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Scientists Build New Type of Photon Gun
KentuckyFC writes "Single photons are surprisingly difficult to generate. But since they are crucial for quantum communication, a number of research groups are working on photon guns that fire single photons on demand. The problem they have come up against is that making the photons identical is proving harder than expected. Now a group in Cambridge, UK, has cracked the problem using a quantum dot on a transistor to emit single photons that are essentially identical. In the process, the group has developed an entirely new technique to trigger photon emission (abstract on the physics arxiv)."
No folly is more costly than the folly of intolerant idealism. - Winston Churchill
Also (much more difficult to control) what the "phase" of it is. Lasers achieve tremendous frequency-uniformity, which is quite nice, but the amazing thing is that their photons are essentially mostly phase-locked, so each is identical to the last. It means that one can get tremendous power a large distance with them. But lasers are inherently producers of large populations of photons (in a sense, you need a lot of photons to control the mechanisms which produce more photons) at the same time. The ability to produce single photons of a given frequency and random phase is relatively easy; producing single frequencies and single phases is much more difficult.
One model for secure communication uses quantum cryptography to exchange a key that is actually pairs of entangled photons. In rough terms, you have a source that generates entangled photon pairs, and you keep one and send the other whoever you're trying to communicate with. You use this stream of photons to generate cryptographic keys, with the added quantum bonus that you can detect whether someone else has intercepted the key exchange (because, if so, the entanglement will be broken so the correlations between the two sets of photons will be "wrong").
For this to work, you need a way to reliably generate single photons or single photon pairs, and a way to transmit these photons without them losing their entanglement. This paper helps address the first part, by generating single photons on demand. Better yet, they generate 'indistinguishable' photons, which is necessary because the objective is to interfere two photons with each other to generate entangled pairs.
The figure I had quoted to me was that you have a 50% chance of seeing a single photon in an otherwise completely dark room, unfortunately the guy who said it was a grad student and can't be cited. Here's a link, though. http://www.accessexcellence.org/AE/AEC/CC/vision_background.html paragraph ten.
Because of the Heisenberg Uncertainty relations between photon number and phase, being able to produce exactly one photon (which is what they do in this case) means that the photons have and undefined phase. However, this is not important for the applications that they use. If you were to send such a photon into an interferometer, you would still see interference as the photon interferes with itself and a relative phase can develop between the two modes/arms of the interferometer. Almost all classical interference is just single photon interference scaled up. Here the challenge is to ensure that each of the successive photons emitted are indistinguishable. The photons must have the same wavelength, there wavefunctions the same shape, and there emission time must be short enough to make sure that dephasing mechanisms within the quantum dot do not affect when the photons are emitted (low jitter). With an ideal source of indistinguishable photons (which is very different from a laser source), it is possible to combine the photons and obtain higher order quantum interference effects where two different photons interfere with one another as opposed to just themselves. This effect can be used to create simple logic gates used in quantum computing.