Photons for quantum computing

A reliable source of photons for quantum computers could be a step closer to realization, thanks to scientists at the National Institute of Standards and Technology (NIST) who have found that calculations impossible or impractical to achieve with the conventional computers used today can be performed using quantum mechanics.

In two papers published in Optics Express on Jan. 14, 2011 (doi: 10.1364/OE.19.001484 and doi: 10.1364/OE.19.001470), the researchers described one of the many challenges faced in preparing a practical quantum computer. They identified the need for a device that can produce photons in ready quantities, but only one at a time, and only when the computer’s processor is ready to receive it. They found that, if a computer processor is not expecting or ready to receive a particle, the calculation can be ruined.

To combat the problem, the team addressed the need to be certain that a photon is in fact on the way when the processor expects it, and that none show up unpredictably. As detailed in their first paper, the researchers explained that many single-photon sources create a pair of photons, and that one of them is sent to a detector, which tips off the processor that the information-bearing photon is en route. Because of inaccuracy, the detector often misses the “herald” photon, which ends up entering the processor with the other photon, jamming the system.

A gated photon source starts with the bright-green 532-nm-wavelength laser beam that strikes a crystal (bright-green spot, center) and is converted into pairs of photons at 810 nm (false-colored blue here, at the end of the red spectrum) and 1550 nm (in the infrared, false-colored red here). The herald channel is the “blue” beam; the “red” beam goes through a spool of optical fiber, shown right, to delay it long enough for the gate to open or shut. Courtesty of G. Brida, INRIM.

The researchers teamed up with scientists from L’Istituto Nazionale di Ricerca Metrologica (INRIM) in Italy to build a simple gate for the source, which would eliminate the confusion. The device will open for the second photon to pass through when the herald photon reaches the detector. While the solution seems obvious and was proposed long ago, the collaborators were the first to build it.

In the NIST team’s second paper, the researchers described a photon source to address two other requirements. First, quantum computers require many sources working in parallel, so sources must be readily built and reliably operated. Second, the computer must be able to tell photons apart, so the source must be able to create multiple individual photons at differing wavelengths. To address these requirements, they concluded that the source should be made of silicon, the standard material used for creating computer chips. Their findings indicated that their design would allow photons to be produced at a number of regular and distinct wavelengths simultaneously, all from a single source.

Compatible with microfabrication techniques, their accomplishment is the first step in creating sources that are part of integrated circuits that could one day be more than just a laboratory prototype.