Electrical charge is the key to modern computing. We can generate, detect, and control current with accuracy and precision. As we contemplate a world of quantum computing, it’s important to note researchers have not really coalesced around an equivalent basic architecture. Each architecture we’ve looked at has a set of disadvantages that are sufficient to keep all the other architectures alive despite their problems.
This is true for optical quantum computers. They require single photons, and sources that generate single photons kind of… well, suck. Ideally, what we want is a device that generates single photons on demand. To put not too fine a point on it, at the press of a button, we want an electrical current to reliably generate a single photon. And, while we haven’t had it, it’s exactly what a group of researchers from Germany is now claiming to have accomplished.
Single photons: Accidentally on purpose
There are two basic ways to make single photon sources: accidentally and on purpose. The accidental method is to use a parametric amplifier, which takes a light source of one color and uses it to generate light of another color. If the amplifier is set up correctly, then each incoming light pulse has sufficient power to generate just a single pair of photons, which can then be separated by polarization. The single photon production is accidental in the sense that the process is subject to a lot of random noise. Mostly you get one photon, often you get no photons, and sometimes you get two or more photons. The saving grace of the whole thing is that you can use one of the photon pair to announce that the other one is a single photon with which you can perform a calculation.
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Source: Ars Technica – We demand single photons; carbon nanotube delivers