get the professors to look at this john

Speed-of-light computing comes a step closer
18 July 2007
NewScientist.com news service
Saswato Das
Advertisement Computers that operate at the speed of light have come a step closer. Researchers have devised a light-based transistor made of semiconducting nanowires that could be a key building block of machines that are hundreds of times faster than today's supercomputers.

Until now, optical transistors, in which one beam of light controls the state of another, have required large bursts of photons to switch states, making them unfeasibly power-hungry. Now Mikhail Lukin and colleagues at Harvard University have come up with a technique that uses a single photon to switch the state of a light beam. This is the first workable suggestion for building an optical computer, they say.

An electrical transistor's speed is limited by the speed at which an electric current flows. In theory, because photons travel much faster than an electric current, substituting photons for electrons would speed things up. In reality, however, finding the optical equivalent of a transistor has proved difficult.

Like an electric current, light can be pulsed on or off. The difficulty arises in controlling the switch between these two states. Unlike electrons, whose flow can be controlled by an electric field, photons are electrically neutral and do not normally interact with each other. This makes it hard to use one light beam to control another.

A possible way to get light beams to interact is via a rippling "sea" of loose electrons known as a surface plasmon, which forms when light is shone between a metal and a non-conductor. The plasmon changes according to the intensity of the light, which in turn affects the intensity of the light emerging from the plasmon. If two light beams are shone at a surface plasmon at the same time, the first, known as the control beam, will change the plasmon, which will then change the intensity of the other light source, known as the signal beam.

Last year, Anatoly Zayats and colleagues at Queen's University Belfast in the UK used the technique to build an optical-computing component. However, a burst of photons was required to control the signal beam, making the device power-hungry. Now Lukin has proposed an approach that is more efficient.

While Zayats shone light from two directions on a polymer-coated gold film to create surface plasmons, Lukin suggests using a semiconducting nanowire that resembles a miniature optical fibre, and sending both beams down it. Because the nanowire confines the rippling plasmon to a smaller space than the gold film, it should be more sensitive to changes in the intensity of the control beam. Lukin says that this increased sensitivity is enough to allow just a single photon to switch the state of the signal beam.

Zayats agrees that this approach is "much more subtle and practical". John Howell, a physicist at the University of Rochester in New York, calls it "one of the best proposals I have seen" for an optical computer. Co-author Darrick Chang says the team has started testing the idea.

Related Articles
Optical amplifier to light-up super-fast computers
http://www.newscientisttechnology.com/article/dn9719
11 August 2006