Tuesday, April 13, 2010

Single Photon Quantum Erasing: A Demonstration Experiment

OK, I had every intention to read this paper first and then do a report on it afterwards. But y'know, work and life got in the way, and the paper is still on my desk and remained unread. So I'm going to just highlight it here and point to the link at the IoP website now so that it is within the 30-day online print window, and you can get the paper yourself for free (upon registration).

Abstract: In the conventional interpretation of quantum mechanics the interference of particles in a two-beam interferometer is closely related to the problem of which-way information. One of the mysteries of quantum mechanics relies on the assumption that the wavefunction of each photon propagates simultaneously along both classically allowed paths, and that interference arises as a consequence of the indistinguishability of those paths. Any attempt to obtain which-way information by putting individual labels on the photons in each pathway inevitably destroys interference. However, even in cases in which the photons carry which-way labels, it is possible to erase those labels after the particle has left the interferometer. The erasing process (partly or completely) destroys the which-way information, and thereby restores interference. This phenomenon is known as quantum erasing. Here we present a lecture demonstration experiment of quantum erasing based on a Mach–Zehnder interferometer operated with single photons.

T.L. Dimitrova and A. Weis, Eur. J. Phys. v.31, p.625 (2010).

These authors have published another paper that I liked and highlighted before on the so-called wave-particle duality. I'm guessing this is as good as the previous one, but like I said, I haven't read it yet. But that shouldn't stop you from looking over it! :)


1 comment:

Peter said...

Perhaps this is acceptable as an explanation of a demonstration for undergraduates, but I think it's misleading enough to be worrying to say that by decreasing the intensity of a coherent light source one can produce a single photon state. Shouldn't we give Greenstein & Zajonc level material to undergraduates?

The analysis that is given, in terms of a single-particle Hilbert space, will work well enough for this experiment, but with this measurement scheme one can't distinguish a coherent state from a state created by the action of a single creation operator on the vacuum state.

This experiment is different from a double-slit demonstration only in that it demonstrates the orthogonality of the interference of polarized components and uses transformations of polarization. Otherwise it shows, as usual, that if we decrease the light intensity enough, and we use the kind of apparatus that is engineered to manifest discrete events in the presence of very low intensity light, then we observe discrete events. Unsurprisingly, the calculation of light intensity at a given point is identical to the calculation of the expected rate of events at the same point.