In quantum information and communication two fundamental nonclassical features that one exploits in harnessing the true power of composite quantum systems are the interference and quantum correlations. We show how a concept akin to interference can be connected to entanglement and quantum correlation. In particular, we prove that the difference in the squared visibility for the density operator before and after a complete measurement, averaged over all unitary evolutions, is related to the quantum correlation measure based on the measurement disturbance. For pure and mixed bipartite states the unitary average of the squared visibility is related to the entanglement measure. This may provide a method to detect entanglement and quantum correlation with quantum interference setups. Furthermore, we prove that for a fixed purity of the subsystem state, there is a complementarity relation between the linear entanglement of formation and the measurement disturbance. This brings out a quantitative difference between two kinds of quantum correlations and suggests that the origins of these two can be different.

We demonstrate deceleration and trapping of methylfluoride $\left({\mathrm{CH}}_3\mathrm{F}\right)$ molecules in the low-field-seeking component of the $J=1,K=1$ state using a combination of a conventional Stark decelerator and a traveling wave decelerator. The methylfluoride molecules are detected by nonresonant multiphoton ionization using a femtosecond laser. Subsequent mass and velocity selection of the produced ions enables us to eliminate most background signal resulting from thermal gas in our vacuum chamber. This detection method can be applied to virtually any molecule, thereby enhancing the scope of molecules that can be Stark decelerated. Methylfluoride is so far the heaviest and most complex molecule that has been decelerated to rest. Typically we trap $2\times {10}^4 {\text{CH}}_3\text{F}$ molecules at a peak density of $4.5\times {10}^7 {\text{cm}}^{-3}$ and a temperature of 40 mK.

We theoretically analyze a discrete Schrödinger chain with hopping to the first and second neighbors, as can be realized with zigzag arrangements of optical waveguides or lattice sites for cold atoms. Already at moderate values, second-neighbor hopping has a strong impact on the band structure, leading to the emergence of a new extremum located inside the band, accompanied by a van Hove singularity in the density of states. The energy band is then divided into a subcritical regime, with the usual unique correspondence between wave number and energy of the traveling waves, and a supercritical regime, in which waves of different wave number are degenerate in energy. We study the consequences of these features in a scattering setup, introducing a defect that locally breaks the translational invariance. The notion of a local probability current is generalized beyond the nearest-neighbor approximation and bound states with energies outside the band are discussed. At subcritical energies inside the band, an evanescent mode coexists with the traveling plane wave, giving rise to resonance phenomena in scattering. At weak coupling to the defect, we identify a prototypical Fano-Feshbach resonance of tunable shape and provide analytical expressions for its profile parameters. At supercritical energies, we observe coupling of the degenerate traveling waves, leading to an intricate wave-packet fragmentation dynamics. The corresponding branching ratios are analyzed.

We investigate the celebrated problem of the one-dimensional tight-binding model in the presence of disorder leading to Anderson localization from a novel perspective. A binary disorder is assumed to be created by immobile, heavy particles that affect the motion of the lighter, mobile species in the limit of no interaction between mobile particles. Fast, periodic modulations of interspecies interactions allow us to produce an effective model with small diagonal and large off-diagonal disorder previously unexplored in cold-atom experiments. We present an expression for an approximate Anderson localization length and verify the existence of the well-known, extended resonant mode. We also analyze the influence of nonzero next-nearest-neighbor hopping terms. We point out that periodic modulation of interaction allows disorder to work as a tunable bandpass filter for momenta.

We study drag dynamics of several fermions in a fermion cloud in one-dimensional continuous systems, with particular emphasis on the nontrivial quantum many-body effects in systems whose parameters change gradually in real time. We adopt the Fermi-Hubbard model and the time-dependent density-matrix renormalization group (DMRG) method to calculate the drag force on a trapped fermion cluster in a cloud of another fermion species with contact interaction. The simulation result shows that a nontrivial peak in the resistance force is observed in the high-cloud-density region, which implies a criterion of effective ways in diffusive transport in a fermion cloud. We compare the DMRG simulation result with a mean-field result, and it is suggested that some internal degrees of freedom have a crucial role in the excitation process when the cloud density is high. This work emphasizes the difference between the full-quantum calculation and the semiclassical calculation, which is the quantum effects, in slow dynamics of many-body systems bound in a fermion cloud.

We study irreversible ultrafast dynamics caused by interaction of a semiconductor quantum-dot–metallic-nanorod system with an infrared laser field. We show that when this system supports exciton-plasmon coupling, by just varying the amplitude of this laser for a short period of time (several nanoseconds), one can decide the instance when the plasmon field of the nanorod becomes significant and its duration. This is done by showing that a sudden rise in the amplitude of the infrared laser (positive pulse) can induce irreversible transition from one of the collective molecular states of this system to another, making the plasmon field significant. When this amplitude reduces for a short period of time (negative pulse), the system returns back to its initial state, suppressing this field. We provide a detailed description of how, depending on the location, the infrared-induced dynamics can lend itself to different time-dependent plasmon fields around the nanorod. Our results show that at a given moment of time at each location we can have dramatically different types of dynamics for the phase and amplitude of the plasmon field. Using these we show that a quantum-dot–metallic-nanoparticle system can act as an all-optical and logic gate.

We develop a simple analytical model for calculating the vibrational contribution to the nonlinear refractive index $n_2$ (Kerr coefficient) of a crystal in terms of known crystalline parameters such as the linear refractive index, the coefficient of thermal expansion, the atomic density, and the reduced mass and the natural oscillation frequency of the vibrational modes of the crystal lattice. We show that the value of this contribution in the terahertz spectral region can exceed the value of the nonlinear refractive index $n_2$ in the visible and near-IR spectral ranges (which is largely electronic in origin) by several orders of magnitude. For example, for crystal quartz the value of the Kerr coefficient in the low-frequency limit is $n_2=2.2\times {10}^{-9}$ esu or, equivalently, $4.4\times {10}^{-16}{\mathrm{m}}^2$/W, which is very much larger than its value of $3\times {10}^{-20}{\mathrm{m}}^2$/W in the visible range. Furthermore, we present an analysis of the dispersion of $n_2$ in the terahertz spectral range and show that even larger values of $n_2$ occur at frequencies close to the vibrational resonances.

The program of classifying symmetry-protected topological (SPT) phases in one dimension has been recently completed and has opened the doors to studying closely the properties of systems belonging to these phases. It was recently found that being able to constrain the form of ground states of SPT order based on symmetry properties also makes it possible to explore novel resource states for processing of quantum information. In this paper, we generalize the consideration of Else et al. [Phys. Rev. Lett. 108, 240505 (2012)], where it was shown that the ground-state form of spin-1 chains protected by ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry supports perfect operation of the identity gate, important also for long-distance transmission of quantum information. We develop a formalism to constrain the ground-state form of SPT phases protected by any arbitrary finite symmetry group and use it to examine examples of ground states of SPT phases protected by various finite groups for similar gate protections. We construct a particular Hamiltonian invariant under $A_4$ symmetry transformation, which is one of the groups that allows protected identity operation, and examine its ground states. We find that there is an extended region where the ground state is the AKLT state, which not only supports the identity gate but also arbitrary single-qubit gates.

We provide an upper bound on the maximal rate at which irreversible quantum dynamics can generate entanglement in a bipartite system. The generator of irreversible dynamics consists of a Hamiltonian and dissipative terms in Lindblad form. The relative entropy of entanglement is chosen as a measure of entanglement in an ancilla-free system. We provide an upper bound on the entangling rate which has a logarithmic dependence on a dimension of a smaller system in a bipartite cut. We also investigate the rate of change of quantum mutual information in an ancilla-assisted system and provide an upper bound independent of dimension of ancillas.

Employing advanced methods in the relativistic coupled-cluster framework, the electric dipole moments (EDM) of ${}^{225}\mathrm{Ra}$ due to parity- and time-reversal-violating tensor-pseudotensor (T-PT) and nuclear Schiff moment (NSM) interactions are obtained as $d_A=-10.01\times {10}^{-20}{C}_T\langle {\sigma }_n\rangle \left|e\right|$ cm and $d_A=-6.79\times {10}^{-17}S\left(\right|e|{\mathrm{fm}}^3{)}^{-1}\left|e\right|$ cm, respectively, with $C_T$ being the T-PT coupling constant and $S$ being NSM. These values for the corresponding T-PT and NSM interactions are reduced by about 45% and 23%, respectively, compared to the previous calculations. The validity of our calculations is proved by comparing our results with the earlier studies using the zeroth-order Dirac-Fock method and all-order random-phase approximation. The first measurement of ${}^{225}\mathrm{Ra}$ EDM was reported recently [R. H. Parker et al., Phys. Rev. Lett. 114, 233002 (2015)], and in that study the authors also anticipate obtaining the result with an improvement in systematics and the statistical sensitivity of the experiment, which could possibly lead to the best limit for an atomic EDM. Thus, it offers considerable hope to extract more accurate limits for the electron-quark T-PT interaction and the ${\theta }_{QCD}$ parameter in particle physics in the future.

We present the results of a theoretical study of dissociative electron attachment (DEA) of low-energy electrons to ${\mathrm{CF}}_2$. We carried out electron scattering calculations using the complex Kohn variational method at the static-exchange and relaxed self-consistent field (SCF) level at the equilibrium geometry and compare our differential cross sections to other results. We then repeated these calculations as a function of the three internal degrees of freedom to obtain the resonance energy surfaces and autoionization widths. We use this data as input to form the Hamiltonian relevant to the nuclear dynamics. The multidimensional wave equation is solved using the multiconfiguration time-dependent Hartree (MCTDH) approach within the local approximation.

The cold weakly bound ${\mathrm{Rb}}_2$ molecule dressed by a standing linear polarized laser wave is investigated. In such a case the light-induced conical intersections (LICIs) occur. We demonstrate that when the LICI conditions are fulfilled the transition from the weakly trapped rubidium dimer in the red-detuned optical lattice to the strongly trapped rubidium dimer in the blue-detuned optical lattice takes place. A significant localization enhancement of the molecule is observed.

We experimentally construct a three-qubit entangled $W$ superposition $\left(W\overline{W}\right)$ state on an NMR quantum information processor. We give a measurement-based filtration protocol for the invertible local operation (ILO) that converts the $W\overline{W}$ state to the Greenberger-Horne-Zeilinger (GHZ) state, using a register of three ancilla qubits. Further we implement an experimental protocol to reconstruct full information about the three-party $W\overline{W}$ state using only two-party reduced density matrices. An intriguing fact unearthed recently is that the $W\overline{W}$ state, which is equivalent to the GHZ state under an ILO, is in fact reconstructible from its two-party reduced density matrices, unlike the GHZ state. We hence demonstrate that, although the $W\overline{W}$ state is interconvertible with the GHZ state, it stores entanglement very differently.

Interference phenomena of quantum systems have been studied in the context of fundamental aspects of quantum physics and are considered a necessary resource for quantum information. Here we investigate the interference of multiparticle wave packets in terms of modular variables, which is a natural and convenient way to describe two or more interfering wave functions. Through the modular-variable description, interesting phenomena appear such as the complementarity between the number of wave packets and the width of the peaks of the momentum distribution. In the multipartite case, this effect produces quantum entanglement. We derive entanglement criteria that test for bipartite entanglement in generic bipartitions of a multipartite quantum state and use these criteria to test for genuine $D$-partite entanglement.

We present and define a privacy-preserving problem called the oblivious set-member decision problem, which allows a server to decide whether a private secret of a user is a member of his private set in an oblivious manner. Namely, if the secret belongs to his private set, he does not know which member it is. We propose a quantum solution to the oblivious set-member decision problem. Compared to classical solutions, the proposed quantum protocol achieves an exponential reduction in communication complexity, since it only needs $O\left(1\right)$ communication cost.

Iridium ions near $4f-5s$ level crossings are the leading candidates for a new type of atomic clocks with a high projected accuracy and a very high sensitivity to the temporal variation of the fine structure constant $\alpha$. To identify spectra of these ions in experiment accurate calculations of the spectra and electromagnetic transition probabilities should be performed. Properties of the $4f\text{-core-excited}$ states in ${\mathrm{Ir}}^{16+},{\mathrm{Ir}}^{17+}$, and ${\mathrm{Ir}}^{18+}$ ions are evaluated using relativistic many-body perturbation theory and Hartree-Fock-relativistic method (COWAN code). We evaluate excitation energies, wavelengths, oscillator strengths, and transition rates. Our large-scale calculations included the following set of configurations: $4f^{14}5s,4f^{14}5p,4f^{13}5s^2,4f^{13}5p^2,4f^{13}5s5p,4f^{12}5s^{2}5p$, and $4f^{12}5s5p^2$ in Pm-like ${\mathrm{Ir}}^{16+}$; $4f^{14},4f^{13}5s,4f^{13}5p,4f^{12}5s^2,4f^{12}5s5p$, and $4f^{12}5p^2$ in Nd-like ${\mathrm{Ir}}^{17+}$; and $4f^{13},4f^{12}5s,4f^{12}5p,4f^{11}5s^2$, and $4f^{11}5s5p$ in Pr-like ${\mathrm{Ir}}^{18+}$. The $5s-5p$ transitions are illustrated by the synthetic spectra in the 180–200 Å range. Large contributions of magnetic-dipole transitions to lifetimes of low-lying states in the region below 2.5 Ry are demonstrated.

We study the dynamics of a single-photon pulse traveling through a linear atomic chain coupled to a one-dimensional (1D) single mode photonic waveguide. We derive a time-dependent dynamical theory for this collective many-body system which allows us to study the real time evolution of the photon transport and the atomic excitations. Our analytical result is consistent with previous numerical calculations when there is only one atom. For an atomic chain, the collective interaction between the atoms mediated by the waveguide mode can significantly change the dynamics of the system. The reflectivity of a photon can be tuned by changing the ratio of coupling strength and the photon linewidth or by changing the number of atoms in the chain. The reflectivity of a single-photon pulse with finite bandwidth can even approach $100\%$. The spectrum of the reflected and transmitted photon can also be significantly different from the single-atom case. Many interesting physical phenomena can occur in this system such as the photonic band-gap effects, quantum entanglement generation, Fano-like interference, and superradiant effects. For engineering, this system may serve as a single-photon frequency filter, single-photon modulation, and may find important applications in quantum information.

Laser interferometric gravitational-wave detectors implement Fabry-Pérot cavities to increase their peak sensitivity. However, this is at the cost of reducing their detection bandwidth, which originates from the propagation phase delay of the light. The “white-light-cavity” idea, first proposed by Wicht et al. [Opt. Commun. 34, 431 (1997)], is to circumvent this limitation by introducing anomalous dispersion, using a double-pumped gain medium, to compensate for such a phase delay. In this article, starting from the Hamiltonian of the atom-light interaction, we apply an input-output formalism to evaluate the quantum noise of the system. We find that apart from the additional noise associated with the parametric amplification process noted by others, the stability condition for the entire system poses an additional constraint. By surveying the parameter regimes where the gain medium remains stable (not lasing) and stationary, we find that there is no net enhancement of the shot-noise-limited sensitivity. Therefore, other gain media or different parameter regimes should be explored for realizing the white-light cavity.

We study the problem of closing the detection loophole in three-qubit Bell tests, the experimentally most relevant case beyond the usual bipartite scenario, and show that the minimal detection efficiencies required can be considerably lowered compared to the two-qubit case. The lowest reported detection efficiency thresholds for two and three qubits so far are $\sim 66.7$ and $60\%$, respectively. Using the three-qubit $W$ state and a three-setting Bell inequality, we beat these thresholds and with an eight-setting Bell inequality we reach $50.13\%$. We also investigate generic three-qubit states which allow us to attain a detection efficiency of $50\%$ in a four-setting Bell test. We conjecture that the limit of $50\%$ is unbeatable using three-qubit states and any number of measurements.

Entanglement can effectively increase communication channel capacity as evidenced by dense coding that predicts a capacity gain of $1\text{bit}$ when compared to entanglement-free protocols. However, dense coding relies on Bell states and when implemented using photons the capacity gain is bounded by $0.585\text{bits}$ due to one's inability to discriminate between the four optically encoded Bell states. In this paper we study the following question: Are there alternative entanglement-assisted protocols that rely only on linear optics, coincidence photon counting, and separable single-photon input states and at the same time provide a greater capacity gain than $0.585\text{bits}$? We show that besides the Bell states there is a class of bipartite four-mode two-photon entangled states that facilitate an increase in channel capacity. We also discuss how the proposed scheme can be generalized to the case of two-photon $N$-mode entangled states for $N=6,8$.