The theory of scattering of scalar stochastic fields from deterministic and random media is generalized to the electromagnetic domain under the first-order Born approximation. The analysis allows for determining the changes in spectrum, coherence, and polarization of electromagnetic fields produced on their propagation from the source to the scattering volume, interaction with the scatterer, and propagation from the scatterer to the far field. An example of scattering of a field produced by a $\delta$-correlated partially polarized source and scattered from a $\delta$-correlated medium is provided.

We propose a scheme for storing arbitrary states of two superconducting charge qubits in a single ${}^{87}\mathrm{Rb}$ cold-atom ensemble. The information of the two charge qubits is transferred sequentially into the single atomic ensemble, via a transmission line resonator serving as “data bus.” We simulate the storage process by integrating the master equation that takes into account all kinds of decoherence mechanisms, and show that the mean storage fidelity is very high with the present experimental parameters. Moreover, the storage efficiency of the single ensemble in our scheme is double that of previous schemes. Finally, we also briefly discuss the possibilities and difficulties of extending it to multiqubit cases.

A possible experimental explanation for the unusual temperature dependence of water line broadening by molecular hydrogen recently observed by Dick et al. [Phys. Rev. A 81, 022706 (2010)] is proposed. This temperature dependence differs radically from both theoretical predictions and the higher temperature behavior, but remains unexplained. It is suggested that the cause is a change in sample composition due to preferential adsorption of orthohydrogen onto the cell walls at low temperature. As parahydrogen has collision cross sections with water molecules up to an order of magnitude smaller than those with orthohydrogen, an enrichment of the gas sample by parahydrogen would lead to a decrease in broadening parameters at low temperatures, as observed in the experiment.

This response describes the authors’ reaction to a critique of recent work on the ultracold physics of water. The possibility of spin-selective adsorption occurring in the context of the collisional cooling experiment is discussed.

We show that fundamental versions of the Deutsch-Jozsa and Bernstein-Vazirani quantum algorithms can be performed using a small entangled cluster state resource of only six qubits. We then investigate the minimal resource states needed to demonstrate general $n$-qubit versions and a scalable method to produce them. For this purpose, we propose a versatile photonic on-chip setup.

We explain the origin of the unusual large isotopic dependence found in charge-transfer cross sections for $\mathrm{H}(\mathrm{D},\mathrm{T}){}^{+}+\mathrm{Be}$ collisions. We show that this large effect appears in a semiclassical treatment as a consequence of the mass dependence of the charge-transfer transition probabilities, which is due to the variation of the radial velocity in the region where the nonadiabatic transitions take place. The possibility of finding such a large isotope effect in other collision systems is discussed.

We propose local strategies to protect global quantum information. The protocols, which are quantum error-correcting codes for dissipative systems, are based on environment measurements, direct feedback control, and simple encoding of the logical qubits into physical qutrits whose decaying transitions are indistinguishable and equally probable. The simple addition of one extra level in the description of the subsystems allows for local actions to fully and deterministically protect global resources such as entanglement. We present codes for both quantum jump and quantum state diffusion measurement strategies and test them against several sources of inefficiency. The use of qutrits in information protocols suggests further characterization of qutrit-qutrit disentanglement dynamics, which we also give together with simple local environment measurement schemes able to prevent distillability sudden death and even enhance entanglement in situations in which our feedback error correction is not possible.

We study the problem of performing orthogonal qubit measurements simultaneously. Since these measurements are incompatible, one has to accept additional imprecision. An optimal joint measurement is the one with the least possible imprecision. All earlier considerations of this problem have concerned only joint measurability of observables, while in this work we also take into account conditional state transformations (i.e., instruments). We characterize the optimal joint instrument for two orthogonal von Neumann instruments as being the Lüders instrument of the optimal joint observable.

We study entanglement generation in a pair of qubits interacting with an initially correlated system. Using time-independent perturbation theory and the adiabatic theorem, we show conditions under which the qubits become entangled as the joint system evolves into the ground state of the interacting theory. We then apply these results to the case of qubits interacting with a scalar quantum field. We study three different variations of this setup; a quantum field subject to Dirichlet boundary conditions, a quantum field interacting with a classical potential, and a quantum field that starts in a thermal state.

Random walks describe diffusion processes, where movement at every time step is restricted to only the neighboring locations. We construct a quantum random walk algorithm, based on discretization of the Dirac evolution operator inspired by staggered lattice fermions. We use it to investigate the spatial search problem, that is, to find a marked vertex on a $d$-dimensional hypercubic lattice. The restriction on movement hardly matters for $d>2$, and scaling behavior close to Grover’s optimal algorithm (which has no restriction on movement) can be achieved. Using numerical simulations, we optimize the proportionality constants of the scaling behavior, and demonstrate the approach to that for Grover’s algorithm (equivalent to the mean-field theory or the $d\to \infty$ limit). In particular, the scaling behavior for $d=3$ is only about $25\%$ higher than the optimal $d\to \infty$ value.

We investigate the spatial search problem on the two-dimensional square lattice, using the Dirac evolution operator discretized according to the staggered lattice fermion formalism. $d=2$ is the critical dimension for the spatial search problem, where infrared divergence of the evolution operator leads to logarithmic factors in the scaling behavior. As a result, the construction used in our accompanying article [A. Patel and M. A. Rahaman, Phys. Rev. A 82, 032330 (2010)] provides an $O(\sqrt{N}\ln N)$ algorithm, which is not optimal. The scaling behavior can be improved to $O(\sqrt{N\ln N})$ by cleverly controlling the massless Dirac evolution operator by an ancilla qubit, as proposed by Tulsi [Phys. Rev. A 78, 012310 (2008)]. We reinterpret the ancilla control as introduction of an effective mass at the marked vertex, and optimize the proportionality constants of the scaling behavior of the algorithm by numerically tuning the parameters.

As an intermediate state of matter between the free monomeric gas phase and the solid state, clusters may exhibit a specific electronic structure and photoionization dynamics that can be unraveled by different types of electron spectroscopies. From mass-selected ion yield scans measured for photoionization of ($R$)-camphor, the ionization potentials (IPs) of the monomer ($8.66\pm 0.01$ eV), and of the homochiral dimer ($⩽8.37\pm 0.01$ eV) and trimer ($⩽8.30\pm 0.01$ eV) were obtained. These spectra, combined with threshold photoelectron spectroscopy and velocity map ion imaging, allow us to show that the camphor monomer and dimer photoionization channels are decoupled, i.e., that the highest occupied molecular orbital (HOMO) of the dimer does not undergo a dissociative ionization process that would lead to a spurious contribution to the monomer ion channel. Therefore mass selection, as achieved in our imaging photoelectron-photoion coincidence experiments, leads to size selection of the nascent monomer or dimer species. Since both the monomer and dimer are chiral, their photoelectron angular distribution (PAD) not only involves the usual $\beta$ anisotropy parameter but also a chiral asymmetry parameter $b$${}_1$ that can generate a forward-backward asymmetry in the PAD. This has been investigated using circularly polarized light (CPL) to record the photoelectron circular dichroism (PECD) in the near-threshold vacuum-ultraviolet (VUV) photoionization region. Analysis of size-selected electron images recorded with left- and right-handed CPL shows that over the first $1.5$ eV above the HOMO orbital ionization potentials (IPs), the $\beta$ parameter is not affected by the dimerization process, while the chiral $b$${}_1$ parameter shows clear differences between the monomer and the dimer, confirming that PECD is a subtle long-range probe of the molecular potential.

Recent high-resolution spectroscopic experiments demonstrated the possibility of measuring the Boltzmann constant directly from the Doppler width of atomic or molecular lines; however, both experimental setups and data analysis methods still need to be improved to compete with the best acoustic methods. In this paper, the influence of choice of spectral line-shape model for data analysis on systematical error of the Doppler-width determination is analyzed on the example of the oxygen line near $\lambda =687$ nm. Profile simulations were performed for many different models at pressures ranging from 0.012 Pa to 120 kPa, taking into account such line-shape effects as the speed dependence of collisional broadening and Dicke narrowing. We discuss limitations of an applicability of the Voigt profile for purposes of the Boltzmann constant determination. The influence of the model line-shape profile and the gas pressure range on the Doppler width extrapolated to the zero-pressure value is investigated.

The method of analytic continuation in the coupling constant in combination with use of the statistical Padé approximation is applied to the determination of resonance energy and width of the ${}^2{\Pi }_g$ state of ${{\mathrm{N}}_2}^{-}$. It is shown that standard quantum chemistry codes provide accurate data which can be used for analytic continuation in the coupling constant, and the resonance energy and width can be inferred with an accuracy comparable to other more elaborate methods.

We show that the quantum reflection coefficient of ultracold heavy atoms scattering off a dielectric surface can be tuned in a wide range by suitable choice of surface and environment temperatures. This effect results from a temperature-dependent repulsive part of the van der Waals-Casimir-Polder-Lifshitz atom-surface interaction potential.

We measure the role of field-induced alignment on the dipole force for molecules using a single focused nonresonant laser beam. We show that through the alignment process we can modify the effective polarizability by field polarization and thus control the center-of-mass motion of the molecule. We observe a maximum change of $20\%$ in the dipole force on CS${}_2$ molecules when changing from linearly to circularly polarized light. Additionally, the effect of the dipole force on different vibrational states is also studied.

In analogy with Bell’s inequality for two-qubit quantum states, we propose an inequality criterion for the nonseparability of the spin-orbit degrees of freedom of a laser beam. A definition of separable and nonseparable spin-orbit modes is used in consonance with the one presented in Phys. Rev. Lett. 99, 160401 (2007). As the usual Bell’s inequality can be violated for entangled two-qubit quantum states, we show both theoretically and experimentally that the proposed spin-orbit inequality criterion can be violated for nonseparable modes. The inequality is discussed in both the classical and quantum domains.

We study the phase diagram of a parity- and time-reversal- ($\mathcal{P}\mathcal{T}$-) symmetric tight-binding chain with $N$ sites and hopping energy $J$ in the presence of two impurities with imaginary potentials $\pm i\gamma$ located at arbitrary ($\mathcal{P}$-symmetric) positions $(m,\mathrm{m̄}=N+1-m)$ on the chain where $m⩽N/2$. We find that except in the two special cases where impurities are either the farthest or the closest, the $\mathcal{P}\mathcal{T}$-symmetric region is algebraically fragile. We analytically and numerically obtain the critical impurity potential ${\gamma }_{\mathrm{PT}}$ and show that ${\gamma }_{\mathrm{PT}}\propto 1/N\to 0$ as $N\to \infty$ except in the two special cases. When the $\mathcal{P}\mathcal{T}$ symmetry is spontaneously broken, we find that the maximum number of complex eigenvalues is given by $2m$. When the two impurities are the closest, we show that ${\gamma }_{\mathrm{PT}}$ in the limit $N\to \infty$ approaches $J$ ($J/2$) provided that $N$ is even (odd). For an even $N$, the $\mathcal{P}\mathcal{T}$ symmetry is maximally broken, whereas for an odd $N$, it is sequentially broken. Our results show that the phase diagram of a $\mathcal{P}\mathcal{T}$-symmetric tight-binding chain is extremely rich and that, in the continuum limit, this model may give rise to hitherto unexplored $\mathcal{P}\mathcal{T}$-symmetric Hamiltonians.

We propose a physical model for generating multipartite entangled states of spin-$s$ particles that have important applications in distributed quantum information processing. Our protocol is based on a process where mobile spins induce the interaction among remote scattering centers. As such, a major advantage lies in the management of stationary and well-separated spins. Among the generable states, there is a class of $N$-qubit singlets allowing for optimal quantum telecloning in a scalable and controllable way. We also show how to prepare Aharonov, $W$, and Greenberger-Horne-Zeilinger states.

Models of quantum computation (QC) are important because they change the physical requirements for achieving universal QC. For example, one-way QC requires the preparation of an entangled “cluster” state, followed by adaptive measurement on this state, a set of requirements which is different from the standard quantum-circuit model. Here we introduce a model based on one-way QC but without measurements (except for the final readout), instead using adiabatic deformation of a Hamiltonian whose initial ground state is the cluster state. Our results could help increase the feasibility of adiabatic schemes by using tools from one-way QC.