We show that, for any system with a number of levels which can be identified with $n$ qubits, there is an inequality for the correlations between three compatible dichotomic measurements which must be satisfied by any noncontextual theory, but is violated by any quantum state. Remarkably, the violation grows exponentially with $n$, and the tolerated error per correlation also increases with $n$, showing that state-independent quantum contextuality is experimentally observable in complex systems.

It was recently found that thermodynamic anomalies which arise in the Casimir effect between metals described by the Drude model can be attributed to the interaction of fluctuating Foucault (or eddy) currents [F. Intravaia and C. Henkel, Phys. Rev. Lett. 103, 130405 (2009).] We focus on the transverse electric (TE) polarization, where the anomalies occur, and show explicitly that the two leading terms of the low-temperature correction to the Casimir free energy of interaction between two plates are identical to those pertaining to the Foucault current interaction alone, up to a correction which is very small for good metals. Moreover, a mode density along real frequencies is introduced, showing that the TE contribution to the Casimir free energy, as given by the Lifshitz theory, separates in a natural manner into contributions from eddy currents and propagating cavity modes, respectively. The latter have long been known to be of little importance to the low-temperature Casimir anomalies. This convincingly demonstrates that eddy current modes are responsible for the large temperature correction to the Casimir effect between Drude metals, predicted by the Lifshitz theory, but not observed in experiments.

We show that the nonadiabatic orientation of diatomic polar molecules can be controlled by polarization-gated ultrashort laser pulses. By finely adjusting the time interval between two circularly polarized pulses of different wavelengths but the same helicity, the orientation direction of the molecules can be twirled. A cloverlike potential is created by using two circularly polarized laser pulses of different wavelengths and opposite helicity, leading to multidirectional molecular orientation along the potential wells, which can be well revealed by a high-order statistics metric of $\langle \langle \cos {}^3{\theta }_d\rangle \rangle$.

We study high-order harmonic generation in argon driven by an intense 800 nm laser field and a small fraction of its second harmonic. The intensity and divergence of the emitted even and odd harmonics are strongly modulated as a function of the relative delay between the two fields. We provide a detailed analysis of the underlying interference effects. The interference changes drastically when approaching the cutoff region due to a switch of the dominant trajectory responsible for harmonic generation.

We develop a mean-field treatment of a polarized trapped Fermi gas with dipole-dipole interactions. Our approach is based on self-consistent semiclassical Hartree-Fock theory that accounts for direct and exchange interactions. We discuss our procedure for numerically implementing the calculation. We study the thermodynamic and the first- and second-order correlation properties of the system. We find that the system entropy depends on the trap geometry, allowing the system to be cooled as the trap aspect ratio is increased, and that exchange interactions cause the correlation functions to be anisotropic in the low-temperature regime. We also find that many uniform gas thermodynamic predictions, for which direct interaction effects vanish, are qualitatively unreliable for trapped systems, most notably for oblate traps. We develop a simplified Hartree formalism that is applicable to anisotropic harmonic traps.

We study ground-state properties and dynamics of a dilute ultracold atomic gas in a double-well potential. The Gaussian barrier separating the two wells derives from the interaction between the atoms and a quantized field of a driven Fabry-Perot cavity. Due to intrinsic atom-field nonlinearity, several interesting phenomena arise which are the focus of this work. For the ground state, there is a critical pumping amplitude in which the atoms self-organize and the intra-cavity-field amplitude drastically increases. In the dynamical analysis, we show that the Josephson oscillations depend strongly on the atomic density and may be greatly suppressed within certain regimes, reminiscent of self-trapping of Bose-Einstein condensates in double-well setups. This pseudo-self-trapping effect is studied within a mean-field treatment valid for large atom numbers. For small numbers of atoms, we consider the analogous many-body problem and demonstrate a collapse-revival structure in the Josephson oscillations.

We develop a generalized principle of electromagnetically induced transparency (EIT) vector magnetometry based on high-contrast EIT resonances and the symmetry of atom-light interaction in the linearly polarized bichromatic fields. Operation of such vector magnetometer on the $D_1$ line of ${}^{87}\mathrm{Rb}$ has been demonstrated. The proposed compass-magnetometer has an increased immunity to shifts produced by quadratic Zeeman and ac-Stark effects, as well as by atom-buffer gas and atom-atom collisions. In our proof-of-principle experiment the detected angular sensitivity to magnetic field orientation is ${10}^{-3}$ deg/Hz${}^{1/2}$, which is limited by laser intensity fluctuations, light polarization quality, and magnitude of the magnetic field.

We report an experimental observation of all-optical switching in a cavity QED system consisting of multiple three-level atoms confined in a cavity mode. A signal light is coupled into the normal mode of the cavity-atom system and its transmission through the cavity is switched on or off by a control laser coupled to the atoms from free space. Our experiment shows that the all-optical switching in the cavity QED system can be performed for the cavity mode tuned on resonance with or off resonance from the atomic transition frequency.

The total internal reflection of a weak signal pulse from a powerful reference pulse at another frequency in a dispersive nonlinear medium is demonstrated. As a result of the two-wave collision, the signal pulse changes its frequency and group velocity, and time delay occurs. We develop both a wave and a pulse-tracing theory of the total reflection from the moving inhomogeneity induced by the optical pulse. An analytical expression for the critical value of group-velocity mismatch as a function of pump intensity and the group-velocity dispersion are obtained. The possibility of signal pulse reflection from a bright soliton in a cubic medium is considered.

We study the nonlinear dynamics of an ensemble of cold trapped atoms with a hyperfine transition magnetically coupled to a resonant microwave cavity mode. Despite the minute single-atom coupling, one obtains strong coupling between collective hyperfine qubits and microwave photons, enabling coherent transfer of an excitation between the long-lived atomic qubit state and the mode. Evidence of strong coupling can be obtained from the cavity transmission spectrum even at finite thermal photon number. The system makes it possible to study further prominent collective phenomena such as superradiant decay of an inverted ensemble or the building of a narrowband stripline micromaser locked to an atomic hyperfine transition.

Dissipation is generally thought to affect the quantum nature of the system in an adverse manner; however, we show that dissipatively coupled nanosystems can be prepared in states which beat the standard quantum limit of the mechanical motion. We show that the reactive coupling between the waveguide and the microdisk resonator can generate the squeezing of the waveguide by injecting a quantum field and laser into the resonator through the waveguide. The waveguide can show about $70–75\%$ of maximal squeezing for temperature about 1–10 mK. The maximum squeezing can be achieved with an incident pump power of only 12 $\mu$W for a temperature of about 1 mK. Even for temperatures of 20 mK, achievable by dilution refrigerators, the maximum squeezing is about $60\%$.

We report on plasmonic waveguides made of a thin metal stripe surrounded on one or both sides by a Kerr nonlinear medium. Using an iterative numerical method, we investigate the stationary long-range plasmons that exist for self-focusing and self-defocusing Kerr-type nonlinearities. The solutions are similar to the well-known case of infinitely wide nonlinear waveguides—they are strongly power-dependent and can experience symmetry-breaking bifurcations under appropriate conditions.

Dakic, Vedral, and Brukner [arXiv:1004.0190 (2010)] introduced a geometric measure of quantum discord and derived an explicit formula for any two-qubit state. This measure is significant in capturing quantum correlations from a geometric perspective. In this brief report, we evaluate the geometric measure of quantum discord for an arbitrary state and obtain an explicit and tight lower bound. Furthermore, we reveal an intrinsic feature of geometric measure of quantum discord by showing that it actually coincides with a simpler quantity based on von Neumann measurements.

Gedanken experiments are used to explore properties of quantum vacuum energy that are currently challenging to explore experimentally. A constant lateral Casimir force is predicted to exist between two overlapping finite parallel plates at 0 K; otherwise it would be possible to extract an arbitrary amount of energy from the quantum vacuum. A rigid unpowered object cannot be accelerated by the quantum vacuum because of the translational symmetry of space. By considering systems in which vacuum energy and other forms of energy are exchanged, we demonstrate that a change $\Delta E$ in vacuum energy, whether positive or negative with respect to the free field, corresponds to an equivalent inertial mass and equivalent gravitational mass $\Delta M=\Delta E/c^2.$ We consider the possibility of a gravitational shield and show that, if it exists, the energy to operate it would have to cancel the net energy extracted from the gravitational field; otherwise we could extract an arbitrary amount of energy from the field.

We present simple protocols for oblivious transfer and password-based identification which are secure against general attacks in the noisy-quantum-storage model as defined in R. König, S. Wehner, and J. Wullschleger [e-print arXiv:0906.1030]. We argue that a technical tool from König et al. suffices to prove security of the known protocols. Whereas the more involved protocol for oblivious transfer from König et al. requires less noise in storage to achieve security, our “canonical” protocols have the advantage of being simpler to implement and the security error is easier control. Therefore, our protocols yield higher OT rates for many realistic noise parameters. Furthermore, a proof of security of a direct protocol for password-based identification against general noisy-quantum-storage attacks is given.

We show that entanglement between two local spins which are initially independent and act as qubits can be established by applying an electron current through them. The setup can be realized by using spin-$\frac{1}{2}$ quantum dots as the qubits and equally connecting them to leads with the tip of scanning tunnel microscopy. The tunneling of electrons through two dots is in the sequential regime and the entanglement between dots originates from the quantum coherence of two tunneling channels. We calculate the effect of the electron current on the entanglement of local qubits fully quantum-mechanically by using extended scattering-matrix theory and analyze the favorite conditions for the realization of entangling gates.

We consider a class of quantum systems with spin-flip symmetry and derive the quantum correlation measured by the quantum discord (QD). As an illustration, we investigate the QD in a three-body interaction model and an XYZ interaction model, whose ground states can be expressed as matrix product states, and the QD is exactly soluble. We show that the QD behaves differently than the quantum entanglement (QE) in many ways; for example, they may show opposite monotonicity and completely different finite-size effects. Furthermore, we compare the capability of the QD and the QE to detect quantum phase transitions (QPTs) and find that the QD is more reliable than the QE for signaling QPTs in these models: In the three-body interaction model, the QE is singular at the quantum critical point, however, it exhibits an additional singularity in the noncritical region, while the analyticity of the QD can be used to identify the quantum critical point perfectly; and in the XYZ interaction model, the QE vanishes in the thermodynamic limit, thus losing its ability to detect QPTs, while the QD still functions very well.

A condition for fault-tolerant quantum computation (FTQC) with cavity schemes is discussed. It is shown that the condition is very hard if the standard error threshold of FTQC is simply applied. To relax the condition, we propose to combine the cavity-quantum-electrodynamics (QED) scheme proposed by Duan et al. [Phys. Rev. A 72, 032333 (2005)] and Xiao et al. [Phys. Rev. A 70, 042314 (2004)] with the recently proposed FTQC scheme with probabilistic two-qubit gates [Goto and Ichimura, Phys. Rev. A 80, 040303(R) (2009)]. It is shown that the condition for FTQC is dramatically relaxed compared to the case of the standard threshold. The optimization of the cavity-QED scheme is also discussed.

Recently we demonstrated the formation of ultracold polar LiCs molecules in deeply bound levels of the $X^1{\Sigma }^{+}$ ground state, including the rovibrational ground state [J. Deiglmayr et al., Phys. Rev. Lett. 101, 133004 (2008)]. Here we report on an experimental determination of the permanent electric dipole moment of deeply bound LiCs molecules. For $X^1{\Sigma }^{+}$, $v^{″}=2$, and $v^{″}=3$ we measure values of $\mu =5.5(2)$ D and 5.3(2) D, respectively.

We analyze the double ionization of He under the impact of 100 keV ${\mathrm{He}}^{2+}$ projectiles. The process is described within the framework of the impact parameter and frozen-correlation approximations where the one-electron events are treated by the continuum distorted wave method. Correlation between the emitted electrons, which plays an important role in forming the shape of the differential distribution of the electron emission, is described by the Coulomb density of states approximation (CDS). Special attention is paid to the region of the two-electron cusp that has been observed in a recent experiment for 100 keV ${\mathrm{He}}^0+\mathrm{He}$ collisions [L. Sarkadi and A. Orbán, Phys. Rev. Lett. 100, 133201 (2008)]. In the cusp region the correlated motion of the two electrons is influenced dominantly by the outgoing projectile, that is, the correlation function of the CDS treatment is expected to depend on the electron momenta measured relative to the projectile rather than to the target nucleus. A qualitative agreement with the experiment is achieved with a CDS model based on the use of such a projectile-centered correlation function that applies effective charges as given in the dynamically screened three-Coulomb wave function.