We consider the signatures of the integer quantum Hall effect in a degenerate gas of electrically neutral atomic fermions. An effective magnetic field is achieved by applying two incident light beams with a high orbital angular momentum. We show how states corresponding to completely filled Landau levels are obtained and discuss various possibilities to measure the incompressible nature of the trapped two-dimensional gas.

A dressed-state approach to mixing of bosonic matter waves is presented. Two cases are studied using this formalism. In the first, two macroscopically populated modes of atoms (two-wave mixing) are coupled through the presence of light. In the second case, three modes of Bogoliubov quasiparticles (three-wave mixing) are coupled through $s$-wave interaction. In both cases, wave mixing induces oscillations in the population of the different modes that decay due to interactions. Analytic expressions for the dressed basis spectrum and the evolution of the mode populations in time are derived both for resonant mixing and nonresonant mixing. Oscillations in the population of a given mode are shown to lead to a splitting in the decay spectrum of that mode, in analogy with the optical Autler-Townes splitting in the decay spectrum of a strongly driven atom. These effects cannot be described by a mean-field approximation.

We consider a three-level $\Lambda$-type atomic medium in an electromagnetically induced transparency configuration interacting with two independent broadband squeezed baths. We show that absorptive and dispersive properties of the medium can be controlled using squeezed bath parameters and coupling field strength such that the medium can have subluminal to superluminal group velocity for the probe pulse.

Recently Barrett and Kok proposed an elegant method for entangling separated matter qubits. They outlined a strategy for using their entangling operation (EO) to build graph states, the resource for one-way quantum computing. Here I argue that their strategy considerably underrates the power and utility of their EO. By viewing their EO as a graph fusion event, one perceives that each successful event introduces an ideal redundant graph edge, which growth strategies should exploit. For example, if each EO succeeds with probability $p\gtrsim 0.4$ then a highly connected graph can be formed with an overhead of only about ten EO attempts per graph edge. The Barrett and Kok (BK) scheme then becomes competitive with the more elaborate entanglement procedures designed to permit $p$ to approach unity [Phys. Rev. Lett. 95, 030505 (2005)].

Distributions of ejected electrons following collisions of slow ${\mathrm{He}}^{+}$ and ${\mathrm{He}}^{2+}$ ions and a He target were measured for projectile energies of 20 and 40 keV, respectively. The electrons were detected at angles of 30° and 90° with respect to the incident beam direction. Superimposed on a continuous background originating from target ionization, small amplitude, high-frequency oscillations are revealed. The frequency of these oscillations is found to be nearly independent of the projectile charge and observation angle. In view of recent experiments and calculations, the origin of such oscillations is discussed. Processes such as autoionization following the production of highly excited states, Fermi-shuttle ionization, or coherent electron emission caused by interference between the target centered and projectile centered amplitudes, are considered.

We examine two exactly solvable models of decoherence—a central spin-system, (i) with and (ii) without a self-Hamiltonian, interacting with a collection of environment spins. In the absence of a self-Hamiltonian we show that in this model (introduced some time ago to illustrate environment-induced superselection) generic assumptions about the coupling strengths can lead to a universal (Gaussian) suppression of coherence between pointer states. On the other hand, we show that when the dynamics of the central spin is dominant a different regime emerges, which is characterized by a non-Gaussian decay and a dramatically different set of pointer states. We explore the regimes of validity of the Gaussian decay and discuss its relation to the spectral features of the environment and to the Loschmidt echo (or fidelity).

We propose a protocol that allows both the creation and the distribution of entanglement, resulting in two distant parties (Alice and Bob) conclusively sharing a bipartite Bell state. The system considered is a graph of three-level objects (“qutrits”) coupled by SU(3) exchange operators. The protocol begins with a third party (Charlie) encoding two lattice sites in unentangled states, and allowing unitary evolution under time. Alice and Bob perform a projective measurement on their respective qutrits at a given time, and obtain a maximally entangled Bell state with a certain probablility. We also consider two further protocols, one based on simple repetition and the other based on successive measurements and conditional resetting, and show that the cumulative probability of creating a Bell state between Alice and Bob tends to unity.

In order to make a unified treatment for estimation problems of a very small noise or a very weak signal in a quantum process, we introduce the notion of a low-noise quantum channel with one noise parameter. It is known in several examples that prior entanglement together with nonlocal output measurement improves the performance of the channel estimation. In this paper, we study this “ancilla-assisted enhancement” for estimation of the noise parameter in a general low-noise channel. For channels on two level systems we prove that the enhancement factor, the ratio of the Fisher information of the ancilla-assisted estimation to that of the original one, is always upper bounded by $3∕2$. Some conditions for the attainability are also given with illustrative examples.

The three most important electron-impact processes dissociating ${\mathrm{CH}}_4$ molecule into neutral fragments are $e+{\mathrm{CH}}_4\to {\mathrm{CH}}_3+\cdots$, $e+{\mathrm{CH}}_4\to {\mathrm{CH}}_2+\cdots$, and $e+{\mathrm{CH}}_4\to \mathrm{CH}+\cdots$. Neither experimental nor theoretical cross sections for the processes in a broad range of energy are available, except for the first process for which reliable measured cross sections for energies up to $500\mathrm{eV}$ are available. Only two measured values (at a single impact energy) of the cross section for the second process are available, and only two measured values (also at a single energy) of the cross section for the third process are available in the literature. Therefore, we derive in this work analytical dissociation cross sections for the second and the third processes mentioned above in the energy range up to $500\mathrm{eV}$. At higher (but still nonrelativistic) energies, we calculate the parameters of the Bethe-Born cross section for the second and the third processes.

The effect of external fields on directional states of a linear molecule trapped in a crystal field of octahedral symmetry is studied numerically. Adiabatic field-dressed energy levels are obtained by solving the time-independent Schrödinger equation for the rotational degrees of freedom of the confined molecule. In the absence of external fields, the internal octahedral crystal field serves to transform free-rotor states to angularly confined librational states of defined parity which arrange in near-degenerate sets of high multiplicity. Interaction of a linearly polarized, nonresonant laser field with the polarizability or of a static electric field with the dipole moment create alignment or orientation of the molecular axis, respectively. In the latter case, the combined effect of the internal (octahedral) and external static field is instrumental in creating orientation by coupling different tunneling states. Depending on the polarization direction of the external fields with respect to the symmetry axes provided by the crystal field, cooperative and competitive effects are distinguished. If the direction of the external field coincides with the minima of the crystal field, high degrees of alignment or orientation can be achieved for specific states, even for low field strengths. Otherwise, high efficiency of this mechanism is restricted to high fields and low temperatures. Strategies for an experimental realization are outlined.

The parity of photonic number states is known to be an important observable for quantized electromagnetic fields with applications to quantum information processing and to Heisenberg-limited measurement of phase shifts in quantum interferometry performed with maximally entangled states and with twin number states. In this paper we describe an approach to the quantum nondemolition measurement of parity for quantized optical fields. The method proposed involves the use of a cross-Kerr interaction where we assume a large Kerr nonlinearity is available through the techniques of electromagnetically induced transparency. Our proposed method does not require the measurement of photon number but rather measures parity directly. The method not only allows for the quantum nondemolition measurement of parity but also allows for the von Neumann projection of parity eigenstates from an arbitrary field state. The generation and detection of higher-order parity eigenstates is also discussed. Losses from dissipation and the effects of detector efficiency are considered.

Using sum rules, the dipolar terms can be eliminated from the commonly-used sum-over-states (SOS) expression for nonlinear susceptibilities. This new dipole-free expression is more compact, converges to the same results as the common SOS equation, and is more appropriate for analyzing certain systems such as octupolar molecules. The dipole-free theory can be used as a tool for analyzing the uncertainties in quantum calculations of susceptibilities, can be applied to a broader set of quantum systems in the three-level model where the standard SOS expression fails, and more naturally leads to fundamental limits of the nonlinear susceptibilities.

To find the criterion of a set of positive-definite matrices which can be written as reduced density matrices of a multipartite matrix is a hard and important problem. When the problem is concerned with multiparty density matrices, it is much more significant for computational many-body physics and many-body quantum entanglement which is one of the focuses of current quantum information theory. We give several results on the necessary compatibility relations between a set of two-party reduced density matrices and a global state in Hilbert space $N_A\times N_B\times N_C$ where $N_A$, $N_B$, and $N_C$ are arbitrary.

We prove that by communicating $N-1$ times a single qubit, instead of $N-1$ classical bits, the success probability for solving some $N$ partner communication complexity tasks is strongly enhanced. The superiority, as measured by the quantum-to-classical fidelity ratio, of the quantum scheme grows exponentially with $N$. We report an experimental implementation of these tasks for $N=5$. Even without correcting for any inefficiencies of the state-of-the-art setup, our multiparty quantum protocol still outperforms the best classical protocols.

We analyze a method that uses fixed, minimal physical resources to achieve generation and nested purification of quantum entanglement for quantum communication over arbitrarily long distances and discuss its implementation using realistic photon emitters and photonic channels. In this method, we use single-photon emitters with two internal degrees of freedom formed by an electron spin and a nuclear spin to build intermediate nodes in a quantum channel. State-selective fluorescence is used for probablistic entanglement generation between electron spins in adjacent nodes. We analyze in detail several approaches which are applicable to realistic, homogeneously broadened single-photon emitters. Furthermore, the coupled electron and nuclear spins can be used to efficiently implement entanglement swapping and purification. We show that these techniques can be combined to generate high-fidelity entanglement over arbitrarily long distances. We present a specific protocol that functions in polynomial time and tolerates percent-level errors in entanglement fidelity and local operations. The scheme has the lowest requirements on physical resources of any current scheme for fully fault-tolerant quantum repeaters.

We study the connection between the Hilbert-Schmidt measure of entanglement (that is the minimal distance of an entangled state to the set of separable states) and entanglement witness in terms of a generalized Bell inequality which distinguishes between entangled and separable states. A method for checking the nearest separable state to a given entangled one is presented. We illustrate the general results by considering isotropic states, in particular two-qubit and two-qutrit states—and their generalizations to arbitrary dimensions—where we calculate the optimal entanglement witnesses explicitly.

Photonic quantum-information processing schemes, such as linear optics quantum computing, and other experiments relying on single-photon interference, inherently require complete photon indistinguishability to enable the desired photonic interactions to take place. Mode-mismatch is the dominant cause of photon distinguishability in optical circuits. Here we study the effects of photon wave-packet shape on tolerance against the effects of mode mismatch in linear optical circuits, and show that Gaussian distributed photons with large bandwidth are optimal. The result is general and holds for arbitrary linear optical circuits, including ones which allow for postselection and classical feed forward. Our findings indicate that some single photon sources, frequently cited for their potential application to quantum-information processing, may in fact be suboptimal for such applications.

In this paper, the classical Keldysh approximation for evaluating the tunnel ionization rate of a bound electron under the action of an intense radiation field is revisited. We transform the original Keldysh expression for the transition amplitude to a simplified one used by Gribakin and Kuchiev [Phys. Rev. A 55, 3760 (1997)], and show that the two expressions are equivalent up to a phase factor which does not contribute to ionization. Using the residue theorem to perform the contour integral, rather than the saddle point method employed by Keldysh and others, a revised closed form analytical formula for the photoionization rate is obtained. The formula also avoids using the redundant assumption of small kinetic momentum employed by Keldysh. This paper thus realizes a long-sought goal to calculate the transition rate within only the Keldysh approximation and provides an “exact” formula to compare with other approaches using different gauges at various levels of approximation.