We analyze strong correlation effects and topological properties of interacting fermions with a Falicov-Kimball-type interaction in circularly shaken hexagonal optical lattices, which can be effectively described by the Haldane-Falicov-Kimball model, using the real-space Floquet dynamical mean-field theory (DMFT). The Haldane model, a paradigmatic model of the Chern insulator, is experimentally relevant, because it has been realized using circularly shaken hexagonal optical lattices. We show that in the presence of staggering a charge density wave emerges, which is affected by interactions and resonant tunneling. We demonstrate that interactions smear out the edge states by introducing a finite life time of quasiparticles. Even though a general method for calculating the topological invariant of a nonequilibrium steady state is lacking, we extract the topological invariant using a Laughlin charge pump setup. We find and attribute to the dissipations into the bath connected to every lattice site, which is intrinsic to real-space Floquet DMFT methods, that the pumped charge is not an integer even for the noninteracting case at very low reservoir temperatures. Furthermore, using the rate equation based on the Floquet-Born-Markov approximation, we calculate the charge pump from the rate equations for the noninteracting case to identify the role of the spectral properties of the bath. Starting from this approach we propose an experimental protocol for measuring quantized charge pumping.

In this paper we examine the spectral changes in a white light laser filament due to different pulse shapes generated by a pulse-shaping setup. We particularly explore how the properties of the filament spectra can be controlled by parametrically tailored white light pulses. The experiments are carried out in a gas cell with up to 9 bars of argon. Plasma generation and self-phase modulation strongly affect the pulse in the spectral and temporal domains. By exploiting these effects we show that the pulse spectrum can be modified in a desired way by either using second-order parametric chirp functions to shift the filament spectrum to higher or lower wavelengths, or by optimizing pulse shapes with a genetic algorithm to generate more complex filament spectra. This paper is an example of the application of complex, parametrically shaped white light pulses.

We introduce collective geometric phases of bosons and fermions interfering on a linear unitary multiport, where each phase depends on the internal states of identical particles (i.e., not affected by the multiport) and corresponds to a cycle of the symmetric group. We show that quantum interference of $N$ particles in generic pure internal states, i.e., with no pair being orthogonal, is governed by $(N-1)(N-2)/2$ independent triad phases (each involving only three particles). The deterministic distinguishability, preventing quantum interference with two or three particles, allows for the genuine $(N\ge 4)$-particle phase (interference) on a multiport: setting each particle to be deterministically distinguishable from all others except two by their internal states allows for a novel (circle-dance) interference of $N\ge 4$ particles governed by a collective $N$-particle phase, while simultaneously preventing the $R$-particle interference for $3\le R\le N-1$. The genuine $N$-particle interference manifests the $N\mathrm{th}$-order quantum correlations between identical particles at a multiport output; it does not appear in the marginal probability for a subset of the particles, e.g., it cannot be detected if at least one of the particles is lost. This means that the collective phases are not detectable by the usual “quantumness” criteria based on the second-order quantum correlations. The results can be useful for quantum computation, quantum information, and other quantum technologies with single photons.

Quantum Darwinism and spectrum broadcast structure describe the emergence of objectivity in quantum systems. However, it is unclear whether these two frameworks lead to consistent predictions on the objectivity of the state of a quantum system in a given scenario. In this paper, we jointly investigate quantum Darwinism and spectrum broadcasting, as well as the subdivision of quantum Darwinism into accessible information and quantum discord, in a two-level system interacting with an $N$-level environment via a random matrix coupling. We propose a partial trace method to suitably and consistently partition the effective $N$-level environment and compare the predictions with those obtained using the partitioning method proposed by Perez [Phys. Rev. A 81, 052326 (2010)]. We find that quantum Darwinism can apparently emerge under the Perez trace even when spectrum broadcast structure does not emerge, and the majority of the quantum mutual information between system and environment fractions is in fact quantum in nature. This work therefore shows there can be discrepancies between quantum Darwinism and the nature of information and spectrum broadcast structure.

We study single-qutrit gates composed of Clifford and $T$ gates, using the qutrit version of the $T$ gate proposed by Howard and Vala [M. Howard and J. Vala, Phys. Rev. A 86, 022316 (2012)]. We propose a normal form for single-qutrit gates analogous to the Matsumoto-Amano normal form for qubits. We prove that the normal form is optimal with respect to the number of $T$ gates used and that any string of qutrit Clifford+$T$ operators can be put into this normal form in polynomial time. We also prove that this form is unique and provide an algorithm for exact synthesis of any single-qutrit Clifford+$T$ operator.

We theoretically investigate the two-dimensional photoelectron momentum distributions (PMDs) of ${\mathrm{F}}^{-}$ ions in an orthogonal two-color laser field with equal intensities. The PMDs for different atomic orbitals are simulated by an exact solution to the three-dimensional time-dependent Schrödinger equation and the strong-field approximation method, respectively. Through the comparison of the calculations of these methods, we confirm that the asymptotic behavior of initial bound-state wave function plays a crucial role in forming the main shape of PMDs at large momenta. Based on the saddle-point method and the imaginary time theory, we show that the PMDs of ${\mathrm{F}}^{-}$ ions can be decoded to reveal the definite imprint of the photoelectron sub-barrier phase from the subcycle interference structures. We demonstrate the sub-barrier phases from different atomic orbitals have different impacts on the subcycle interference structures.

Since the dawn of quantum theory, coherence has been attributed as a key to understand the weirdness of fundamental concepts such as, e.g., the wave-particle duality and the Stern-Gerlach experiment. Recently, based on a resource theory approach, the notion of quantum coherence was revisited and a plethora of coherence quantifiers was proposed. In this work, we address such issues employing the language of coherence orders developed by the NMR community. This allowed us to investigate the role played by different subspaces of the Hilbert-Schmidt space into physical processes and quantum protocols. We found some links between decoherence and each coherence order. Moreover, we propose a sufficient and straightforward method to testify to the usefulness of a given state for quantum enhanced phase estimation, relying on a minimal set of elements belonging to the density matrix.

We propose here a method for the construction of the Lewis and Riesenfeld time-dependent invariants. Instead of directly invoking the equation that defines the invariant associated with a time-dependent Hamiltonian, our method allows the step-by-step construction of the invariant as a product of operators, and mainly a straightforward derivation of its eigenvalue equation.

We undertake experimental detection of the entanglement present in arbitrary three-qubit pure quantum states on an NMR quantum information processor. Measurements of only four observables suffice to experimentally differentiate between the six classes of states which are inequivalent under stochastic local operation and classical communication. The experimental realization is achieved by mapping the desired observables onto Pauli $z$ operators of a single qubit, which is directly amenable to measurement. The detection scheme is applied to known entangled states as well as to states randomly generated using a generic scheme that can construct all possible three-qubit states. The results are substantiated via direct full quantum state tomography as well as via negativity calculations and the comparison suggests that the protocol is indeed successful in detecting tripartite entanglement without requiring any a priori information about the states.

In the presence of symmetry, entanglement measures of quantum many-body states can be decomposed into contributions from distinct symmetry sectors. Here we investigate the decomposability of negativity, a measure of entanglement between two parts of a generally open system in a mixed state. While the entanglement entropy of a subsystem within a closed system can be resolved according to its total preserved charge, we find that negativity of two subsystems may be decomposed into contributions associated with their charge imbalance. We show that this charge-imbalance decomposition of the negativity may be measured by employing existing techniques based on creation and manipulation of many-body twin or triple states in cold atomic setups. Next, using a geometrical construction in terms of an Aharonov-Bohm-like flux inserted in a Riemann geometry, we compute this decomposed negativity in critical one-dimensional systems described by conformal field theory. We show that it shares the same distribution as the charge-imbalance between the two subsystems. We numerically confirm our field theory results via exact calculations for noninteracting particles based on a double-Gaussian representation of the partially transposed density matrix.

We present an efficient decoy-state protocol measurement-device-independent quantum key distribution without vacuum source, with the heralded single-photon source (HSPS) and global optimization. This protocol is able to give a high key rate and avoid the security problem caused by the vacuum source or extremely weak source. In the protocol, the heralded outcome pulse pairs in $X$ basis are used as decoy pairs and all outcome pairs in $Z$ basis are used as signal pairs. Each side uses three different intensities for the outcome pulses, two in $X$ basis and one in $Z$ basis. In order to efficiently optimize the key rate globally, we present an efficient method to estimate what values we will observe by using the HSPS. This efficient method greatly reduces the computation complexity from the fourfold summation to a single-fold summation in the numerical simulation and makes it operable to seek the optimization points globally.

We report cross sections for electron-impact dissociative excitation of the $B{}^1{\mathrm{\Sigma }}_u^{+}$, $C{}^1{\mathrm{\Pi }}_u$, $D{}^1{\mathrm{\Pi }}_u$, $B^{\prime }{}^1{\mathrm{\Sigma }}_u^{+}$, and $E,F{}^1{\mathrm{\Sigma }}_g^{+}$ singlet states of molecular hydrogen from all $v_i=0–14$ vibrational levels of the ground $X{}^1{\mathrm{\Sigma }}_g^{+}$ state. Calculations are performed using the adiabatic-nuclei convergent close-coupling method formulated in prolate spheroidal coordinates from threshold to 500 eV. Agreement with previous calculations varies with transition and impact energy, ranging from excellent to poor. Agreement with available experiment is generally good.

The weak-field asymptotic theory (WFAT) of tunneling ionization in a static electric field is limited by the assumption that tunneling occurs from an isolated state in a compact atomic or molecular system. In this paper, we generalize the WFAT in two directions: to the cases when tunneling ionization occurs from (i) nearly degenerate states separated by a small energy distance $\mathrm{\Delta }E$ in compact systems, and (ii) an isolated state in heteronuclear diatomic molecules at large internuclear distances $R$. The weak-field asymptotic formulas for the ionization rates in these two cases are obtained. The asymptotics are uniform with respect to $\mathrm{\Delta }E$ and $R$ and for sufficiently large $\mathrm{\Delta }E$ and small $R$, respectively, reduce to the previously known WFAT rate formula. By combining the two asymptotics, we obtain a formula for the ionization rates from the nearly degenerate $1s{\sigma }_g$ and $2p{\sigma }_u$ states of a homonuclear molecular ion ${{\mathrm{H}}_2}^{+}$ at large internuclear distances, which is of particular interest for applications in strong-field physics. The analytical results are illustrated by calculations for several atomic and molecular models.

We have observed collective spontaneous emission (superfluorescence) from ionized nitrogen molecules generated by 800-nm laser pulses. The superfluorescence intensity around 391 nm was found to be greatly enhanced when injecting an external seed with various wavelengths. Our finding provides insights into controlling the coherent emission from air filamentation induced by intense femtosecond laser fields, which has great potential application in remote atmospheric sensing.

We consider two spin-1/2 fermions inside an optical cavity which supports a single-mode quantized light field. We demonstrate that the atom-light coupling (ALC) gives rise to the two-atom polariton states, where the two atoms are highly entangled with cavity photons. We focus on the case where the cavity light is on resonant with the bare atomic transition. We show that in the absence of interatomic interaction, the polariton is unbound, has finite center-of-mass momentum, and contains no atomic spin-singlet fraction in its ground state. When strong attractive interatomic contact interaction is present, a stable bound polariton state exists when the ALC strength is below a critical value. When the ALC strength exceeds the critical value, a first-order transition is observed and the bound polariton becomes unbound. The first-order transition is characterized by abrupt changes of various quantities associated with the polariton and should be readily detectable.

Quantum interface of coherent optical field and atomic excitations plays an important role in quantum metrology and quantum information science. The electromagnetically-induced-transparency (EIT) technique has shown versatile and powerful capability in many applications during the last decades. By using efficient EIT-based memory, we directly observed single-photon-level Ramsey interference and also high-contrast spectral transparency comb with a visibility up to about 0.81 at around the resonant line in cold atoms. The interference fringes are mainly influenced by the pulse repetition period, the waveform overlap of the slowed and retrieved signal pulses, and the coherent lifetime of the EIT memory. All these factors have been investigated in detail. Such high-contrast interference fringe may offer us methods for precision measurement and potential applications in areas of atomic clock and magnetometry.

Protocols for testing or exploiting quantum correlations—such as entanglement, Bell nonlocality, and Einstein-Podolsky-Rosen steering—generally assume a common reference frame between two parties. Establishing such a frame is resource intensive and can be technically demanding for distant parties. While Bell nonlocality can be demonstrated with high probability for a large class of two-qubit entangled states when the parties have one or no shared reference direction, the degree of observed nonlocality is measurement-orientation dependent and can be arbitrarily small. In contrast, we theoretically prove that steering can be demonstrated with $100\%$ probability for a larger class of states, in a rotationally invariant manner, and experimentally demonstrate rotationally invariant steering in a variety of cases. We also show, by comparing with the steering inequality of Cavalcanti et al. [J. Opt. Soc. Am. B 32, A74 (2015)], that the steering inequality we derive is the optimal rotationally invariant one for the case of two settings per side and maximally mixed local qubit states.

We propose a scheme to generate polarized hybrid entanglement in the form of $|H\rangle |\alpha \rangle +|V\rangle |-\alpha \rangle$ by interference between a superposition of coherent states and a polarization entangled photon pair. In the scheme two sets of interferences with perpendicular polarizations are designed to erase “which path” information. The joint output fields of the two sets of interferences are used to herald the generation of the polarized hybrid entanglement. The advantage of the protocol is its experimental feasibility since it not only realizes effective interference but also retains the polarization information in the output fields of interferences. We also analyze the effect of imperfect experimental conditions on the generated hybrid state, including inefficiency on-off single-photon detectors with dark counts and available approximate resource states. The theoretical analysis shows that the fidelity of the heralded hybrid state with small amplitude can be maintained at a high level under actual experimental conditions.