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AIP conference proceedings

Single spontaneous photon as a coherent beamsplitter for an atomic matter-wave

Jiří Tomkovič, Michael Schreiber, Joachim Welte, Martin Kiffner, Markus K. Oberthaler, Jörg Schmiedmayer

Published Jan 1, 2014• 1 citation
Publisher

Abstract

In free space the spontaneous emission of a single photon destroys motional coherence. Close to a mirror surface the reflection erases the which-path information and the single emitted photon can be regarded as a coherent beam splitter for an atomic matter-wavewhich can be verified by atom interferometry. Our experiment is a realization of the recoiling slit Gedanken experiment by Einstein.

DOI
10.1063/1.4903084

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In free space the spontaneous emission of a single photon destroys motional coherence. Close to a mirror surface the reflection erases the which-path information and the single emitted photon can be regarded as a coherent beam splitter for an atomic matter-wavewhich can be verified by atom interferometry. Our experiment is a realization of the recoiling slit Gedanken experiment by Einstein.
The key elements for atom interferometers are atomic beam splitters and recombiners. An atomic beam splitter separates a single atomic wave function into a superposition state corresponding to two center of mass wavepackets that propagate in different spatial directions and a recombiner reunites the atomic beams. We report here on the investigation of 1) an atomic beam splitter with large separation angle based on the optical Stern Gerlach effect, 2) an atomic mirror based on the reflection of atoms by different types of evanescent waves (a simple evanescent wave and a surface plasmon wave), 3) an atomic beam splitter realized by the partial spatial reflection of an atom by an evanescent wave and 4) diffraction of atoms by a standing evanescent wave.
In 1927, during the fifth Solvay Conference, Einstein and Bohr described a double-slit interferometer with a "movable slit" that can detect the momentum recoil of one photon. Their debate centered on this Gedankenexperiment has provided profound insights into the central concepts of quantum mechanics. Despite many efforts to reproduce this conceptual experiment, a linear-optical version with a movable, quantum-limited slit and tunable momentum width remained challenging. Here, we report a realization of the Einstein-Bohr interferometer using a single atom in an optical tweezer, which is cooled to the motional ground state in three dimensions such that its momentum uncertainty is comparable to that of a single photon. We design an interferometric configuration where the single atom serves as an ultralight, quantum-limit beam splitter that is momentum entangled with the input photon. By varying the trap depth of the optical tweezer, we dynamically tune the atom's intrinsic momentum uncertainty, thus enabling the observation of a gradual shift in the visibility of single-photon interference. The interferometer also allows us to distinguish the classical noise caused by atom heating from the quantum-limited noise due to the momentum transfer, illustrating a quantum-to-classical transition.
Atom interferometers are powerful tools for the study of fundamental issues in quantum mechanics. This paper describes the use of our atom interferometer [1] for an experimental realization of Feynman's gedanken experiment in which the observation of photons scattered off of particles emerging from a double slit is used to obtain which path information. This determination, in principal, of the particle's path, destroys any interference effects downstream. The interference can be regained by observing only those particles which scatter a photon into a small range of final directions.
We have demonstrated the loss of transverse spatial coherence of an atomic wave function after a single spontaneous emission. He∗{\mathrm{He}}^{*}He∗ atoms were both diffracted and excited by a standing light wave with a variable period. After the interaction, the excited atoms decay by a single spontaneously emitted photon. By changing the period of the standing light wave, we have mapped the loss of spatial coherence as a function of the transverse coordinate. By detecting the emitted photon one could "erase" the position information available and recover the transverse coherence in a correlation experiment, or realize a Heisenberg microscope.
Two recent experiments demonstrating novel atomic lenses—one using a free standing Fresnel zone plate and the other using an optical standing wave with large (40 μm) period—will be discussed. In addition, an experiment demonstrating the splitting of an atomic beam by the optical Stern–Gerlach effect will be described. In this effect, a beam of (two-level) atoms is coherently split into two paths by an exactly resonant optical field gradient, each path containing atoms in one of the two dressed states. The field gradient was derived from a large period standing wave, produced by bouncing a laser beam from a glass surface at grazing incidence. The experiments were carried out with metastable He atoms interacting through the 23S1to 23P2 transition with an optical field at 1.083 μm. The interaction times were significantly less than the 23P natural lifetime of 100 ns, so spontaneous emission played no role during the interaction.