We determine the radial dipole moment between the $2S$ and $2P$ states of atomic lithium by analyzing the long-range vibrational eigenenergies of the singly excited diatomic molecule. The result can be expressed in terms of the $2P_{\frac{1}{2}}$ radiative lifetime of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, which is found to be 27.102(2)(7) ns. This result agrees with most current atomic-structure calculations and resolves the long-standing disagreement with previous experiment. The current level of precision is sensitive to relativistic effects in the atomic-structure calculation and to non-Born-Oppenheimer and radiation retardation effects in the molecule.

Reduced Stokes parameters of the $2{}_{}{}^2{}_{}{}^{}P$ state of lithium are measured using superelastic electron-scattering techniques and calculated using the convergent close-coupling method. The measurements and calculations are in excellent agreement at all scattering angles. This is particularly significant at the large scattering angles where there has been a long-standing discrepancy between theory and electron-photon coincidence measurements in the corresponding electron-hydrogen scattering problem.

The content of phase information of an arbitrary phase-sensitive measurement is evaluated using the maximum likelihood estimation. The phase distribution is characterized by the relative entropy—a nonlinear functional of input quantum state. As an explicit example, the multiple measurement of the quadrature operator is interpreted as quantum phase detection achieving the ultimate resolution predicted by the Fisher information. © 1996 The American Physical Society.

The quantum-mechanical problem of N fermions with δ-function interaction in a one-dimensional potential well of finite depth is solved. It is shown that there exists exact wave function of Bethe-ansatz form in the case that a single particle tunnels outside of the well. The Bethe-ansatz-like secular equations for the spectrum are obtained. © 1996 The American Physical Society.

The Larmor clock which was proposed to determine tunneling times of particles with spin through barriers is studied. Tunneling of wave packets is numerically derived from the time-dependent Schrödinger-Pauli equation. The Larmor clock often does not proceed unidirectionally in time. The asymptotic readings are well defined. For wide wave packets the asymptotic Larmor times agree with the stationary-state predictions; deviations are seen for narrow wave packets. Büttiker’s dwell time is investigated and its connection with the Larmor times is confirmed in the case of wave packets. © 1996 The American Physical Society.

It is shown that an ideal beam splitter has a small probability of splitting a single photon into a pair of secondary photons, conserving energy in the process. These nonlinear effects are a fundamental consequence of the quantization of the field and are analogous to other nonlinear effects in QED, such as the scattering of one photon by another. © 1996 The American Physical Society.

The behavior of light clocks which undergo constant acceleration in flat space-time is considered. Time dilation, the Doppler shift, and clock synchronization are shown to be different from that of comoving unaccelerated clocks. These results are discussed in relation to the nature of time in accelerating systems. © 1996 The American Physical Society.

In the Heisenberg picture, the generalized invariant and exact quantum motions are found for a time dependent forced harmonic oscillator. We find the eigenstate and the coherent state of the invariant and show that the dispersions of these quantum states do not depend on the external force. Our formalism is applied to two interesting cases. © 1996 The American Physical Society.

The quantum theory for measurements of energy is introduced and its consequences for the average position of monitored dynamical systems are analyzed. It turns out that energy measurements lead to a localization of the expectation values of other observables. This is manifested, in the case of position, as a damping of the motion without classical analog. Quantum damping of position for an atom bouncing on a reflecting surface in the presence of a homogeneous gravitational field is dealt with in detail and the connection with an experiment already performed in the classical regime is studied. We show that quantum damping is testable provided that the same measurement strength obtained in the experimental verification of the quantum Zeno effect in atomic spectroscopy [W. M. Itano et al., Phys. Rev. A 41, 2295 (1990)] is made available. © 1996 The American Physical Society.

We present a possible scheme to tamper with nonlocal quantum correlations in a way that is consistent with relativistic causality, but goes beyond quantum mechanics. A nonlocal ‘‘jamming’’ mechanism, operating within a certain space-time window, would not violate relativistic causality and would not lead to self-contradictory causal loops. The results presented in this paper do not depend on any model of how quantum correlations arise and apply to any jamming mechanism. © 1996 The American Physical Society.

We start from the reasonable assumption that a quantum measurement can be properly described only when the measurement apparatus itself is also treated quantum mechanically. Using idealized optical measurements as an example and introducing a small loss into the measurement apparatus, we show that the combined density matrix of the system and apparatus decoheres in a manner that the outcome of the measurement can be described classically. The loss can be arbitrarily small when the classical limit of the measurement apparatus is taken, i.e., the gain is made to approach infinity. Further, we show that the von Neumann postulate of the collapse of the wave function of the measured system into an eigenstate of the measured observable can be deduced from the analysis of two quantum nondemolition experiments in cascade and need not be introduced as a separate postulate. © 1996 The American Physical Society.

We look for the solution to the eigenvalue problem, in a recently introduced phase-space representation, for the quantum particle in a linear potential. We find that the solution is not unique and that some of these functions correspond to the eigenfunctions in coordinate space and that others correspond to the classical limit. © 1996 The American Physical Society.

The exactness of the semiclassical method for three-dimensional problems in quantum mechanics is analyzed. The wave equation appropriate in the quasiclassical region is derived. It is shown that application of the standard leading-order WKB quantization condition to this equation reproduces exact energy eigenvalues for all solvable spherically symmetric potentials. © 1996 The American Physical Society.

An extension of a model proposed by Wilczek and Zee [Phys. Rev. Lett. 52, 2111 (1984)] is studied in this paper. It is shown that slow variation of an adiabatic parameter causes both geometric and nongeometric (i.e., rate dependent) effects in the model that do not vanish in the adiabatic limit. The latter effect is due to the fact that small perturbations of order ε may accumulate over the period of +ε[0,1/ε] if motion of the system is unbounded. Interesting relations between these two effects are discussed. © 1996 The American Physical Society.

A measurement process is logically reversible if the premeasurement density operator of the measured system is uniquely determined from the postmeasurement density operator and the outcome of the measurement. This paper analyzes the necessary and sufficient condition for a measurement process to be logically reversible and discusses specific examples on quantum-nondemolition measurements, quantum counting, and measurement of spin systems. It is shown that for any sharp measurement we can construct a logically reversible measurement that continuously approaches the sharp measurement with a decrease in the measurement error. A general condition for a measurement process to be reversed by another with a nonzero probability of success is given, and the implications of such physical reversibility are discussed. © 1996 The American Physical Society.

I discuss the generation of certain kinds of four-photon coherent states in the context of dispersive cavity QED. These states, eigenstates of fourth power of the photon annihilation operator, may be expressed as various superpositions of four coherent states symmetrically located on a circle in phase space. I also point out that for low amplitude initial coherent states, number states containing one, two, or three photons can be produced in the cavity. The decay of these states can be monitored by doing parity measurements on the field from which the lifetime of a number state can be obtained. © 1996 The American Physical Society.

Using discrete displacement-operator expansion, s-parametrized phase-space functions associated with the operators in a finite-dimensional Hilbert space are introduced and their properties are studied. In particular, the phase-space functions associated with the density operator can be regarded as quasidistributions whose properties are similar to those of the well-known quasidistributions in the continuous phase space. So the Q function (s=-1) is non-negative and can be measured directly in particular experiments, whereas the P function (s=1) corresponds to the diagonal form of the density operator in an overcomplete basis. Except for the W function (s=0), the introduction of discrete phase-space functions requires the choice of a special reference state. We finally present a simple model for measuring the discrete Q function. © 1996 The American Physical Society.

A class of squeezed states for the su(1,1) algebra is found and expressed by the exponential and Laguerre-polynomial operators acting on the vacuum states. As a special case it is proved that Perelomov’s coherent state is a ladder-operator squeezed state and therefore a minimum uncertainty state. The theory is applied to the two-particle Calogero-Sutherland model. We find some squeezed states and compared them with the classical trajectories. The connection with some su(1,1) quantum optical systems (amplitude-squared realization, Holstein-Primakoff realization, the two-mode realization, and a four-mode realization) is also discussed. © 1996 The American Physical Society.

The electronic structure of the ${\mathrm{Sr}}_2$ molecule has been investigated by use of a two-valence-electron pseudopotential, configuration-interaction calculations with the four active electrons via the CIPSI (configuration interaction by perturbation of a multiconfiguration wave function selected iteratively) algorithm, and addition of a core-polarization potential. Potential-energy curves for the ground and 21 excited states of ${\mathrm{Sr}}_2$ as well as for the ground state of ${\mathrm{Sr}}_2^{+}$ have been determined, and subsequent spectroscopic constants have been obtained. Quite a good agreement with the few available experimental values is displayed for the unambiguously assigned states X${\mathrm{}}^1$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$, A${\mathrm{}}^1$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ of ${\mathrm{Sr}}_2$ and X${\mathrm{}}^2$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ of ${\mathrm{Sr}}_2^{+}$, while present predictions lead to a different assignment for the observed B${\mathrm{}}^1$${\mathrm{\Pi }}_{\mathit{u}}$ state which is found to correspond to the adiabatic (3${)}^1$${\mathrm{\Pi }}_{\mathit{u}}$ state. © 1996 The American Physical Society.

The generalization of the highly accurate exponential variational expansion in the relative coordinates ${\mathit{r}}_{32}$, ${\mathit{r}}_{31}$, and ${\mathit{r}}_{21}$ is presented for the computation of bound states with arbitrary values of the total angular momentum L and space parity π for three-body nonrelativistic systems with arbitrary particle masses. For all matrix elements explicit analytical formulas are presented which have relatively simple structures and contain L and π as parameters. © 1996 The American Physical Society.