Researchers shed light on photons

Back in 1956, three Americans – William Shockley, John Bardeen and Walter Brattain – won a Nobel prize in physics for a discovery which led to the dawn of the information age. Back when they discovered the transistor effect, they had absolutely no idea their discovery would change the world. In fact, at the time, they viewed the effect as an interesting physics phenomenon. There are hundreds of physicists around the world today who are working on the cutting edge. Some of the research will undoubtedly end in dusty archives, while other findings may be a source of inspiration for future technology which will change the way we live.

Aephraim Steinberg, a University of Toronto physicist, teamed with fellow researchers Kevin Resch and Jeff Lundeen, is working on a photon optical switch which could play a role in the future of quantum computing. But like all cutting-edge research, it could also amount to nothing.

“Like most pure research, you don’t know if it is going to have any application,” he said in explaining his work.

As most readers know, the reason today’s transistors work is because electrons interact. Were it not for this, there would be no transistors, no chips, no supercomputers. But in the world of quantum computing, (see “Scientists explore the next frontier”, ComputerWorld Canada May 4, 2001, pg.6), excessive interaction is a death knell. The quantum computer environment has to be extremely isolated to keep itself in a quantum state.

The Photon Switch

The basic idea behind a photon switch (please see accompanying diagram) is to have one photon interfere with another, thereby preventing it from passing through a piece of crystal. In essence, turning it off. If the photon passes unhindered through the crystal, it would be deemed to be on.

In theory, not a huge a problem. But there is one caveat. Photons don’t like to interact with each other. They are the loners of the atomic world.

If you shine a light on someone, and then shine a second light, the person still looks the same, Steinberg explained. This is linear optics – the photons are essentially independent and don’t care about one another. “That is what is not good enough for computation. We want something where, when I add a second photon, it knows that the first one is there,” he said.

In non-linear optics, the area in which Steinberg is working, the photons do interact, however imperceptibly. Light has inherent frequencies – red being about half that of blue. Two red photons can collide and form one blue one. This non-linear attribute is usually only noticeable when you have billions and billions of photons streaming along at a time.

But photons can easily be scattered due to a speck of dust or some defect in the glass. Unlike in today’s computing, where a lost electron amounts to nothing, in the quantum world it is disastrous.

“One photon being scattered away destroys the whole operation,” Steinberg explained. Essentially everything has to be accounted for.

So Steinberg and his fellow researchers are working to narrow down the non-linear effect to an action between just two photons. They use a normal laser, and with a series of filters can essentially reduce the photons down to one. They are also achieving this effect by doing a little cheating. Photons, though independent, are kind of like kids in high school. They like being in the same state as those photons around them.

“By shooting the blue beam (2w in the diagrams) into the back of the crystal, we are telling the photons: here is a state you might want to be in.” Essentially, follow the leader.

“We have shown that in the right circumstances we can actually get two photons to collide with each other,” he said. “So basically I can use one beam to shut off another beam,” he said.

Though potentially decades off, this is one of the first steps to building something as influential to future technological development as the transistor turned out to be.