Researchers put atoms in front of the mirror

Quantum overlay states generated

Does the light come directly from the atom or from its reflection? A mirror generates a quantum mechanical overlay. © TU Vienna
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Anyone standing in front of a mirror certainly has no problem distinguishing themselves from their reflection. The mirror has no influence on our possibilities of movement. This is more complicated with quantum physical particles. This has now been demonstrated by an international team of scientists. The researchers succeeded in continuing Einstein's thought experiment in the laboratory and blurring the difference between individual particles and their mirror images, reports the journal Nature Physics.

When an atom spontaneously emits a particle of light in a particular direction, it recovers in the opposite direction. If one measures the light particle, one therefore knows the state of motion of the atom. The research team now placed atoms a few millionths of a meter in front of a gilded mirror - in this case, there are two possible ways for a particle of light to reach the observer: it may have come directly from the atom to the observer, or it may have been sent in the opposite direction, has hit the mirror and then reached the observer. If one can not distinguish between these two cases, the atom is in a superposition of both ways.

Movement state of the atom in the visor

"With a very small distance between atom and mirror, it is fundamentally impossible to distinguish between the two possibilities, " explains Jiri Tomkovic from the University of Heidelberg. The origin particle and the mirror image are physically indistinguishable. The atom simultaneously moves towards the mirror and away from the mirror. What sounds paradoxical and impossible for macroscopic particles has long been known in quantum physics.

"This uncertainty about the state of motion of the atom does not mean that we have not measured accurately enough, " emphasizes Professor Jörg Schmiedmayer from the Technical University (TU) Vienna. "This is a fundamental property of quantum physics: The particle is in both states of motion at the same time, it is in a superposition state."

In the experiment, the states of motion that the atom occupies at the same time - towards the mirror and away from the mirror - are recombined by so-called Bragg scattering on a grid of laser light. This proves that the atom was actually in a superposition state. display

Mirror ensures undecidability

This is reminiscent of the famous double-slit experiment in which a particle is shot at a plate with two openings and due to its quantum mechanical wave properties occurs simultaneously through both openings.

Einstein already considered that this is only possible if no possible measurement can decide which path the particle took, not even by measuring tiny movements of the double-slit plate, As soon as it is theoretically ascertainable by any experiment which way the particle has decided, it is over with the quantum superposition.

"In our case, the light particles play a similar role as a double slit, " says Professor Markus Oberthaler from the University of Heidelberg. If, in principle, light can provide information about the direction in which the atom is moving, then the state of the atom is also fixed. Only if this is fundamentally undecidable, the atom is in a superposition state that unites both possibilities. And precisely this undecidability is guaranteed by the mirror.

Quantum physical relationship

It is an important research question in quantum physics to determine the conditions under which such quantum superpositions can be detected: this is the only way to exploit these effects. The idea for this experiment was developed by Schmiedmayer and Oberthaler several years ago.

The fascinating thing about it, diethe researchers, the possibility of creating a superposition state simply by the presence of a mirror, without any intervention by external fields. The particle and its mirror image naturally come into a quantum physical relationship with each other without the scientists having to spend a lot of time and effort. (Nature Physics, 2011; doi: 10.1038 / nphys1961)

(University of Heidelberg / TU Vienna, 06.04.2011 - DLO)