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Physical Review A

Diffraction in time of particles released from spherical traps

Salvador Godoy

Published Jan 9, 2003• 16 citations
Publisher

Abstract

We extend the theory of diffraction in time to particles with angular momentum. We consider the problem of the sudden release of a particle which initially was inside a spherical trap. The particle's free time evolution is described by spherical waves, which have amplitudes diffracted in time. The time-dependent density pattern is, for high or low energies, one of Fresnel or Fraunhofer diffraction by a single slit respectively.

DOI
10.1103/physreva.67.012102

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We extend the theory of diffraction in time to particles with angular momentum. We consider the problem of the sudden release of a particle which initially was inside a spherical trap. The particle's free time evolution is described by spherical waves, which have amplitudes diffracted in time. The time-dependent density pattern is, for high or low energies, one of Fresnel or Fraunhofer diffraction by a single slit respectively.
Moshinsky's work on diffraction in time is recognized as a particular case of monoenergetic, quantum dispersion process. Diffraction in time is extended to include new initial conditions, which under free evolution exhibit temporal quantum interference patterns in close analogy to spatial diffraction patterns found in optics. We show that free propagation of states, which initially are stationary states of infinite potential wells, diffract in time similarly as a plane wave by a double slit. We introduce the concepts of mass transport by transient Fraunhofer and Fresnel dispersion currents.
Quantum transient currents are obtained after the sudden release of a particle initially confined in a nanoscopic circuit. We extend the 1D theory of diffraction in time to particles released from nanoscopic circuits with cylindrical cross‐sections. For a circular cross‐section, the particle’s free time evolution is described by cylindrical waves with their amplitudes showing diffraction in time. For large observation distances, the time‐dependent probability density current looks similar to the Fraunhofer diffraction pattern by a circular aperture. A temporal quantum Airy disk can be defined.
In analogy to the well-known diffraction of waves at spatial structures, one can also define diffraction in time when a wave is temporarily modulated. In the present paper we investigate this phenomenon starting from a Green's-function approach. Diffraction in time appears for Schr\"odinger waves and not for light in vacuum. Specific cases of diffraction in time investigated here are diffraction at an ``edge in time'' and a Fresnel lens in time. We then investigate in detail the simultaneous diffraction both in space and in time representing a general solution. Then we analyze in detail diffraction at an edge, both in space and in time, of a single slit in space and in time, a double slit in space, and a single slit in time, a single slit in space and a double slit in time, and, finally, a double slit both in space and in time. In all cases we analyze the possibilities of various approximations which can be made, and show the limits and validity of the Fraunhofer approximation both in space and in time. We give explicit results for a gedanken experiment with very cold atoms.
A theoretical model of diffraction based on the concept of quantum measurement is presented. It provides a general expression of the state vector of a particle after its passage through an aperture of any shape in a plane screen (diaphragm). This model meets the double requirement of compatibility with the Huygens-Fresnel principle and with the kinematics of the particle-diaphragm interaction. This led to assume that the diaphragm is a device for measuring the three spatial coordinates of the particle and that the transition from the initial state (incident wave) to the final state (diffracted wave) consists of two specific projections which occur successively in a sequence involving a transitional state. In the case of the diffraction at infinity (Fraunhofer diffraction), the model predicts the intensity of the diffracted wave over the whole angular range of diffraction (0 ∘ - 90 ∘ ). The predictions of the quantum model and of the theories of Fresnel-Kirchhoff (FK) and Rayleigh-Sommerfeld (RS1 and RS2) are close at small diffraction angles but significantly different at large angles, a region for which specific experimental studies are lacking. A measurement of the particle flow intensity in this region should make it possible to test the FK and RS1-2 theories and the suggested quantum model.
A theoretical model of diffraction based on quantum mechanics is presented. It provides a general expression of the quantum state of a particle after its passage through an aperture in an opaque plane (diaphragm). In this model, the diaphragm is considered as a measurement device not only of the transverse coordinates but also of the longitudinal coordinate of the particle. Moreover, the change of state from the initial state (incident wave) to the final state (diffracted wave) results from two successive projections involving a transitional state which corresponds to the state of the particle when it is localized inside the aperture. These features of the model results from the need to ensure compatibility with the Huygens-Fresnel principle and with the kinematics constraints of the particle-diaphragm interaction. In the case of the diffraction at infinity (Fraunhofer diffraction), the predictions of the quantum model and of the classical theories based on the integrals of Fresnel-Kirchhoff (FK) and Rayleigh-Sommerfeld (RS1 and RS2) are different. They are close at small diffraction angles but contradictory at large angles. In this latter region, only the quantum and RS1 models predict the probable decrease in the intensity of the diffracted wave to zero when the diffraction angle reaches 90{\deg}. However, this decrease is different according to the model and it turns out that the discrepancies become significant beyond 60{\deg}. A measurement of the intensity of the particle flow in this region should allow to test the two theories.