A general framework of optimal eavesdropping on BB84 protocol was provided by Fuchs et al. [Phys. Rev. A 56, 1163 (1997)]. An upper bound on mutual information was derived, which could be achieved by a specific type of interaction and the corresponding measurement. However, uniqueness of optimal interaction was posed as an unsolved problem there and it has remained open for almost two decades now. In this paper, we solve this open problem and establish the uniqueness of optimal interaction up to rotation. The specific choice of optimal interaction by Fuchs et al. is shown to be a special case of the form derived in our work.

Trapped charged particles have been at the forefront of quantum information processing (QIP) for a few decades now, with deterministic two-qubit logic gates reaching record fidelities of $99.9\%$ and single-qubit operations of much higher fidelity. In a hybrid system involving trapped charges, quantum degrees of freedom of macroscopic objects such as bulk acoustic resonators, superconducting circuits, or nanomechanical membranes, couple to the trapped charges and ideally inherit the coherent properties of the charges. The hybrid system therefore implements a “quantum transducer,” where the quantum reality (i.e., superpositions and entanglement) of small objects is extended to include the larger object. Although a hybrid quantum system with trapped charges could be valuable both for fundamental research and for QIP applications, no such system exists today. Here we study theoretically the possibilities of coupling the quantum-mechanical motion of a trapped charged particle (e.g., an ion or electron) to the quantum degrees of freedom of superconducting devices, nanomechanical resonators, and quartz bulk acoustic wave resonators. For each case, we estimate the coupling rate between the charged particle and its macroscopic counterpart and compare it to the decoherence rate, i.e., the rate at which quantum superposition decays. A hybrid system can only be considered quantum if the coupling rate significantly exceeds all decoherence rates. Our approach is to examine specific examples by using parameters that are experimentally attainable in the foreseeable future. We conclude that hybrid quantum systems involving a single atomic ion are unfavorable compared with the use of a single electron because the coupling rates between the ion and its counterpart are slower than the expected decoherence rates. A system based on trapped electrons, on the other hand, might have coupling rates that significantly exceed decoherence rates. Moreover, it might have appealing properties such as fast entangling gates, long coherence, and flexible topology that is fully electronic in nature. Realizing such a system, however, is technologically challenging because it requires accommodating both a trapping technology and superconducting circuitry in a compatible manner. We review some of the challenges involved, such as the required trap parameters, electron sources, electrical circuitry, and cooling schemes in order to promote further investigations towards the realization of such a hybrid system.

We implement faster-than-adiabatic two-qubit phase gates using smooth state-dependent forces. The forces are designed to leave no final motional excitation, independently of the initial motional state in the harmonic small-oscillations limit. They are simple, explicit functions of time and the desired logical phase of the gate, and are based on quadratic invariants of motion and Lewis-Riesenfeld phases of the normal modes.

L. Lamata et al. use an inductive approach to classify the entangled pure states of four qubits under stochastic local operations and classical communication (SLOCC) [Phys. Rev. A 75, 022318 (2007)]. The inductive method yields a priori ten entanglement superclasses, of which they discard three as empty. One of the remaining superclasses is split into two, resulting in eight superclasses of genuine four-qubit entanglement. Here, we show that two of the three discarded superclasses are in fact nonempty and should have been retained. We give explicit expressions for the canonical states of those superclasses, up to SLOCC and qubit permutations. Furthermore, we confirm that the third discarded superclass is indeed empty, yielding a total of ten superclasses of genuine four-qubit entanglement under the inductive classification scheme.

The role of entanglement and quantum correlations in complex physical systems and quantum information processing devices has become a topic of intense study in the past two decades. In this work we present tools for learning about entanglement and quantum correlations in dynamical systems where the quantum states are mixed and the eigenvalue spectrum is highly degenerate. We apply these results to the deterministic quantum computing with one qubit (DQC1) computation model and show that the states generated in a DQC1 circuit have an eigenvalue structure that makes them difficult to entangle, even when they are relatively far from the completely mixed state. Our results strengthen the conjecture that it may be possible to find quantum algorithms that do not generate entanglement and yet still have an exponential advantage over their classical counterparts.

Using ab initio methodologies, we investigate the lowest electronic states of ${\mathrm{AlS}}^{2+}$ dications. These computations are performed using standard and explicitly correlated methods. Here we predict the metastability of this particular dication, where several potential wells are located. For the bound states, we report equilibrium distances and vibrational and rotational spectroscopic parameters. These computations show that the ground state of ${\mathrm{AlS}}^{2+}$ is of ${}^2{\mathrm{\Sigma }}^{-}$ space symmetry. We also derive vertical and adiabatic charge-stripping energies of ${\mathrm{AlS}}^{+}$ and vertical and adiabatic double-ionization energies of AlS. A comparison with the isovalent molecular species is carried out. Also, we show that the electronic properties of sulfur-containing dications cannot be predicted from those of the corresponding oxygenated dications.

Multiphoton free-free transitions were observed in laser-assisted elastic electron scattering (LAES) by Xe atoms in a femtosecond near-infrared intense laser field. The distinct peak structures at the energy shifts of $n$-photons $(n=+1,+2,+3,+4,+5$, and $+6)$ were identified in the observed energy spectrum, and the energy and angular distributions of the LAES signals were in good agreement with those obtained by numerical simulations based on the Kroll-Watson theory. The LAES signal intensities at the scattering angles at 9.1° and 11.8° exhibited a clear plateau structure as a function of the harmonic order $n$, and the mechanism of these nonperturbative LAES processes was interpreted by a classical mechanical description.

Aiming at efficient numerical analysis of time-dependent (TD) many-electron dynamics of atoms involving multielectron continua, the TD restricted-active-space self-consistent-field theory with space partition (TD-RASSCF-SP) is presented. The TD-RASSCF-SP wave function is expanded in terms of TD configuration-interaction coefficients with Slater determinants composed of two kinds of TD orbitals: $\widehat{M}$ orbitals are defined to be nonvanishing in the inner region ($\widehat{V}$), a small volume around the atomic nucleus, and $\check{M}$ orbitals are nonvanishing in the large outer region ($\check{V}$). For detailed discussion of the SP strategy, the equations of motion are derived by two different formalisms for comparison. To ensure continuous differentiability of the wave function across the two regions, one of the formalisms makes use of the property of the finite-element discrete-variable-representation (FEDVR) functions and introduces additional time-independent orbitals. The other formalism is more general and is based on the Bloch operator as in the $R$-matrix theory, but turns out to be less practical for numerical applications. Hence, using the FEDVR-based formalism, the numerical performance is tested by computing double-ionization dynamics of atomic beryllium in intense light fields. To achieve high accuracy, $\widehat{M}$ should be set large to take into account the strong many-electron correlation around the nucleus. On the other hand, $\check{M}$ can be set much smaller than $\widehat{M}$ for capturing the weaker correlation between the two outgoing photoelectrons. As a result, compared with more accurate multiconfigurational TD Hartree-Fock (MCTDHF) method, the TD-RASSCF-SP method may achieve comparable accuracy in the description of the double-ionization dynamics. There are, however, difficulties related to the stiffness of the equations of motion of the TD-RASSCF-SP method, which makes the required time step for this method smaller than the one needed for the MCTDHF approach.

We propose a coupled Dirac sea approach to examine numerically the electron-positron pair creation process with interfermionic interactions in a supercritical background field. In this approach the Hilbert space is doubled, leading to two separate Dirac equations for the dynamics of the electrons and the positrons. It permits us to remove the unphysical self-repulsion while the internal electric fields that couple different fermions are determined from the Maxwell equations. The time evolution of the yield and the spatial distribution, as well as the energy spectra of the particles, show that the attractive force between the created particles decreases the pair creation probability, independent of the spatial characteristics of the force-intermediating internal fields. This decrease is accompanied by shifts in the kinetic energy spectrum of the emitted particles. These findings are supported by an independent model that predicts the creation rate based on the quantum mechanical transmission coefficient.

The manipulation of neutral atoms and molecules via their de Broglie wave properties, also referred to as de Broglie matter wave optics, is relevant for several fields ranging from fundamental quantum mechanics tests and quantum metrology to measurements of interaction potentials and new imaging techniques. However, there are several challenges. For example, for diffractive focusing elements, the zero-order beam provides a challenge because it decreases the signal contrast. Here we present the experimental realization of a zero-order filter, also referred to as an order-sorting aperture for de Broglie matter wave diffractive focusing elements. The zero-order filter makes it possible to measure even at low beam intensities. We present measurements of zero-order filtered, focused, neutral helium beams generated at source stagnation pressures between 11 and 81 bars. We show that for certain conditions the atom focusing at lower source stagnation pressures (broader velocity distributions) is better than what has previously been predicted. We present simulations with the software ray-tracing simulation package mcstas using a realistic helium source configuration, which gives very good agreement with our measurements.

We present a theoretical study based upon quantum Monte Carlo methods of the Bose polaron in one-dimensional systems with contact interactions. In this instance of the problem of a single impurity immersed in a quantum bath, the medium is a Lieb-Liniger gas of bosons ranging from the weakly interacting to the Tonks-Girardeau regime, whereas the impurity is coupled to the bath via a different contact potential, producing both repulsive and attractive interactions. Both the case of a mobile impurity, having the same mass as the particles in the medium, and the case of a static impurity with infinite mass are considered. We make use of numerical techniques that allow us to calculate the ground-state energy of the impurity, its effective mass, and the contact parameter between the impurity and the bath. These quantities are investigated as a function of the strength of interactions between the impurity and the bath and within the bath. In particular, we find that the effective mass rapidly increases to very large values when the impurity gets strongly coupled to an otherwise weakly repulsive bath. This heavy impurity hardly moves within the medium, thereby realizing the “self-localization” regime of the Landau-Pekar polaron. Furthermore, we compare our results with predictions of perturbation theory valid for weak interactions and with exact solutions available when the bosons in the medium behave as impenetrable particles.

In this paper, a Bose-Hubbard extension of a Weyl semimetal is proposed that can be realized for ultracold atoms using laser-assisted tunneling and Feshbach resonance technique in three-dimensional optical lattices. The global phase diagram is obtained consisting of a superfluid phase and various Mott insulator phases by using Landau theory. The Bogoliubov excitation modes for the weakly interacting case have nontrivial properties (Weyl nodes, bosonic surface arc, etc.), analogs of those in Weyl semimetals of electronic systems, which are smoothly carried over to those of Bloch bands for the noninteracting case. The properties of the insulating phases for the strongly interacting case are explored by calculating both the quasiparticle and quasihole dispersion relation, which shows that two quasiparticle spectra touch at Weyl nodes.

We present a quantum theory for a fully coupled hybrid optomechanical system where all mutual couplings between a two-level atom, a confined photon mode, and a mechanical oscillator mode are considered. In such a configuration, new quantum interference effects and correlations arise due to the interplay and competition between the different physical interactions. We present an analytical diagonalization of the related fully coupled Hamiltonian, showing the nature and energy spectra of the tripartite dressed excitations. We determine the driven-dissipative dynamics of such hybrid systems and study phonon blockade effects under resonant excitation. We also study the statistical properties of the photon emission obtained under incoherent pumping of the two-level atom, which is particularly relevant for experiments with solid-state two-level emitters.

We present a physical setup for realizing all-real-spectrum optical potentials with arbitrary gain-and-loss distributions in a coherent medium consisting of a cold three-level atomic gas driven by control and probe laser fields. We show that by the interference of Raman resonances and the Stark shift induced by a far-detuned laser field, tunable, non-parity-time (non-$\mathcal{PT})$-symmetric optical potentials with all-real spectra proposed recently by Nixon and Yang [Phys. Rev. A 93, 031802(R) (2016)] can be actualized physically. We also show that when the real parts of the non-$\mathcal{PT}$-symmetric optical potentials are tuned cross certain thresholds, phase transitions—where the eigenspectrum of the system changes from all real to complex—may occur and hence the stability of the probe-field propagation is altered. Our scheme can also be extended to high dimensions and to a nonlinear propagation regime, where stable optical solitons with power of the order of nano-Watts may be generated in the system.

We consider the quantum harmonic oscillator in contact with a finite-temperature bath, modeled by the Caldeira-Leggett master equation. Applying periodic kicks to the oscillator, we study the system in different dynamical regimes between classical integrability and chaos, on the one hand, and ballistic or diffusive energy absorption, on the other. We then investigate the influence of the heat bath on the oscillator in each case. Phase-space techniques allow us to simulate the evolution of the system efficiently. In this way, we calculate high-resolution Wigner functions at long times, where the system approaches a quasistationary cyclic evolution. Thereby, we perform an accurate study of the thermodynamic properties of a nonintegrable, quantum chaotic system in contact with a heat bath at finite temperature. In particular, we find that the heat transfer between harmonic oscillator and heat bath is governed by Fourier's law.

To explain the conceptual gap between classical and quantum and other, hypothetical descriptions of the world, several principles have been proposed. So far, all these principles have not explicitly included the uncertainty relation. Here we introduce an information content principle (ICP) which represents a constrained uncertainty principle. The principle, by taking into account the encoding and decoding properties of a single physical system, is capable of separation, both classicality and quanta from a number of potential physical theories, including hidden variable theories. The ICP, which is satisfied by both classical and quantum theory, states that the amount of nonredundant information which may be extracted from a given system is bounded by a perfectly decodable information content of the system. We show that ICP allows one to discriminate theories which do not allow for correlations stronger than Tsirelson's bound. We show also how to apply the principle to composite systems, ruling out some theories despite that their elementary constituents behave quantumly.

We present a study of the ultrafast dissociation dynamics of the lowest-lying electronic excited states in ${\mathrm{CO}}_2$ by using ultraviolet (UV) and extreme-ultraviolet (XUV) pulses from high-order harmonic generation. We observe two primary dissociation channels: a direct dissociation channel along the ${}^1{\mathrm{\Pi }}_g$ electronically excited manifold, and a second channel which results from the mixing of electronic states. The direct dissociation channel is found to have a lifetime which is shorter than our experimental resolution, whereas the second channel has a significantly longer lifetime of nearly 200 fs. In this long-lived channel we observe a beating of the vibrational populations with a period of $\sim 133$ fs.

Photoelectron angular distributions from helium atoms are measured using the circularly polarized extreme ultraviolet (XUV) vortex produced by a helical undulator as the higher harmonics of its radiation. The XUV vortex has a helical wave front and carries orbital angular momentum as well as the spin angular momentum associated with its circular polarization. While the violation of the electric dipole transition rules has been predicted for interactions between vortices and atoms, the photoelectron angular distributions are well reproduced by assuming electric dipole transitions only. This observation can be explained by the localized nature of the helical phase effect of the vortex on the interaction with atoms, and demonstrates that nondipole interactions induced by the XUV vortex are hardly observable in conventional gas-phase experiments.

In this paper we study the coarsening dynamics of a spinor condensate quenched into an easy-axis ferromagnetic phase by a sudden change in the quadratic Zeeman energy. We show that applying a spin rotation prior to changing the Zeeman energy accelerates the development of local order and reduces heating. We examine the longitudinal spin ordering and the superfluid ordering of the system and show that the respective order parameter correlation functions exhibit dynamic scaling in the late-time dynamics. Our results also demonstrate that these two types of order grow at different rates, i.e., with different dynamic critical exponents. The spin domain area distribution is calculated and is shown to have power-law scaling behavior expected from percolation theory.