In the Unruh effect an observer with constant acceleration perceives the quantum vacuum as thermal radiation. The Unruh effect has been believed to be a pure quantum phenomenon, but here we show theoretically how the effect arises from the correlation of noise, regardless of whether this noise is quantum or classical. We demonstrate this idea with a simple experiment on water waves where we see the first indications of a Planck spectrum in the correlation energy.

Entanglement distillation is the process of concentrating entanglement from a given quantum state. We present a technique for distillation of bipartite polarization entanglement using interferometry. This technique can be optimized to extract maximal entanglement from any pure or mixed entangled state. A model for this method is presented, and, in particular, we present experimental results for pure states when using polarizing Mach-Zehnder interferometers. These experimentally distilled states always demonstrate an increased violation of Bell's inequality.

Reverse annealing is a variant of quantum annealing that starts from a given classical configuration of spins (qubits). In contrast to the conventional formulation, where one starts from a uniform superposition of all possible states (classical configurations), quantum fluctuations are first increased and only then decreased. One then reads out the state as a proposed solution to the given combinatorial optimization problem. We formulate a mean-field theory of reverse annealing using the fully connected ferromagnetic $p$-spin model, with and without random longitudinal fields, and analyze it in order to understand how and when reverse annealing is effective at solving this problem. We find that the difficulty arising from the existence of a first-order quantum phase transition, which leads to an exponentially long computation time in conventional quantum annealing, is circumvented in the context of this particular problem by reverse annealing if the proximity of the initial state to the (known) solution exceeds a threshold. Even when a first-order transition is unavoidable, the difficulty is mitigated due to a smaller jump in the order parameter at a first-order transition, which implies a larger rate of quantum tunneling. Reverse annealing has not been studied analytically before, and this study paves the way toward a systematic understanding of this relatively unexplored protocol in a broader context.

The adiabatic theorem of quantum mechanics states that the error between an instantaneous eigenstate of a time-dependent Hamiltonian and the state given by quantum evolution of duration $\tau$ is upper bounded by $C/\tau$ for some positive constant $C$. It has been known for decades that this error can be reduced to $C_k/{\tau }^{k+1}$ if the Hamiltonian has vanishing derivatives up to order $k$ at the beginning and end of the evolution. Here we extend this result to open systems described by a time-dependent Liouvillian superoperator. We find that the same results holds provided the Liouvillian has vanishing derivatives up to order $k$ only at the end of the evolution. This asymmetry is ascribable to the arrow of time inherent in open system evolution. We further investigate whether it is possible to satisfy the required assumptions by controlling only the system, as required for realistic implementations. Surprisingly, we find the answer to be affirmative. We establish this rigorously in the setting of the Davies-Lindblad adiabatic master equation, and numerically in the setting of two different time-dependent Redfield-type master equations we derive. The results are shown to be stable with respect to imperfections in the preparation. Finally, we prove that the results hold also in a fully Hamiltonian model.

Two main obstacles for observing quantum advantage in noisy intermediate-scale quantum computers (NISQ) are the finite-precision effects due to control errors, or disorders, and decoherence effects due to thermal fluctuations. It has been shown that dissipative quantum computation is possible in the presence of an idealized fully engineered bath. However, it is not clear, in general, what performance can be achieved by NISQ when internal bath degrees of freedom are not controllable. In this work, we consider the task of quantum search of a marked node on a complete graph of $n$ nodes in the presence of both static disorder and nonzero coupling to an environment. We show that, given fixed and finite levels of disorder and thermal fluctuations, there is an optimal range of bath temperatures that can significantly improve the success probability of the algorithm. Remarkably for a fixed disorder strength $\sigma$, the system relaxation time decreases for higher temperatures within a robust range of parameters. In particular, we demonstrate that for strong disorder, the presence of a thermal bath increases the success probability from $1/\left(n{\sigma }^2\right)$ to at least $1/2$. While the asymptotic running time is approximately maintained, the need to repeat the algorithm many times and issues associated with unitary over-rotations can be avoided as the system relaxes to an absorbing steady state. Furthermore, we discuss for what regimes of disorder and bath parameters quantum speedup is possible and mention conditions for which similar phenomena can be observed in more general families of graphs. Our work highlights that in the presence of static disorder, even nonengineered environmental interactions can be beneficial for a quantum algorithm.

Electron interactions with the nuclear-spin-dependent (NSD) parity-nonconserving (PNC) anapole moment are strongly enhanced within heteronuclear diatomic molecules. A low-energy optical rotation experiment is being proposed with the aim of observing NSD PNC interactions in HgH. Based on the relativistic coupled cluster method we present a sophisticated numerical calculation of the circular polarization parameter $P=2\mathrm{Im}\left(E{1}_{\mathrm{PNC}}\right)/M1\approx 3\times {10}^{-6}\kappa$ for the ${}^2\mathrm{\Sigma }_{1/2}\to {}^2\mathrm{\Pi }_{1/2}$ optical transition of HgH, where $\kappa$ is a dimensionless constant determined by the nuclear anapole moment. This provides an improvement in sensitivity to NSD PNC by 2–3 orders of magnitude over the leading atomic Xe, Hg, Tl, Pb, and Bi optical rotation experiments. Therefore we show that the proposed measurement should be sensitive enough to extract the ${}^{199}\mathrm{Hg}$ anapole moment and shed light on the underlying theory of hadronic parity violation.

We extend the semiclassical two-step model [Phys. Rev. A 94, 013415 (2016)] to include a multielectron polarization-induced dipole potential. Using this model we investigate the imprints of multielectron effects in the momentum distributions of photoelectrons ionized by a linearly polarized laser pulse. We predict narrowing of the longitudinal momentum distributions due to electron focusing by the induced dipole potential. We show that the polarization of the core also modifies interference structures in the photoelectron momentum distributions. Specifically, the number of fanlike interference structures in the low-energy part of the electron momentum distribution may be altered. We analyze the mechanisms underlying this interference effect. The account of the multielectron dipole potential seems to improve the agreement between theory and experiment.

We investigate theoretically the above-threshold ionization (ATI) of localized atomic targets by intense few-cycle Bessel pulses that carry orbital angular momentum (OAM), known also as twisted light. More specifically, we use the strong-field approximation (SFA) to compute the photoelectron energy spectra. While for plane-wave laser pulses the outgoing photoelectron is typically described by Volkov states within the SFA, no equivalent is known for an electron in a twisted laser field. Here, we therefore introduce a local dipole approximation for the (continuum) state of the photoelectron that is justified for few-cycle pulses. Based on this approximation, we demonstrate that the photoelectrons can also be emitted into the propagation direction of the pulse. When measured in propagation direction, moreover, we show that the magnitude of the ATI peaks depend on the opening angle and the (projection of) total angular momentum of the Bessel pulse.

We demonstrate that a bichromatic standing-wave laser field can exert a significantly larger force on a molecule than ordinary radiation pressure. Our experiment measures the deflection of a pulsed supersonic beam of CaF molecules by a two-frequency laser field detuned symmetrically about resonance with the nearly closed $X(v=0)\to B(v^{\prime }=0)$ transition. The inferred force as a function of relative phase between the two counterpropagating beams is in reasonable agreement with numerical simulations of the bichromatic force in this multilevel system. The large magnitude of the force, coupled with the reduced rate of spontaneous emission, indicates its potential utility in the production and manipulation of ultracold molecules.

The calculation of partial two-photon ionization cross sections in the above-threshold energy region is discussed in the framework of exterior complex scaling. It is shown that with a minor modification of the usual procedure, which is based on the calculation of the outgoing partial waves of the second-order scattering wave function, reliable partial ionization amplitudes can be obtained. The modified procedure relies on a few-term least-squares fit of radial functions pertaining to different partial waves. To test the procedure, partial and total two-photon ionization cross sections of the helium atom have been calculated for a broad range of incident photon energies. The calculated cross sections may be seen to agree well with the results found in the literature. Furthermore, it is shown that, using a similar approach, partial photoionization cross sections of an atom in an autoionizing (resonance) state may be calculated in a relatively straightforward way. Such photoionization cross sections may find their use in enhanced few-parameter models describing the atom-light interaction in cases where a direct solution of the time-dependent Schrödinger equation becomes too resource intensive.

Our investigation considers the amended strong-field approximation (SFA) ionization of rare-gas atoms in a strong laser field accompanied by excitation of the atom due to laser-induced recollision. This process can be viewed as inelastic above-threshold ionization and corresponds to the recollision-excitation step of the laser-induced nonsequential double ionization via RESI (recollision excitation with subsequent ionization) mechanism. Within the SFA framework, up to now, Born approximation has been used to calculate the recollision-excitation process during RESI, which is not a good approximation at low-energy recollisions. In this work, we improve this model by employing Coulomb-Born approximation, where the recolliding electron is described with a Coulomb continuum wave function. We calculate the photoelectron momentum distributions (PMD) of the rescattered electron for helium and argon atoms by considering different excitation states. The Coulomb effect is shown to play a significant role for shaping the recollision-excitation probability, visible as in the fully differential PMD, as well as in the PMD integrated over transverse momenta.

Spontaneous breaking of continuous time translation symmetry into a discrete one is related to time crystal formation. While the phenomenon is not possible in the ground state of a time-independent many-body system, it can occur in an excited eigenstate. Here, we concentrate on bosons on a ring with attractive contact interactions and analyze a quantum quench from the time crystal regime to the noninteracting regime. We show that dynamical quantum phase transitions can be observed where the return probability of the system to the initial state before the quench reveals a nonanalytical behavior in time. The problem we consider constitutes an example of the dynamical quantum phase transitions in a system where both time and space continuous translation symmetries are broken.

We develop a highly efficient method to numerically simulate thermal fluctuations and correlations in nonrelativistic continuous bosonic one-dimensional systems. The method is suitable for arbitrary local interactions as long as the system remains dynamically stable. We start by proving the equivalence of describing the systems through the transfer matrix formalism and a Fokker-Planck equation for a distribution evolving in space. The Fokker-Planck equation is known to be equivalent to a stochastic differential (Itō) equation. The latter is very suitable for computer simulations, allowing the calculation of any desired correlation function. As an illustration, we apply our method to the case of two tunnel-coupled quasicondensates of bosonic atoms. The results are compared to the predictions of the sine-Gordon model for which we develop analytic expression directly from the transfer matrix formalism.

Recent experiments on a one-dimensional chain of trapped alkali-metal atoms [Bernien et al., Nature (London) 551, 579 (2017)] have observed a quantum transition associated with the onset of period-3 ordering of pumped Rydberg states. This spontaneous ${\mathbb{Z}}_3$ symmetry breaking is described by a constrained model of hard-core bosons proposed by Fendley et al. [Phys. Rev. B 69, 075106 (2004)]. By symmetry arguments, the transition is expected to be in the universality class of the ${\mathbb{Z}}_3$ chiral clock model with parameters preserving both time-reversal and spatial-inversion symmetries. We study the nature of the order–disorder transition in these models and numerically calculate its critical exponents with exact diagonalization and density-matrix renormalization-group techniques. We use finite-size scaling to determine the dynamical critical exponent $z$ and the correlation length exponent $\nu$. Our analysis presents the only known instance of a strongly coupled generic transition between gapped states with $z\ne 1$, implying an underlying nonconformal critical-field theory.

We have studied manifestation of optical activity for higher-order modes that propagate in optical fibers with multihelical distribution of the refractive index profile. We have shown that due to form-induced intermodal coupling between the fundamental modes and the higher-order modes the hybrid modes of such fibers are symmetric (antisymmetric) combinations of circularly polarized optical vortices with opposite orbital angular momentum. We have demonstrated that because of the spin-orbit interaction the system features circular birefringence for such hybrid modes. We have obtained analytical expressions for the coefficients of optical activity related to propagation of such hybrid higher-order modes. We have shown that for those modes the effect of optical activity greatly surpasses the one exhibited for the fundamental modes. The obtained results might be useful for predicting possible new phenomena concerned with propagation of higher-order fields in twisted photonic crystal fibers and for understanding the nature of optical activity for fundamental modes in them.

Optical analog experiments have captured a lot of interest in recent years by offering a strategy to test theoretical models and concepts that would be otherwise untestable. The approach relies on the similarity between the mathematical model for light propagation in nonlinear optical media and the model to be mimicked. In particular, the analogy between light and a quantum fluid with superfluidlike properties has been studied extensively. Still, while most of these studies use thermo-optical media to perform these experiments, the possibility of using nematic liquid crystals to perform such optical analog experiments remains to be analyzed. This work explores how this medium can constitute an alternative to materials more commonly used in optical analogs, such as thermo-optical media, and how its tunable properties can be advantageous to explore and better control fluidlike properties of light. Moreover, we explore the analogy between the propagation of light and a quantum fluid, and propose a pump-probe experiment to measure the dispersion relation of the superfluid analog.

The classification of band structures by topological invariants provides a powerful tool for understanding phenomena such as the quantum Hall effect. This classification was originally developed in the context of electrons but can also be applied to photonic crystals. In this paper we study the topological classification of the refractive index surfaces of two-dimensional photonic crystals. We consider crystals formed from birefringent materials in which the constitutive relation provides an optical spin-orbit coupling. We show that this coupling, in conjunction with optical activity, can lead to a gapped set of index surfaces with nonzero Chern numbers. This method for designing photonic Chern insulators exploits birefringence rather than lattice structure and does not require band crossings originating from specific lattice geometries.

We report the formation of bound states in the continuum in a whispering gallery resonator coupled to a one-dimensional waveguide. We find that an incident photon wave packet is partially stored in the bound state in the continuum due to a roughness-induced symmetry breaking. We discuss quantum interference effects such as Fano in the transmission spectra and the photonic density of states. The conditions under which the bound state can be accessed through single-photon transport processes are investigated.

It has recently been shown [Rossi et al., Phys. Rev. Lett. 119, 123603 (2017); Phys. Rev. Lett. 120, 073601 (2018)] that feedback-controlled in-loop light can be used to enhance the efficiency of optomechanical systems. We analyze the theoretical ground at the basis of this approach and explore its potentialities and limitations. We discuss the validity of the model, analyze the properties of in-loop cavities, and show how they can be used to observe coherent optomechanical oscillations also with a weakly coupled system, improve the sideband cooling performance, and increase ponderomotive squeezing.