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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)."
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.