Thursday, February 23, 2012

Interference time

This story isn’t new, but it’s kind of cool. Everybody knows about double slit interference with light: since light is waves you get interference if it can go through two openings. Electron can do the same sort of thing; they behave both as particles and as waves. And there are some non-intuitive features of measurement.

You can get interference fringes in space with electrons, which is how we know they can act like waves and where quantum mechanics comes from. It turns out there can be interference in time too.

The idea was clever, though the implementation was tricky. The team had to generate laser pulses so short that there were only a few wavelengths. (There are some ambiguities about defining the wavelength when the train is that short, but that doesn’t matter for the purpose.) In particular they could reproducibly generate wave trains about 1 1/2 wavelengths long, and for a given polarization direction have 2 peaks or 2 valleys.

The team was able to control the output of the laser so that all the pulses were identical. The researchers could, for example, ensure that each pulse contained two maxima of the electric field (that is, two peaks with large positive values) and one minimum (a peak with a large negative value). There was a small probability that an atom would be ionized by one or other of the maxima, which therefore played the role of the slits, with the resulting electron being accelerated towards a detector. If the atom was ionized by the minimum, the electron traveled in the opposite direction towards a second detector.

The team registered the arrival times of the electrons at both detectors and then plotted the number of electrons as a function of energy. The researchers observed interference fringes at the first detector because it was impossible to know if an electron counted by the detector was produced during the first or second maximum.

There was no interference pattern at the second detector because all the electrons were produced at the same time at the minimum. However, when the phase of the laser was changed so that there was one maximum and two minima, interference fringes were seen at the second detector but not at the first.

So with two different "time apertures" you get interference too.

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