We present a path-integral formulation of ’t Hooft’s derivation of quantum physics from classical physics. The crucial ingredient of this formulation is Gozzi et al.’s supersymmetric path integral of classical mechanics. We quantize explicitly two simple classical systems: the planar mathematical pendulum and the Rössler dynamical system.

We compute the energy-level displacement of the excited $np$ states of kaonic hydrogen within the quantum field theoretic and relativistic covariant model of strong low-energy $\overline{K}N$ interactions suggested by [Ivanov et al.Eur. Phys. J. A 21, 11 (2004)]. For the width of the energy-level of the excited $2p$ state of kaonic hydrogen, caused by strong low-energy interactions, we find ${\Gamma }_{2p}=2\mathrm{meV}=3\times {10}^{12}{\mathrm{s}}^{-1}$. This result is important for the theoretical analysis of the x-ray yields in kaonic hydrogen.

We present the results of a study of elastic scattering and vibrational excitation of NO by electron impact in the low-energy $(0–2\mathrm{eV})$ region where the cross sections are dominated by resonance contributions. The ${}^3\Sigma ^{-}$, ${}^1\Delta$, and ${}^1\Sigma ^{+}$ $\mathrm{N}{\mathrm{O}}^{-}$ resonance lifetimes are taken from our earlier study [Phys. Rev. A 69, 062711 (2004)], but the resonance energies used here are obtained from different configuration-interaction studies. Here we employ a more elaborate nonlocal treatment of the nuclear dynamics, which is found to remedy the principal deficiencies of the local complex potential model we employed in our earlier study, and gives cross sections in better agreement with the most recent experiments. We also present cross sections for dissociative electron attachment to NO leading to ground-state products, ${\mathrm{O}}^{-}\left({}^2\mathrm{P}\right)+\mathrm{N}\left({}^4\mathrm{S}\right)$. The calculations show that, while the peak cross sections starting from NO in its ground vibrational state are very small $(\sim {10}^{-20}{\mathrm{cm}}^2)$, the cross sections are extremely sensitive to vibrational excitation of the target and should be readily observable for target NO molecules excited to $\nu =10$ and above.

The preparation of a sample of ultra cold caesium atoms in an $m_F=0$ Zeeman sublevel of the $F=3$ ground state is demonstrated, using cooling in an optical lattice and adiabatic passage. 97% of the atoms finally detected in the experiment populate the $m_F=0$ sublevel at a temperature of $300\mathrm{nK}$. Microwave spectroscopy is used to measure the adiabatic transfer directly and its dependence on various experimental parameters. A theoretical model is provided to estimate the upper limit for the efficiency of adiabatic transfer in a multilevel system with nonresonant excitation. The results constitute significant progress towards constructing a caesium fountain frequency standard operating in the nK temperature range.

We provide a theory for spatial solitons due to the two-photon photorefractive effect based on the Castro-Camus model [Opt. Lett. 28, 1129 (2003)]. We present the evolution equation of one-dimensional spatial solitons in two-photon photorefractive media. In steady state and under appropriate external bias conditions, we obtain the dark and bright soliton solutions of the optical wave evolution equation, and also discuss the self-deflection of the bright solitons theoretically by taking into account the diffusion effect.

Berry’s phase of the atom in the state with a positive or negative $g$ factor for partial cycles of a rotating magnetic field was determined free from the dynamical phase shift using a time-domain atom interferometer. The experimental phase shift is in good agreement with the prediction of Berry’s phase for partial cycles. It was found that the sense of Berry’s phase depends on the sign of the magnetic quantum number, the sense of the rotating magnetic field, and the sign of the $g$ factor of the state.

We show that, one may take advantage of the asymmetry of channel noise. With appropriate modifications to the standard protocols, both the key rate and the tolerable total channel noise can be increased if the channel noise is asymmetric.

The Ekert 1991 quantum key distribution (QKD) protocol appears to be secure regardless of whatever devices legitimate users adopt for the protocol, as long as the devices give a result that violates Bell’s inequality. However, this is not the case if they ignore nondetection events because Eve can make use of the detection loophole, as Larrson showed [Quantum Inf. Comput. 2, 434 (2002)]. We show that even when legitimate users take into account nondetection events Eve can successfully eavesdrop if the QKD system has been appropriately designed by the manufacturer. A loophole utilized here is that of “free choice” (or “real randomness”). Local QKD devices with a pseudorandom sequence generator installed in them can apparently violate Bell’s inequality.

A joint experimental and theoretical study of the transfer ionization process $p+\mathrm{He}\to {\mathrm{H}}^0+{\mathrm{He}}^{2+}+e^{-}$ is presented for $630\text{−}\mathrm{keV}$ proton impact energy, where the electron is detected in a plane perpendicular to the proton beam direction. With this choice of kinematics we find the triple-differential cross section to be particularly sensitive to angular correlation in the helium target. There is a good agreement between the experimental data and theoretical calculations.

A low-energy effective theory based on a microscopic multichannel description of the atom-atom interaction is derived for the scattering of alkali-metal atoms in different hyperfine states. This theory describes all scattering properties, including medium effects, in terms of the singlet and triplet scattering lengths and the range of the atom-atom potential and provides a link between a microscopic description of Feshbach scattering and more phenomenological approaches. It permits the calculation of medium effects on the resonance coming from the occupation of closed channel states. The examination of such effects is demonstrated to be of particular relevance to an experimentally important Feshbach resonance for ${}^{40}\mathrm{K}$ atoms. We analyze a recent rethermalization rate experiment on ${}^{40}\mathrm{K}$ and demonstrate that a measurement of the temperature dependence of this rate can determine the magnetic moment of the Feshbach molecule. Finally, the energy dependence of the Feshbach interaction is shown to introduce a negative effective range inversely proportional to the width of the resonance. Since our theory is based on a microscopic multichannel picture, it allows the explicit calculation of corrections to commonly used approximations such as the neglect of the effective range and the treatment of the Feshbach molecule as a point boson.

The motion of polar molecules can be controlled by time-varying inhomogeneous electric fields. In a Stark decelerator, this is exploited to select a fraction of a molecular beam that is accelerated, transported, or decelerated. Phase stability ensures that the selected bunch of molecules is kept together throughout the deceleration process. In this paper an extended description of phase stability in a Stark decelerator is given, including higher-order effects. This analysis predicts a wide variety of resonances that originate from the spatial and temporal periodicity of the electric fields. These resonances are experimentally observed using a beam of OH $({}^2\Pi _{3∕2},v=0,J=3∕2)$ radicals passing through a Stark decelerator.

We calculate the density profiles and density correlation functions of the one-dimensional Bose gas in a harmonic trap, using the exact finite-temperature solutions for the uniform case, and applying a local density approximation. The results are valid for a trapping potential that is slowly varying relative to a correlation length. They allow a direct experimental test of the transition from the weak-coupling Gross-Pitaevskii regime to the strong-coupling, “fermionic” Tonks-Girardeau regime. We also calculate the average two-particle correlation which characterizes the bulk properties of the sample, and find that it can be well approximated by the value of the local pair correlation in the trap center.

Using numerical solutions of a time-dependent Schrödinger equation for a hydrogen atom in a linearly polarized few-cycle laser field, we calculate the left-right photoelectron kinetic-energy spectra measured by two opposing detectors placed along the laser polarization vector, with laser focus in the center. The fastest electrons show huge asymmetries strongly dependent on the laser carrier-envelope (CE) phase which confirms the recent theoretical results [D. B. Milosevic et al., Opt. Express 11, 1418 (2003)], obtained from a modified strong field approximation model which includes rescattering by the Coulomb potential. This asymmetry can also be explained by a simple semiclassical model in which the electron after tunneling through a potential barrier returns to the proton and is elastically backscattered in the presence of the laser field thus acquiring energy close $10U_p$ where $U_p$ is the electron ponderomotive energy in the laser field. We also present a semiclassical interpretation of counterintuitive left-right asymmetries of slow electrons discussed in our previous work [Phys. Rev. A. 70, 013815 (2004)]. Our analysis shows that the Coulomb attraction from the proton must be included in the standard tunneling model in order to account for the CE phase dependent angular asymmetry seen in our previous numerical calculations.

We describe a scheme that performs a SWAP gate between two photons at different wavelengths with near 100% fidelity. The essential idea is the preparation of a near-maximal atomic coherence using electromagnetically induced transparency.

We present an iterative algorithm that finds the optimal measurement for extracting the accessible information in any quantum communication scenario. The maximization is achieved by a steepest-ascent approach toward the extremal point, following the gradient uphill in sufficiently small steps. We apply it to a simple ad hoc example, as well as to a problem with a bearing on the security of a tomographic protocol for quantum key distribution.

We report observation of molecular fusion of $\left({\mathrm{C}}_{60}\right)_N{}^{+}$ clusters to form large fullerene molecules after excitation by 800-nm femtosecond laser pulses. The fused species decay by undergoing ${\mathrm{C}}_2$ fragmentation. Such fragmentation behavior for excited clusters of fullerene molecules can be understood in terms of the energetics of molecular fusion as determined previously from fullerene-fullerene collision experiments. Within a certain excitation energy window, there is sufficient time for a substantial atomic rearrangement to take place within the clusters before they dissociate. Below this energy window there is not sufficient internal energy for molecular fusion to occur while above the energy window multiple ionization followed by breakup of the clusters dominates.

A test of Lorentz invariance for electromagnetic waves was performed by comparing the resonance frequencies of two optical resonators as a function of orientation in space. In terms of the Robertson-Mansouri-Sexl theory, we obtain $\beta -\delta -1∕2=(+0.5\pm 3\pm 0.7)\times {10}^{-10}$, a tenfold improvement compared to the previous best results. We also set a first upper limit for a parameter of the standard model extension test theory, $\mid {\left({\tilde{\kappa }}_{e-}\right)}^{ZZ}\mid <2\times {10}^{-14}$.

I show how probabilities arise in quantum physics by exploring the implications of environment-assisted invariance or envariance, a recently discovered symmetry exhibited by entangled quantum systems. Envariance of perfectly entangled “Bell-like” states can be used to rigorously justify complete ignorance of the observer about the outcome of any measurement on either of the members of the entangled pair. For more general states, envariance leads to Born’s rule $p_k\propto {\mid {\psi }_k\mid }^2$ for the outcomes associated with Schmidt states. The probabilities derived in this manner are an objective reflection of the underlying state of the system—they represent experimentally verifiable symmetries, and not just a subjective “state of knowledge” of the observer. This envariance-based approach is compared with and found to be superior to prequantum definitions of probability including the standard definition based on the “principle of indifference” due to Laplace and the relative frequency approach advocated by von Mises. Implications of envariance for the interpretation of quantum theory go beyond the derivation of Born’s rule: Envariance is enough to establish the dynamical independence of preferred branches of the evolving state vector of the composite system and, thus, to arrive at the environment-induced superselection (einselection) of pointer states, which was usually derived by an appeal to decoherence. The envariant origin of Born’s rule for probabilities sheds light on the relation between ignorance (and, hence, information) and the nature of quantum states.

Equilibrium states of infinite extended lattice systems at high temperature are studied with respect to their entanglement. Two notions of separability are offered. They coincide for finite systems but differ for infinitely extended ones. It is shown that for lattice systems with localized interaction for high enough temperature there exists no entanglement localized in a finite region of given size. Even more quasifree states at high temperature are also not distillably entangled for all local regions of arbitrary size. For continuous systems entanglement survives at all temperatures. In mean-field theories it is possible that local regions are not entangled but we have still entanglement hidden in the non local fluctuation algebra.

Motivated by the idea of entanglement loss along renormalization group flows, analytical majorization relations are proven for the ground state of $(1+1)$-dimensional conformal field theories. For any of these theories, majorization is proven to hold in the spectrum of the reduced density matrices in a bipartite system when changing the size $L$ of one of the subsystems. Continuous majorization along uniparametric flows is also proven as long as part of the conformal structure is preserved under the deformation and some monotonicity conditions hold as well. As particular examples of our derivations, we study the cases of the $XX$, Heisenberg, and $XY$ quantum spin chains. Our results provide in a rigorous way explicit proofs for all the majorization conjectures raised by Latorre, Lütken, Rico, Vidal, and Kitaev in previous papers on quantum spin chains.