We have performed time-dependent density functional theory calculations on the molecule vinyl bromide under ionization by a laser and have found long-lived coherent oscillations in the electron density created by a superposition of orbitals. The superposition of the molecular orbitals is quite pronounced for some intensities and is compared to varied alignments and strengths of the exciting laser. The coherence between the orbitals is also shown to be strongly dependent upon atomic motion.

Efficient transduction of electromagnetic signals between different frequency scales is an essential ingredient for modern communication technologies as well as for the emergent field of quantum information processing. Recent advances in waveguide photonics have enabled a breakthrough in light-matter coupling, where individual two-level emitters are strongly coupled to individual photons. Here we propose a scheme that exploits this coupling to boost the performance of transducers between low-frequency signals and optical fields operating at the level of individual photons. Specifically, we demonstrate how to engineer the interaction between quantum dots in waveguides to enable efficient transduction of electric fields coupled to quantum dots. Owing to the scalability and integrability of the solid-state platform, our transducer can potentially become a key building block of a quantum internet node. To demonstrate this, we show how it can be used as a coherent quantum interface between optical photons and a two-level system like a superconducting qubit.

In this paper, we explore the time-energy domain quantum-classical transition comparing a classical pump-probe experiment on a diatomic molecule to its quantum enhanced counterpart, where the pump and probe pulses are substituted by the signal and idler beams of a spontaneous parametric down conversion (SPDC) source. Absorption of biphotons generated with SPDC exploits quantum time-energy entanglement to enhance the overall yield and selectivity of the process, when compared with a classical pump-probe setup, while maintaining femtosecond time resolution. We systematically study the effects of correlation strength on process efficiency and selectivity, comparing the results to classical pump-probe spectra. An excitation scheme to improve the yield based on spectral narrowing of biphotons is shown. The results indicate that the quantum improvements in yield are caused by a more efficient use of the total power available for the process.

We present a method, based on Feynman path integrals, to describe the propagation and properties of the quantized electromagnetic field in an arbitrary, nonlinear medium. We provide a general theory, valid for any order of optical nonlinearity, and we then specialize the case of second-order nonlinear processes. In particular, we show that second-order nonlinear processes in arbitrary media, under the undepleted pump approximation, can be described by an effective free electromagnetic field, propagating in a vacuum, dressed by the medium itself. Moreover, we show that the probability of such processes to occur is related to the biphoton propagator, which contains information about the structure of the medium, its nonlinear properties, and the structure of the pump beam.

We define an uncertainty observable acting on several replicas of a continuous-variable state, whose measurement induces phase-space uncertainty relations for a single copy of the state. By exploiting the Schwinger representation of angular momenta in terms of bosonic operators, this observable can be constructed so as to be invariant under symplectic transformations (rotation and squeezing in phase space). We first design a two-copy uncertainty observable, which is a discrete-spectrum operator vanishing with certainty if and only if it is applied on (two replicas of) any pure Gaussian state centered at the origin. The non-negativity of its variance translates into the Schrödinger-Robertson uncertainty relation. We then extend our construction to a three-copy uncertainty observable, which exhibits additional invariance under displacements (translations in phase space) so that it vanishes on every pure Gaussian state. The resulting invariance under all Gaussian unitaries makes this observable a natural tool to capture the phase-space uncertainty (or the deviation from pure Gaussianity) of continuous-variable bosonic states. In particular, it suggests that the Shannon entropy associated with the measurement of this observable provides a symplectic-invariant entropic measure of uncertainty in position-momentum phase space.

We present an algorithm to simulate genuine, measurement-conditioned quantum trajectories for a class of non-Markovian systems, using a collision model for the environment. We derive two versions of the algorithm: the first corresponding to photodetection and the second to homodyne detection with a finite local oscillator amplitude. We use the algorithm to simulate trajectories for a system with delayed coherent feedback, as well as a system with a continuous memory.

Quantum circuits solely comprising matchgates can perform nontrivial (but nonuniversal) quantum algorithms. Because matchgates can be mapped to noninteracting fermions, these circuits can be efficiently simulated on a classical computer. Universal quantum computation is attainable by adding any nonmatchgate parity-preserving gate, from which one may infer that interacting fermions are natural candidates for universal quantum computation. We consider the quantum computational power of fermions hopping on a one-dimensional double-well lattice within the context of matchgates. In particular, we show that universal quantum computation can be implemented using spinless (spin-polarized) fermions and nearest-neighbor interactions, as well as with spin-half fermions with on-site interactions (i.e., the Hubbard model). We suggest that these schemes are currently within reach in the context of ultracold atomic gases.

Three-qubit mixed states are used as a channel for controlled quantum teleportation (CQT) of single-qubit states. The connection between different channel parameters to achieve maximum controlled teleportation fidelity is investigated. We show that for a given multipartite entanglement and mixedness a class of non-maximally-entangled mixed $X$ states ($X$-NMEMS) achieves optimum controlled quantum teleportation fidelity; interestingly a class of maximally entangled mixed $X$ states ($X$-MEMS) fails to do so. This demonstrates, for a given spectrum and mixedness, that $X$-MEMS are not sufficient to attain optimum controlled quantum teleportation fidelity, which is in contradiction with the traditional quantum teleportation of single qubits. In addition, we show that biseparable $X$-NMEMS, for a certain range of mixedness, are useful as a resource to attain high controlled quantum teleportation fidelity, which essentially lowers the requirements of quantum channels for CQT.

Formation mechanism of ${\mathrm{H}}_3^{+}$ ions from doubly charged hydrocarbons ($\mathrm{C}{\mathrm{H}}_4$ and ${\mathrm{C}}_2{\mathrm{H}}_4$) is investigated by combining charged particle (300 eV electrons and 3 keV/u ${\mathrm{Ar}}^{8+}$ ions) collision experiments and quantum chemistry calculations. The kinetic energy release (KER) distribution for each ${\mathrm{H}}_3^{+}$ loss process was measured with the cold target recoil ion momentum spectroscopy. The good agreement between the mean KER and corresponding theoretical reverse activation energy, related to a transition state and the asymptote of the dissociation products, provides information on the ${\mathrm{H}}_3^{+}$ formation dynamics. A ${\mathrm{H}}_2$ roaming mechanism is proposed for the formation of ${\mathrm{H}}_3^{+}$. Since the hydrocarbons in the interaction with charged particles have direct interests in astronomical environments, the finding of this work would be helpful in the understanding of the energetics and dynamics of ${\mathrm{H}}_3^{+}$ formation in space.

We demonstrate the emergence of a time crystal of atoms in a high-finesse optical cavity driven by a phase-modulated transverse pump field, resulting in a shaken lattice. This shaken system exhibits macroscopic oscillations in the number of cavity photons and order parameters at noninteger multiples of the driving period, which signals the appearance of an incommensurate time crystal. The subharmonic oscillatory motion corresponds to dynamical switching between symmetry-broken states, which are nonequilibrium bond ordered density wave states. Employing a semiclassical phase-space representation for the driven-dissipative quantum dynamics, we confirm the rigidity and persistence of the time crystalline phase. We identify experimentally relevant parameter regimes for which the time crystal phase is long lived, and map out the dynamical phase diagram. We compare and contrast the incommensurate time crystal with the commensurate Dicke time crystal in the amplitude-modulated case.

Recently, a novel kind of hybrid atom-optomechanical system, consisting of atoms in a lattice coupled to a membrane, has been experimentally realized [A. Vochezer et al., Phys. Rev. Lett. 120, 073602 (2018)], which promises a viable contender in the competitive field of simulating nonequilibrium many-body physics. Here we are motivated to investigate a spinor Bose gas coupled to a vibrational mode of a nanomembrane, focusing on analyzing the role of the spinor degrees of freedom therein. Through an adiabatic elimination of the degrees of freedom of the quantum oscillator, we derive an effective Hamiltonian which reveals a competition between the force localizing the atoms and the membrane displacement. We analyze the dynamical stability of the steady state using the Bogoliubov–de Gennes approach and derive the stationary phase diagram in the parameter space. Then, we investigate the first-order nonequilibrium quantum phase transition (NQPT) from a localized symmetric state of the atom cloud to a shifted symmetry-broken state, characterized by the occurrence of a hysteresis. Moreover we present a detailed analysis of the effects of the spin degree of freedom on NQPT. Our work presents a simple way to study the effects of the spinor degree of freedom on the nonequilibrium nonlinear phenomena that is complementary to ongoing experiments on the hybrid atom-optomechanical system.

Via the hierarchy of correlations, we study the Mott insulator phase of the Fermi-Hubbard model in the limit of strong interactions and derive a quantum Boltzmann equation describing its relaxation dynamics. In stark contrast to the weakly interacting case, we find that the scattering cross sections strongly depend on the momenta of the colliding quasiparticles and holes. Therefore, the relaxation towards equilibrium crucially depends on the spectrum of excitations. For example, for particle-hole excitations directly at the minimum of the (direct) Mott gap, the scattering cross sections vanish such that these excitations can have a very long lifetime.

Up to now it remains challenging to couple photons from a circularly polarized emitter into a photonic structure to simultaneously realize strong photon-emitter interaction and unidirectional propagation locked by local helicity of the optical mode at the nanoscale. In this paper we propose a unique approach that combines a photonic crystal and metallic nanoparticle structure to create nanocavities with both strong local-field intensity and high helicity. In this system the rate of circularly polarized photons emitting into the photonic crystal waveguide reaches $148{\gamma }_0$, which is one order of magnitude larger than that without the nanoparticle, and in the ultranarrow band-edge mode the linewidth of Rabi splitting spectra is about one-tenth of that with the nanoparticle only, both with $\approx 95\%$ of photons propagating unidirectionally along the nanoscale waveguide. We suggest that our paper establishes a nanophotonic interface of chiral quantum electrodynamics for on-chip nonreciprocal quantum light sources, quantum circuits, and scalable quantum networks.

We propose an approach to generate strong quantum entanglement by the controllable four-wave-mixing mechanism in a single-cavity, weak-coupling optomechanical system. The optomechanical system is driven by a strong two-tone pump field and a weak signal field, simultaneously. The two-tone pump field consists of a lower and an upper sideband, which couple with the optical cavity and mechanical resonator, and generate the beam-splitter and two-mode squeezing interactions under the rotating-wave approximation. This interaction mechanism modifies the effective susceptibility of the optomechanical cavity and optomechanically induces a four-wave-mixing process. Strong quantum entanglement can be generated between the signal and four-wave-mixing fields in realistic optomechanical systems. The generation scheme of the quantum entanglement is quite robust against thermal-mechanical noise, and significant entanglement can persist at room temperature in the weak-coupling regime.

Floquet engineering is one of the most vigorous fields in periodically driven (Floquet) systems, with which we can control phases of matter usually by high-frequency drives. In this paper, with Floquet engineering by a combination of high-frequency drives and resonant drives, we propose a way to realize nontrivial topological phases protected by a ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry only in the presence of a ${\mathbb{Z}}_2$ symmetry, using a robust emergent ${\mathbb{Z}}_2$ symmetry induced by the resonant drives. Moreover, the symmetry protected topological (SPT) phases are switchable between nontrivial and trivial phases only by the direction of a static transverse field, and even perturbations on the resonant drive can be utilized to realize richer SPT phases. We also discuss the real-time dynamics of the model, and find that which topological phases the system lies in can be distinguished by a period doubling of a nonlocal order parameter, as with discrete time crystals. A realization or a control of nontrivial SPT phases without the required symmetries by resonant drives, proposed in this paper, would shed a light on the observation of topological phenomena in nonequilibrium setups.

To explain aspects of the quantum-to-classical transition, quantum Darwinism explores the fact that, due to interactions between a quantum open system and its surrounding environment, information about the system can be spread redundantly to the environment. Here we recall that there are in the literature two distinct and nonequivalent ways to make this statement precise and quantitative. We first point out the difference with some simple but illustrative examples. We then consider a model where Darwinism can be seen from both perspectives. Moreover, the non-Markovianity of our model can be varied with a parameter. In a recent work [F. Galve et al., Sci. Rep. 6, 19607 (2016)], the authors concluded that quantum Darwinism can be hindered by non-Markovianity. We depart from their analysis and argue that, from both perspectives to quantum Darwinism, there is no clear relationship between non-Markovianity and quantum Darwinism in our model.

Open quantum walks often lead to a classical asymptotic behavior. Here we look for a simple open quantum walk whose asymptotic behavior can be nonclassical. We consider a discrete-time quantum walk on an $n$-cycle subject to a random coin-dependent phase shift at a single position. This finite system, whose evolution is described by only two Kraus operators, can exhibit all kinds of asymptotic behavior observable in quantum Markov chains: It either evolves towards a maximally mixed state, or partially mixed state, or tends to an oscillatory motion on an asymptotic orbit. We find that the asymptotic orbits do not have a product structure; therefore, the corresponding states can manifest entanglement between the position and the coin degrees of freedom, even if the system started in a product state.

It is crucial to know whether the quantum state generated in the experiment is entangled. In the literature, this topic was studied extensively and researchers proposed different approaches for the detection of a mixed bipartite entangled state in arbitrary dimension. Proceeding in this line of research, we propose three different criteria for the detection of mixed bipartite negative partial transpose (NPT) entangled state in arbitrary dimension. Our criteria are based on the method of structural physical approximation (SPA) of partial transposition (PT). We show that the proposed criteria for the detection of a NPT entangled state can be realized experimentally. Two of the proposed criteria are given in terms of the concurrence of the given state in arbitrary dimension so it is essential to find out the concurrence. Thus, we provide lower and upper bounds of concurrence of the quantum state under investigation in terms of average fidelity of two quantum states, and hence these bounds can be realized experimentally. Moreover, we show how to perform a SPA map on a qutrit-qubit system and then explicitly calculate the matrix elements of the density matrix describing the SPA-PT of the qutrit-qubit system. We then illustrate our criteria for the detection of entanglement by considering a class of qubit-qubit system and a class of qutrit-qubit system.