An ab initio method based on a complex-scaling approach and aimed at a rigorous QED description of autoionizing states is worked out. The autoionizing-state binding energies are treated nonperturbatively in $\alpha Z$ and include all the many-electron QED contributions up to the second order. The higher-order electron correlation, nuclear recoil, and nuclear polarization effects are taken into account as well. The developed formalism is demonstrated on the $LL$ resonances in helium-like argon and uranium. The most accurate theoretical predictions for the binding energies are obtained.

Discrete-time quantum walks (DTQWs) provide a convenient platform for a realization of many topological phases in noninteracting systems. They often offer more possibilities than systems with a static Hamiltonian. Nevertheless, researchers are still looking for DTQW symmetries protecting topological phases and for definitions of appropriate topological invariants. Although the majority of DTQW studies on this topic focus on the so-called split-step quantum walk, two distinct topological phases can be observed in more basic models. Here we infer topological properties of the basic DTQWs directly from the mapping of the Brillouin zone to the Bloch Hamiltonian. We show that for translation-symmetric systems they can be characterized by a homotopy relative to special points. We also propose a topological invariant corresponding to this concept. This invariant indicates the number of edge states at the interface between two distinct phases.

In a scenario where data-storage qubits are kept in isolation as far as possible, with minimal measurements and controls, noise mitigation can still be done using additional noise probes, with corrections applied only when needed. Motivated by the case of solid-state qubits, we consider dephasing noise arising from a two-state fluctuator, described by random telegraph process, and a noise probe which is also a qubit, a so-called spectator qubit (SQ). We construct the theoretical model assuming projective measurements on the SQ, and derive the performance of different measurement and control strategies in the regime where the noise mitigation works well. We start with the Greedy algorithm; that is, the strategy that always maximizes the data qubit coherence in the immediate future. We show numerically that this algorithm works very well, and find that its adaptive strategy can be well approximated by a simpler algorithm with just a few parameters. Based on this, and an analytical construction using Bayesian maps, we design a one-parameter ($\mathrm{\Theta }$) family of algorithms. In the asymptotic regime of high noise sensitivity of the SQ, we show analytically that this $\mathrm{\Theta }$ family of algorithms reduces the data qubit decoherence rate by a divisor scaling as the square of this sensitivity. Setting $\mathrm{\Theta }$ equal to its optimal value, ${\mathrm{\Theta }}^{★}$, yields the Map-based Optimized Adaptive Algorithm for Asymptotic Regime (MOAAAR). We show, analytically and numerically, that MOAAAR outperforms the Greedy algorithm, especially in the regime of high noise sensitivity of SQ.

We theoretically propose an optical polarization-state engineering based on a cavity optomagnonic platform in the full quantum regime. Here, the optomagnonic interaction can be realized in an yttrium iron garnet (YIG) sphere, which supports both two traveling-photon modes with orthogonal linear polarizations [i.e., horizontally ($H$-) and vertically ($V$-) polarized photon modes] and a Kittel magnon mode. In our scheme, the magnons mediate the photons during polarization conversions, and break the time-reversal symmetry in mutual interconversions. Through a triple resonance between the magnon mode, $H$-polarized, and $V$-polarized photon modes within the YIG optomagnonic cavity, we demonstrate an all-optical scheme to manipulate the optical polarization behaviors by adjusting an external driving laser, finding that the polarization states of the output light can be well mapped to the whole Poincaré sphere and a large polarization rotation of the output light with photon antibunching and superbunching can be achieved easily for a range of realistic parameters. We find the strong discrepancy in the polarization response between the exact numerical calculation using the full quantum master equation and the semiclassical approximation. In addition, we reveal the magnon-induced broken time-reversal symmetry, which connects the quantum cavity optomagnonics to the classical magneto-optical effect. Our obtained results have potential application in quantum polarization-state engineering, and also offer a further understanding for cavity optomagnonics at the quantum level.

Exploiting the tension between the two dynamics of quantum theory (QT) in the Wigner's Friend thought experiment, we point out that the standard QT leads to inconsistency in observed probabilities of measurement outcomes between two superobservers: Wigner and his Student. To avoid such inconsistent predictions of QT, we hypothesize two distinct perspectives. The first one is “absoluteness of measurement (AoM),” that is, any measurement process is an absolute event irrespective of other observers and yields a single outcome. The other is “nonabsoluteness of measurement (NoM)” as the negation of AoM. We introduce an operational approach, first with one friend and then with two spatially separated friends, to test the validity of these two perceptions in quantum theory without assuming the details of the experiment. First, we show that the set of probabilities obtainable for NoM is strictly larger than the set obtainable for AoM. We provide the simplest scenario so far, involving a single quantum preparation and one unitary operation by a superobserver that can demonstrate correlations incompatible with AoM. Remarkably, in the scenario with spatially separated observers, we present a strict hierarchy among the sets of probabilities observed in the following three theories: classical or local realist, quantum theory with AoM, and quantum theory with NoM.

Quantum sensors are expected to be a prominent use-case of quantum technologies, but, in practice, noise easily degrades their performance. Quantum sensors can for instance be afflicted with erasure errors. Here, we consider using quantum probe states with a structure that corresponds to classical $[n,k,d]$ binary block codes of minimum distance $d\ge t+1$. We obtain bounds on the ultimate precision that these probe states can give for estimating the unknown magnitude of a classical field after at most $t$ qubits of the quantum probe state are erased. We show that the quantum Fisher information is proportional to the variances of the weight distributions of the corresponding $2^t$ shortened codes. If the shortened codes of a fixed code with $d\ge t+1$ have a nontrivial weight distribution, then the probe states obtained by concatenating this code with repetition codes of increasing length enable asymptotically optimal field sensing that passively tolerates up to $t$ erasure errors.

High-resolution photoelectron spectra of xenon (Xe) were measured in the $4p$ photoionization region. The anisotropy parameters β were obtained for several photoelectron peaks with angle-resolved photoelectron measurements. The various complicated spectral features were observed due to the perturbed $4p^{-1}$ spin-orbit doublet and adjacent satellite structures. An analysis based on a relativistic multiconfiguration calculation has been carried out. The key role of the strong interactions between the $4p^{-1}$ core hole and the $4d^{-2}nl$ configurations has been investigated in detail. Experimental peak energies and intensities are in good agreement with theory. We find that the many of the complicated features are $4d^{-2}nl$ satellite lines, and their intensities are determined mostly by the amount of $4p^{-1}$ configurations that are mixed in. The experimental β values agree quite well with theory, indicating that the photoelectron angular distributions are also strongly influenced by the $4p$ direct-ionization contributions.

Electrostriction, the deformation of dielectric materials under the influence of an electric field, is of continuous interest in optics. The classic experiment by Hakim and Higham [Proc. Phys. Soc. 80, 190 (1962)] for a stationary field supports a different formula of the electrostrictive force density than the recent experiment by Astrath et al. [Light Sci. Appl. 11, 103 (2022)] for an optical field. In this work, we study the origin of this difference by developing a time-dependent covariant theory of optical force densities in photonic materials. When a light pulse propagates in a bulk dielectric, the field-induced force density consists of two parts: (i) The optical wave momentum force density ${\mathbf{f}}_{\mathrm{owm}}$ carries the wave momentum of light and drives forward a mass density wave of the covariant coupled field-material state of light. (ii) The optostrictive force density ${\mathbf{f}}_{\mathrm{ost}}$ arises from the atomic density dependence of the electric and magnetic field energy densities. It represents an optical Lorentz-force-law-based generalization of the electro- and magnetostrictive force densities well known for static electromagnetic fields and derived from the principle of virtual work. Since the work done by ${\mathbf{f}}_{\mathrm{ost}}$ is not equal to the change of the field energy density during the contraction of the material, we have to describe this difference with optostriction-related dissipation terms to fulfill the energy conservation. The detailed physical model of the dissipation is left for further work. The optostrictive force density can be understood in terms of field-induced pair interactions inside the material. Because of the related action and reaction effects, this force density cannot contribute to the net momentum transfer of the optical field. The theory is used to simulate the propagation of a Gaussian light pulse through a dielectric material. We calculate the electric and magnetic fields of the Gaussian light pulse from Maxwell's equations and simultaneously solve Newton's equation of motion of atoms to find how the velocity and displacement fields of atoms develop as a function of time under the influence of the field-induced force density.

The spin-noise mechanisms in a spin-1 system are theoretically and experimentally investigated in detail. Eight different independent spin degrees of freedom are isolated, leading to spin-noise signals at the Larmor frequency or its second harmonic in the presence of a magnetic field. The signatures of these different modes of the detuned probe light beam polarization fluctuations, observed either as Faraday rotation noise or as ellipticity noise, are shown to depend dramatically on the probed transition. In particular, depending on the Zeeman structure of the upper level of the transitions, the different noise modes can be visible or not, and their dependence on the probe polarization can be completely modified. Those predictions are successfully compared with measurements of spin-noise signals using three different transitions of an ensemble of metastable ${}^4\mathrm{He}$ at room temperature.

Spin squeezing is one of the most attractive methods for realizing high-precision metrology. In this paper we propose a protocol for generating spin squeezing in an atomic ensemble via a fully quantum degenerate parametric amplifier. We discuss the properties of generating spin squeezing with and without driving the pump cavity. Numerical simulation results show that the generated spin squeezing strength is sizable and is comparable to that obtained using a two-axis twisting model. Moreover, we demonstrate that the protocol is experimentally feasible by introducing the corresponding experimental parameters. Therefore, the proposed protocol provides a promising approach to realize spin squeezing in photon-spin coupling systems.

Quantum battery (QB) is the miniature energy storage and release device and plays a crucial role in future quantum technology. Here an implementation scheme of a QB is proposed on a superconducting circuit, which is composed by $N$ coupled transmon qubits and a one-dimensional transmission line resonator. We derive the Hamiltonian of the QB system and investigate its charging performance by considering three decay channels. We find that the presence of the decay channels suppresses the high oscillation of the energy storage process, thereby realizing a stable and powerful QB. In particular, compared with the resonator decay and the qubit relaxation, the qubit dephasing shows a counterintuitive advantage in our QB. We show that the nearest-neighbor interaction always has a positive impact on the stable energy and the coupling only significantly influences the maximum charging power in the fully nondegenerate ground-state region. We also demonstrate the feasibility of our approach by evaluating the QB performance under experimental parameters.

A general technique for experimentally characterizing the effect of qubit-preserving impairments on the Clauser-Horne-Shimony-Holt parameter is introduced. This technique is independent of the underlying qubit encoding and is theoretically demonstrated for specific example impairments in polarization-encoded quantum-optical qubits. Included in this analysis is how spectrotemporal impairments can be incorporated into this technique.

Entanglement-breaking channels are quantum channels transforming entangled states to separable states. Despite a detailed discussion of their operational structure, to be found in the literature, studies on dynamical characteristics of this type of maps are yet limited. We consider one of the basic questions: for Lindblad-type dynamics, when does a given channel break entanglement? We discuss the finite-dimensional case where the quantification of entanglement via entanglement witnesses is utilized. For the general setup, we use the method of quantum speed limit to derive lower bounds on entanglement breaking times in terms of an input state, the dynamical map, and the witness operator. Then, with a particular choice of the input state and the entanglement witness, the bounds for the breaking time are turned to solely reflect the characteristics of the dynamics.

Recent progress in quantum optics has led to setups that are able to prepare high-dimensional quantum states for quantum information processing tasks. As such, it is of importance to benchmark the states generated by these setups in terms of their quantum mechanical properties, such as their Schmidt numbers, i.e., the number of entangled degrees of freedom. In this paper, we develop an iterative algorithm that finds Schmidt number witnesses tailored to the measurements available in specific experimental setups. We then apply the algorithm to find a witness that requires the measurement of a number of density matrix elements that scales linearly with the local dimension of the system. As a concrete example, we apply our construction method to an implementation with photonic temporal modes.

Sampling from the stationary distribution is one of the fundamental tasks of Markov-chain-based algorithms and has important applications in machine learning, combinatorial optimization, and network science. For the quantum case, quantum sampling from Markov chains corresponds to preparing quantum states with amplitudes arbitrarily close to the square root of a stationary distribution instead of classical sampling from a stationary distribution. A different quantum sampling algorithm for all reversible Markov chains is constructed by discrete-time quantum walks. We build a quantum sampling algorithm that not only accelerates nonregular graphs, but also keeps the speed up of existing quantum algorithms for regular graphs. In nonregular graphs, the invocation of the quantum fast-forwarding algorithm accelerates previous state-of-the-art quantum sampling algorithms for both discrete-time and continuous-time cases, especially on sparse graphs. Compared to existing algorithms, we decrease the runtime by a factor of $logn$, where $n$ is the number of graph vertices. In regular graphs, our results match other quantum algorithms and the reliance on the gap of Markov chains achieves quadratic speedup compared with classical cases. For both cases, we reduce the number of ancilla qubits required compared with the existing results. In some widely used graphs and a series of sparse graphs where stationary distributions are difficult to reach quickly, our algorithm achieves complete quadratic acceleration (without log factor) over the classical case without any limit. In addition, we construct a different reflection around the stationary state with fewer ancilla qubits and believe it may have an independent application.

We propose and demonstrate a method for Doppler cooling trapped-ion crystals where the distribution of micromotion amplitudes may be large and uneven. The technique uses pulses of Doppler cooling light synchronized with the trap rf that selectively target ions when their velocity is near a node, leading to more uniform cooling across a crystal by a single tone of cooling light. We lay out a theoretical framework that describes where this technique is practical and provides a simple experimental demonstration.

Resolved sideband cooling provides a crucial step in subrecoil cooling the trapped atoms toward their motional ground state, which is essential in atom-based quantum technologies. Here we present an enhanced dark-state sideband cooling in trapped atoms utilizing photon-mediated dipole-dipole interactions among them. By placing the atoms at the magic interparticle distances, we manifest an outperformed cooling behavior in the target atom, which surpasses the limit that a single atom permits. We further investigate various atomic configurations in a multiatom setup with a laser detuning and different light polarization angles, where multiple magic spacings can be identified and a moderate improvement in cooling performance is predicted as the number of atoms increases. Our results provide insights to subrecoil cooling of atoms with collective and light-induced long-range dipole-dipole interactions and pave the way toward implementing genuine quantum operations in multiple quantum registers.