Angular distributions of the ${\mathrm{N}}_2$${\mathrm{O}}^{+}\left(X\right)$ and ${\mathrm{N}}_2$${\mathrm{O}}^{+}\left(A\right)$ photoelectrons were measured as functions of the exciting-photon energy with small bandwidth in the vicinity of the resonant ${\mathrm{N}}_t(1s\to {\pi }^{*})$, ${\mathrm{N}}_t(1s\to {\sigma }^{*})$, and ${\mathrm{N}}_c(1s\to {\pi }^{*})$ core excitations of ${\mathrm{N}}_2\mathrm{O}$. For selected exciting-photon energies in this range, angular distribution parameters of the ${\mathrm{N}}_2$${\mathrm{O}}^{+}(A\to X)$ fluorescence were also determined. These data are interpreted by ab initio calculations. Long-range exciting-photon energy dependencies of the photoelectron angular distribution parameters ${\beta }_X^e\left(\omega \right)$ and ${\beta }_A^e\left(\omega \right)$, recorded vibrationally unresolved, are attributed to an electronic state interference between the direct and the different resonant amplitudes for the population of the final ionic states. In addition to that, strong influence of lifetime vibrational interference is identified in the observed dispersion of the vibrationally resolved fluorescence angular distribution parameter $\beta 2_A^X\left(\omega \right)$ across the electronic resonances.

It is shown using numerical simulations that flow patterns around an obstacle potential moving in a superfluid exhibit hysteresis. In a certain velocity region, there is a bistability between stationary laminar flow and periodic vortex shedding. The bistability exists in two- and three-dimensional systems.

Two-component mixtures in optical lattices reveal a rich variety of different phases. We employ an exact diagonalization method to obtain the relevant correlation functions in hexagonal optical lattices which characterize those phases. We relate the occupation difference of the two species to the magnetic polarization. “Iso” -magnetic correlations disclose the nature of the system, which can be of easy-axis type, bearing phase segregation, or of easy-plane type, corresponding to super-counter-fluidity. In the latter case, the correlations reveal easy-plane segregation, involving a highly entangled state. We identify striking correlated supersolid phases appearing within the superfluid limit.

We study the ground-state properties of ultracold bosonic atoms in a state-dependent graphenelike honeycomb optical lattice, where the degeneracy between the two triangular sublattices A and B can be lifted. We discuss the various geometries accessible with this lattice setup and present a scheme to control the energy offset with external magnetic fields. The competition of the on-site interaction with the offset energy leads to Mott phases characterized by population imbalances between the sublattices. For the definition of an optimal Hubbard model, we demonstrate a scheme that allows for the efficient computation of Wannier functions. Using a cluster mean-field method, we compute the phase diagrams and provide a universal representation for arbitrary energy offsets. We find good agreement with the experimental data for the superfluid to Mott insulator transition.

We have measured the two-particle correlation function of atoms from a Bose-Einstein condensate participating in a superradiance process, which directly reflects the second-order coherence of the emitted light. We compare this correlation function with that of atoms undergoing stimulated emission. Whereas the stimulated process produces correlations resembling those of a coherent state, we find that superradiance, even in the presence of strong gain, shows a correlation function close to that of a thermal state, just as for ordinary spontaneous emission.

Spectral singularities are spectral points of non-Hermitian Hamiltonians with the real energies, that annihilate the completeness of the eigenfunctions. The complex $\mathcal{P}\mathcal{T}$-symmetric potentials with real spectrum have such Hamiltonians. It was identified that the spectral singularities of complex potentials such as an optical gain medium correspond to zero-width resonance modes of the reflected and transmitted beams from the medium. We show that a Fabry-Perot resonator with a gain medium under inclined incident beam exhibits spectral singularities that are measurable by changing the incident angle of the beam to the resonator. The results are realized by theory and simulations.

Photon-subtracted and photon-added Gaussian states are amongst the simplest non-Gaussian states that are experimentally available. It is generally believed that they are some of the best candidates to enhance sensitivity in parameter extraction. We derive here the quantum Cramér-Rao bound for such states and find that for large photon numbers photon subtraction or addition only leads to a small correction of the quantum Fisher information (QFI). On the other hand, a divergence of the QFI appears for very small squeezing in the limit of vanishing photon number in the case of photon subtraction, implying an arbitrarily precise measurement with almost no light. However, at least for the standard and experimentally established preparation scheme, the decreasing success probability of the preparation in that limit exactly cancels the divergence, leading to finite sensitivity per square root of Hz, when the duration of the preparation is taken into account.

In Phys. Rev. A 88, 062313 (2013) we proposed and studied a model for a self-correcting quantum memory in which the energetic cost for introducing a defect in the memory grows without bounds as a function of system size. This positive behavior is due to attractive long-range interactions mediated by a bosonic field to which the memory is coupled. The crucial ingredients for the implementation of such a memory are the physical realization of the bosonic field as well as local five-body interactions between the stabilizer operators of the memory and the bosonic field. Here, we show that both of these ingredients appear in a low-energy effective theory of a Hamiltonian that involves only two-body interactions between neighboring spins. In particular, we consider the low-energy, long-wavelength excitations of an ordered Heisenberg ferromagnet (magnons) as a realization of the bosonic field. Furthermore, we present perturbative gadgets for generating the required five-spin operators. Our Hamiltonian involving only local two-body interactions is thus expected to exhibit self-correcting properties as long as the noise affecting it is in the regime where the effective low-energy description remains valid.

We discuss the formalism of Balian and Duplantier [Balian and Duplantier, Ann. Phys. (NY) 104, 300 (1977); Balian and Duplantier, Ann. Phys. (NY) 112, 165 (1978)] for the calculation of the Casimir energy for an arbitrary smooth compact surface and use it to give some examples: a finite cylinder with hemispherical caps, a torus, an ellipsoid of revolution, a cube with rounded corners and edges, and a drum made of disks and part of a torus. We propose a model function that approximately captures the shape dependence of the Casimir energy.

Pulse-amplitude and fluence-specified optimal control simulation with linear (dipole) and nonlinear (polarizability) interactions is applied to find the best way to orient CO molecules with a combination of THz and laser pulses. The optimal pulses are numerically designed with a specified maximum amplitude of the THz pulse and a specified fluence of the laser pulse within the ranges of $E_{\max }=50\mathrm{MV}/\mathrm{m}\sim 150\mathrm{MV}/\mathrm{m}$ and $f_0=2.5\mathrm{J}/{\mathrm{cm}}^2\sim 4.5\mathrm{J}/{\mathrm{cm}}^2$ at temperature $T=0\mathrm{K}\sim 10\mathrm{K}$. The optimal pulse almost always consists of a near-single-cycle THz pulse of zero area and a laser pulse that is mainly composed of three subpulses. There is no temporal overlap between the THz pulse and the laser subpulses. The THz pulse, which virtually contains no dc components, induces rotational transitions in a resonant way, whereas the three laser subpulses efficiently induce multiple rotational transitions through Raman scattering. Comparing the optimal control simulations with and without laser pulses, the laser pulses effectively increase the degrees of orientation by approximately half through the polarizability interaction with inversion symmetry.

We investigate the steady-state cooling dynamics of vibrational degrees of freedom related to a nanomechanical oscillator coupled with a laser-pumped quantum dot in an optical resonator. Correlations between phonon-cooling and quantum-dot photon emission processes occur, respectively, when a photon laser absorption together with a vibrational phonon absorption is followed by photon emission in the optical resonator. Therefore, the detection of photons generated in the cavity mode concomitantly contributes to phonon cooling detection of the nanomechanical resonator.

We present a theoretical investigation on the saturation of stimulated Raman scattering (SRS) and propose an application of it to break the diffraction limit in SRS microscopy. In our proposed scheme, a donut-shaped Stokes beam is used to saturate SRS at the rim of a focused Gaussian pump beam; thus the addition of another Gaussian Stokes beam can only induce additional stimulated Raman loss to the pump beam in a small area inside the donut-shaped beam. Resembling stimulated-emission-depletion microscopy, this method can significantly improve the lateral imaging resolution. Compared with the diffraction-limited resolution, theoretical simulations show that it may be possible to double the spatial resolution with a few ${\mathrm{TW/cm}}^2$ of laser intensity. Such super-resolution could greatly enhance the advantage of SRS microscopy for potential applications.

The propagation of a step pulse through a thick resonant absorber with a homogeneously broadened absorption line is considered. It is shown that a specific subradiant state is naturally developed in the absorber due to the formation of the spatial domains of the atomic coherence with opposite phases. It is proposed to divide the absorber into slices in accordance with these domains and place the phase shifters in front of the first slice and between the other slices. If the phase shifters are switched on simultaneously at a particular moment of time, elapsed from the beginning of the step pulse, a strong sharp pulse is generated at the output of the last slice of the absorber. The effect is explained by the phasing of the atomic coherence along all slices of the absorber, which transforms the subradiant state of the atom-field system to a superradiant state.

In measurement-based quantum computing, there is a natural “causal cone” among qubits of the resource state, since the measurement angle on a qubit has to depend on previous measurement results in order to correct the effect of by-product operators. If we respect the no-signaling principle, by-product operators cannot be avoided. Here we study the possibility of acausal measurement-based quantum computing by using the process matrix framework [Oreshkov, Costa, and Brukner, Nat. Commun. 3, 1092 (2012)]. We construct a resource process matrix for acausal measurement-based quantum computing restricting local operations to projective measurements. The resource process matrix is an analog of the resource state of the standard causal measurement-based quantum computing. We find that if we restrict local operations to projective measurements the resource process matrix is (up to a normalization factor and trivial ancilla qubits) equivalent to the decorated graph state created from the graph state of the corresponding causal measurement-based quantum computing. We also show that it is possible to consider a causal game whose causal inequality is violated by acausal measurement-based quantum computing.

We show that the synthetic spin-orbit coupling created in current ultracold atom experiments provides physicists a unique tool to control the Luttinger liquid parameter $K$ of weakly interacting bosons in one dimension. At a critical value of the Raman coupling strength ${\Omega }_c$, $K$ is suppressed down to zero, and the characteristic quasi-long-range order for ordinary one-dimensional quantum systems disappears. Consequently, the single-particle correlation function decays exponentially at the ground state, signifying the rise of a one-dimensional quantum many-body state beyond the standard Luttinger liquid paradigm. Momentum distribution, as well as scaling relations for various quantities in the vicinity of the critical point, can be used as a direct diagnosis of this non-Luttinger quantum liquid.

The quantum reality problem is that of finding a mathematically precise definition of a sample space of configurations of beables, events, histories, paths, or other mathematical objects, and a corresponding probability distribution, for any given closed quantum system. Given a solution, we can postulate that physical reality is described by one randomly chosen configuration drawn from the sample space. For a physically sensible solution, this postulate should imply quasiclassical physics in realistic models. In particular, it should imply the validity of Copenhagen quantum theory and classical dynamics in their respective domains. A Lorentzian solution applies to relativistic quantum theory or quantum field theory in Minkowski space and is defined in a way that respects Lorentz symmetry. We outline a solution to the nonrelativistic and Lorentzian quantum reality problems and associated generalizations of quantum theory.

Postselected weak measurement has been widely used in experiments to observe weak effects in various physical systems. However, it is still unclear how large the amplification ability of a weak measurement can be and what determines the limit of this ability, which is fundamental to understanding and applying weak measurements. The limitation of the conventional weak-value formalism for this problem is the divergence of weak values when the pre- and postselections are nearly orthogonal. In this paper, we study this problem by a variational approach for a general Hamiltonian $H_{\mathrm{int}}=gA\otimes \Omega \delta (t-t_0),g\ll 1$. We derive a general asymptotic solution and show that the amplification limit is essentially independent of $g$ and is determined by only the initial state of the detector and the number of distinct eigenvalues of $A$ or $\Omega$. An example of spin-$\frac{1}{2}$ particles with a pair of Stern-Gerlach devices is given to illustrate the results. The limiting case of continuous-variable systems is also investigated to demonstrate the influence of system dimension on the amplification limit.

We study the performance of composite pulses in the presence of time-varying control noise on a single qubit. These protocols, originally devised only to correct for static, systematic errors, are shown to be robust to time-dependent non-Markovian noise in the control field up to frequencies as high as $\sim 10\%$ of the Rabi frequency. Our study combines a generalized filter-function approach with asymptotic dc-limit calculations to give a simple analytic framework for error analysis applied to a number of composite-pulse sequences relevant to nuclear magnetic resonance as well as quantum information experiments. Results include examination of recently introduced concatenated composite pulses and dynamically corrected gates, demonstrating equivalent first-order suppression of time-dependent fluctuations in amplitude and/or detuning, as appropriate for the sequence in question. Our analytic results agree well with numerical simulations for realistic $1/f$ noise spectra with a roll-off to $1/f^2$, providing independent validation of our theoretical insights.

It has been known that, among pure states, $N$-qubit $W$ states cannot be uniquely determined by their arbitrary $(N-1)$ bipartite reduced density matrices. Parashar and Rana proved that among arbitrary states, $(N-1)$ bipartite reduced density matrices that the pairs of qubits constitute a star graph or a line graph can uniquely determine stochastic local operations and classical communication (SLOCC) equivalent $W$ states, and we generalize this conclusion into tree graph. In this paper, we show that all SLOCC equivalent $W$ states can be uniquely determined (among pure, mixed states) by their $(N-1)$ bipartite reduced density matrices, if the $(N-1)$ pairs of qubits constitute a tree graph on $N$ vertices, where each pair of qubits represents an edge.

We apply the quantum optimal control theory based on the Krotov method to implement single-qubit $X$ and $Z$ gates and two-qubit cnot gates for inductively coupled superconducting flux qubits with fixed qubit transition frequencies and fixed off-diagonal qubit-qubit coupling. Our scheme shares an advantage with other direct-coupling schemes in that it requires no additional coupler subcircuit and control lines. The control lines needed are only for the manipulation of individual qubits (e.g., a time-dependent magnetic flux or field applied on each qubit). The qubits are operated at the optimal coherence points and the gate operation times (less than 1 ns for single-qubit gates and $\sim 2$ ns for cnotgates) are much shorter than the corresponding qubit decoherence time. A cnot gate or other general quantum gates can be implemented in a single run of the pulse sequence rather than being decomposed into several single-qubit and several entangled two-qubit operations in series by composite pulse sequences. Quantum gates constructed via our scheme are all with very high fidelity (very low error) as our optimal control scheme takes into account the fixed qubit detuning and fixed two-qubit interaction as well as all other time-dependent magnetic-field-induced single-qubit interactions and two-qubit couplings. The effect of leakage to higher-energy-level states and the effect of qubit decoherence on the quantum-gate operations are also discussed.