Physicists from the University of Toronto say they have taken a step toward making a logic gate using single photons, something they suggest could be an essential building block of quantum computers.
While photons are said to have many advantages in quantum computing, the challenge is to get them to interact. "We've seen the effect of a single particle of light on another optical beam," said Aephraim Steinberg, a researcher at the University of Toronto's Centre for Quantum Information and Quantum Computing. "Normally, light beams pass through each other with no effect at all. To build technologies like optical quantum computers, you want your beams to talk to one another. That's never been done before using a single photon."
In their work, the researchers shot one photon at rubidium atoms cooled almost to absolute zero. The photon became 'entangled' with the atoms, which then affected the way in which the rubidium interacted with a separate optical beam. Because the photon changes the atoms' refractive index, there is a small, but measurable, 'phase shift' in the beam.
This process, says the team, could be used as an all optical quantum logic gate, allowing for inputs, information processing and outputs.
"Quantum logic gates are the most obvious application of this advance," said Steinberg. "But being able to see these interactions is the starting page of an entirely new field of optics. Most of what light does is so well understood that you wouldn't think of it as a field of modern research. But two big exceptions are 'what happens when you deal with light one particle at a time?' and 'what happens when there are media like our cold atoms that allow different light beams to interact with each other?'"
Both questions have been studied, he saids, but never together until now.
