We review and develop an algorithm to determine arbitrary quantum bounds based on the seminal work of Tsirelson [Lett. Math. Phys. 4, 93 (1980)]. The potential of this algorithm is demonstrated by both deriving marginal-involving number-valued quantum bounds and identifying a generalized class of function-valued quantum bounds. Those results facilitate an eight-dimensional volume analysis of quantum mechanics which extends the work of Cabello [Phys. Rev. A 72, 012113 (2005)]. We contrast the quantum volume defined by these bounds to that of macroscopic locality, defined by the inequalities corresponding to the first level of the hierarchy of Navascués et al. [New J. Phys. 10, 073013 (2008e)], proving our function-valued quantum bounds to be more complete.

We investigate the scattering of a quantum-mechanical particle on an oscillating rectangular potential barrier. In the first step, we show the calculation for a classical potential revealing coherent, quantized energy exchange between the particle and potential, respectively, the likewise classical field that generates the latter. In order to explore the quantized energy transfer in more detail, the transition to a quantum field is performed and the Schrödinger equation for the composite quantum system consisting of particle and field is solved analytically as well. The result allows a physically very intuitive explanation of the underlying energy exchange processes and explicitly displays the occurring entanglement between particle and field due to their interaction, thus offering the possibility to use this combined system for quantum information experiments. Depending on the initial field state, incoherent or coherent energy exchange is observed. In the limit of high mean photon numbers the classical results can be fully retrieved.

We use the theory of quantum estimation in two different qubit-boson coupling models to demonstrate that the temperature of a quantum harmonic oscillator can be estimated with high precision by quantum-limited measurements on the qubit. The two models that we address embody situations of current physical interest due to their connection with ongoing experimental efforts on the control of mesoscopic dynamics. We show that population measurements performed over the qubit probe are near optimal for a broad range of temperatures of the harmonic oscillator.

We present a formalized perturbation theory for Markovian master equations in the language of a generalized Schrieffer-Wolff (SW) transformation. A nonunitary rotation decouples the unperturbed steady states from all fast degrees of freedom, in order to obtain an effective Liouvillian that reproduces the exact low excitation spectrum of the system. The transformation is derived in a constructive way, yielding a perturbative expansion of the effective Liouville operator. The presented formalism realizes an adiabatic elimination of fast degrees of freedom to arbitrary order. We exemplarily employ the SW formalism to two generic open systems and discuss general properties of the different orders of the perturbation.

We report opposite statistical-mechanical behaviors of the two major paradigms in which quantum correlation measures are defined, viz., the entanglement-separability paradigm and the information-theoretic one. We show this by considering the ergodic properties of such quantum correlation measures in transverse quantum $XY$ spin-$\frac{1}{2}$ systems in low dimensions. While entanglement measures are ergodic in such models, the quantum correlation measures defined from an information-theoretic perspective can be nonergodic.

We extend the criteria for $k$-particle entanglement from the spin-squeezing parameter presented in Sørensen and Mølmer [Phys. Rev. Lett. 86, 4431 (2001)] to systems with a fluctuating number of particles. We also discuss how other spin-squeezing inequalities can be generalized to this situation. Further, we show how, by employing additional degrees of freedom, it is possible to extend the bounds to the case when the individual particles cannot be addressed. As a by-product, this allows us to show that in spin-squeezing experiments with cold gases the particles are typically distinguishable in practice. Our results justify the application of the Sørensen-Mølmer bounds in recent experiments on spin squeezing in Bose-Einstein condensates.

The momentum distributions of the recoil ions were measured for the single ionization of atomic targets (Ne, Ar) and of molecular targets (CH${}_4$, N${}_2$, CO${}_2$) at incident electron energies ranging from 80 to 220 eV by using the reaction microscope. It was found that there are a remarkable number of recoil ions with large longitudinal momenta, and the relative contribution of these ions for Ne ($e$, 2$e$) reactions decreases faster than that for Ar ($e$, 2$e$) reactions with the incident electron energy increasing. This observed feature can be qualitatively interpreted by the different effective potential of the target core experienced by the electrons. The appearance of the large longitudinal momentum recoil ions for CH${}_4$, N${}_2$, and CO${}_2$ ($e$, 2$e$) reactions indicates that the strong interaction between the electron and the component-atoms target core in the molecules plays a significant role in the single ionization of small molecular targets. Depending on the independent atom model, we can intuitively understand the characteristics of the momentum distributions of the recoil ions CH${}_4^{+}$, N${}_2^{+}$, and CO${}_2^{+}$, respectively.

A fascinating aspect of Rydberg atoms is their ability to form huge but very weakly bound molecules with a ground-state atom, only held together by a scattering process between the latter and the Rydberg electron. Beyond the usual way of creating such molecules by laser excitation from two ground-state atoms with a distance of less than the Rydberg radius, we demonstrate that Rydberg molecules can also be formed by capturing a ground-state atom which is initially located outside the range of the Rydberg atom when it comes in contact with it. To demonstrate this effect, we investigate the scattering process between the Rydberg electron and the ground-state atom within a quantum-classical framework. In this picture, the capture results from a dissipative finite-mass correction term in the classical equations of motion. We show that, and under which conditions the capturing takes place.

We report the results of ab initio calculations on valence photoionization of the acetylene monocation in its ground ($X$) and electronically excited ($A$) states using the complex Kohn variational method. In contrast to the $K$-shell molecular frame photoelectron angular distributions (MFPADs), which are only sensitive to the molecular geometry, these results show that the valence MFPADs are also sensitive to the electronic state of the target. Hence, the isomerization path from acetylene to vinylidene, which is mediated by a conical intersection responsible for a change in electronic state of the monocation, could, in principle, be traced through valence photoionization.

We consider the extension of ultrafast laser alignment schemes to surface-adsorbed molecules, where the laser field coerces the molecule to reorient itself relative to the surface. When probed by a scanning tunneling microscope tip, this reorientation modifies the tip-molecule distance, and thus the tunneling current, suggesting a route to an ultrafast, nanoscale current switch. In addition to exploring the controllability of adsorbed molecules by moderately intense laser fields and discussing the fundamental differences of alignment behavior between surface-adsorbed molecules and gas phase molecules, we computationally investigate the quality of orientation with respect to field intensity, field duration, and the location of the tip. Overall, the molecule moves directly to its oriented configuration, which is reasonably insensitive to the tip location. These results collectively suggest the efficacy of using laser alignment schemes to control electron transport through junctions.

Temporal Bragg-diffraction-induced laser-pulse splitting into two pulses propagating with different group velocities is observed in multilayered linear photonic crystals (PCs). This phenomenon originates from spatially inhomogeneous light localization within the PCs at the Laue scheme of the dynamical Bragg diffraction. In a homogeneous medium at the PC output each pulse is spatially separated into two pulses, propagating in the transmission and diffraction directions, respectively. The experiments are carried out for a one-dimensional porous silicon-based PC consisting of 375 spatial periods of 800 nm thickness using a femtosecond Ti:sapphire laser as a probe. A linear dependence of the time splitting of each pair of transmitted and diffractively reflected pulses on the crystal thickness is demonstrated and is supported by theoretical estimations.

The ability to directly store optical images in atomic vapor has many potential applications, but the diffusion of atoms severely limits the image resolution and storage duration. In this paper, we report an experimental demonstration on diffusion-free storage of optical images in hot atomic vapor. By encoding two-dimensional images in incoherent light and making use of the correlation imaging technique, we overcome the effect of atomic diffusion on the all-optical image storage in atomic vapor.

Helicity is a familiar concept in particle physics and also appears in the physics of fluids and plasmas. In this paper, we present the optical helicity in a form readily applicable to both quantum and classical problems. We examine the relationship between the optical helicity and the more familiar optical spin and show that the conservation of helicity is an expression of the electric-magnetic symmetry for light. We show that helicity is distinct from Lipkin's 00-zilch; a simple relationship exists between the two only for monochromatic fields. It is only the optical helicity that has the correct dimensions of an angular momentum, thereby accurately describing the helicity of light. To illustrate the physical significance of the optical helicity, we consider a circularly polarized plane wave and the field emitted by a rotating electric dipole.

We consider the electric- and magnetic-field fluctuations in the vacuum state in the region external to a half space filled with a homogeneous nondissipative dielectric. We discuss an appropriate limit to an ideal metal and concentrate our interest on the renormalized field fluctuations, or equivalently on the renormalized electric and magnetic energy densities, in the proximity of the dielectric-vacuum interface. We show that surface divergences of field fluctuations arise at the interface in an appropriate ideal conductor limit, and that our limiting procedure allows us to discuss their structure in detail. Field fluctuations close to the surface can be investigated through the retarded Casimir-Polder interaction with an appropriate polarizable body.

We study a single two-level atom interacting with a reservoir of modes defined by a reservoir structure function with a frequency gap. Using the pseudomodes technique, we derive the main features of a trapping state formed in the weak coupling regime. Utilizing different entanglement measures we show that strong correlations and entanglement between the atom and the modes are in existence when this state is formed. Furthermore, an unexpected feature for the reservoir is revealed. In the long time limit and for weak coupling the reservoir spectrum is not constant in time.

The spatial search problem on regular lattice structures in integer number of dimensions $d\ge 2$ has been studied extensively, using both coined and coinless quantum walks. The relativistic Dirac operator has been a crucial ingredient in these studies. Here, we investigate the spatial search problem on fractals of noninteger dimensions. Although the Dirac operator cannot be defined on a fractal, we construct the quantum walk on a fractal using the flip-flop operator that incorporates a Klein-Gordon mode. We find that the scaling behavior of the spatial search is determined by the spectral (and not the fractal) dimension. Our numerical results have been obtained on the well-known Sierpinski gaskets in two and three dimensions.

We analyze simple quantum error detection and quantum error correction protocols relevant to current experiments with superconducting qubits. We show that for qubits with energy relaxation the repetitive $N$-qubit codes cannot be used for quantum error correction, but can be used for quantum error detection. In the latter case it is sufficient to use only two qubits for the encoding. In the analysis we demonstrate a useful technique of unraveling the qubit energy relaxation into “relaxation” and “no relaxation” scenarios. Also, we propose and numerically analyze several two-qubit algorithms for quantum error detection and correction, which can be readily realized at the present-day level of the phase qubit technology.

Any practical experiment utilizing the innate $D$-dimensional entanglement of the orbital angular momentum (OAM) of photons is subject to the generation capacity of the entangled photon source and the modal capacity of the detection system. We report an experimental spontaneous parametric-down-conversion system able to generate and detect tunable high-dimensional OAM entanglement. By tuning the phase matching, we demonstrate a factor of 2 increase on the half-width of the OAM-correlation spectrum, from 10 to 20. In terms of quantum mutual information capacity, this is an increase from $3.18$ to $4.95$ bits/photon. Furthermore, we measure correlations in the conjugate variable, angular position, and obtain concurrence values $0.96$ and $0.90$. The good entanglement measures in both OAM and angular position bases indicate bipartite, $D$-dimensional entanglement where $D$ is tunable.

Recent work on approximate quantum error correction (QEC) has opened up the possibility of constructing subspace codes that protect information with high fidelity in scenarios where perfect error correction is impossible. Motivated by this, we investigate the problem of approximate subsystem codes. Subsystem codes extend the standard formalism of subspace QEC to codes in which only a subsystem within a subspace of states is used to store information in a noise-resilient fashion. Here, we demonstrate easily checkable sufficient conditions for the existence of approximate subsystem codes. Furthermore, for certain classes of subsystem codes and noise processes, we prove the efficacy of the transpose channel as a simple-to-construct recovery map that works nearly as well as the optimal recovery channel. This work generalizes our earlier approach [H. K. Ng and P. Mandayam, Phys. Rev. A 81, 062342 (2010)] of using the transpose channel for the approximate correction of subspace codes to the case of subsystem codes, and brings us closer to a unifying framework for approximate QEC.

The time-dependent close-coupling method for the double photoionization of atoms is extended to compute fully differential nuclear-recoil cross sections. Excellent agreement is found between our calculated double-photoionization total cross sections of He and the measurements at all photon energies. Differential cross-section results are presented for the single-photon double ionization of He at photon energies of 99, 125, and 225 eV. The single-differential cross-section results at 99 eV agree with previous theory and experiment. Symmetric momentum distributions are found in the plane perpendicular to the polarization direction, while dipolelike momentum distributions are found in the other two planes. The variation of the nuclear-recoil triple-differential cross sections of He${}^{2+}$ with the nuclear-recoil momenta are presented. The total cross sections for the nuclear-recoil of He${}^{2+}$ match the corresponding ones for the double electron ejection of He for all the studied photon energies.