[David @ Jan 21st 2007 8:43AM]

While the team may have allowed only a single photon to pass through the stencil, they don't mention how they retrieved the final image shown in the article. Generally, when information is imprinted on the wave function of a photon, you can't retrieve this information from just that single photon alone. An example is the famous double-slit experiment. All physics students know that if you shine a beam of light through a pair of slits and only a screen, you obtain a series of bright and dark lines ... an interference pattern due to constructive and destructive interference of light passing through each slit. Now, if you reduce this light down to one photon at a time, it turns out that you still get this interference pattern since the wave function of even a single photon "sees" both slits. However, that single photon, when it hits the screen, doesn't create the full interference pattern. It merely creates a spot somewhere consistent with the pattern. This is because the information about the slits is carried by the photon's wave function, which describes the probability of detecting the photon in one physical location or another. For any individual. observation, you merely observe the photon in one place, consistent with this probability. You thus need to allow many photons, albeit one at a time, to continually pass through the slits and onto the screen. Over time, a pattern of dots build up that merge into the interference pattern. It's likely that the same thing applies here. One photon carries the "UR" imprint, but detection may require many of these imprinted photons. I wish the article went into more detail.

[John @ Jan 23rd 2007 6:17AM]

Hello David,
you are exactly right. Quantum mechanics is an interesting field. I suggest reading the paper as it clears up many of the issues discussed in this blog. In reality we used a stream of weak coherent pulses, which are superpositions of all possible number states of the electromagnetic field, but the probability of measuring two photons in the detector at once was about 1 part in a milliion. It did require an ensemble (a large number of identical events) of pulses to see the image. It should also be pointed out that if the image is an eigenstate of the measuring device you can determine the image in a single event. We want to work on that.