Double-slit with single atom shows quantum to classical transition
A single atom as a movable slit: Researchers realize Einstein's thought experiment in original form, observing quantum-classical transition.
(Image: vchal / Shutterstock.com)
Almost a century after the legendary debate between Albert Einstein and Niels Bohr, a team led by Jian-Wei Pan from the University of Science and Technology of China has brought the famous "recoiling-slit" thought experiment into the laboratory in a particularly faithful form. The work published in the December issue of Physical Review Letters (a pre-print version is available on arXiv.org) thus closes a clearly defined gap: for the first time, a linear-optical single-photon interferometer with a quantum-limited, tunable "movable slit" has been realized – as close to Einstein's original idea as never before.
The Double-Slit Experiment: "The Only Mystery"
Richard Feynman once called the double-slit experiment "the heart of quantum mechanics" and "in reality, the only mystery." If you send individual particles through two narrow slits, they hit a point like classical particles – but over many passes, an interference pattern emerges as if each particle were a wave that passes through both slits simultaneously and interferes with itself. However, as soon as you try to find out which path the particle took, the interference pattern disappears. This complementarity – wave or particle properties, but never both simultaneously – was the starting point of the famous Bohr-Einstein debates at the Solvay Conference in 1927.
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At the time, Einstein proposed a thought experiment that was intended to circumvent complementarity: a "movable slit" on sensitive springs was to register the recoil when a photon passed through it and was deflected. The photon's path could be determined from the recoil – and yet the interference pattern was supposed to be preserved. Bohr countered with Heisenberg's uncertainty principle: if you want to measure the momentum of the slit precisely enough, its position uncertainty becomes so large that it blurs the interference pattern.
(Image:Â Pan et al. / Bearbeitung heise medien)
Gap in previous experiments closed
Although there had already been experimental approximations to this thought experiment – for example with molecules and X-rays or as a theoretical ion trap proposal. However, these approaches either destroyed the photon state, used additional degrees of freedom, or did not represent a true linear-optical interferometer.
The Chinese team has now realized a conceptually "purist" setup: a single atom of the radioisotope Rubidium-87, cooled to its motional ground state, in an optical tweezer serves as the movable slit. A photon is scattered by it without being absorbed, and the scattered light then interferes. Crucially, neither internal atomic states nor thermal motion provide additional path information – the atom functions as an almost ideal quantum mechanical beam splitter.
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The transition between quantum and classical physics
By varying the trap depth of the optical tweezer, the momentum uncertainty of the atom can be adjusted between 0.78 and 1.60 photon momenta, thus systematically controlling the interference visibility. At low trap depth, the atomic recoil contains a lot of information about the photon's path – the photon and atom are strongly entangled, and interference disappears. At deep trap depth, the recoil "disappears" into quantum fluctuations, and interference is preserved. The measured visibilities precisely follow the theoretical prediction for minimal Heisenberg uncertainty.
Furthermore, the researchers were able to separate quantum mechanical noise from classical heating and thus directly observe the transition from the quantum-limited to the classically dominated regime – experimental confirmation of Bohr's position.
The result is not physically revolutionary – the connection between visibility, entanglement, and complementarity had long been theoretically clarified. However, in this specific combination of a single-photon interferometer, a quantum-limited atom as a movable slit, and optically adjustable momentum uncertainty, there is no direct precedent. It is likely to be the "cleanest textbook version" of Einstein's thought experiment to date.
(vza)
