Physicists in Germany and Austria have shown that individual atoms can move forwards and backwards at the same time, thanks to photon emission and a carefully placed mirror. They say that this result improves our understanding of quantum coherence and could perhaps help to build a workable quantum computer.

Lying at the heart of quantum mechanics, superposition is the idea that a particle can be in two states at the same time. A simple example of this occurs when single photons pass through a double slit and build up an interference pattern on a screen beyond the slits. This demonstrates that individual photons pass through both slits at the same time.

An analogous result can be achieved by splitting a beam of atoms such that each of the atoms travels in two directions at the same time. To date, such a superposition of atomic momentum states has needed a macroscopic beam splitter such as a solid diffraction grating. But now superposition using a scheme based on single photons has been achieved by Markus Oberthaler and colleagues at the University of Heidelberg along with physicists at the Technical University of Vienna, Technical University of Munich and the Ludwig Maximilians University.

Very slight kick

To do this, Oberthaler's group passes a slow-moving, narrow beam of argon atoms very close to a mirror and then excites the atoms with a laser beam. As each atom drops back down to a lower energy level it emits a photon – and some photons bounce off the mirror. Each departing photon provides a very slight kick to the atom in the opposite direction to which the photon is emitted. As a result the photon's trajectory reveals the direction of the atom's recoil.

However, for those photons emitted at right angles to the mirror's surface, it is impossible to tell the difference between a photon that travels away from the mirror as it leaves the atom and one that initially moves towards the mirror but then bounces off its surface. Quantum mechanics tell us that this indistinguishability places the atom into a superposition – it does not recoil either towards or away from the mirror but both towards and away from the mirror at the same time.

To prove that they created this superposition state, Oberthaler's team took advantage of the fact that a beam of atoms has wave-like properties. The physicists exposed the argon atoms to a second laser beam, which was bounced off a second mirror to create a standing light wave across the argon beam. This standing wave acted like a diffraction grating and meant that after the atoms had passed the first laser and had their trajectories simultaneously bent very slightly towards and away from the first mirror, the two atom-states were each split into an undisturbed forward-travelling wave and a diffracted wave.

Interference spotted

The researchers then used an atom detector to measure the interference of the undisturbed wave from the first atom-state with the diffracted wave from the second atom-state, and vice-versa in a second detector. They find that the counts in both detectors rise and fall in a regular sinusoidal-like way as they changed the position of the second mirror. This means that the waves are interfering with one another coherently and that therefore they are coming from a single source – in other words, that the atom is indeed in the two momentum states simultaneously.

This experiment is analogous to the quantum-mechanical double-slit experiment, since the two undistinguishable photon trajectories play the part of the two slits – the atom responding to both at the same time. And like the double-slit experiment, this latest work shows that by determining which paths the particle took you destroy the superposition. Oberthaler and team demonstrate this by moving the beam far enough away from the first mirror so that in effect the mirror isn't there. This means that the photons leaving the atom in opposite directions can be unambiguously distinguished. In this case the detectors no longer measured a series of peaks and troughs but rather a slightly noisy constant count rate. This indicates that the different atom waves arriving at the second laser are not coherent because they are associated with different atoms.

A path to stable qubits?

According to team member Jirí Tomkovic, physicists usually think of spontaneous emission from an atom as destroying coherence. This is because this emission acts like a measurement that tells you unambiguously what energy and momentum state the atom is currently in. But he says that the latest work shows how spontaneous emission of a single photon can create a superposition of states. By improving our understanding of quantum coherence, he believes this research may help in the creation of stable quantum-mechanical bits (qubits) for quantum computers. However, Tomkovic cautions that the work has more relevance for fundamental, rather than applied, physics.

The research is published in Nature Physics: doi:10.1038/nphys1961