The demonstration of a single-particle interferometer opens the door to a number of applications, from investigations of gravity to surface impact physics.
Since the early days of quantum mechanics, matter-wave interferences with their mind-boggling implications have fascinated generations of physicists. Today we can coherently split and combine beams of electrons, neutrons, atoms, or even large clusters, and make them interfere. The study of these interferometers is motivated by a wide variety of applications, ranging from fundamental physics, such as the study of gravity, to applications, such as global positioning and geodesy, and also provided part of the research honored with the Nobel Prize in physics this year (see 12 October 2012 Focus). The usual technique involves operating an interferometer with sources providing a high flux of particles to increase the signal, while reducing the shot noise. For this and other reasons, many people still think about coherent beam splitting in statistical terms.
However, quantum mechanics teaches us that one could operate an interferometer with single particles rather than beams because particles can interfere with themselves [1, 2]. Now, writing in Physical Review Letters, Paul Parazzoli and colleagues [3] at Sandia National Laboratories, New Mexico, have realized a new scheme for an interferometer that demonstrates single-particle control in a free-space atom interferometer (Fig. 1). The principle of self-interfering atoms has even been demonstrated in traps [4], for example; however, demonstration of this effect in a free-space interferometer has not been reported until now.
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