When a gain system is coupled to a loss system, the energy usually flows from the gain system to the loss one. We here present a counterintuitive theory for the ground-state cooling of a mechanical resonator in an optomechanical system via a gain cavity. The energy flows first from the mechanical resonator into the loss cavity and then into the gain cavity and finally localizes there. The energy localization in the gain cavity dramatically enhances the cooling rate of the mechanical resonator. Moreover, we show that an unconventional optical spring effect, e.g., a giant frequency shift and optically induced damping of the mechanical resonator, can be realized. Those feature a precooling-free ground-state cooling, i.e., the mechanical resonator in thermal excitation at room temperature can directly be cooled to its ground state. This cooling approach has potential application in fundamental tests of quantum physics without complicated cryogenic setups.

We performed a series of experiments to examine the arrival of phase nonanalytical points in fast- and slow-light media, using a Gaussian-shaped temporal pulse and encoding phase nonanalytical points at various positions within the pulse envelope. For the phase nonanalytical points, the amplitude of the slowly varying pulse envelope, as well as any order of the derivatives, is continuous, but the phase of the carrier wave is discontinuous. The phase nonanalytical points were neither advanced nor delayed, but appeared at the same instance as they entered the fast- and slow-light media, in good accordance with the idea that the information velocity was equal to the velocity of light in a vacuum, $c$, or in the background medium.

We propose to generate the multiphoton subradiant states and investigate their fluorescences in an array of two-level atoms. These multiphoton states are created initially from the timed Dicke states. Then we can use either a Zeeman or Stark field gradient pulse to imprint linearly increasing phases on the atoms, and this phase-imprinting process unitarily evolves the system to the multiphoton subradiant states. The fluorescence engages a long-range dipole-dipole interaction which originates from a system-reservoir coupling in the dissipation. We locate some of the subradiant multiphoton states from the eigenmodes and show that an optically thick atomic array is best for the preparation of the state with the most reduced decay rate. This phase-imprinting process enables quantum-state engineering of the multiphoton subradiant states and realizes a potential quantum storage of the photonic qubits in the two-level atoms.

We theoretically investigate the scattering of few-photon light on Bose-Hubbard lattices using diagrammatic scattering theory. We explicitly derive general analytical expressions for the lowest-order photonic correlation functions, which we apply numerically to several different lattices. We focus specifically on nonlinear effects visible in the intensity-intensity correlation function and explain bunching and antibunching effects in dimers, chains, rings, and planes. The numerical implementation can be applied to arbitrary Bose-Hubbard graphs, and we provide it as an attachment to this publication.

An alternative non-Markovianity measure for open quantum processes is proposed, which takes advantage of the nonincreasing property of relative entropy of coherence under the incoherent completely positive and trace-preserving maps in the extended Hilbert space constituted by the open system and its ancillary. By applying the proposed measure to some typical noisy channels, we find that for phase damping and amplitude damping channels it is equivalent to the three previous measures of non-Markovianity, i.e., the measures based on the quantum trace distance, dynamical divisibility, and quantum mutual information. For the random unitary channel, however, these measures do not coincide exactly, and the proposed measure in the witness of Markovianity is more general than the measures based on quantum trace distance and dynamical divisibility but overlaps partly with the measure based on quantum mutual information.

We discuss common properties and reasons for integrability in the class of multistate Landau-Zener models with all diabatic levels crossing at one point. Exploring the Stokes phenomenon, we show that each previously solved model has a dual one, whose scattering matrix can be also obtained analytically. For applications, we demonstrate how our results can be used to study conversion of molecular into atomic Bose condensates during passage through the Feshbach resonance, and provide purely algebraic solutions of the bowtie and special cases of the driven Tavis-Cummings model.

We propose a protocol for complete Bell-state analysis for two superconducting-quantum-interference-device qubits. The Bell-state analysis could be completed by using a sequence of microwave pulses designed by the transitionless tracking algorithm, which is a useful method in the technique of shortcut to adiabaticity. After the whole process, the information for distinguishing four Bell states will be encoded on two auxiliary qubits, while the Bell states remain unchanged. One can read out the information by detecting the auxiliary qubits. Thus the Bell-state analysis is nondestructive. The numerical simulations show that the protocol possesses a high success probability of distinguishing each Bell state with current experimental technology even when decoherence is taken into account. Thus, the protocol may have potential applications for the information readout in quantum communications and quantum computations in superconducting quantum networks.

We consider opto- and electromechanical quantum systems composed of a driven cavity mode interacting with a set of mechanical resonators. It has been proposed that the latter can be initialized in arbitrary cluster states, including universal resource states for measurement-based quantum computation (MBQC). We show that, despite the unavailability in this setup of direct measurements over the mechanical resonators, computation can still be performed to a high degree of accuracy. In particular, it is possible to indirectly implement the measurements necessary for arbitrary Gaussian MBQC by properly coupling the mechanical resonators to the cavity field and continuously monitoring the leakage of the latter. We provide a thorough theoretical analysis of the performances obtained via indirect measurements, comparing them with what is achievable when direct measurements are instead available. We show that high levels of fidelity are attainable in parameter regimes within reach of present experimental capabilities.

The past several years have seen renewed interest in the use of symmetry-projected Hartree-Fock for the description of strong correlations. Unfortunately, these symmetry-projected mean-field methods do not adequately account for dynamic correlation. Presumably, this shortcoming could be addressed if one could combine symmetry-projected Hartree-Fock with a many-body method such as coupled-cluster theory, but this is by no means straightforward because the two techniques are formulated in very different ways. However, we have recently shown that the singlet $S^2$-projected unrestricted Hartree-Fock wave function can in fact be written in a coupled-cluster-like wave function. That is, the spin-projected unrestricted Hartree-Fock wave function can be written as a polynomial of a double-excitation operator acting on some closed-shell reference determinant. Here, we extend this result and show that the spin-projected generalized Hartree-Fock wave function (which has both $S^2$ and $S_z$ projection) is likewise a polynomial of low-order excitation operators acting on a closed-shell determinant and provide a closed-form expression for the resulting polynomial coefficients. The spin projection of the generalized Hartree-Fock wave function introduces connected triple and quadruple excitations which are absent when spin-projecting an unrestricted Hartree-Fock determinant. We include a few preliminary applications of the combination of this spin-projected Hartree-Fock and coupled-cluster theory to the Hubbard Hamiltonian and comment on generalizations of the methodology. Results here are not for production level, but a similarity-transformed theory that combines the two offers the promise of being accurate for both weak and strong correlation, and may offer significant improvements in the intermediate correlation regime where neither projected Hartree-Fock nor coupled cluster is particularly accurate.

The XCHEM approach interfaces well established quantum chemistry packages with scattering numerical methods in order to describe single-ionization processes in atoms and molecules. This should allow one to describe electron correlation in the continuum at the same level of accuracy as quantum chemistry methods do for bound states. Here we have applied this method to study multichannel photoionization of Ne in the vicinity of the autoionizing states lying between the $2s^{2}2p^5$ and $2s2p^6$ ionization thresholds. The calculated total photoionization cross sections are in very good agreement with the absolute measurement of Samson et al. [J. Electron Spectrosc. Relat. Phenom. 123, 265 (2002)], and with independent benchmark calculations performed at the same level of theory. From these cross sections, we have extracted resonance positions, total autoionization widths, Fano profile parameters, and correlation parameters for the lowest three autoionizing states. The values of these parameters are in good agreement with those reported in earlier theoretical and experimental work. We have also evaluated $\beta$ asymmetry parameter and partial photoionization cross sections and, from the latter, partial autoionization widths and Starace parameters for the same resonances, not yet available in the literature. Resonant features in the calculated $\beta$ parameter are in good agreement with the experimental observations. We have found that the three lowest resonances preferentially decay into the $2p^{-1}\epsilon d$ continuum rather than into the $2p^{-1}\epsilon s$ one [Phys. Rev. A 89, 043415 (2014)], in agreement with previous expectations, and that in the vicinity of the resonances the partial $2p^{-1}\epsilon s$ cross section can be larger than the $2p^{-1}\epsilon d$ one, in contrast with the accepted idea that the latter should amply dominate in the whole energy range. These results show the potential of the XCHEM approach to describe highly correlated process in the ionization continuum of many-electron systems, in particular molecules, for which the XCHEM code has been specifically designed.

A theoretical study devoted to suppression of magnetic systematic effects in ${\mathrm{HfF}}^{+}$ cation for an experiment to search for the electron electric dipole moment is reported. The $g$ factors for $J=1$, $F=3/2$, $|M_F|=3/2$ hyperfine levels of the ${}^3\mathrm{\Delta }_1$ state are calculated as functions of the external electric field. The minimal value for the difference between the $g$ factors of $\mathrm{\Omega }$-doublet levels, $\mathrm{\Delta }\mathrm{g}=3\times {10}^{-6}$, is attained at the electric field 7 V/cm. The body-fixed $g$ factor, $G_{\parallel }$, was obtained both within the ab initio electronic structure calculations and with our fit of the experimental data [H. Loh, K. C. Cossel, M. C. Grau, K.-K. Ni, E. R. Meyer, J. L. Bohn, J. Ye, and E. A. Cornell, Science 342, 1220 (2013)]. For the electronic structure calculations we used a combined scheme to perform correlation calculations of ${\mathrm{HfF}}^{+}$, which includes both the direct four-component all-electron and generalized relativistic effective core potential approaches. The electron correlation effects were treated using the coupled cluster methods. The calculated value $G_{\parallel }=0.0115$ agrees very well with the $G_{\parallel }=0.0118$ obtained with our fitting procedure. The calculated ab initio value $D_{\parallel }=-1.53$ a.u. for the molecule-frame dipole moment (with the origin in the center of mass) is in agreement with the experimental datum $D_{\parallel }=-1.54\left(1\right)$ a.u. [H. Loh, Ph.D. thesis, Massachusetts Institute of Technology, 2006.].

The recently developed wave-packet continuum-discretization approach [I. B. Abdurakhmanov, A. S. Kadyrov, and I. Bray, Phys. Rev. A 94, 022703 (2016)] is extended to antiproton-helium collisions. The helium target is treated as a three-body Coulomb system using a frozen-core approximation, in which the electron-electron correlation within the target is accounted for through the static interaction. The Schrödinger equation for the helium target is solved numerically to yield bound and continuum states of the active electron. The resulting continuum state is used to construct wave-packet pseudostates with arbitrary energies. The energies of the pseudostates are chosen in a way that is ideal for detailed differential ionization studies. Two-electron target wave functions, formed from the bound and continuum wave-packet states of the active electron and the $1s$ orbital of ${\mathrm{He}}^{+}$, are then utilized in the single-center semiclassical impact-parameter close-coupling scheme. A comprehensive set of benchmark results, from angle-integrated to fully differential cross sections for antiproton impact single ionization of helium in the energy range from 1 keV to 1 MeV, is provided. Furthermore, we use our single-center convergent close-coupling approach to study fully differential single ionization of helium by 1-MeV proton impact. The calculated results are in good agreement with recent experimental measurements [H. Gassert, O. Chuluunbaatar, M. Waitz, F. Trinter, H.-K. Kim, T. Bauer, A. Laucke, C. Müller, J. Voigtsberger, M. Weller et al., Phys. Rev. Lett. 116, 073201 (2016)] for all considered geometries.

A momentum-dependent Coulomb factor in the probability for nonlinear ionization of atoms by a strong low-frequency laser field is calculated analytically in the adiabatic approximation. Expressions for this Coulomb factor, valid for an arbitrary laser pulse waveform, are obtained and analyzed in detail for the cases of linear and circular polarizations. The dependence of the Coulomb factor on the photoelectron momentum is shown to be significant in both cases. Using a similar technique, the Coulomb factor for emission of high-order harmonics by an atom in a bichromatic laser field is also calculated. In contrast to the case of a single-frequency field, for bichromatic fields the Coulomb factor depends significantly on the harmonic energy.

We chart out the phases of ultracold “spin-half” bosons in a one-dimensional optical lattice in the presence of Aubry-André (AA) potential and with spin-orbit (SO) and Raman couplings. We investigate the superfluid (SF) and localized phases and demonstrate the existence of density wave phase for nearest-neighbor interaction (NNI) between the bosons. We show that the presence of SO coupling and AA potential leads to a spin-split momentum distribution of the bosons in the localized phase near the boundary with the SF phase, which can act as a signature of the SF-localized phase transition. We also obtain the level statistics of the bosons in the superfluid phase with finite NNI and demonstrate its change from Gaussian unitary ensemble (GUE) to Gaussian orthogonal ensemble (GOE) as a function of the Raman coupling. We discuss experiments which can test our theory.

We experimentally study a protocol of using the broadband high-frequency squeezed vacuum to detect the low-frequency signal. In this scheme, the lower sideband field of the squeezed light carries the low-frequency modulation signal, and the two strong coherent light fields are applied as the bichromatic local oscillator in the homodyne detection to measure the quantum entanglement of the upper and lower sideband for the broadband squeezed light. The power of one of the local oscillators for detecting the upper sideband can be adjusted to optimize the conditional variance in the low-frequency regime by subtracting the photocurrent of the upper sideband field of the squeezed light from that of the lower sideband field. By means of the quantum correlation of the upper and lower sideband for the broadband squeezed light, the low-frequency signal beyond the standard quantum limit is measured. This scheme is appropriate for enhancing the sensitivity of the low-frequency signal by the aid of the broad squeezed light, such as gravitational waves detection, and does not need to directly produce the low-frequency squeezing in an optical parametric process.

We demonstrate a reconfigurable lateral optical force (OF) on a plasmonic nanoparticle immersed in a simple optical field invariant along the lateral direction and formed by two interfering plane waves. This lateral OF is shown, from the multipolar expansion technique, attributed to several coupling channels established between multiple multipoles excited on a plasmonic nanoparticle, in particular, the adjacent electric multipole modes that bring about the Fano interferences, which can substantially enhance the lateral scattering asymmetry, leading to an augmented lateral OF comparable to the longitudinal OF. More importantly, by engineering Fano interference either intrinsically through particle size or extrinsically through selectively exciting narrow plasmonic dark modes the direction of the lateral OF is reversibly switchable. The lateral OF can even be modulated continuously from positive to negative by controlling the incident angle of the interfering plane waves due to the variation of relative phase of the excited plasmonic dark modes near Fano resonance, facilitating the plasmonic nanoparticle as a controllable conveyor as well as the optical selection and separation. Besides, a fundamental and counterintuitive physical consequence emerges in that the simple proportional relation between the lateral OF and the Belinfante spin momentum derived in the small particle limit breaks down when the Fano interference comes into play, in particular, a negative lateral OF opposite the Belinfante spin momentum can be induced by properly controlling the selective excitation.

We use homodyne detection to monitor the radiative decay of a superconducting qubit. According to the classical theory of conditional probabilities, the excited-state population differs from an exponential decay law if it is conditioned upon a later projective qubit measurement. Quantum trajectory theory accounts for the expectation values of general observables, and we use experimental data to show how a homodyne detection signal is conditioned upon both the initial state and the finally projected state of a decaying qubit. We observe, in particular, how anomalous weak values occur in continuous weak measurement for certain pre- and postselected states. Subject to homodyne detection, the density matrix evolves in a stochastic manner, but it is restricted to a specific surface in the Bloch sphere. We show that a similar restriction applies to the information associated with the postselection, and thus bounds the predictions of the theory.

We introduce a method to determine whether a given generalized quantum measurement is isolated or if it belongs to a family of measurements having the same prescribed symmetry. The technique proposed reduces to solving a linear system of equations in some relevant cases. As a consequence, we provide a simple derivation of the maximal family of symmetric informationally complete positive operator-valued measure SIC-POVM in dimension 3. Furthermore, we show that the following remarkable geometrical structures are isolated, so that free parameters cannot be introduced: (a) maximal sets of mutually unbiased bases in prime power dimensions from 4 to 16, (b) SIC-POVM in dimensions from 4 to 16, and (c) contextual Kochen-Specker sets in dimension 3, 4, and 6, composed of 13, 18, and 21 vectors, respectively.