A temporally two-dimensional overtone vibrational dephasing spectroscopy, which is based on the fifth-order nonlinearity, is proposed to investigate correlation between fluctuations of different vibrational frequencies in liquids. The experiment is done for the C–D stretching of neat ${\mathrm{CDCl}}_3$ liquid. We found that a signal decay as a function of the second-pulse interval gets faster as the first-pulse interval increases.

This work studies the interference of electrons in the presence of a line of magnetic flux surrounded by a normal-conducting mesoscopic cylinder at low temperature. It is found that, while there is a supplementary phase contribution from each electron of the mesoscopic cylinder, the sum of these individual supplementary phases is equal to zero, so that the presence of a normal-conducting mesoscopic ring at low temperature does not change the Aharonov-Bohm interference pattern of the incident electron. It is shown that it is not possible to ascertain by experimental observation that the shielding electrons have responded to the field of an incident electron, and at the same time to preserve the interference pattern of the incident electron. It is also shown that the measuring of the transient magnetic field in the region between the two paths of an electron interference experiment with an accuracy at least equal to the magnetic field of the incident electron generates a phase uncertainty which destroys the interference pattern.

Another laser-induced resonant transition in the v=n-l-1=2 metastable cascade of antiprotonic ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atoms has been found by using a double-resonance technique. This was done by setting the first laser to the already known 470.724-nm resonance [(n,l)=(37,34)→(36,33)], while the (38,35)→(37,34) transition was searched for with the second laser. The resonant transition was found at a wavelength of 529.622±0.003 nm, showing excellent agreement with a recent prediction of Korobov.

It is known that fluorescence of trapped ${\mathrm{Yb}}^{+}$ ions fades with continuous driving of the ${}_{}{}^2{}_{}{}^{}$ ${\mathrm{S}}_{1\mathrm{/}2}$${\mathrm{-}}^2$${\mathrm{P}}_{1\mathrm{/}2}$ transition in the presence of buffer gases. This phenomenon has been interpreted as being caused by population trapping in the ${}_{}{}^2{}_{}{}^{}$ ${\mathrm{F}}_{7\mathrm{/}2}$ state of ${\mathrm{Yb}}^{+}$ . As another cause of the fluorescence disappearance, we found the production of ${\mathrm{YbH}}^{+}$ by chemical reaction of ${\mathrm{Yb}}^{+}$ in excited states with ${\mathrm{H}}_2$ gas. We found that the wavelengths of 369.482, 369.202, and 368.947 nm, formerly assigned as wavelengths for deexcitation of ${\mathrm{Yb}}^{+}$ in the ${}_{}{}^2{}_{}{}^{}$ ${\mathrm{F}}_{7\mathrm{/}2}$ state to the ${}_{}{}^2{}_{}{}^{}$ ${\mathrm{S}}_{1\mathrm{/}2}$ ground state, are actually photodissociation lines of ${\mathrm{YbH}}^{+}$ . The change of the resonance point of rf resonance absorption signals indicates the production and dissociation of ${\mathrm{YbH}}^{+}$ ions. The production of ${\mathrm{YbH}}^{+}$ may be a problem in establishing frequency standards using buffer-gas-cooled ${\mathrm{Yb}}^{+}$ , owing to difficulty in removing ${\mathrm{H}}_2$ gas from vacuum systems.

We measured the cusp electron production associated with target ionization at the impact of a 400-keV pure2${\mathrm{}}^3$S He beam and a mixed beam containing all three long-lived He states, i.e., 1${\mathrm{}}^1$S,n2${\mathrm{}}^1$S, and 2${\mathrm{}}^3$S. Using the results of an earlier experiment [Kuzel et al., Phys. Rev. A 48, R1745 (1993)], we estimated the cross section for both metastable states of He. We found that the cusp for the 2${\mathrm{}}^1$S state is much larger and sharper than for the 2${\mathrm{}}^3$S state. The peaks are manifestations of excitation of low-lying virtual or weakly bound states of the ${\mathrm{He}}^{\mathrm{-}}$ ion at the 2${\mathrm{}}^1$S and 2${\mathrm{}}^3$S thresholds.

We find scaling solutions for the evolution of a Bose gas in time-dependent anisotropic traps and reveal the phenomenon of stochastization in the evolution of a condensate. In contrast to collisionless gases, the evolution of the thermal gas in the hydrodynamic regime proves to be in many aspects similar to that of the condensate. We emphasize that clear signatures of Bose-Einstein condensation and the effects of the loss of coherence can be found in local correlation properties of the evolving gas through the measurement of the rates of intrinsic or light-induced inelastic collisional processes.

The behavior of the ionization cross section of atoms is known classically in the limits of threshold energy and at high energies. These two limits are used to construct a simple analytical formula for the ionization cross section that depends upon two parameters: the magnitude of the maximum of the cross section and its position in energy. The parametrization has been tested for electron- and positron-impact ionization as well as for proton- and antiproton-impact ionization. It reproduces in all cases the shape of the cross section and offers a unified treatment for ionization by bare projectiles irrespectively of their charge and mass.

A vacuum ultraviolet photoionization study of atomic Ir in the region between 40 and 70 eV was performed with the use of monochromatized synchrotron radiation and photoion detection. The atomic beam of Ir was obtained with the technique of evaporating thin Ir wires. The photoion signals are compared with the theoretical photoionization cross section in the region of the 5p and 4f excitation calculated in the framework of overlapping Fano resonances.

We present an alternative and experimentally feasible scheme to realize the teleportation of a cavity mode in a coherent superposition of zero-, one-, and two-photon field states. Particular nonmaximally entangled states of two-level atoms and particular cavity modes are employed and their preparation is discussed. A Ramsey-type arrangement, together with pointer atoms, is required to prepare the atomic entangled states as well as to read out the cavity system. Basically, the pointer atoms induce the state-vector collapse of the field in the cavity system and indicate, through their detection, the measurement result. © 1996 The American Physical Society.

We consider Gell-Mann and Hartle’s consistent histories formulation of quantum cosmology in the interpretation in which one history, chosen randomly according to the decoherence functional probabilities, is realized from each consistent set. We show that in this interpretation, if one assumes that an observed quasiclassical structure will continue to be quasiclassical, one cannot infer that it will obey the predictions of classical or Copenhagen quantum mechanics. © 1996 The American Physical Society.

We study the problem of computing the probability for the time of arrival of a quantum particle at a given spatial position. We consider a solution to this problem based on the spectral decomposition of the particle’s (Heisenberg) state into the eigenstates of a suitable operator, which we denote as the ‘‘time-of-arrival’’ operator. We discuss the general properties of this operator. We construct the operator explicitly in the simple case of a free nonrelativistic particle and compare the probabilities it yields with the ones estimated indirectly in terms of the flux of the Schrödinger current. We derive a well-defined uncertainty relation between time of arrival and energy; this result shows that the well-known arguments against the existence of such a relation can be circumvented. Finally, we define a ‘‘time representation’’ of the quantum mechanics of a free particle, in which the time of arrival is diagonal. Our results suggest that, contrary to what is commonly assumed, quantum mechanics exhibits a hidden equivalence between independent (time) and dependent (position) variables, analogous to the one revealed by the parametrized formalism in classical mechanics. © 1996 The American Physical Society.

A group-theoretic quantization method is applied to the ‘‘complete symmetry group’’ describing the motion of a point charge in a constant magnetic field. Within the regular ray representation, the Schrödinger operator is obtained as the Casimir operator of the extended Lie algebra. Configuration ray representations of the complete group cast the Schrödinger operator into the familiar space-time differential operator. Next, ‘‘group quantization’’ yields the superselection rules, which produce irreducible configuration ray representations. In this way, the Schrödinger operator becomes diagonalized, together with the angular momentum. Finally, the evaluation of an invariant integral, over the group manifold, gives rise to the Feynman propagation kernel 〈t′,x′|t,x〉 of the system. Everything stems from the assumed symmetry group. Neither canonical quantization nor the path-integral method is used in the present analysis. © 1996 The American Physical Society.

We consider a realization of quantum computing using polarized photons. The information is encoded in two polarization directions of the photons and two-quantum-bit operations are performed using a conditional Faraday effect. We investigate the performance of the system as a computing device. © 1996 The American Physical Society.

We derive a general limit on the fidelity of a quantum channel conveying an ensemble of pure states. Unlike previous results, this limit applies to arbitrary coding and decoding schemes. This establishes in full generality the quantum noiseless coding theorem, formerly obtained only for the special case of unitary decoding schemes. © 1996 The American Physical Society.

A scheme for amplifying phase shifts is presented, based on ideal photon number deamplification. For high-sensitivity measurements phase amplification sizeably reduces the bit-error rate and enhances the mutual information retrieved from the measurement. Phase-coherent states preserve their coherence under amplification, and achieve the best amplifier performance. © 1996 The American Physical Society.

The detection process of photons is studied at the limit of a vanishing Planck’s constant. The detectors are conceived as particular cases of systems composed of atomic entities, called sources, acting in some given regions of space-time. When these regions are small and when their number is large with respect to the number of atoms of a real detector, we speak of a many-source system. The observable detection probabilities at given times are defined in general in an S-matrix formalism. It is shown that, in the case of a many-source system, these probabilities satisfy the same relations as those valid for a classical statistical system. When Planck’s constant goes to zero, the energy density of the radiation remaining finite, an approximate expression of the amplitudes is constructed, which preserves the unitarity of the S matrix, and therefore the conservation of probability. Consequently, no recourse to the attenuation of the radiation field is necessary to ensure a coherent set of probabilities. Due to a factorization property of the amplitudes, it is then shown that the behavior of a real detector is nearly the same as that of a many-source system, namely, a classical behavior. © 1996 The American Physical Society.

Methods of finding good quantum error-correcting codes are discussed, and many example codes are presented. The recipe ${\mathit{C}}_2^{\mathrm{\perp }}$⊂${\mathit{C}}_1$, where ${\mathit{C}}_1$ and ${\mathit{C}}_2$ are classical codes, is used to obtain codes for up to 16 information quantum bits (qubits) with correction of small numbers of errors. The results are tabulated. More efficient codes are obtained by allowing ${\mathit{C}}_1$ to have reduced distance, and introducing sign changes among the code words in a systematic manner. This systematic approach leads to single-error-correcting codes for 3, 4, and 5 information qubits with block lengths of 8, 10, and 11 qubits, respectively. © 1996 The American Physical Society.

Time dependence for barrier penetration is considered in the phase space. An asymptotic phase-space propagator for nonrelativistic scattering on a one-dimensional barrier is constructed. The propagator has a form universal for various initial state preparations and local potential barriers. It is manifestly causal and includes time-lag effects and quantum spreading. Specific features of quantum dynamics which disappear in the standard semiclassical approximation are revealed. The propagator may be applied to calculation of the final momentum and coordinate distributions, for particles transmitted through or reflected from the potential barrier, as well as for elucidating the tunneling time problem. © 1996 The American Physical Society.