Photons for quantum computing
GAITHERSBURG, Md. – 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
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
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.
- quantum mechanics
- The science of all complex elements of atomic and molecular spectra, and the interaction of radiation and matter.
MORE FROM PHOTONICS MEDIA