Quantum electrodynamics (QED) provides a highly accurate description of phenomena involving the interaction of atoms with light. We argue that the quantum theory describing the interaction of cold atoms with a vibrating membrane—quantum acoustodynamics (QAD)—shares many issues and features with QED. Specifically, the adsorption of an atom on a vibrating membrane can be viewed as the counterpart to QED radiative electron capture. A calculation of the adsorption rate to lowest order in the atom-phonon coupling is finite; however, higher-order contributions suffer from an infrared problem mimicking the case of radiative capture in QED. Terms in the perturbation series for the adsorption rate diverge as a result of massless particles in the model (flexural phonons of the membrane in QAD and photons in QED). We treat this infrared problem in QAD explicitly to obtain finite results by regularizing with a low-frequency cutoff that corresponds to the inverse size of the membrane. Using a coherent-state basis for the soft-phonon final state, we then sum the dominant contributions to derive a new formula for the multiphonon adsorption rate of atoms on the membrane that gives results that are finite, nonperturbative in the atom-phonon coupling, and consistent with the Kinoshita-Lee-Nauenberg theorem. For micromembranes, we predict a reduction with increasing membrane size for the low-energy adsorption rate. We discuss the relevance of this to the adsorption of a cold gas of atomic hydrogen on suspended graphene.

Recently a paper on the construction of consistent Wigner functions for cylindrical phase spaces ${\mathbb{S}}^1\times \mathbb{R}$, i.e., for the canonical pair angle and orbital angular momentum, was published [H. A. Kastrup, Phys. Rev. A 94, 062113 (2016)] in which the main properties of these functions are derived and discussed and their usefulness is illustrated with examples. The present paper is a continuation which compares the properties of the Wigner functions for cylindrical phase spaces with those of the well-known Wigner functions for planar phase spaces in more detail. Furthermore, the mutual (Weyl) correspondence between Hilbert space operators and their phase-space functions is discussed. The $☆$ product formalism is shown to be completely implementable. In addition, basic dynamical laws for Wigner and Moyal functions are derived as generalized Liouville and energy equations. They are very similar to those in the planar case but also show characteristic differences.

Quantum mechanics is known to provide significant improvements in information processing tasks when compared to classical models. These advantages range from computational speedups to security improvements. A key question is where these advantages come from. The toy model developed by Spekkens [R. W. Spekkens, Phys. Rev. A 75, 032110 (2007)] mimics many of the features of quantum mechanics, such as entanglement and no cloning, regarded as being important in this regard, despite being a local hidden variable theory. In this work, we study several protocols within Spekkens' toy model where we see it can also mimic the advantages and limitations shown in the quantum case. We first provide explicit proofs for the impossibility of toy bit commitment and the existence of a toy error correction protocol and consequent $k$-threshold secret sharing. Then, defining a toy computational model based on the quantum one-way computer, we prove the existence of blind and verified protocols. Importantly, these two last quantum protocols are known to achieve a better-than-classical security. Our results suggest that such quantum improvements need not arise from any Bell-type nonlocality or contextuality, but rather as a consequence of steering correlations.

We present a theoretical study of a four-electron four-quantum-dot system based on molecular-orbital methods, which hosts a pair of singlet-triplet spin qubits. We took into account the admixture of electron wave functions in all dots, and we found that this mixing of wave functions has consequences to the energy spectrum, exchange interaction, and gate crosstalk of the system. Specifically, we found that when the two singlet-triplet qubits are close enough, some of the states are no longer dominated by the computational basis states, and the exchange interaction cannot simply be understood as the energy difference between the singlet and triplet states. Using the Hund-Mulliken calculation of the Hubbard parameters, we characterized the effective exchange interaction of the system, and we found good agreement with results calculated by taking energy differences where applicable. We studied the two commonly conceived schemes coupling two qubits, namely the exchange and capacitive coupling, and we found that when the interqubit distance is at certain intermediate values, the two kinds of coupling are comparable in strength, complicating the analyses of the evolution of the two qubits. We also investigated the gate crosstalk in the system due to the quantum-mechanical mixing of electron states, and we found that while this effect is typically very weak, it should not be ignored if the spacing between the qubits is similar to or less than the distance between the double dots that constitute the qubit.

Motivated by Buscemi's semiquantum nonlocal game [F. Buscemi, Phys. Rev. Lett. 108, 200401 (2012)], we propose an entanglement witness game, a quantum game based on entanglement witness. Similar to the semiquantum nonlocal game, the existence of entanglement shared by the players is necessary and sufficient for obtaining a positive average payoff in our entanglement witness game. Two explicit examples are constructed to demonstrate how to play our entanglement witness game and its relations with the Clauser-Horne-Shimony-Holt nonlocal game.

In the context of quantum tomography, we recently introduced a quantity called a partial determinant [C. Jackson and S. J. van Enk, Phys. Rev. A 92, 042312 (2015)]. Partial determinants (PDs) are explicit functions of the collected data that are sensitive to the presence of state preparation and measurement correlated errors. As such, PDs bypass the need to estimate state preparation or measurement parameters individually. In the present work, we suggest a theoretical perspective for the PD. We show that the PD is a holonomy and that the notions of state, measurement, and tomography can be generalized to nonholonomic constraints. To illustrate and clarify these abstract concepts, direct analogies are made to parallel transport, thermodynamics, and gauge field theory. This paper is the first of a two-part series where the following paper is about scalable generalizations of the PD in multiqudit systems, with possible applications for debugging a quantum computer.

In the context of quantum tomography, quantities called partial determinants [C. Jackson and S. J. van Enk, Phys. Rev. A 92, 042312 (2015)] were recently introduced. Partial determinants (PDs) are explicit functions of the collected data that are sensitive to the presence of state preparation and measurement (SPAM) correlations. In this paper we demonstrate further applications of the PD and its generalizations. In particular, we construct methods for detecting various types of SPAM correlation in multiqudit systems, e.g., measurement-measurement correlations. The relationship between the PDs of each method and the correlations they are sensitive to is topological. We give a complete classification scheme for all such methods but focus on the explicit details of only the most scalable methods, for which the number of settings scales as $O\left(d^4\right)$. This paper is the second of a two-part series where the preceding paper [C. Jackson and S. van Enk, preceding paper, Phys. Rev. A 95, 052327 (2017)] is about a theoretical perspective for the PD, particularly its interpretation as a holonomy.

Ultracold atoms placed in a tight cigar-shaped trap are usually described in terms of the Lieb-Liniger model. We study the extensions of this model which arise when van der Waals interaction between atoms is taken into account. We find that the corrections induced by the finite range of interactions can become especially important in the vicinity of narrow Feshbach resonances and suggest realistic schemes of their experimental detection. The interplay of confinement and interactions can lead to effective transparency where the one-dimensional interactions are weak in a wide range of parameters.

Dynamical computations demonstrate considerable selectivity over the fragmentation channels of the LiH molecule via the polarization and the carrier envelope phase (CEP) of a single ultrashort one-cycle strong IR pulse. For aligned molecules, control of the CEP allows building nonstationary coherent electronic wave packets of contrasting ionic character, either $\mathrm{L}{\mathrm{i}}^{\delta +}{\mathrm{H}}^{\delta -}$ or $\mathrm{L}{\mathrm{i}}^{\delta -}{\mathrm{H}}^{\delta +}$. The complementary coherences are maintained all the way to dissociation. The direction of the electric field at its maximum points either towards the Li or towards the H atom, which selectively steers the nuclear dynamics to specific dissociation products.

The convergent close-coupling method for atomic photoionization is modified by treating the singularity in the Green's function analytically. The resultant close-coupling equations are then free of any singularities, and can be solved at all incident energies including at the exact thresholds. The utility and superiority of the approach is demonstrated by considering single photoionization of the ground state of helium at photoelectron energies from the ionic $n=1,2,3$ thresholds through to 1 keV. For completeness, the double photoionization cross section is also presented.

We investigate vortex excitations in dilute Bose-Einstein condensates in the presence of complex $\mathcal{PT}$-symmetric potentials. These complex potentials are used to describe a balanced gain and loss of particles and allow an easier calculation of stationary states in open systems than in a full dynamical calculation including the whole environment. We examine the conditions under which stationary vortex states can exist and consider transitions from vortex to nonvortex states. In addition, we study the influences of $\mathcal{PT}$ symmetry on the dynamics of nonstationary vortex states placed at off-center positions.

We study the (pseudo-) anyon Hubbard model on a one-dimensional lattice without the presence of a three-body hardcore constraint. In particular, for the pseudo-fermion limit of a large statistical angle $\theta \approx \pi$, we observe a wealth of exotic properties, including a first-order transition between different superfluid phases and a two-component partially paired phase for large fillings without need of an additional three-body hardcore constraint. In this limit, we analyze the effect of an induced hardcore constraint, which leads to the stabilization of superfluid ground states for vanishing or even small attractive onsite interactions. For finite statistical angles, we study the unconventional broken-symmetry superfluid peaked at a finite momentum, resulting in an interesting beat phenomenon of single-particle correlation functions. We show how some features of various ground-state phases, including an analog of the partially paired phase in the pseudo-fermion limit, may be reproduced in a naive mean field frame.

For incidence in the critical region, the propagation of Gaussian lasers through dielectric blocks is characterized by the joint action of angular deviations and lateral displacements. This mixed effect, known as the composite Goos-Hänchen shift, produces a lateral displacement that is dependent on the axial coordinate, recently confirmed by a weak measurement experiment. We discuss under which conditions this axial lateral displacement, which only exists for the composite Goos-Hänchen shift, presents an oscillatory behavior. This oscillation phenomenon shows a peculiar behavior of light for critical incidence and, if experimentally tested, could stimulate further theoretical studies and lead to interesting optical applications.

A proposed phase-estimation protocol based on measuring the parity of a two-mode squeezed-vacuum state at the output of a Mach-Zehnder interferometer shows that the Cramér-Rao sensitivity is sub-Heisenberg [P. M. Anisimov et al., Phys. Rev. Lett. 104, 103602 (2010)]. However, these measurements are problematic, making it unclear if this sensitivity can be obtained with a finite number of measurements. This sensitivity is only for a phase near zero, and in this region there is a problem with ambiguity because measurements cannot distinguish the sign of the phase. Here, we consider a finite number of parity measurements and show that an adaptive technique gives a highly accurate phase estimate regardless of the phase. We show that the Heisenberg limit is reachable, where the number of trials needed for mean photon number $\overline{n}=1$ is approximately 100. We show that the Cramér-Rao sensitivity can be achieved approximately, and the estimation is unambiguous in the interval ($-\pi /2,\pi /2$).

In this work we consider quantum cascade networks in which quantum systems are connected through unidirectional channels that can mutually interact giving rise to interference effects. In particular we show how to compute master equations for cascade systems in an arbitrary interferometric configuration by means of a collisional model. We apply our general theory to two specific examples: the first consists of two systems arranged in a Mach-Zender-like configuration and the second is a three system network where it is possible to tune the effective chiral interactions between the nodes exploiting interference effects.

We explore the spin-orbit nature of the optical torque exerted on chiral liquid-crystal microspheres exhibiting circular Bragg reflection. Experimental investigation relies on the direct optomechanical observation of spinning liquid-crystal droplets immersed in water and held in a circularly polarized laser levitator. More generally, we anticipate that the total angular momentum transfer per photon may depart from the commonly assumed spin-only $\pm 2\hslash$ contribution, when the topological features of the illuminated microsphere are taken into account.

We analyze a photon transport through a one-dimensional open waveguide side coupled to the $N$-photon microwave cavity with embedded an artificial two-level atom (qubit). The qubit state is probed by a weak signal at the fundamental frequency of the waveguide. Within the formalism of projection operators and a non-Hermitian Hamiltonian approach we develop a one-photon approximation scheme to obtain the photon wave function, which allows for the calculation of the probability amplitudes of the spontaneous transitions between the levels of two Rabi doublets in an $N$-photon cavity. We obtain analytic expressions for the transmission and reflection factors of the microwave signal through a waveguide which contains the information of the qubit parameters. We show that for a small number of cavity photons the Mollow spectrum consists of four spectral lines, which is a direct manifestation of the quantum nature of light. The results obtained in the paper are of a general nature and can be applied to any type of qubits. The specific properties of the qubit are only encoded in the two parameters: the energy $\mathrm{\Omega }$ of the qubit and its coupling $\lambda$ to the cavity photons.

Caldeira and Leggett (CL), in a seminal paper, derived a master equation describing Markovian quantum Brownian motion. Such an equation suffered from not being completely positive, and many efforts have been made to solve this issue. We show that when a careful mathematical analysis is performed, the model considered by CL leads to a nondissipative master equation. We argue that the correct way to understand the master equation derived in the CL regime is to consider it non-Markovian. Moreover, we show that if one wants to provide a microscopic description of quantum Brownian motion with the CL model, one always needs to consider a non-Markovian dynamics. We conclude that dissipation is a genuinely non-Markovian feature.