We report the first measurements of the index of refraction of gases for lithium waves. Using an atom interferometer, we have measured the real and imaginary parts of the index of refraction $n$ for argon, krypton, and xenon as a function of the gas density for several velocities of the lithium beam. The linear dependence of ($n-1$) with the gas density is well verified. The total collision cross section deduced from the imaginary part of ($n-1$) is in very good agreement with traditional measurements of this quantity. Finally, the real and imaginary parts of ($n-1$) and their ratio $\rho$ exhibit glory oscillations, in good agreement with calculations.

In this Letter we propose a robust quantum repeater architecture building on the original Duan-Lukin-Cirac-Zoller (DLCZ) protocol [L.-M. Duan, M. D. Lukin, J. I. Cirac, and P. Zoller, Nature (London) 414, 413 (2001)]. The architecture is based on two-photon Hong-Ou-Mandel–type interference which relaxes the long-distance stability requirements by about 7 orders of magnitude, from subwavelength for the single photon interference required by DLCZ to the coherence length of the photons. Our proposal provides an exciting possibility for robust and realistic long-distance quantum communication.

We obtain the limit on the space-time variation of the ratio of the proton mass to the electron mass, $\mu =m_p/m_e$, based on comparison of quasar absorption spectra of ${\mathrm{NH}}_3$ with CO, ${\mathrm{HCO}}^{+}$ and HCN rotational spectra. For the inversion transition in ${\mathrm{NH}}_3$ ($\lambda \approx 1.25{\mathrm{cm}}^{-1}$) the relative frequency shift is significantly enhanced: $\delta \omega /\omega =-4.46\delta \mu /\mu$. This enhancement allows one to increase sensitivity to the variation of $\mu$ using ${\mathrm{NH}}_3$ spectra for high redshift objects. We use published data on microwave spectra of the object $B0218+357$ to place the limit $\delta \mu /\mu =(0.6\pm 1.9)\times {10}^{-6}$ at redshift $z=0.6847$; this limit is several times better than the limits obtained by different methods and may be significantly improved. Assuming linear time dependence we obtain $\dot{\mu }/\mu =(-1\pm 3)\times {10}^{-16}{\mathrm{yr}}^{-1}$.

We propose a Raman spectroscopy technique which is able to probe the one-particle Green function, the Fermi surface, and the quasiparticles of a gas of strongly interacting ultracold atoms. We give quantitative examples of experimentally accessible spectra. The efficiency of the method is validated by means of simulated images for the case of a usual Fermi liquid as well as for more exotic states: specific signatures of, e.g., a $d$-wave pseudogap are clearly visible.

The momentum distribution (MD) dynamics of a Tonks-Girardeau (TG) gas is studied in the context of Bragg reflections of a many-body wave packet. We find strong suppression of a Bragg reflection peak for a large and dense TG wave packet; our observation illustrates the dependence of the MD on the interactions and wave function symmetry. The MD is calculated from the reduced single-particle density matrix (RSPDM). We develop a method for calculating the RSPDM of a TG gas, which is operative for a large number of particles, and does not depend on the external potential and the state of the system. The method is based on a formula expressing the RSPDM via a dynamically evolving single-particle basis.

We consider the three-body Casimir-Polder interaction between three atoms during their dynamical self-dressing. We show that the time-dependent three-body Casimir-Polder interaction energy displays nonlocal features related to quantum properties of the electromagnetic field and to the nonlocality of spatial field correlations. We discuss the measurability of this intriguing phenomenon and its relation with the usual concept of stationary three-body forces.

We propose a semiconductor device that can electrically generate entangled electron spin-photon states, providing a building block for entanglement of distant spins. The device consists of a $p\mathrm{\text{−}}i\mathrm{\text{−}}n$ diode structure that incorporates a coupled double quantum dot. We show that electronic control of the diode bias and local gating allow for the generation of single photons that are entangled with a robust quantum memory based on the electron spins. Practical performance of this approach to controlled spin-photon entanglement is analyzed.

Thermodynamics of a three-level maser was studied in the pioneering work of Scovil–Schulz-DuBois [Phys. Rev. Lett. 2, 262 (1959)]. In this Letter we consider the same three-level model, but we give a full thermodynamic analysis based on Hamiltonian and dissipative Lindblad superoperators. The first law of thermodynamics is obtained using a recently developed alternative [Phys. Rev. A 74, 063823 (2006)] to Alicki’s definitions for heat flux and power [J. Phys. A 12, L103 (1979)]. Using a novel variation on Spohn’s entropy production function [J. Math. Phys. (N.Y.) 19, 1227 (1978)], we obtain Carnot’s efficiency inequality and the Scovil–Schulz-DuBois maser efficiency formula when the three-level system is operated as a heat engine (amplifier). Finally, we show that the three-level system has two other modes of operation—a refrigerator mode and a squanderer mode —both of which attenuate the electric field.

We propose a scheme for generating two-mode squeezing in high-$Q$ resonators using a beam of atoms with random arrival times, which acts as a reservoir for the field. The scheme is based on four-wave mixing processes leading to emission into two cavity modes, which are resonant with the Rabi sidebands of the atomic dipole transition, driven by a saturating classical field. At steady state the cavity modes are in an Einstein-Podolsky-Rosen state, whose degree of entanglement is controlled by the intensity and the frequency of the transverse field. This scheme is robust against stochastic fluctuations in the atomic beam, does not require atomic detection nor velocity selection, and can be realized by presently available experimental setups with microwave resonators.

We propose an experimentally feasible architecture with controllable long-range couplings built up from local exchange interactions. The scheme consists of a spin bus, with strong, always-on interactions, coupled dynamically to external qubits of the Loss and DiVincenzo type. Long-range correlations are enabled by a spectral gap occurring in a finite-size chain. The bus can also form a hub for multiqubit entangling operations. We show how multiqubit gates may be used to efficiently generate $W$ states (an important entanglement resource). The spin bus therefore provides a route for scalable solid-state quantum computation, using currently available experimental resources.

We study the dynamic critical behavior of the local bond-update (Sweeny) dynamics for the Fortuin-Kasteleyn random-cluster model in dimensions $d=2$, 3 by Monte Carlo simulation. We show that, for a suitable range of $q$ values, the global observable ${\mathcal{S}}_2$ exhibits “critical speeding-up”: it decorrelates well on time scales much less than one sweep. In some cases the dynamic critical exponent for the integrated autocorrelation time is negative. We also show that the dynamic critical exponent $z_{\exp }$ is very close (possibly equal) to the rigorous lower bound $\alpha /\nu$ and quite possibly smaller than the corresponding exponent for the Chayes-Machta-Swendsen-Wang cluster dynamics.

We apply a stochastic center manifold method to the calculation of noise-induced phase transitions in the stochastic Swift-Hohenberg equation. This analysis is applied to the reduced mode equations that result from Fourier decomposition of the field variable and of the temporal noise. The method shows a pitchfork bifurcation at lower perturbation order, but reveals a novel additive-noise-induced postponement of the Turing bifurcation at higher order. Good agreement is found between the theory and the numerics for both the reduced and the full system. The results are generalizable to a broad class of nonlinear spatial systems.

We present the optimal collective attack on a quantum key distribution protocol in the “device-independent” security scenario, where no assumptions are made about the way the quantum key distribution devices work or on what quantum system they operate. Our main result is a tight bound on the Holevo information between one of the authorized parties and the eavesdropper, as a function of the amount of violation of a Bell-type inequality.

We show that an arbitrary basis of a multipartite quantum state space consisting of $K$ distant parties such that the $k$th party has local dimension $d_k$ always contains at least $N=\sum _{k=1}^K(d_k-1)+1$ members that are unambiguously distinguishable using local operations and classical communication (LOCC). We further show that this lower bound is optimal by analytically constructing a special product basis having only $N$ members unambiguously distinguishable by LOCC. Interestingly, such a special product basis not only gives a stronger form of the weird phenomenon “nonlocality without entanglement,” but also implies the existence of a locally distinguishable entangled basis.

We consider a high-frequency response of electrons in a single miniband of superlattice subject to dc and ac electric fields. We show that Bragg reflections in miniband result in a parametric resonance which is detectable using ac probe field. We establish theoretical feasibility of phase-sensitive THz amplification at the resonance. The parametric amplification does not require operation in conditions of negative differential conductance. This prevents a formation of destructive domains of high electric field inside the superlattice.

We study the entanglement between a qubit and its environment from the spin-boson model with Ohmic dissipation. Through a mapping to the anisotropic Kondo model, we derive the entropy of entanglement of the spin $E(\alpha ,\Delta ,h)$, where $\alpha$ is the dissipation strength, $\Delta$ is the tunneling amplitude between qubit states, and $h$ is the level asymmetry. For $1-\alpha \gg \Delta /{\omega }_c$ and $(\Delta ,h)\ll {\omega }_c$, we show that the Kondo energy scale $T_K$ controls the entanglement between the qubit and the bosonic environment (${\omega }_c$ is a high-energy cutoff). For $h\ll T_K$, the disentanglement proceeds as $(h/T_K{)}^2$; for $h\gg T_K$, $E$ vanishes as $(T_K/h{)}^{2-2\alpha }$, up to a logarithmic correction. For a given $h$, the maximum entanglement occurs at a value of $\alpha$ which lies in the crossover regime $h\sim T_K$. We emphasize the possibility of measuring this entanglement using charge qubits subject to electromagnetic noise.

In Bell experiments, one problem is to achieve high enough photodetection to ensure that there is no possibility of describing the results via a local hidden-variable model. Using the Clauser-Horne inequality and a two-photon nonmaximally entangled state, a photodetection efficiency higher than 0.67 is necessary. Here we discuss atom-photon Bell experiments. We show that, assuming perfect detection efficiency of the atom, it is possible to perform a loophole-free atom-photon Bell experiment whenever the photodetection efficiency exceeds 0.50.

The problem of closing the detection loophole with asymmetric systems, such as entangled atom-photon pairs, is addressed. We show that, for the Bell inequality $I_{3322}$, a minimal detection efficiency of 43% can be tolerated for one of the particles, if the other one is always detected. We also study the influence of noise and discuss the prospects of experimental implementation.

By applying a new technique for dynamic nuclear polarization involving simultaneous excitation of electronic and nuclear transitions, we have enhanced the nuclear polarization of the nitrogen nuclei in ${}^{15}\mathrm{N}@{\mathrm{C}}_{60}$ by a factor of ${10}^3$ at a fixed temperature of 3 K and a magnetic field of 8.6 T, more than twice the maximum enhancement reported to date. This methodology will allow the initialization of the nuclear qubit in schemes exploiting $\mathrm{N}@{\mathrm{C}}_{60}$ molecules as components of a quantum information processing device.

We discuss how the presence of gauge subsystems in the Bacon-Shor code [D. Bacon , Phys. Rev. A 73, 012340 (2006)] leads to remarkably simple and efficient methods for fault-tolerant error correction (FTEC). Most notably, FTEC does not require entangled ancillary states, and it can be implemented with nearest-neighbor two-qubit measurements. By using these methods, we prove a lower bound on the quantum accuracy threshold, $1.94\times {10}^{-4}$ for adversarial stochastic noise, that improves previous lower bounds by nearly an order of magnitude.