We discuss a protocol based on quenching a purified quantum system that allows one to capture bulk spectral features. It uses an infinite temperature initial state and an interferometric strategy to access the Loschmidt amplitude, from which the spectral features are retrieved via Fourier transform, providing coarse-grained approximation at finite times. It involves techniques available in current experimental setups for quantum simulation, at least for small systems. We illustrate possible applications in testing the eigenstate thermalization hypothesis and the physics of many-body localization.

We investigate the thermodynamic consistency of the master equation description of heat transport through an optomechanical system attached to two heat baths, one optical and one mechanical. We employ three different master equations to describe this scenario: (i) The standard master equation used in optomechanics, where each bath acts only on the resonator that it is physically connected to; (ii) the so-called dressed-state master equation, where the mechanical bath acts on the global system; and (iii) what we call the global master equation, where both baths are treated nonlocally and affect both the optical and mechanical subsystems. Our main contribution is to demonstrate that, under certain conditions including when the optomechanical coupling strength is weak, the second law of thermodynamics is violated by the first two of these pictures. In order to have a thermodynamically consistent description of an optomechanical system, therefore, one has to employ a global description of the effect of the baths on the system.

Contextuality describes the local consistency but global inconsistency of data. In this paper, we review and generalize the bundle diagram representation introduced in [S. Abramsky, R. S. Barbosa, K. Kishida, R. Lal, and S. Mansfield, Contextuality, Cohomology and Paradox, 24th EACSL Annual Conference on Computer Science Logic, Vol. 41 (CSL, 2015), p. 211-228 ] to graphically demonstrate the contextuality of diverse empirical models (a collection of probability distributions for jointly measurable observables).

Quantum coherence inherently affects the dynamics and the performances of a quantum machine. Coherent control can, at least in principle, enhance the work extraction and boost the velocity of evolution in an open quantum system. Using advanced tools from the calculus of variations and reformulating the control problem in the instantaneous Hamiltonian eigenframe, we develop a general technique for minimizing a wide class of cost functionals when the external control has access to full rotations of the system Hamiltonian. The method is then applied both to time and heat loss minimization problems and explicitly solved in the case of a two-level system in contact with either bosonic or fermionic thermal environments.

Characterizing tradeoffs between simultaneous violations of multiple Bell inequalities in a large network has important physical consequences but is computationally demanding. We propose a graph-theoretic approach to efficiently produce Bell monogamy relations in arbitrary arrangements of qubits. All the relations obtained for bipartite Bell inequalities are tight and leverage only a single Bell monogamy relation. This feature is unique to bipartite Bell inequalities, as we show that there is no finite set of such elementary monogamy relations for multipartite inequalities. Nevertheless, many tight monogamy relations for multipartite inequalities can be obtained with our method as shown in explicit examples.

We numerically obtain the full time-evolution of a parametrically driven-dissipative Bose-Einstein condensate in an optical cavity and investigate the implications of driving for the phase diagram. Beyond the normal and superradiant phases, a third nonequilibrium phase emerges as a many-body parametric resonance. This dynamical phase switches between two symmetry-broken superradiant configurations. The switching is accompanied by a substantial occupation of other momentum states not retained in the mapping to the Dicke model. The emergent phase shows features of nonintegrability and thermalization.

In a recent experiment [Peng et al., Phys. Rev. A 97, 012702 (2018)], it has been shown that the $p$-wave Feshbach resonance can be shifted toward the $s$-wave Feshbach resonance by a laser field. Based on this experiment, we study the universal relations and the normal-state properties in an ultracold Fermi gas with coexisting $s$- and $p$-wave interactions under optical control of a $p$-wave magnetic Feshbach resonance. Within the operator-product expansion, we derive the high-momentum tail of various observable quantities in terms of contacts. We find that the high-momentum tail becomes anisotropic. Adopting the quantum virial expansion, we calculate the normal-state contacts with and without a laser field for ${}^{40}\mathrm{K}$ atoms using typical experimental parameters. We show that the contacts are dependent on the laser dressing. We also reveal the interplay of laser dressing and different partial-wave interactions on various contacts. In particular, we demonstrate that the impact of the laser dressing in the $p$-wave channel can be probed by measuring the $s$-wave contacts, which is a direct manifestation of few-body effects on the many-body level. Our results can be readily checked experimentally.

We show that the analog of the geometric phase for non-Hermitian coupled waveguides with $\mathcal{PT}$ symmetry and at least one periodically varying parameter can be purely imaginary, is hence no longer a true phase and instead a real multiplier, and will consequently result in the amplification of Floquet sidebands in the system. The sideband peaks seen in the spectrum of the system's eigenstates after evolution along the waveguides can be directly mapped to the spectrum of the derivative of the geometric function. The sidebands are magnified (becoming virtually unstable) as the exceptional point of the system is approached, and nonadiabatic effects begin to appear. Because the system cannot evolve adiabatically in the vicinity of the exceptional point, $\mathcal{PT}$ symmetry will be observed breaking earlier than theoretically predicted.

The interaction among the components of a hybrid quantum system is often neglected when considering the coupling of these components to an environment. However, if the interaction strength is large, this approximation leads to unphysical predictions, as has been shown for cavity-QED and optomechanical systems in the ultrastrong-coupling regime. To deal with these cases, master equations with dissipators retaining the interaction between these components have been derived for the quantum Rabi model and for the standard optomechanical Hamiltonian. In this article, we go beyond these previous derivations and present a general master equation approach for arbitrary hybrid quantum systems interacting with thermal reservoirs. Specifically, our approach can be applied to describe the dynamics of open hybrid systems with harmonic, quasiharmonic, and anharmonic transitions. We apply our approach to study the influence of temperature on multiphoton vacuum Rabi oscillations in circuit QED. We also analyze the influence of temperature on the conversion of mechanical energy into photon pairs in an optomechanical system, which has been recently described at zero temperature. We compare our results with previous approaches, finding that these sometimes overestimate decoherence rates and underestimate excited-state populations.

In this paper we explore the entanglement of two relativistic spin-$1/2$ particles with continuous momenta. The spin state is described by the Bell state and the momenta are given by Gaussian distributions of product form. Transformations of the spins are systematically investigated in different boost scenarios by calculating the orbits and concurrence of the spin degree of freedom. By visualizing the behavior of the spin state we get further insight into how and why the entanglement changes in different boost situations.

Adiabatic quantum computing (AQC) is an approach for solving optimization problems and has advantages against environmental decoherence and certain kinds of random unitary perturbations; however, long coherence time of quantum systems is required to meet the adiabatic criteria for the implementation of AQC. To break the dilemma of evolution time in AQC, the method of shortcuts to adiabaticity (STA) is employed to speed up the evolution process and suppress the nonadiabatic transitions. In this work we demonstrate the application of STA in AQC by implementing a fast two-bit Grover's algorithm with STA in an ion trap system, and the method here is also applicable for realizing Grover's algorithm with more entries.

In this paper, we propose a scheme to implement the two-qubit controlled-$Z$ gate via the Stark-tuned Förster interaction of Rydberg atoms, where the Förster defect is driven by a time-dependent electric field of a simple sinusoidal function while the matrix elements of the dipole-dipole interaction are time independent. It is shown that when the system is initially in a specific state, it makes a cyclic evolution after a preset interaction time, returning to the initial state, but picks up a phase, which can be used for realizing a two-atom controlled-$Z$ gate. Due to the interference of sequential Landau-Zener transitions, the population and phase of the state is quasideterministic after the cyclic evolution and therefore the gate fidelity is insensitive to fluctuations of the interaction time and the dipole-dipole matrix elements. Feasibility of the scheme realized with Cs atoms is discussed in detail, which shows that the two-qubit gate via Landau-Zener control can be realized with the state-of-the-art experimental setup.

We report the ${}^{209}\mathrm{Bi}$ magnetic moment derived from NMR experiments and first-principles calculated shielding constants of ${\mathrm{Bi}}^{3+}$ ions in aqueous solutions of $\mathrm{Bi}{\left({\mathrm{NO}}_3\right)}_3$ and $\mathrm{Bi}{\left({\mathrm{ClO}}_4\right)}_3$ salts. Our values represent an independent confirmation of the ${}^{209}\mathrm{Bi}$ magnetic moment recently determined by Skripnikov et al. [L. V. Skripnikov et al., Phys. Rev. Lett. 120, 093001 (2018)], which is of utmost importance for the interpretation of ${\mathrm{Bi}}^{82+}$ ion hyperfine splitting experiments. The accuracy limit of the ${}^{209}\mathrm{Bi}$ magnetic moment set by the present computational chemistry methods is discussed.

We apply point-particle effective field theory to electronic and muonic ${}^4\mathrm{He}^{+}$ ions, and use it to identify linear combinations of spectroscopic measurements for which the theoretical uncertainties are much smaller than for any particular energy levels. The error is reduced because these combinations are independent of all short-range physics effects up to a given order in the expansion in the small parameters $R/a_B$ and $Z\alpha$ (where $R$ and $a_B$ are the ion's nuclear and Bohr radii). In particular, the theory error is not limited by the precision with which nuclear matrix elements can be computed, or compromised, by the existence of any novel short-range interactions, should these exist. These combinations of ${}^4\mathrm{He}$ measurements therefore provide particularly precise tests of quantum electrodynamics. The restriction to ${}^4\mathrm{He}$ arises because our analysis assumes a spherically symmetric nucleus, but the argument used is more general and extendable to both nuclei with spin, and to higher orders in $R/a_B$ and $Z\alpha$.

Non-Markovian quantum evolution of the electronic subsystem in a laser-driven molecule is characterized through the appearance of negative decoherence rates in the canonical form of the electronic master equation. For a driven molecular system described in a bipartite Hilbert space $\mathcal{H}={\mathcal{H}}_{el}⨂{\mathcal{H}}_{\mathrm{vib}}$ of dimension $2\times N_v$, we derive the canonical form of the electronic master equation, deducing the canonical measures of non-Markovianity and the Bloch volume of accessible states. We find that one of the decoherence rates is always negative, accounting for the inherent non-Markovian character of the electronic evolution in the vibrational environment. Enhanced non-Markovian behavior, characterized by two negative decoherence rates, appears if there is a coupling between the electronic states $g,e$, such that the evolution of the electronic populations obeys $d\left(P_g{P}_e\right)/dt>0$. Non-Markovianity of the electronic evolution is analyzed in relation to temporal behaviors of the electronic-vibrational entanglement and electronic coherence, showing that enhanced non-Markovian behavior accompanies entanglement increase. Taking as an example the coupling of two electronic states by a laser pulse in the ${\mathrm{Cs}}_2$ molecule, we analyze non-Markovian dynamics under laser pulses of various strengths, finding that the weaker pulse stimulates the bigger amount of non-Markovianity. Our results show that increase of the electronic-vibrational entanglement over a time interval is correlated to the growth of the total amount of non-Markovianity calculated over the same interval using canonical measures and connected with the increase of the Bloch volume. After the pulse, non-Markovian behavior is correlated to electronic coherence, such that vibrational motion in the electronic potentials which diminishes the nuclear overlap, implicitly increasing the linear entropy of entanglement, brings a memory character to dynamics.

We study the spreading of correlations in space and time after a quantum quench in the Bose-Hubbard model. We derive equations of motion for the single-particle Green's function within the contour-time formalism, allowing us to study dynamics in the strong-coupling regime. We discuss the numerical solutions of these equations and calculate the single-particle density matrix for quenches in the Mott phase. We demonstrate light-cone-like spreading of correlations in the Mott phase in one, two, and three dimensions and calculate the propagation velocities in each dimension.

Novel fermionic quasiparticles with integer pseudospins in some energy bands, such as pseudospin-1 triple-point fermions, are attracting increasing interest since they are beyond the conventional spin-$1/2$ Dirac and Weyl counterparts. In this paper, we propose a class of pseudospin-1 fermionic excitations emerging in topological metal bands, dubbed double-triple-point (DTP) fermions. We first present a general three-band continuum model with $C_4$ symmetry in three dimensions, which has three types of threefold degenerate points in the bands classified by their topological charges $C=\pm 4,\pm 2,0$, respectively. They are dubbed DTPs as spin-1 generalization of double-Weyl points. We then construct two-dimensional and three-dimensional tight-binding lattice models of topological metal bands with exotic DTP fermions near the DTPs. In two dimensions, the bands close at a trivial DTP with zero Berry phase, which occurs at the transition between the normal and topological insulator phases. In three dimensions, the topological properties of three different DTP fermions in lattice systems are further investigated, and the effects of breaking $C_4$ symmetry are also studied, which generally leads to splitting each quadratic DTP into two linear triple points and gives topological phase diagrams. By using ultracold fermionic atoms in optical lattices, the proposed models can be realized and the topological properties of the DTP fermions can be detected.

Atomic interferometry in optical lattices is a new trend for developing a practical quantum gravimeter. Here, we propose a compact and portable gravimetry scheme with an ensemble of ultracold atoms in gravitationally tilted spin-dependent optical lattices. The fast coherent separation and recombination of atoms can be realized via polarization-synthesized optical lattices. The input atomic wave packet is coherently split into two parts by a spin-dependent shift and a subsequent $\frac{\pi }{2}$ pulse. Then the two parts are held for accumulating a relative phase related to the gravity. Lastly the two parts are recombined for interference by a $\frac{\pi }{2}$ pulse and a subsequent spin-dependent shift. The $\frac{\pi }{2}$ pulses not only preclude the spin-dependent energies in the accumulated phase, but also avoid the error sources, such as dislocation of optical lattices in the holding process. In addition, we develop an analytical method for the sensitivity in multipath interferometry.

This work analyzes the quantum state of twin photons and twin beams generated by parametric down-conversion in a hexagonally poled photonic crystal, characterized by the simultaneous presence of two nonlinear processes sustained by two vectors of the reciprocal lattice. In those special points of the fluorescence spectrum where the two processes coexist, we show that a tripartite entangled state is realized, equivalent to a single parametric process followed by a beam splitter. By proper angle tuning, a peculiar resonance condition is reached, with a transition to a four-mode entanglement, dominated by the golden ratio of the segment $\varphi =(1+\sqrt{5})/2$. A maximal coherence between the two nonlinear processes is established here, as the overall process is shown to be equivalent to two independent parametric processes followed by a beam splitter. We offer an interpretation of the occurrence of the golden ratio in this system based on an analogy between the evolution of the light modes and the Fibonacci sequence.