flow in one and two impurity qubits trapped in a double-well potential and interacting with a one-dimensional ultracold Bose-Bose mixture reservoir. For a single qubit immersed in a binary Bose mixture, we show that the system maintains coherence over finite timescales and exhibits non-Markovian dynamics, particularly in the upper branch of the environment. We explore the transition from Markovian to non-Markovian dephasing through the Ohmicity of the spectral density functions, which are significantly influenced by interspecies interactions. In the case of two spatially separated qubits coupled to the Bose-Bose mixture reservoir, we demonstrate that collective decoherence affects the system dynamics, leading to prolonged coherence survival in both branches of the mixture. The intricate evolution of the decoherence factors is reflected in the density spectral functions and their Ohmicity characteristics. We find that the decoherence functions and spectra oscillate with increasing distance between the qubits, modifying the information flow dynamics. Additionally, we conduct a thorough investigation of the entanglement dynamics between the two qubits induced by the binary Bose mixture reservoir in both branches, underscoring the critical role of interspecies interactions.

In this paper, we derive universal nonadiabatic passages in a general $M+N$-dimensional discrete system, where $M$ and $N$ denote the degrees of freedom for the assistant and working subspaces, respectively, that could be separated by rotation or energy and coupled through driving. A systematic method is provided to construct parametric ancillary bases by the von Neumann equation with the time-dependent system Hamiltonian. The resulting universal passages set up connections between arbitrary initial and target states. In applications, a transitionless dynamics can be formulated to entangle distant qubits, as a crucial prerequisite for practical quantum networks. Using tunable longitudinal interaction between distant qubits and driving frequency, the superconducting qubits can be prepared from the ground state to the single-excitation Bell state with a fidelity as high as $\mathcal{F}=0.997$ and be further converted to the double-excitation Bell state with $\mathcal{F}=0.982$. Moreover, our protocol is extended to generate the Greenberger-Horne-Zeilinger state for an $N$-qubit system with $N$ steps. Our work develops a full-fledged theory for nonadiabatic state engineering, which is flexible in target selection and robust against both external noises and systematic errors in quantum information processing.

The variational quantum simulation technique on gate-based quantum processors can be made more efficient if symmetries of the Hamiltonian system of interest are factored into the design construction of the underlying parameterized quantum circuits (PQC) needed by the technique. In the first part of this work, we present an easy and practical approach that can be used to construct symmetric quantum circuits. The method we present can directly be adapted for any symmetry, though in the present work we focus explicitly on the examples of ${\mathbb{Z}}_2$, ${\mathbb{Z}}_3$, and particle number conservation. We show how to construct symmetric gates that respects the aforementioned symmetries. In the second part, we focus on numerical analysis of parameterized particle-conserving gates. In variational quantum simulation, for example, the variational quantum eigensolver (VQE), the symmetric gates of the PQC are parameterized. Since there are often many possible inequivalent parametrizations, it is often not immediately obvious which gate parametrization would be most efficient in general for a wide range of Hamiltonians that respect the symmetry. We streamline our analysis to particle-conserving circuits and Hamiltonians. We numerically analyze the efficiency of different particle-conserving gate parametrizations by studying two examples, namely, a physics problem of learning the ground state of Heisenberg spin chains, and an artificial problem of learning the random arbitary states sampled within the particle number conserved subspace.

In a previous paper we showed that an inhomogeneous body in vacuum will experience a spontaneous force if it is not in thermal equilibrium with its environment. This is due to the asymmetric asymptotic radiation pattern such an object emits. We demonstrated this self-propulsive force by considering an expansion in powers of the electric susceptibility: A torque arises in first order, but only if the material constituting the body is nonreciprocal. No force arises in first order. A force does occur for bodies made of ordinary (reciprocal) materials in second order. Here we extend these considerations to the torque. As one would expect, a spontaneous torque will also appear on an inhomogeneous chiral object if it is out of thermal equilibrium with its environment. Once a chiral body starts to rotate, it will experience a small quantum frictional torque, but much more important, unless a mechanism is provided to maintain the nonequilibrium state, is thermalization: The body will rapidly reach thermal equilibrium with the vacuum, and the angular acceleration will essentially become zero. For a small, or even a large, inhomogeneous chiral body, a terminal angular velocity will result, which seems to be in the realm of observability.

High-order harmonic generation in the presence of a strong co-propagating THz pulse (up to 7.5 MV/cm) is studied experimentally and compared to numerical simulations based on strong-field approximation. The time-resolved study of the high harmonic spectra together with the temporal profile of the THz electric field suggest a transient generation of even high harmonics along with odd-harmonic intensity suppression. The observed even- and odd-harmonic intensity modulation is explained by the coherent control of the subcycle interference between the adjacent attosecond bursts. A considerable cutoff extension of up to $I_p+9U_p$ is obtained numerically under application of the THz field with the field strength up to 120 MV/cm due to the additional electron drift arising from the electron motion in the synthesized field. The obtained results represent an experimental step towards ultrafast manipulation of the high-order harmonic yield and cutoff by the optical waveform synthesis with long-wavelength fields, which may considerably extend the EUV spectrum keeping high generation efficiency inherent to near-IR drivers.

Spin masers with optical detection and artificial feedback are widely used in fundamental and practical applications. However, a full picture of the maser dynamics is still absent. By solving the feedback-driven Bloch equations, we simulated the dynamics of an ideal spin maser in a broad parameter space. Rich nonlinear dynamics including high-order-harmonic generation, nonperiodic spin oscillations, and frequency comb were revealed when the artificial feedback interaction exceeds the normal spin-field interaction. We also propose a pulse feedback spin maser protocol, which constructs an ultralow field magnetic frequency comb and could be useful in precision atomic magnetometers in searching for spin-dependent exotic interactions.

We discuss impurities in a one-dimensional Bose gas with arbitrary boson-boson and boson-impurity interactions. To fully account for quantum effects, we employ numerical simulations based on the density-matrix renormalization group (DMRG) and—in the regime of strong boson-boson interactions—the mapping to weakly interacting fermions. A mean-field description of the Bose polaron based on coupled Gross–Pitaevskii Schrödinger equations predicts the existence of a self-localized polaron. We here show that such a solution does not exist and is an artifact of the underlying decoupling approximation. To this end we consider a mobile impurity in a box potential. Our work demonstrates that correlations between the impurity position and the bosons are important even in the limit where mean-field approaches are expected to work well. Furthermore, we derive analytical approximations for the energy of a single polaron formed by a heavy impurity for arbitrary interaction strengths and large but finite boson-boson couplings which accurately reproduce DMRG results. This demonstrates that the polaron problem of a heavy impurity in a one-dimensional Bose gas can be accurately approximated by a proper mean-field description plus a linearized treatment of quantum fluctuations for arbitrary boson-boson and impurity-boson couplings. Finally, we determine the polaron-polaron interaction potential $V\left(r\right)$ in the Born-Oppenheimer approximation for small and intermediate distances $r$, which in the Tonks gas limit is oscillatory due to Friedel oscillations in the Bose gas.

We propose a technique that makes it possible to transform the intensity of a quasimonochromatic single-photon wave packet, emitted by a radioactive Mössbauer γ-ray source, into a sequence of short bursts with an arbitrary number of bursts, including a single burst. In addition, the technique allows one to individually and independently control, on demand, the moments of the burst appearance, as well as the peak intensity, duration, and shape of each burst in the sequence. The technique is based on the transmission of Mössbauer (recoilless) photons through a resonantly absorbing medium, which is rapidly displaced at some moments of time relative to the source (or vice versa) along the photon propagation direction at a distance less than the photon wavelength, and returned to its original position. The burst duration can be comparable to the duration of the single-photon pulses that are produced by synchrotrons but have the controlled spectral-temporal characteristics. We show that the proposed technique can be implemented on the basis of currently available equipment with the use of 14.4-keV recoilless photons, emitted by a ${}^{57}\mathrm{Co}$ source and a ${}^{57}\mathrm{Fe}$ absorber, which opens up prospects for its applications in Mössbauer spectroscopy and x-ray quantum optics.

We introduce a hierarchy of tests satisfied by any probability distribution $P$ that represents the quantum correlations generated in prepare-and-measure (P&M) quantum multichain-shaped networks, assuming only the inner-product information of prepared states. The P&M quantum multichain-shaped networks involve multiple measurement parties, with each measurement party potentially having multiple sequential receivers. We adapt the original Navascués, Pironio, and Acín (NPA) hierarchy by incorporating a finite number of linear and positive semi-definite constraints to characterize the quantum correlations in P&M quantum multichain-shaped networks. These constraints in each hierarchy are derived from sequential measurements and the inner-product matrix of prepared states. The adapted NPA hierarchy is further applied to tackle some quantum information tasks, including sequential quantum random access codes (QRACs) and semi-device-independent randomness certification. First, we derive the optimal trade-off between the two sequential receivers in the $2\to 1$ sequential QRACs, and investigate randomness certification in its double violation region. Second, considering the presence of an eavesdropper in actual communication, we show how much local and global randomness can be certified using the optimal trade-off of $2\to 1$ sequential QRACs. We also quantify the amount of local and global randomness that can be certified from the complete set of probabilities generated by the two sequential receivers. Our conclusion is that utilizing the full set of probabilities certifies more randomness than relying solely on the optimal trade-off relationship.