Since the beginning of the 20th century, physicists have known that light must be considered as both a wave and a particle. Some properties of light — such as diffraction — can only be explained by treating it as waves, while others — like the photoelectric effect — can only be explained by treating it as particles. In the 1920s Louis de Broglie suggested that this wave–particle duality, which lies at the heart of the counterintuitive laws of quantum mechanics, could be extended to massive particles.
In other words, electrons, atoms and molecules should diffract and interfere just like light does — and a variety of tools have been developed to split these so-called matter waves into different parts as prisms and mirrors do for light. Unlike these optical elements, however, devices capable of manipulating matter waves need exotic structures such as mechanical and laser gratings. Armed with such devices, we can build a matter-wave interferometer.
Interferometers are a vital tool in physics. By analyzing the interference pattern produced when two or more waves that have travelled along different paths interfere, an interferometer can reveal information about the physical environments of those paths or “arms”. In an optical interferometer, for example, the separation of fringes in such an interference pattern depends on the relative phases of — and thus the distances travelled by — the waves in each arm of the device. Most interferometers use electromagnetic waves, but the principle is the same for sound and matter waves too. The latter, for instance, experience a phase shift that depends on the refractive index of the medium they are travelling through.
The crucial difference between a matter-wave and an optical interferometer, however, is that matter waves have mass while photons are massless. As a result, the phase of a matter wave is affected by gravity to a much greater extent, thereby offering a highly accurate way to test Newtonian gravity on very small scales and to measure certain fundamental constants.
In the November issue of Physics World, Franck Pereira Dos Santos and Arnaud Landragin explain why atom interferometers also have much more practical applications based on their ability to provide an absolute measure of rotation and acceleration. Indeed, argue the authors, it may not be long before such atom interferometers are used to guide aircraft, submarines and even spacecraft.
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