We describe how quasiclassical relative positions of particles emerge in an initially delocalized quantum system as scattering of a probe beam is observed. We show that in the multiparticle case this localization in position space occurs via intermediate states that are quantum superpositions of spatial configurations. These superpositions are robust to consecutive scattering events, and scattering alone does not lead to their decoherence. Instead the free evolution of the system combines with scattering to destroy these superpositions, leading the particles to adopt classical-like positions relative to one another.

We have developed the capability to determine accurate harmonic spectra for multielectron atoms within time-dependent $R$-matrix (TDRM) theory. Harmonic spectra can be calculated using the expectation value of the dipole length, velocity, or acceleration operator. We assess the calculation of the harmonic spectrum from He irradiated by 390-nm laser light with intensities up to $4\times {10}^{14}$ W cm${}^{-2}$ using each form, including the influence of the multielectron basis used in the TDRM code. The spectra are consistent between the different forms, although the dipole acceleration calculation breaks down at lower harmonics. The results obtained from TDRM theory are compared with results from the helium code, finding good quantitative agreement between the methods. We find that bases which include pseudostates give the best comparison with the helium code, but models comprising only physical orbitals also produce accurate results.

The free-free transition is studied for an electron-hydrogen atom in ground state when a low-energy electron (external) is injected into hydrogenic plasma in the presence of an external homogenous, monochromatic, and linearly polarized laser field. The effect of plasma screening is considered in the Debye-Hückel approximation. The calculations are performed in the soft photon limit. The incident electron is considered to be dressed by the laser field in a nonperturbative manner by choosing the Volkov solutions in both the initial and final channels. The space part of the scattering wave function for the electron is solved numerically by taking into account the electron exchange. The laser-assisted differential and total cross sections are calculated for single-photon absorption or emission and no-photon exchange in the soft photon limit, the laser intensity being much less than the atomic field intensity. The calculations have been carried out for various values of Debye parameter, ranging from 0.005 to 0.12. A strong suppression is noted in the laser-assisted cross sections as compared to the field-free situation. A significant difference is noted for the singlet and triplet cross sections. The suppression is much more in the triplet states.

The general concept of entangled photon holes is based on a correlated absence of photon pairs in an otherwise constant optical background. Here we consider the specialized case when this background is confined to two well-defined time bins, which allows the formation of time-bin-entangled photon holes. We show that when the typical coherent-state background is replaced by a true single-photon (Fock state) background, the basic time-bin-entangled photon-hole state becomes equivalent to one of the time-bin-entangled photon-pair states. We experimentally demonstrate these ideas using a parametric down-conversion photon-pair source, linear optics, and postselection to violate a Bell inequality with time-bin-entangled photon holes.

Extraordinary group delays shorter than the transit time of light propagating at $c$ through an equal distance have been experimentally demonstrated in single-Fabry-Pérot (FP) waveguide systems and cascaded-FP structures under off-resonant conditions. These “superluminal” phenomena are well explained by the multiple-reflection destructive interference that reduces the intracavity stored energy when operating off resonances. Excellent agreement between theory and experiment is obtained when the dispersive effects of reflective boundaries are considered. These results provide further insight into the nature of apparent superluminality in regions of allowed propagation.

We study the behavior of excitations in the tilted one-dimensional Bose-Hubbard model. In the phase with broken symmetry, fundamental excitations are domain walls which show fractional statistics. Using perturbation theory, we derive an analytic model for the time evolution of these fractional excitations, and demonstrate the existence of a repulsively bound state above a critical center-of-mass momentum. The validity of the perturbative analysis is confirmed by the use of time-adaptive density-matrix renormalization group simulations. These findings open the path for the experimental detection of fractional particles in cold atomic gases.

In recent years the paradigm based on entanglement as the unique measure of quantum correlations has been challenged by the rise of new correlation concepts, such as quantum discord, able to reveal quantum correlations that are present in separable states. It is in general difficult to compute quantum discord, because it involves a minimization over all possible local measurements in a bipartition. In the realm of continuous-variable (CV) systems, a Gaussian version of quantum discord has been put forward upon restricting to Gaussian measurements. It is natural to ask whether non-Gaussian measurements can lead to a stronger minimization than Gaussian ones. Here we focus on two relevant classes of two-mode Gaussian states: squeezed thermal states and mixed thermal states, and allow for a range of experimentally feasible non-Gaussian measurements, comparing the results with the case of Gaussian measurements. We provide evidence that Gaussian measurements are optimal for Gaussian states.

We propose a family of error-detecting stabilizer codes with an encoding rate of $1/3$ that permit a transversal implementation of the gate $T=\exp (-i\pi Z/8)$ on all logical qubits. These codes are used to construct protocols for distilling high-quality “magic” states $T\left|+\right.〉$ by Clifford group gates and Pauli measurements. The distillation overhead scales as $O({\log }^{\gamma }(1/\epsilon ))$, where $\epsilon$ is the output accuracy and $\gamma ={\log }_2\left(3\right)\approx 1.6$. To construct the desired family of codes, we introduce the notion of a triorthogonal matrix, a binary matrix in which any pair and any triple of rows have even overlap. Any triorthogonal matrix gives rise to a stabilizer code with a transversal $T$ gate on all logical qubits, possibly augmented by Clifford gates. A powerful numerical method for generating triorthogonal matrices is proposed. Our techniques lead to a twofold overhead reduction for distilling magic states with accuracy $\epsilon \sim {10}^{-12}$ compared with previously known protocols.

We report here oscillator strengths, transition rates, branching ratios, and lifetimes due to the allowed transitions in the potassium (K) atom. We evaluate electric dipole ($E1$) amplitudes using an all-order relativistic many-body perturbation method. The obtained results are compared with previously available experimental and theoretical studies. Using the $E1$ matrix elements mentioned above and estimated from the lifetimes of the $4p{}^2{P}_{1/2,3/2}$ states, we determine precise values of static and dynamic polarizabilities for the first five low-lying states in the considered atom. The static polarizabilities of the ground and $4p{}^2{P}_{1/2,3/2}$ states in the present work are more precise than the available measurements in these states. Only the present work employs relativistic theory to evaluate the polarizabilities in the $3d{}^2{D}_{3/2,5/2}$ states for which no experimental results are known with which to compare. We also reexamine “magic wavelengths” for the $4p{}^2{P}_{1/2}\to 4s{}^2{S}_{1/2}$ and $4p{}^2{P}_{3/2}\to 4s{}^2{S}_{1/2}$ transitions due to the linearly polarized light, which are useful to perform the state-insensitive trapping of K atoms.

We investigate theoretically radio-frequency spectroscopy of weakly bound molecules in an ultracold spin-orbit-coupled atomic Fermi gas. We consider two cases with either equal Rashba and Dresselhaus coupling or pure Rashba coupling. The former system has been realized very recently at Shanxi University [Wang et al., Phys. Rev. Lett. 109, 095301 (2012)] and MIT [Cheuk et al., Phys. Rev. Lett. 109, 095302 (2012)]. We predict realistic radio-frequency signals for revealing the unique properties of anisotropic molecules formed by spin-orbit coupling.

We theoretically investigate a Rashba spin-orbit-coupled Fermi gas near Feshbach resonances, by using mean-field theory and a two-channel model that takes into account explicitly Feshbach molecules in the closed channel. In the absence of spin-orbit coupling, when the channel coupling $g$ between the closed and the open channels is strong, it is widely accepted that the two-channel model is equivalent to a single-channel model that excludes Feshbach molecules. This is the so-called broad resonance limit, which is well satisfied by ultracold atomic Fermi gases of ${}^6$Li atoms and ${}^{40}$K atoms in current experiments. Here, with Rashba spin-orbit coupling we find that the condition for equivalence becomes much more stringent. As a result, the single-channel model may already be insufficient to describe properly an atomic Fermi gas of ${}^{40}$K atoms at a moderate spin-orbit coupling. We determine a characteristic channel coupling strength $g_c$ as a function of the spin-orbit-coupling strength, above which the single-channel and two-channel models are approximately equivalent. We also find that for narrow resonance with small-channel coupling, the pairing gap and molecular fraction are strongly suppressed by SO coupling. Our results can be readily tested in ${}^{40}$K atoms by using optical molecular spectroscopy.

The Mie scattering of a circularly polarized, purely azimuthal Laguerre-Gauss beam carrying orbital angular momentum from a particle small compared with the incident wavelength is treated analytically. Using the vector spherical harmonics, we show angular momentum conservation leads to two physical mechanisms that cause vanishingly small scattering intensity when the dipole moment dominates the response of the particle. The application of this angular-momentum-induced transparency is discussed in the case of a plasmonic crystal whose collective modes possess a well-defined angular momentum content.

The atomic masses of the three most abundant isotopes of strontium, ${}^{86,87,88}$Sr, and of the six most abundant isotopes of ytterbium, ${}^{170,171,172,173,174,176}$Yb, have been obtained from measurements of cyclotron frequency ratios of pairs of ions simultaneously trapped in a Penning trap. Our results, with one standard deviation uncertainty, are $M$(${}^{86}$Sr) $=$ 85.909 260 730 9 (91) u, $M$(${}^{87}$Sr) $=$ 86.908 877 497 0 (91) u, $M$(${}^{88}$Sr) $=$ 87.905 612 257 1 (97) u, and $M$(${}^{170}$Yb) $=$ 169.934 767 241 (18) u, $M$(${}^{171}$Yb) $=$ 170.936 331 514 (19) u, $M$(${}^{172}$Yb) $=$ 171.936 386 655 (18) u, $M$(${}^{173}$Yb) $=$ 172.938 216 213 (18) u, $M$(${}^{174}$Yb) $=$ 173.938 867 539 (18) u, $M$(${}^{176}$Yb) $=$ 175.942 574 702 (22) u. These results have application to photon-recoil determinations of the fine-structure constant.

We demonstrate efficient resolved sideband laser cooling ($99\pm 1\%$ ground-state population) of a single ${}^{40}{\mathrm{Ca}}^{+}$ ion in a large linear Paul trap (electrode spacing of 7 mm) operated at an rf drive frequency of just 3.7 MHz. For ion oscillation frequencies in the range 280–585 kHz, heating rates below or about one motional quantum per second have been measured at room temperature. The results, obtained under these unconventional sideband cooling conditions, pave the way for a range of new types of cold ion experiments, including spectroscopy of molecular ions as well as ultracold chemistry.

Generating nonclassical states of light by optomechanical coupling depends critically on the mechanical and optical properties of micro-oscillators and on the minimization of thermal noise. We present an oscillating micromirror with a mechanical quality factor $Q=2.6\times {10}^6$ at cryogenic temperature and a finesse of 65 000, obtained thanks to an innovative approach to the design and the control of mechanical dissipation. We predict that, already at 4 K with an input laser power of 2 mW, the radiation-pressure quantum fluctuations become the main noise source, overcoming thermal noise. This feature makes our devices particularly suitable for the production of ponderomotive squeezing.

To quantify single-mode nonclassicality, we start from an operational approach. A positive semidefinite observable is introduced to describe a measurement setup. The quantification is based on the negativity of the normally ordered version of this observable. Perfect operational quantumness corresponds to the quantum-noise-free measurement of the chosen observable. Surprisingly, even moderately squeezed states may exhibit perfect quantumness for a properly designed measurement. The quantification is also considered from an axiomatic viewpoint, based on the algebraic structure of the quantum states and the quantum superposition principle. Basic conclusions from both approaches are consistent with this fundamental principle of the quantum world.

We investigate the implementation of a measurement for binary optical coherent states that minimizes the error probability with a fixed rate of an inconclusive result. As a special case, the optimal measurement without inconclusive results, which is expressed as a projective measurement, can be implemented using only a beam splitter, a local coherent light source, a photon detector, and a feedback circuit. We find that the optimal measurement for binary optical coherent states with any probability of an inconclusive result can be implemented by the same optical components, even though the measurement is generally not projective.

In this paper we continue and extend the investigations of the ensembles of random physical states introduced in Hamma et al. [Phys. Rev. Lett. 109, 040502 (2012)]. These ensembles are constructed by finite-length random quantum circuits (RQC) acting on the (hyper)edges of an underlying (hyper)graph structure. The latter encodes for the locality structure associated with finite-time quantum evolutions generated by physical, i.e., local, Hamiltonians. Our goal is to analyze physical properties of typical states in these ensembles; in particular here we focus on proxies of quantum entanglement as purity and $\alpha$-Renyi entropies. The problem is formulated in terms of matrix elements of superoperators which depend on the graph structure, choice of probability measure over the local unitaries, and circuit length. In the $\alpha =2$ case these superoperators act on a restricted multiqubit space generated by permutation operators associated to the subsets of vertices of the graph. For permutationally invariant interactions the dynamics can be further restricted to an exponentially smaller subspace. We consider different families of RQCs and study their typical entanglement properties for finite time as well as their asymptotic behavior. We find that area law holds in average and that the volume law is a typical property (that is, it holds in average and the fluctuations around the average are vanishing for the large system) of physical states. The area law arises when the evolution time is $O\left(1\right)$ with respect to the size $L$ of the system, while the volume law arises as is typical when the evolution time scales like $O\left(L\right)$.

We demonstrate a scheme for realizing quantum-information storage (retrieval) into (from) a quantum memory using a hybrid quantum system, which consists of $N$ nitrogen-vacancy (N-$V$) centers (as memory), $N$ transmission line resonators (TLRs) (as data bus), and $N$ current-biased Josephson-junction (CBJJ) superconducting phase qubits (as heads of read and write). By virtually exciting the vibrational modes of the TLRs, this can produce an effective coupling between the N-$V$ centers and the CBJJs in the system. Because the operation time ($\sim {10}^{-8}$ s) is much shorter than the decoherence time of the CBJJ ($\sim {10}^{-5}$ s), the quantum information can be transferred between the N-$V$ centers and the CBJJs easily. Owing to the strong-coupling condition $[\lambda /2\pi \gg ({\gamma }_{eg},\Gamma ,{\gamma }_{\varphi })]$ and the long coherence time ($\sim {10}^{-3}$ s) of the N-$V$ centers, the quantum information can be stored in the memory steadily, and it can be retrieved with a high fidelity ($>96\%$). This scheme provides a scalable way of exchanging quantum information between different quantum interfaces, and has the experimental feasibility with currently available experimental technology.

We investigate a family of qubit-oscillator states as resources for hybrid quantum communication. They result from a mechanism of qubit-controlled displacement on the oscillator. For large displacements, we obtain analytical formulas for entanglement and other nonclassical correlations, such as entropic and geometric discord, in those states. We design two protocols for quantum communication using the considered resource states: a hybrid teleportation and a hybrid remote-state preparation. The latter, in its standard formulation, is shown to have a performance limited by the initial mixedness of the oscillator, echoing the behavior of the geometric discord. If one includes a further optimization over nonunitary correcting operations performed by the receiver, the performance is improved to match that of teleportation, which is directly linked to the amount of entanglement. Both protocols can then approach perfect efficiency even if the oscillator is originally highly thermal. We discuss the critical implications of these findings for the interpretation of general quantum correlations.