Quantum Measurements and Delays in Scattering by Zero-Range Potentials.

Eisenbud-Wigner-Smith delay and the Larmor time give different estimates for the duration of a quantum scattering event. The difference is most pronounced in the case where the de Broglie wavelength is large compared to the size of the scatterer. We use the methods of quantum measurement theory to analyse both approaches and to decide which one of them, if any, describes the duration a particle spends in the region that contains the scattering potential.

A single resonance Regge pole dominates the forward-angle scattering of the state-to-state F + H → FH + H reaction at = 62.09 meV.

The aim of the present paper is to bring clarity, through simplicity, to the important and long-standing problem: does a resonance contribute to the forward-angle scattering of the F + H reaction? We reduce the problem to its essentials and present a well-defined, yet rigorous and unambiguous, investigation of structure in the differential cross sections (DCSs) of the following three state-to-state reactions at a translational energy of 62.09 meV: F + H( = 0, = 0, = 0) → FH( = 3, = 0, 1, 2, = 0) + H, where , , and , , are the initial and final vibrational, rotational and helicity quantum numbers respectively. , we carry out quantum-scattering calculations for the Fu-Xu-Zhang potential energy surface, obtaining accurate numerical scattering matrix elements for indistinguishable H.

Unitary Evolution and Elements of Reality in Consecutive Quantum Measurements.

Probabilities of the outcomes of consecutive quantum measurements can be obtained by construction probability amplitudes, thus implying the unitary evolution of the measured system, broken each time a measurement is made. In practice, the experimenter needs to know all past outcomes at the end of the experiment, and that requires the presence of probes carrying the corresponding records. With this in mind, we consider two different ways to extend the description of a quantum system beyond what is actually measured and recorded.

Wigner's Friend Scenarios and the Internal Consistency of Standard Quantum Mechanics.

Wigner's friend scenarios involve an Observer, or Observers, measuring a Friend, or Friends, who themselves make quantum measurements. In recent discussions, it has been suggested that quantum mechanics may not always be able to provide a consistent account of a situation involving two Observers and two Friends. We investigate this problem by invoking the basic rules of quantum mechanics as outlined by Feynman in the well-known "".

From Quantum Probabilities to Quantum Amplitudes.

The task of reconstructing the system's state from the measurements results, known as the Pauli problem, usually requires repetition of two successive steps. Preparation in an initial state to be determined is followed by an accurate measurement of one of the several chosen operators in order to provide the necessary "Pauli data". We consider a similar yet more general problem of recovering Feynman's transition (path) amplitudes from the results of at least three consecutive measurements.

Quantum Measurements with, and Yet without an Observer.

It is argued that Feynman's rules for evaluating probabilities, combined with von Neumann's principle of psycho-physical parallelism, help avoid inconsistencies, often associated with quantum theory. The former allows one to assign probabilities to entire sequences of hypothetical Observers' experiences, without mentioning the problem of wave function collapse. The latter limits the Observer's (e.

Near-threshold electron attachment as Regge resonances: cross sections for K, Rb, and Cs atoms.

We investigate the near-threshold formation of negative ions as Regge resonances in electron-atom scattering, with specific results obtained for e--K, e--Rb, and e--Cs. The complex angular momentum method, implemented within the Mulholland formulation of the total elastic cross sections, is employed. We demonstrate that for e--K, e--Rb, and e--Cs scattering, the near-threshold electron attachment cross sections are characterized by the Wigner threshold behavior, Ramsauer-Townsend minima, and Regge resonances, all discernible only through Regge partial cross section scrutiny.