We study the quantum dynamics of a noninteracting spin ensemble under the effect of a reservoir by applying the framework of the non-Hermitian Hamiltonian operators. Theoretically, the two-level model describes the quantum spin system and the Bloch vector to establish the dynamical evolution. Experimentally, phosphorous $\left({}^{31}\mathrm{P}\right)$ nuclei with spin $I=1/2$ are used to represent the two-level system and the magnetization evolution is measured and used to compare with the theoretical prediction. At room temperature, the composite dynamics of the radio-frequency pulse plus field inhomogeneities (or unknown longitudinal fluctuations) along the $z$ axis transform the initial quantum state and drives it into a mixed state at the end of the dynamics. The experimental setup shows a higher accuracy when compared with the theoretical prediction (>98%), ensuring the relevance and effectiveness of the non-Hermitian theory at a high-temperature regime.

The act of measuring a system has profound consequences of dynamical and thermodynamic nature. In particular, the degree of irreversibility ensuing from a nonequilibrium process is strongly affected by measurements aimed at acquiring information on the state of a system of interest: the conditional and unconditional entropy production, which quantify the degree of irreversibility of the open system's dynamics, are related to each other by clearly interpreted informational quantities. Building on a recently proposed collisional model framework [G. T. Landi, M. Paternostro, and A. Belenchia, PRX Quantum 3, 010303 (2022)], we investigate the case of continuous-variable information carriers prepared in Gaussian states and undergoing Gaussian processes. We build up a toolbox that fully characterizes the thermodynamics of continuously measured nonequilibrium Gaussian systems and processes, illustrating how the instruments hereby introduced provide key insight into recent experiments on mesoscopic quantum systems [M. Rossi, L. Mancino, G. T. Landi, M. Paternostro, A. Schliesser, and A. Belenchia, Phys. Rev. Lett. 125, 080601 (2020)].

We explore a small quantum refrigerator in which the working substance is made of paradigmatic nearest-neighbor quantum spin models, the $XYZ$ and the $XY$ model with Dzyaloshinskii-Moriya interactions, consisting of two and three spins, each of which is in contact with a bosonic bath. We identify a specific range of interaction strengths which can be tuned appropriately to ensure a cooling of the selected spin in terms of its local temperature in the weak-coupling limit. Moreover, we report that in this domain, when one of the interaction strengths is disordered, the performance of the thermal machine operating as a refrigerator remains almost unchanged instead of degrading, thereby establishing the flexibility of this device. However, to obtain a significant amount of cooling via ordered as well as disordered spin models, we observe that one has to go beyond the weak-coupling limit and compute the figures of merit by using global master equations.

Many-body localization at nonzero temperature is a quantum-phase transition that is of very fundamental interest to both many-body quantum physics and statistical mechanics. Non-Hermitian systems possess rich and unique phenomena that never are found in their Hermitian counterparts, such as unidirectional invisibility and enhanced sensitivity. In the present study, we explore many-body localization in periodically driven non-Hermitian lattices. The lattices are one-dimensional, and the drive is designed in a piecewise two-step form that consists of a uniform coherent tunneling and incommensurate onsite gain and loss. We find that the system can be in localized, delocalized, or mixed phases depending on the driving frequency and the phase shift of the complex potential. Two critical driving frequencies are identified; one corresponds to the largest phase shift of the complex potential that keeps all the quasienergy spectra real and the eigenstates extended, and the other one is established by the vanishing of all real spectra. We also find that a very weak complex potential can lead the system to localization, when the driving frequency is lower than the critical frequency. We show that in the high-frequency limit, the critical phase shift that separates the two regions with respectively a real and complex spectrum tends to a constant, and the constant can be determined by an effective non-Hermitian Hamiltonian.

Entanglement is a valuable quantum resource in quantum information processing. In classical communication over quantum channels, it is known to boost the communication rate drastically. To generalize such a boost to more general scenarios, we provide computable limits on the communication over optical multiple-access channels (MACs) for both the entanglement-assisted and unassisted communication. For the unassisted case, we generalize the coherent-state achievable rate region and outer bound known for the thermal-loss case [B. J. Yen and J. H. Shapiro, Phys. Rev. A 72, 062312 (2005)] to general bosonic Gaussian MACs. For the assisted case, we generalize the two-mode squeezed vacuum rate region and the outer bound for the thermal-loss case [H. Shi et al., npj Quantum Inf. 7, 74 (2021)] to general bosonic MACs. In terms of the total communication rate of all senders, we prove additivity for general MACs, generalizing the two-sender version in M.-H. Hsieh et al., IEEE Trans. Inf. Theory 54, 3078 (2008). Furthermore, for optical communication modeled as phase-insensitive bosonic Gaussian MACs, we prove that the optimal total rate is achieved by Gaussian entanglement and therefore can be efficiently evaluated. The computable limits confirm entanglement's boosts in optical multiple-access communications. Finally, we formulate an entanglement-assisted version of minimum entropy conjecture, which leads to the additivity of the capacity region of phase-insensitive bosonic Gaussian MACs if it is true.

The interaction with the environment is one of the main obstacles to be circumvented in practical implementations of quantum information tasks. The use of local unitaries, while not changing the initial entanglement present in a given state, can enormously change its dynamics through a noisy channel, and consequently its ability to be used as a resource. In this way, local unitaries provide an easy and accessible way to enhance quantum correlations in a variety of different experimental platforms. Given an initial entangled state and a certain noisy channel, what are the local unitaries providing the most robust dynamics? In this paper we solve this question considering two-qubit states, together with paradigmatic and relevant noisy channels, showing its consequences for teleportation protocols and identifying cases where the most robust states are not necessarily the ones imprinting the least information about themselves into the environment. We also derive a general multipartite law relating the interplay between the total correlations in the system and environment with their mutual information built up over the noisy dynamics. Finally, we employ the IBM Quantum Experience to provide a proof-of-principle experimental implementation of our results.

Duality transformations are very important in both classical and quantum physics. They allow one to relate two seemingly different formulations of the same physical realm through clever mathematical manipulations, and offer numerous advantages for the study of many-body physics. In this work, we suggest a method which introduces them to the world of quantum simulation too: a feasible scheme for implementing duality transformations as physical operations, mapping between dual quantum states showing the same observable physics, rather than just a mathematical trick. Demonstrating with Abelian lattice models, we show how duality transformations could be implemented in the laboratory as sequences of single- and two-body operations—unitaries and measurements.

Improving the simulation of quantum circuits on classical computers is important for understanding quantum advantage and increasing development speed. In this paper, we explore a way to express stabilizer states and further improve the speed of simulating stabilizer circuits with a current existing approach. First, we discover a unique and elegant canonical form for stabilizer states based on graph states to better represent stabilizer states and show how to efficiently simplify stabilizer states to canonical form. Second, we develop an improved algorithm for graph state stabilizer simulation and establish limitations on reducing the quadratic runtime of applying controlled Pauli $Z$ gates. We do so by creating a simpler formula for combining two Pauli-related stabilizer states into one. Third, to better understand the linear dependence of stabilizer states, we characterize all linearly dependent triplets, revealing symmetries in the inner products. Using our controlled Pauli $Z$ algorithm, we improve runtime for inner product computation from $O\left(n^3\right)$ to $O\left(n{d}^2\right)$, where $d$ is the maximum degree of the graph encountered during the calculation.

Nowadays, it is accepted that truly quantum correlations can exist even in the absence of entanglement. For the case of symmetric states, a physically trivial unitary transformation can alter a state from entangled to separable, and vice versa. We propose to certify the presence of quantumness via an average of a state's bipartite entanglement properties over all physically relevant modal decompositions. We investigate extremal states for such a measure: SU(2)-coherent states possess the least quantumness, whereas the opposite extreme is inhabited by states with maximally spread Majorana constellations.

Given quantum hardware that enables sampling from a family of natively implemented Hamiltonians, how well can one use that hardware to sample from a Hamiltonian outside that family? A common approach is to minor embed the desired Hamiltonian in a native Hamiltonian. In Phys. Rev. Research 2, 023020 (2020) it was shown that minor embedding can be detrimental for classical thermal sampling. Here we generalize these results by considering quantum thermal sampling in the transverse-field Ising model, i.e., sampling a Hamiltonian with nonzero off diagonal terms. In the quantum case, loosely speaking, it is even harder to preserve the correct distribution properties, since the local transverse fields affect the physical qubits in the embedding in a manner that cannot be lifted by setting an appropriate energy scale, as in the classical case. To study these systems numerically we introduce a modification to standard cluster update quantum Monte Carlo (QMC) techniques, which allows us to much more efficiently obtain thermal samples of an embedded Hamiltonian, enabling us to simulate systems of much larger sizes and larger transverse-field strengths than would otherwise be possible. Our numerics focus on models that can be implemented on current quantum devices using planar two-dimensional lattices, which exhibit a phase transition driven by the transverse-field strength. Our results include the following: (1) We give an estimate on the probability to sample the logical subspace directly as a function of transverse-field, temperature, and total system size, which agrees with QMC simulations. (2) We show that typically measured observables (diagonal energy and magnetization) are biased by the embedding process, in the regime of intermediate transverse-field strength, meaning that the extracted values are not the same as in the native model. (3) By considering individual embedding realizations akin to “realizations of disorder,” we provide numerical evidence suggesting that as the embedding size is increased, the critical point shifts to increasingly large values of the transverse field.

The recent rapid experimental advancement in the engineering of quantum many-body systems opens the avenue to controlled studies of fundamental physics problems via digital or analog quantum simulations. Here, we systematically analyze the capability of analog ion traps to explore relativistic meson spectra on current devices. We focus on the $E_8$ quantum field theory regime, which arises due to longitudinal perturbations at the critical point of the transverse-field Ising model. As we show through exact numerics, for sufficiently strong long-range suppression in experimentally accessible spin chain models, absorption spectroscopy allows for the identification of the low-lying meson excitations with a good degree of accuracy even for small system sizes. Our proposal thus opens a way for probing salient features of quantum many-body systems reminiscent of meson properties in high-energy physics.

A promising venue for hybrid quantum computation involves the strong coupling between impurity spins and superconducting circuits. This coupling can be controlled and enhanced by preparing superconducting resonators in nonclassical states, such as squeezed states. In this work, we theoretically study the effects of these states on the coherence properties of the spin. We develop an analytic approach based on the Schrieffer-Wolff transformation that allows us to quantitatively predict the dynamics of the spin, and we numerically confirm its validity. We find that squeezing can enhance the coupling between the resonator and the spin. However, at the same time, it amplifies the photon noise and enhances the spin decoherence. Our work demonstrates a major impediment in using squeezing to reach the strong-coupling limit.

The three-body fragmentation dynamics of propyne $\left({\mathrm{CH}}_3\mathrm{C}\mathrm{C}\mathrm{H}\right)$ to ${\mathrm{H}}^{+}$ $+$ $\mathrm{C}{\mathrm{H}}^{+}$ $+$ ${\mathrm{C}}_2{\mathrm{H}}_2{}^{+}$ and ${\mathrm{H}}^{+}$ $+$ ${\mathrm{CH}}_2{}^{+}$ $+$ ${\mathrm{C}}_2{\mathrm{H}}^{+}$ is investigated by 50-keV/u ${\mathrm{Ne}}^{8+}$ ion impact. Employing the reaction microscope, all three ionic fragments are detected in coincidence, and the momentum vector as well as kinetic energy of each fragment is obtained. By analyzing the momentum and kinetic energy correlation between different fragments, various fragmentation mechanisms are identified, and the relative ratio of each mechanism is determined. We find that the concerted fragmentation with CH and CC bonds breaking simultaneously and the sequential pathway with CH breakage prior to the CC breakage are the major contributions to both channels. In contrast, the sequential pathway with CC breakage prior to CH breakage was found to make a minor contribution only to the ${\mathrm{H}}^{+}$ $+$ $\mathrm{C}{\mathrm{H}}^{+}$ $+$ ${\mathrm{C}}_2{\mathrm{H}}_2{}^{+}$ channel. In addition, we compare the present results for ${\mathrm{CH}}_3\mathrm{C}\mathrm{C}\mathrm{H}$ with ${\mathrm{CH}}_2{\mathrm{CCH}}_2$ published in Ma et al. [C. Ma, S. Xu, D. Zhao, D. Guo, S. Yan, W. Feng, X. Zhu, and X. Ma, Phys. Rev. A 101, 052701 (2020)]. The location of the CH and CC bonds' breakage, either from distinct C atoms or from the same C atom, leads to similar dissociation mechanisms but different ionic fragments for the two isomers.

Radiative charge transfer (RCT) from cationic to neutral atoms is a fundamental and frequently occurring process in ion-neutral collisions and in van der Waals clusters. In contrast to collisions with only two collision partners, RCT in clusters is more involved as it may depend on the cluster size, the cluster stoichiometry, and the local arrangement of atoms in the cluster. Here, we present a systematic investigation of an RCT photon spectrum as a function of cluster size, stoichometry, and local atom arrangement. For this purpose, we utilize RCT in heterogeneous NeKr and NeXe clusters after Ne $2p$ photoionization. Our results confirm that Ne dimer ions form prior to RCT if enough Ne is available and we observe that different features dominate the photon spectrum depending on the cluster production parameters. Additionally, we find that the lifetime of the radiative decay is sensitive to the cluster stoichiometry, which we explain and interpret by theoretical calculations on the RCT decay width for different local geometric structures of the involved atoms. We conclude that RCT properties such as photon spectrum and lifetime in turn exhibit information on the mean size and stoichiometry of a cluster jet and the arrangement of the involved atoms.

Diamagnetism is the response of dynamical compositions of charged particles, electrons, and nuclei, to an incident magnetic field. In this paper, we study how the finite temperature and finite nuclear masses affect the diamagnetic susceptibilities of selected small atoms and molecules, as limiting cases of dilute gas. We use nonrelativistic path-integral Monte Carlo simulation (PIMC), where both electrons and nuclei are treated on equal footing at finite temperatures in sampling exact Coulomb pair density matrices. The PIMC estimator of diamagnetic susceptibility has been briefly introduced in Ceperley [D. M. Ceperley, Rev. Mod. Phys. 67, 279 (1995)], but here we present a comprehensive derivation, discussion of practical effects, and proof-of-concept results for selected few-body Coulomb systems. Our results are in perfect agreement with high-accuracy literature references, where available, but also demonstrate additional thermal effects of the diamagnetic response of low-mass systems.

Atomic properties of $n=3$ levels for ${\mathrm{W}}^{47+}-{\mathrm{W}}^{55+}$ ions ($Z=74$) are systematically calculated using two different and independent methods, namely, the second-order many-body perturbation theory and the multiconfiguration Dirac-Hartree-Fock method combined with the relativistic configuration interaction approach. Wavelengths and transition rates for electric- and magnetic-dipole transitions involving the $n=3$ levels of ${\mathrm{W}}^{47+}-{\mathrm{W}}^{55+}$ are calculated. In addition, we discuss in detail the importance of the valence and core-valence electron correlations, the Breit interaction, the higher-order frequency-dependent retardation correction, and the leading quantum electrodynamical corrections for transition wavelengths. Spectroscopic accuracy is achieved for the present calculated wavelengths, and most of them agree with experimental values within $0.05\%$. Our calculated wavelengths, combined with collisional radiative model simulations, are used to identify the yet unidentified 25 observed lines in the extremely complex spectrum between $27\AA$ and $34\AA$ measured by Lennartsson $etal.$ [Phys. Rev. A 87, 062505 (2013)]. We provide additional data for 472 strong electric-dipole transitions in the wavelength range of 17–50 Å, and 185 strong magnetic-dipole transitions between $36\AA$ and $4384\AA$, with a line intensity greater than $1\mathrm{photon}/\mathrm{s}$. These can provide benchmark data for future experiments and theoretical calculations.

The three-body fragmentation channel ${\mathrm{H}}^{+}+{\mathrm{H}}^{+}+{\mathrm{C}}_3{\mathrm{H}}_2{}^{+}$ of the allene trication produced by 30-keV/u ${\mathrm{He}}^{2+}$ ion impact is investigated using cold-target recoil-ion momentum spectroscopy. The two protons with different kinetic energies are found emitted sequentially from the parent trication. The presence of such a sequential deprotonation process and the absence of corresponding concerted deprotonation may result from a major molecular deformation process, e.g., ultrafast hydrogen migration, which occurs in the parent ion prior to fragmentation, and finally leads to the formation of two different protons.

We report measurements of the absolute energies of the hyperfine components of the $12s{}^2{S}_{1/2}$ and $13s{}^2{S}_{1/2}$ levels of atomic cesium, ${}^{133}\mathrm{Cs}$. Using the frequency difference between these components, we determine the hyperfine coupling constants for these states, and report these values with a relative uncertainty of $\sim 0.06$%. We also examine the hyperfine structure of the $11d{}^2{D}_J$ ($J=3/2,5/2$) states, and resolve the sign ambiguity of the hyperfine coupling constants from previous measurements of these states. We also derive new, high precision values for the state energies of the $12s{}^2{S}_{1/2}, 13s{}^2{S}_{1/2}$, and $11d{}^2{D}_J$ states of cesium.

The ionization energy of ${\mathrm{D}}_2$ has been determined experimentally from measurements involving two-photon Doppler-free vacuum-ultraviolet pulsed laser excitation and near-infrared continuous-wave laser excitation to yield $E_{\mathrm{I}}\left({\mathrm{D}}_2\right)=124745.393739\left(26\right) {\mathrm{cm}}^{-1}$. From this value, the dissociation energy of ${\mathrm{D}}_2$ is deduced to be $D_0\left({\mathrm{D}}_2\right)=36748.362282\left(26\right) {\mathrm{cm}}^{-1}$, representing a 25-fold improvement over previous values, and it was found to be in good agreement (at $1.6\sigma$) with recent ab initio calculations of the four-particle nonadiabatic relativistic energy and of quantum-electrodynamic corrections up to order $m{\alpha }^6$. This result constitutes a test of quantum electrodynamics in the molecular domain, while a perspective is opened to determine nuclear charge radii from molecules.

An accurate evaluation of the gravity acceleration from the timing of free fall of antihydrogen atoms in the Gravitational Behaviour of Anti-hydrogen at Rest (GBAR) experiment requires one to account for obstacles surrounding the antimatter source. These obstacles reduce the number of useful events but may improve accuracy since the edges of the shadows of obstacles on the detection chamber depend on gravity, bringing additional information about the value of $g$. We perform Monte Carlo simulations to obtain the dispersion and give a qualitative understanding of the results by analyzing the statistics of events close to an edge. We also study the effect of specular quantum reflections of antihydrogen on surfaces and show that they do not degrade the accuracy that much.