Entanglement is a fascinating feature of quantum mechanics and a key ingredient in most quantum information processing tasks. Yet the generation of entanglement is usually hampered by undesired dissipation owing to the inevitable coupling of a system with its environment. Here, we report an experiment on how to entangle two ${}^{13}\mathrm{C}$ nuclear spins via engineered dissipation in a nitrogen-vacancy system. We utilize the electron spin as an ancilla and combine unitary processes together with optical pumping of the ancilla to implement the engineered dissipation and deterministically produce an entangled state of the two nuclear spins, independent of their initial states. Our experiment demonstrates the power of engineered dissipation as a tool for generation of multiqubit entanglement in solid-state systems.

Cross sections and rate coefficients for vibrational excitation and de-excitation of the ${\mathrm{N}}_2\mathrm{O}$ molecule by a low-energy electron for transitions between the lowest vibrational levels are computed using a first-principles approach. The present theoretical approach employs the normal-mode approximation for the description of target vibrational states, the vibrational frame transformation to compute amplitudes of vibrational transitions, and the $R$-matrix method to compute ab initio electronic bound and continuum states. It was found that the nonadiabatic Renner-Teller effect, which couples partial waves of the incident electron with degenerate bending vibrations of ${\mathrm{N}}_2\mathrm{O}$, is responsible for the excitation of the bending mode. Theoretical results obtained agree reasonably well with available experimental data at low energies. Thermally averaged rate coefficients are computed for temperatures in the 10–10 000 K range.

A new kind of radiotherapy scheme based on the resonant Auger–interatomic Coulombic decay mechanism (RA ICD) was proposed in the work of Gokhberg et al. [Nature 505, 661 (2014)], which may effectively reduce the overall radiation dose in traditional x-ray radiotherapy. An electron produced in this scheme carries most of the energy of the primary x-ray photon. Through the present experiment using an energetic electron impact on a NeAr dimer, we demonstrate that three processes, namely, the interatomic Coulombic decay triggered by a Ne $2s$ electron ionization, ICD triggered by $\mathrm{A}{\mathrm{r}}^{+*}\left(3p^{4}nl\ge 5d\right)$, and charge transfer in the $\mathrm{A}{\mathrm{r}}^{2+}\mathrm{Ne}$ ion, can take place and enhance the yields of slow electrons and ions. Therefore, energetic secondary electrons make the x-ray RA ICD scheme more toxic in radiotherapy.

Molecules with identical nuclei exist in nature in the form of nuclear spin isomers. Most known are the spin isomers in molecules having overall rotational symmetry. There are molecules that do not have overall rotational symmetry but instead contain symmetrical groups of atoms that perform torsion tunneling (i.e., internal rotation) around their local symmetry axes. Quantum statistics gives rise in such molecules to a specific type of spin isomers based on the torsion tunneling of symmetrical groups of atoms. Recently, enrichment and conversion of nuclear spin isomers in methanol (${\mathrm{CH}}_3\mathrm{OH}$) that has torsion tunneling of the methyl group was experimentally investigated by Z.-D. Sun et al. [Nat. Commun. 6, 6877 (2015)]. In the present paper a theoretical model for the methanol spin isomers and their conversion is developed. The model is based on the quantum relaxation theory confirmed to be valid for molecules with overall rotational symmetry. The methanol spin isomer conversion induced by hyperfine spin-spin and spin-rotation interactions is considered. The most important level pair for the methanol isomer conversion is found to be the $(J^{\prime }=25,K^{\prime }=2,{\sigma }^{\prime }=1)–(J=24,K=4,p=1)$ pair in the ground torsion state $v_t=0$. The spin-rotation interaction in methanol is estimated to be equal to 1.6 kHz.

While the treatment of conical intersections in molecular dynamics generally requires nonadiabatic approaches, the Born-Oppenheimer adiabatic approximation is still adopted as a valid alternative in certain circumstances. In the context of Mead-Truhlar minimal coupling, this paper presents a closure of the nuclear Born-Oppenheimer equation leading to a regularized molecular dynamics scheme capturing geometric phase effects. Specifically, a semiclassical closure of the nuclear Ehrenfest dynamics is obtained through a convenient prescription for the nuclear Bohmian trajectories. The conical intersections are suitably regularized in the resulting nuclear particle motion and the associated Lorentz force involves a smoothened Berry curvature identifying a loop-dependent geometric phase. In turn, this geometric phase rapidly reaches the usual topological index as the loop expands away from the original singularity. This feature reproduces the phenomenology appearing in recent exact nonadiabatic studies, as shown explicitly in the Jahn-Teller problem for linear vibronic coupling. Likewise, a newly proposed regularization of the diagonal correction term is also shown to reproduce quite faithfully the energy surface presented in recent nonadiabatic studies.

Recent studies of quantum circuit models have theoretically shown that frequent measurements induce a transition in a quantum many-body system, which is characterized by a change in the scaling law of the entanglement entropy from a volume law to an area law. In order to propose a way to experimentally observe this measurement-induced transition, we present numerical analyses using matrix-product states on the quench dynamics of a dissipative Bose-Hubbard model with controllable two-body losses, which has been realized in recent experiments with ultracold atoms. We find that when the strength of dissipation increases, there occurs a measurement-induced transition from volume-law scaling to area-law scaling with a logarithmic correction in a region of relatively small dissipation. We also find that the strong dissipation leads to a revival of the volume-law scaling due to a continuous quantum Zeno effect. We show that dynamics starting with the area-law states exhibits strong suppression of particle transport stemming from ergodicity breaking, which can be used in experiments to distinguish them from the volume-law states.

For describing a coherent quantum two-level system driven by a linearly time-dependent energy separation of the diabatic states, the Landau-Zener model is routine to serve as a textbook model for its dynamics. Along this research line, a particularly intriguing question is whether the framework of Landau-Zener theory can be extended to an intrinsically nonequilibrium quantum system with coherent and dissipative dynamics occurring on an equal footing. In this work, we are motivated to investigate the Landau-Zener problem of polariton condensates in a periodic potential under nonresonant pumping by using the driven-dissipative Gross-Pitaevskii equations coupled to the rate equation. Within the two-mode approximation, a nonequilibrium Landau-Zener model, characterized by coherent and dissipative dynamics occurring on an equal footing, is derived. Fundamentally different from the previous Landau-Zener model, the total density of nonequilibrium Landau-Zener model, in general, is not the conserved quantity anymore due to the dissipative nature. In a surprise, the parameter regimes of the total density still being conserved can still be found. The motion of Hamiltonian of the nonequilibrium Landau-Zener problem in phase space is further discussed. The instability of the band structure can also be studied by the curvatures in phase space, and there may be two loops in the middle of the Brillouin zone. Detailed analysis of the nonequilibrium nature on the tunneling rate will open a new perspective toward understanding the Landau-Zener problem.

We study the ultimate bounds on the sensitivity of a Bloch-oscillation atom interferometer where the external force is estimated from the measurement of the on-site atomic density. For external forces such that the energy difference between lattice sites is smaller than the tunneling energy, the atomic wave function spreads over many lattice sites, increasing the separation between the occupied modes of the lattice and naturally enhancing the sensitivity of the interferometer. To investigate the applicability of this scheme we estimate the effect of uncontrolled fluctuations of the tunneling energy and the finite resolution of the atom detection. Our analysis shows that a horizontal lattice combined with a weak external force allow for high sensitivities. Therefore, this setup is a promising solution for compact devices or for measurements with high spatial resolution.

The virial expansion characterizes the high-temperature approach to the quantum-classical crossover in any quantum many-body system. Here, we calculate the virial coefficients up to the fifth order of Fermi gases in one, two, and three dimensions, with attractive contact interactions, as relevant for a variety of applications in atomic and nuclear physics. To that end, we discretize the imaginary-time direction and calculate the relevant canonical partition functions. In coarse discretizations, we obtain analytic results featuring relationships between the interaction-induced changes $\mathrm{\Delta }{b}_3, \mathrm{\Delta }{b}_4$, and $\mathrm{\Delta }{b}_5$ as functions of $\mathrm{\Delta }{b}_2$, the latter being exactly known in many cases by virtue of the Beth-Uhlenbeck formula. Using automated-algebra methods, we push our calculations to progressively finer discretizations and extrapolate to the continuous-time limit. We find excellent agreement for $\mathrm{\Delta }{b}_3$ with previous calculations in all dimensions and we formulate predictions for $\mathrm{\Delta }{b}_4$ and $\mathrm{\Delta }{b}_5$ in one and two dimensions. We also provide, for a range of couplings, the subspace contributions $\mathrm{\Delta }{b}_{31}, \mathrm{\Delta }{b}_{22}, \mathrm{\Delta }{b}_{41}$, and $\mathrm{\Delta }{b}_{32}$, which determine the equation of state and static response of polarized systems at high temperature. As a performance check, we compare the density equation of state and Tan contact with quantum Monte Carlo calculations, diagrammatic approaches, and experimental data where available. Finally, we apply Padé and Padé-Borel resummation methods to extend the usefulness of the virial coefficients to approach and in some cases go beyond the unit-fugacity point.

This paper presents a theoretical study of optomechanical systems in which the mechanical resonator modulates both the resonant frequency (dispersive coupling) and the decay rates (dissipative coupling) of the optical cavity. The generic dispersive framework is extended to a more general case in which the dissipative coupling is split between its external and intrinsic contribution. We report a complete analysis of the influence of both external and intrinsic optical losses on each of the three coupling mechanisms, highlighting the interest of each optical loss regime. A presentation of the basic model to experimentally identify the three couplings and their relative influence on the optical response is proposed. We also extend the basic tools by analyzing the mechanical dynamics and demonstrating the general expression of the optical spring effect and of optomechanical damping. A comparison between our theoretical model and experimental measurements in photonic crystal systems from the literature yields good agreement.

We present a direct-frequency-comb spectroscopy method able to resolve the spectral comb structure with an instrumental frequency resolution limited only by the coherence properties of the comb source itself. The proposed technique, based on a 50 000-finesse scanning Fabry-Pérot microcavity resonator combined with a low-resolution diffraction grating, is characterized using a 250-MHz Er-fiber frequency comb demonstrating an ultimate frequency resolution as low as 50 kHz over an optical bandwidth up to 4 THz and with tens-of-milliseconds measurement times.

We consider light scattering by dielectric gratings supporting optical bound states in the continuum. Due to the presence of instantaneous Kerr nonlinearity critical field enhancement in the spectral vicinity of the bound state triggers the effect of optical bistability. The onset of bistability is explained theoretically in the framework of the temporal coupled-mode theory. As the central result we cast the problem in the form of a single field-driven nonlinear oscillator. The theoretical results are verified in comparison with numerical simulations.

Usually, when investigating exceptional points (EPs) of an open Markovian bosonic system, one deals with spectral degeneracies of a non-Hermitian Hamiltonian (NHH), which can correctly describe the system dynamics only in the semiclassical regime. A recently proposed quantum Liouvillian framework [Minganti et al., Phys. Rev. A 100, 062131 (2019)] enables the complete determination of the dynamical properties of such systems and their EPs (referred to as Liouvillian EPs, or LEPs) in the quantum regime by taking into account the effects of quantum jumps, which are ignored in the NHH formalism. Moreover, the symmetry and eigenfrequency spectrum of the NHH become a part of much larger Liouvillian eigenspace. As such, the EPs of an NHH form a subspace of the LEPs. Here we show that once an NHH of a dissipative linear bosonic system exhibits an EP of a certain finite order $n$, it immediately implies that the corresponding LEP can become of any higher order $m\ge n$ defined in the infinite Hilbert space. These higher-order LEPs can be identified by the coherence and spectral functions at the steady state. The coherence functions can offer a convenient tool to probe extreme system sensitivity to external perturbations in the vicinity of higher-order LEPs. As an example, we study a linear bosonic system of a bimodal cavity with incoherent mode coupling to reveal its higher-order LEPs; particularly, of second and third order via first- and second-order coherence functions, respectively. Accordingly, these LEPs can be additionally revealed by squared and cubic Lorentzian spectral lineshapes in the power and intensity-fluctuation spectra. Moreover, we demonstrate that these EPs can also be associated with spontaneous parity-time ($\mathcal{PT}$) and anti-$\mathcal{PT}$-symmetry breaking in the system studied. These symmetries can be switched in the output fields (the so-called supermodes) of an additional linear coupler with a properly chosen coupling strength. Thus, we show that the initial loss-loss dynamics for the supermodes can be equivalent to the balanced gain-loss evolution.

We propose the effective simulation of light-matter ultrastrong-coupling phenomena with strong-coupling systems. Recent theory and experiments have shown that the quantum Rabi Hamiltonian can be simulated by a Jaynes-Cummings system with the addition of two classical drives. This allows one to implement nonlinear processes that do not conserve the total number of excitations. However, parity is still a conserved quantity in the quantum Rabi Hamiltonian, which forbids a wide family of processes involving virtual transitions that break this conservation. Here, we show that these parity-nonconserving processes can be simulated and that this can be done in an even simpler setup: a Jaynes-Cummings-type system with the addition of a single classical drive. By shifting the paradigm from simulating a particular model to simulating a particular process, we are able to implement a much wider family of nonlinear coherent protocols than in previous simulation approaches, doing so with fewer resources and constraints. We focus our analysis on three particular examples: a single atom exciting two photons, frequency conversion, and a single photon exciting two atoms.

Magnetically induced optical transparency (MIOT) is a technique to realize the narrow transmission spectrum in a cavity quantum electrodynamics system, which is demonstrated in a recent experiment of cold ${}^{88}\mathrm{Sr}$ atoms in an optical cavity [M. N. Winchester, M. A. Norcia, J. R. K. Cline, and J. K. Thompson, Phys. Rev. Lett. 118, 263601 (2017)]. In this experiment, MIOT induces a new narrow transmission window for the probe beam, which is highly immune to the fluctuation of the cavity mode frequency. The linewidth of this transmission window approaches the decay rate of the electronic ${}^{3}P_1$ state [about $\left(2\pi \right)7.5\mathrm{kHz}]$ and is much less than the uncertainty of the cavity mode frequency [about $\left(2\pi \right)150\mathrm{kHz}]$. In this paper, we propose an approach to further reduce the linewidth of this MIOT-induced transmission window, with the help of two Raman beams which couple the electronic ${}^{3}P_1$ state to the ${}^{3}S_1$ state, and the ${}^{3}S_1$ state to the ${}^{3}P_0$ state, respectively. With this approach, one can reduce the transmission linewidth by orders of magnitude. Moreover, the peak value of the relative transmission power or the transmission rate of the probe beam is almost unchanged by the Raman beams, and the peak position is insensitive to the average frequency of the two Raman beams.

In this paper, we study bound states and their influence upon the dynamics of a one-dimensional tight-binding system coupled to an environment. We identify three specific kinds of bound states; the first is a discrete bound state (DBS), for which the energy level exhibits a gap from the continuum. The DBS exhibits similar localization features to the edge states of the system and can therefore suppress its decay. The second is a bound state in the continuum (BIC), which can also suppress system decay. The BIC states are found to be strongly connected to the edge mode of the system, since they both show almost the same localization and energy features. The third bound state displays a large gap from the continuum and exhibits extendible (i.e., not localized) behavior. The population of the system in this state decays partially but not entirely, unlike the other bound states. The time evolution of a single excitation in the system is studied to illustrate the influence of the bound states. We find that both the DBS and the BIC play important roles in time evolution; for example, the excitation becomes localized and does not decay depending on the overlap between the initial state and the DBS or the BIC. Furthermore, we observe that the single excitation can show a long-range hopping in a system when the system falls into the strong localizations regime. This feature can be understood by the interplay of system localizations and the bath-induced long-range correlation.

Various quantum thermodynamic bounds are shown to stem from a single tighter and more general inequality, consequence of the operator concavity of the logarithmic function. Such an inequality, which we call the “thermodynamic reverse bound,” is compactly expressed as a quantum relative entropy, from which it inherits mathematical properties and meaning. As concrete examples, we apply our bound to evaluate the thermodynamic length for open processes, the heat exchange in erasure processes, and the maximal energy outflow in general quantum evolutions.

Recently, much attention have been paid to the constructions of nonlocal multipartite orthogonal product states. Among the existing results, some are relatively complex in structure while others have many constraint conditions. In this paper, we first give a simple method to construct a nonlocal set of orthogonal product states in ${\otimes }_{j=1}^n{\mathbb{C}}^d$, where $d,n$ are two positive integers, $d\ge 2$ and $n\ge 3$. Then we give a proof for local indistinguishability of the set constructed by our method. According to the characteristics of this construction method, we get a new construction of nonlocal set with fewer states in the same quantum system. Furthermore, we generalize these two results to a more general ${\otimes }_{j=1}^n{\mathbb{C}}^{d_j}$ quantum system, where $n,d_j$ are two positive integers, $n\ge 3$ and $d_j\ge 2$. Compared with the existing results, the nonlocal set of multipartite orthogonal product states constructed by our method has fewer elements and is simpler. Most of all, our multipartite constructions are more general than the ones reported earlier.

It is widely recognized that entanglement generation and dynamical chaos are intimately related in semiclassical models via the process of decoherence. In this paper, we propose a unifying framework which directly connects the bipartite and multipartite entanglement growth to the quantifiers of classical and quantum chaos. In the semiclassical regime, the dynamics of the von Neumann entanglement entropy, the spin squeezing, the quantum Fisher information, and the out-of-time-order square commutator are governed by the divergence of nearby phase-space trajectories via the local Lyapunov spectrum, as suggested by previous conjectures in the literature. General analytical predictions are confirmed by detailed numerical calculations for two paradigmatic models, relevant in atomic and optical experiments, which exhibit a regular-to-chaotic transition: the quantum kicked top and the Dicke model.

We consider the problem of selectively controlling couplings in a practical quantum processor with always-on interactions that are diagonal in the computational basis, using sequences of local not gates. This methodology is well known in nuclear magnetic resonance implementations, but previous approaches do not scale efficiently for the general fully connected Hamiltonian, where the complexity of finding time-optimal solutions makes them only practical up to a few tens of qubits. Given the rapid growth in the number of qubits in cutting-edge quantum processors, it is of interest to investigate the applicability of this control scheme to much larger-scale systems with realistic restrictions on connectivity. Here we present an efficient scheme to find near time-optimal solutions that can be applied to engineered qubit arrays with local connectivity for any number of qubits, indicating the potential for practical quantum computing in such systems.