In the present work we report on our experimental total cross section and positronium formation cross-section results for positron scattering from pyrimidine for energies from 1 to 180 eV. In addition, the total inelastic integral cross sections (for electronic excitations plus direct ionization) have been measured up to 21.5 eV. We also report quasi-elastic total and differential cross sections at energies up to 21.5 eV. Our results are compared to recent theoretical and experimental data for positron scattering, as well as recent work on electron scattering from this target.

We present an ab initio determination of the shear viscosity for the unitary Fermi gas based on finite temperature quantum Monte Carlo (QMC) calculations and the Kubo linear-response formalism. The results are confronted with the bound for the shear viscosity originating from hydrodynamic fluctuations. Assuming smoothness of the frequency dependent shear viscosity $\eta \left(\omega \right)$, we show that the bound is violated in the low temperature regime and the violation occurs simultaneously with the onset of the Cooper pairing in the system. In order to preserve the hydrodynamic bound in QMC $\eta \left(\omega \right)$ has to possess a sharp structure located in the vicinity of zero frequency which is not resolved by an analytic continuation procedure.

We investigate propagation of few-photon pulses in waveguides coupled to a two-level system by means of the method of distribution functions in coordinate-momentum space that provides a detailed description of photon systems. We find that the distribution function of the transmitted pulse can be negative for the nonclassical input (i.e., single-photon Fock state). This reveals the quasiprobability nature of photon distribution. Analytical expressions for photon densities in the momentum space as well as in the coordinate space are obtained for the mentioned single-photon Gaussian input. We also study the evolution of the multimode coherent-state input for an arbitrary photon number. Time-dependent differential equations describing average densities and fluctuations of outgoing photons are derived and solved. The influence of the number of input photons, pulse width, and radiation-atom interaction strength on the statistical properties of the fluctuations is investigated.

In this Rapid Communication, we consider the open problem of the minimum cost of two-qubit gates for simulating the Toffoli gate and show that five two-qubit gates are necessary. Before our work, it was known that five two-qubit gates are sufficient to implement the Toffoli gate, and numerical evidence indicates that five two-qubit gates are also necessary. The idea introduced here can also be used to solve the problem of optimal simulation of Deutsch three-qubit gates.

We introduce a multipartite communication scheme, with the aim to enable the senders to remotely and obliviously provide the receivers with an arbitrary amount of multipartite entanglement. The scheme is similar to remote state preparation (RSP). However, we show that even though the receivers are restricted to local unitary operations, the required resources for remote entanglement preparation are less than for RSP. In order to do so we introduce a canonical form of arbitrary multipartite pure states describing an arbitrary number of qubits. Moreover, we show that if the receivers are enabled to perform arbitrary local operations and classical communication, the required resources can drastically be reduced. We employ this protocol to derive robust entanglement purification protocols for arbitrary pure states and show that it can also be used for sending classical information.

The ambiguity of the macroscopic description of light pressure on a continuous medium originates from the uncertainty of dividing the energy-momentum tensor of electromagnetically excited matter into a material and field parts or, equivalently, the total acting force into pressure and deformation terms. We show that, although there exists a continuum of formally correct formulations, one can adopt the appropriate form of the macroscopic field stress tensor that allows a unified description of pressure during elementary light-matter interactions, such as reflection, refraction, absorption, and nonlinear conversion. The derived simple expressions for the pressure force are compatible with the polariton momentum $\hslash \mathbf{k}$. The corresponding relation for the electromagnetic momentum density generalizes Rytov's definition for right-handed and left-handed frequency dispersive media.

We study the problem of particle indistinguishability for the three cases known in nature: identical classical particles, identical bosons, and identical fermions. By exploiting the fact that different types of particles are associated with Hilbert space vectors with different symmetries, we establish some relations between the expectation value of several different operators, as the particle number one and the interparticle correlation one, evaluated for states of a pair of identical classical particles, bosons, and fermions. We find that the quantum behavior of a pair of identical classical particles has exactly half fermionic and half bosonic characteristics.

Quantum key distribution over probabilistic quantum repeaters is addressed. We compare, under practical assumptions, two such schemes in terms of their secret key generation rates per quantum memory. The two schemes under investigation are the one proposed by Duan et al. [Nature (London) 414, 413 (2001)] and that of Sangouard et al. [Phys. Rev. A 76, 050301 (2007)]. We consider various sources of imperfection in both protocols, such as nonzero double-photon probabilities at the sources, dark counts in detectors, and inefficiencies in the channel, photodetectors, and memories. We also consider memory decay and dephasing processes in our analysis. For the latter system, we determine the maximum value of the double-photon probability beyond which secret key distillation is not possible. We also find crossover distances for one nesting level to its subsequent one. We finally compare the two protocols in terms of their achievable secret key generation rates at their optimal settings. Our results specify regimes of operation where one system outperforms the other.

Confinement-induced resonances (CIR) in quasi-one-dimensional (quasi-1D) systems have been theoretically predicted and observed in various ultracold atomic gases. Here a regularized local frame transformation method is developed to treat CIR in a quasi-1D system that has an arbitrary transverse trap shape in general. The method is applied to predict the CIR position in a system having a square transverse geometry with hard walls.

We investigate resonant dipole-dipole interactions between two superatoms of different angular momentum, consisting of two Rydberg-blockaded atom clouds where each cloud carries initially a coherently shared single excitation. We demonstrate that the dipole-dipole interaction breaks up the superatoms by removing the excitations from the clouds. The dynamics is akin to an ensemble average over systems where only one atom per cloud participates in entangled motion and excitation transfer. Our findings should thus facilitate the experimental realization of adiabatic exciton transport in Rydberg systems by replacing single sites with atom clouds.

In this paper, we study the influence of time-dependent phase delay on the emission of few photons from a single molecule in a solid matrix. The results demonstrate that a time-dependent phase of the pulses can be used to control the photon emission and strongly affects the spectrum of single molecules. The spectrum of the photon emission probabilities reflects the fast dynamics of the states of the system when the duration of the driven laser pulses is short. Also, it is shown that the coupling between the system and the rf field strongly affects the spectrum of the photon emission probabilities in space.

We consider the noncondensate density and density fluctuations of a trapped dipolar Bose-Einstein condensate, focusing on the regime where a rotonlike excitation spectrum develops. Our results show that a characteristic peak in the noncondensate density occurs at trap center due to the rotons. In this regime we also find that the anomalous density becomes positive and peaked, giving rise to enhanced density fluctuations. We calculate the noncondensate density in momentum space and show that a small momentum halo is associated with the roton excitations.

Enforcing a nonclassical behavior in mesoscopic systems is important for the study of the boundaries between the quantum and the classical world. Recent experiments have shown that optomechanical devices are promising candidates to pursue such investigations. Here we consider two different setups where the indirect coupling between a three-level atom and the movable mirrors of a cavity is achieved. The resulting dynamics is able to conditionally prepare a nonclassical state of the mirrors by means of projective measurements operated over a pure state of the atomic system. The nonclassical features are persistent against incoherent thermal preparation of the mechanical systems and their dissipative dynamics.

We propose an approach for the optical encryption of ghost images reconstructed from the spatially correlated classical two-beam pairs. This approach has the capability of encrypting ghost images by flexibly manipulating the position correlation of a pair of “signal” and “idler” beams. We experimentally demonstrate that the perfect ghost image can be extracted by decrypting the encrypted information of an object, in particular, our approach under eavesdropping has a higher security for encryption compared with previous approaches.

We present a perturbative approach to derive the semiclassical equations of motion for the two-dimensional electron dynamics under the simultaneous presence of static electric and magnetic fields, where the quantized Hall conductance is known to be directly related to the topological properties of translationally invariant magnetic Bloch bands. In close analogy to this approach, we develop a perturbative theory of two-dimensional photonic transport in gyrotropic photonic crystals to mimic the physics of quantum Hall systems. We show that a suitable permittivity grading of a gyrotropic photonic crystal is able to simulate the simultaneous presence of analog electric and magnetic field forces for photons, and we rigorously derive the topology-related term in the equation for the electromagnetic energy velocity that is formally equivalent to the electronic case. A possible experimental configuration is proposed to observe a bulk photonic analog to the quantum Hall physics in graded gyromagnetic photonic crystals.

A method based on optical interferences for measuring optical nonlinearities is presented. In a proof-of-principle experiment, the technique is applied to the experimental determination of the intensity dependence of the photoionization process. It is shown that it can also be used to control and optimize the nonlinear process itself at constant input energy. The presented strategy leads to enhancements that can reach several orders of magnitude for highly nonlinear processes.

The problems of optimally estimating a phase, a direction, and the orientation of a Cartesian frame (or trihedron) with general pure states are addressed. Special emphasis is put on estimation schemes that allow for inconclusive answers or abstention. It is shown that such schemes enable drastic improvements, up to the extent of attaining the Heisenberg limit in some cases, and the required amount of abstention is quantified. A general mathematical framework to deal with the asymptotic limit of many qubits or large angular momentum is introduced and used to obtain analytical results for all the relevant cases under consideration. Parameter estimation with abstention is also formulated as a semidefinite programming problem, for which very efficient numerical optimization techniques exist.

Protecting quantum correlation from decoherence is one of the crucial issues in quantum information processing. It has been commonly recognized that any initial quantum correlation of a composite system diminishes asymptotically or abruptly to zero under local Markovian decoherence. Here we show that, contrary to this recognition, a noticeable Gaussian quantum discord of a continuous-variable bipartite system can be frozen in the steady state in the non-Markovian dynamics if each of the subsystems forms a localized mode with its local reservoir. The condition for this frozen quantum discord can be reached by appropriately engineering the structure of the reservoirs. The possible realization of our results in a coupled cavity array system formed by a photonic crystal is proposed.

Cluster states for measurement-based quantum computing can be created with entangling operations of arbitrary low success probability. This will place a lower bound on the minimum lifetime of the qubits used in the implementation. Here, we present a simple scaling argument for the $T_2$ time of the qubits for various cluster-state geometries, and we estimate the size of the qubit reservoir that is needed for the creation of miniclusters.