We define a quantity called a refbit, which allows one to quantify the resource of sharing a reference frame in quantum communication protocols. By considering both asymptotic and nonasymptotic protocols we find relations between refbits and other communication resources. We also consider the same resources in encoded, reference-frame-independent, form. This allows one to rephrase and unify previous work on phase references, reference frames, and superselection rules.

Solid-state quantum-computing proposals rely on adiabatic operations of the exchange gate among localized spins in nanostructures. We study corrections to the Heisenberg interaction between lateral semiconductor quantum dots in an external magnetic field. Using exact diagonalization we obtain the regime of validity of the adiabatic approximation. We also find qualitative corrections to the Heisenberg model at high magnetic fields and in looped arrays of spins. Looped geometries of localized spins generate flux-dependent multispin terms which go beyond the basic Heisenberg model.

The role of mixed-state entanglement in liquid-state nuclear magnetic resonance (NMR) quantum computation is not yet well understood. In particular, despite the success of quantum-information processing with NMR, recent work has shown that quantum states used in most of those experiments were not entangled. This is because these states, derived by unitary transforms from the thermal equilibrium state, were too close to the maximally mixed state. We are thus motivated to determine whether a given NMR state is entanglable—that is, does there exist a unitary transform that entangles the state? The boundary between entanglable and nonentanglable thermal states is a function of the spin system size $N$ and its temperature $T$. We provide bounds on the location of this boundary using analytical and numerical methods; our tightest bound scales as $N\sim T$, giving a lower bound requiring at least $N\sim 22000$ proton spins to realize an entanglable thermal state at typical laboratory NMR magnetic fields. These bounds are tighter than known bounds on the entanglability of effective pure states.

Total cross sections (TCSs) are experimentally investigated for 0.8–600-eV electron and 0.7–600-eV positron scattering from $c\text{−}{\mathrm{C}}_4{\mathrm{F}}_8$ molecules using a linear transmission time-of-flight method, and a comparative study of the results is carried out in this paper. Electron-scattering differential cross sections (DCS) measurements carried out for energies 1.5–100 eV are used for a combined TCS and DCS discussion. These DCS results help to offer a better understanding of some of the structures observed in the TCSs. Fingerprints of a low-energy resonance, consistent with large electron attachment cross sections near 0 eV, have been observed below 2 eV where TCSs show a continually rising trend. Another resonance peak at $\sim 8.5\mathrm{eV}$, attributed to dissociative electron attachment with the production of fragmented ions, and a broader one at 16–40 eV, have also been observed. Except for the pronounced shoulder at $\sim 40\mathrm{eV}$, there is good qualitative agreement between the TCS result and the integrated cross-section results. Although electron TCSs are found to be generally larger than positron TCSs in the 3–120-eV ranges, these two TCSs, however, show a tendency towards merging above 120 eV.

We present results for direct ionization of the heavy noble gases neon, argon, krypton, and xenon by positron impact. The computational method used is a hybrid approach, describing the projectile by a distorted wave but the initial bound state and the ejected-electron–residual-ion interaction by an $R$-matrix (close-coupling) expansion. A relatively small sensitivity of the theoretical predictions on the details of the computational model is noticed. Overall, fair agreement with recent experimental data is obtained.

Dielectronic recombination via the Ba $6p_{3∕2}n\ell$ states from a continuum of finite bandwidth is resonantly enhanced by a 38.8-GHz microwave field at the energies for which the $\Delta n=1,2,\dots ,k$ transitions of the intermediate $6p_{1∕2}n\ell$ states are resonant with the microwave frequency. The mechanism is resonant Stark mixing of high and low $\ell$ states by the microwave field. While static field enhancement of dielectronic recombination occurs at fields scaling as $n^{-5}$, the field required for enhancement of the $\Delta n=k,k⩾2$ resonances is roughly $n$ independent.

We investigate the rovibrational dynamics of heteronuclear diatomic molecules exposed to a strong external static and homogeneous electric field. We encounter in the presence of the field the effect of induced adiabatic coupling among the vibrational and hybridized rotational motions. Exact results are compared to the predictions of the adiabatic rotor approach as well as to the previously established effective rotor approximation. A detailed analysis of the impact of the electric field is performed: the hybridized and oriented rotational motion, the mixing of angular momenta, and the squeezing of the vibrational motion are observed. It is demonstrated that these effects can well be accounted for by the adiabatic rotor approximation.

We investigate the recently found stationary vortex cluster states in dilute atomic Bose-Einstein condensates confined by a nonrotating trap, and also present a stationary three-vortex cluster. We find the stationary states by minimizing directly an error norm for the stationary Gross-Pitaevskii equation, and study the dynamic and energetic stability of the resulting states by solving the corresponding Bogoliubov equations for the elementary excitations. The results are verified by integrating the time-dependent Gross-Pitaevskii equation. Contrary to previously reported results, the stationary states were observed to be both energetically and dynamically unstable. The dynamical decay rate of the clusters is typically very slow, but it should be experimentally observable. The most promising circumstances to experimentally generate and observe these structures and their dynamics is in weakly dissipative condensate systems, using phase-imprinting techniques.

Quantum-state sharing is a protocol where perfect reconstruction of quantum states is achieved with incomplete or partial information in a multipartite quantum network. Quantum-state sharing allows for secure communication in a quantum network where partial information is lost or acquired by malicious parties. This protocol utilizes entanglement for the secret-state distribution and a class of “quantum disentangling” protocols for the state reconstruction. We demonstrate a quantum-state sharing protocol in which a tripartite entangled state is used to encode and distribute a secret state to three players. Any two of these players can collaborate to reconstruct the secret state, while individual players obtain no information. We investigate a number of quantum disentangling processes and experimentally demonstrate quantum-state reconstruction using two of these protocols. We experimentally measure a fidelity, averaged over all reconstruction permutations, of $\mathcal{F}=0.73\pm 0.02$. A result achievable only by using quantum resources.

Correlations in twin beams composed of many photon pairs are studied using an intensified CCD camera. Joint signal-idler photon-number distribution and quantum phase-space quasidistributions determined from experimental data have nonclassical features.

We have examined the physical meaning of the geometric gauge associated with the photon position operator and find that localized photon states are not spherically symmetric and may have orbital angular momentum and optical vortices determined by the choice of gauge.

The geometric conditions for violation of Bell’s inequality, in its original form dealing with a pair of spin one-half particles formed in the singlet spin state, are discussed. The parameters $x_1,x_2$, and $x_3$ are introduced as the cosines of angles ${\theta }_{ab}=\angle (\mathbf{a},\mathbf{b})$, ${\theta }_{ac}=\angle (\mathbf{a},\mathbf{c})$, and ${\theta }_{bc}=\angle (\mathbf{b},\mathbf{c})$ between the unit vectors $\mathbf{a},\mathbf{b}$, and $\mathbf{c}$ defining the orientation of the corresponding Stern-Gerlach magnets. A Monte Carlo experiment shows that in order to obtain violation, the three parameters must belong to a definite region in the parametric space that encompasses $1∕3$ of all possible triplets of unit vectors generated randomly from the isotropic distribution in the ordinary space. By introducing a measure of violation $D\equiv \mid x_1-x_2\mid +x_3-1$, it is found that $0⩽D⩽1∕2$ and that the maximal violation $D=1∕2$ is obtained only in two isolated cases, when the relevant angles are ${\theta }_{ab}=2\pi ∕3$, ${\theta }_{ac}={\theta }_{bc}=\pi ∕3$, or when ${\theta }_{ab}={\theta }_{bc}=\pi ∕3$, ${\theta }_{ac}=2\pi ∕3$. In both cases the unit vectors $\mathbf{a},\mathbf{b}$, and $\mathbf{c}$ are coplanar.

Entanglement distance is the maximal separation between two entangled electrons in a degenerate electron gas. Beyond that distance, all entanglement disappears. We relate entanglement distance to degeneracy pressure both for extreme relativistic and nonrelativistic systems, and estimate the entanglement distance in a white dwarf. Treating entanglement as a thermodynamical quantity, we relate the entropy of formation and concurrence to relative electron distance, pressure, and temperature, to form an equation of state for entanglement.

The maximum rates for information transmission through noisy quantum channels has primarily been developed for memoryless channels, where the noise on each transmitted state is treated as independent. Many real-world communication channels experience noise which is modeled better by errors that are correlated between separate channel uses. In this Brief Report, upper bounds on the classical information capacities of a class of quantum memory channels are derived. The class of channels consists of indecomposable quantum memory channels, a generalization of classical indecomposable finite-state channels.

The concurrence of two alternate qubits in a four-qubit Heisenberg $XX$ chain is investigated when a uniform magnetic field $B$ is included. It is found that there is no thermal entanglement between alternate qubits if $B$ is close to zero. A magnetic field can induce entanglement in a certain range both for the antiferromagnetic and ferromagnetic cases. Near zero temperature, the entanglement undergoes two sudden changes with increasing value of the magnetic field $B$. This is due to the changes in the ground state. This novel property may be used as a quantum entanglement switch. The anisotropy in the system can also induce the entanglement between two alternate qubits.

We study quantum correlations and quantum noise in soliton collisions described by a general two-soliton solution of the nonlinear Schrödinger equation by using the back-propagation method. Our results include the standard case of a sech-shaped initial pulse analyzed earlier. We reveal that double-hump initial pulses can get more squeezed and the squeezing ratio enhancement is due to the long collision period in which the pulses are more stationary. These results offer promising possibilities of using higher-order solitons to generate strongly squeezed states for the quantum information process and quantum computation.

We use microwave-induced adiabatic passages for selective spin flips within a string of optically trapped individual neutral Cs atoms. We position-dependently shift the atomic transition frequency with a magnetic field gradient. To flip the spin of a selected atom, we optically measure its position and sweep the microwave frequency across its respective resonance frequency. We analyze the addressing resolution and the experimental robustness of this scheme. Furthermore, we show that adiabatic spin flips can also be induced with a fixed microwave frequency by deterministically transporting the atoms across the position of resonance.

We propose an experimentally feasible scheme to generate various types of entangled states of light fields by using beam splitters and single-photon detectors. Two beams of light fields are incident on two beam splitters respectively with each beam being asymmetrically split into two parts in which one part is supposed to be so weak that it contains at most one photon. We let the two weak output modes interfere at a third beam splitter. A conditional joint measurement on both weak output modes may result in an entanglement between the other two output modes. The conditions for the maximal entanglement are discussed based on the concurrence. Several specific examples are also examined.

It is known that quantum fidelity, as a measure of the closeness of two quantum states, is operationally equivalent to the minimal overlap of the probability distributions of the two states over all possible positive-operator-valued measures (POVM’s); the POVM realizing the minimum is optimal. We consider the ability of homodyne detection to distinguish two single-mode Gaussian states and investigate to what extent it is optimal in this information-theoretic sense. We completely identify the conditions under which homodyne detection makes an optimal distinction between two single-mode Gaussian states of the same mean and show that, if the Gaussian states are pure, they are always optimally distinguished.