Researchers build an all-optical transistor

Optical computing — using light rather than electricity to perform calculations — could pay dividends for both conventional computers and quantum computers, largely hypothetical devices that could perform some types of computations exponentially fa
ster than classical computers. 
But optical computing requires light particles — photons — to modify each other’s behavior, something they’re naturally averse to doing: Two photons that collide in a vacuum simply pass through each other.

In the latest issue of the journal Science, researchers at MIT’s Research Laboratory of Electronics — together with colleagues at Harvard University and the Vienna University of Technology — describe the experimental realization of an optical switch that’s controlled by a single photon, allowing light to govern the transmission of light. As such, it’s the optical analog of a transistor, the fundamental component of a computing circuit.

Moreover, since the weird, counterintuitive effects of quantum physics are easier to see in individual particles than in clusters of particles, the ability to use a single photon to flip the switch could make it useful for quantum computing.

The heart of the switch is a pair of highly reflective mirrors. When the switch is on, an optical signal — a beam of light — can pass through both mirrors. When the switch is off, only about 20 percent of the light in the signal can get through.

The paired mirrors constitute what’s known as an optical resonator. “If you had just one mirror, all the light would come back,” explains Vladan Vuletić, the Lester Wolfe Professor of Physics at MIT, who led the new work. “When you have two mirrors, something very strange happens.”

Light can be thought of as particles — photons — but it can also be thought of as a wave — an electromagnetic field. Even though, on the particle description, photons are stopped by the first mirror, on the wave description, the electromagnetic field laps into the space between the mirrors. If the distance between the mirrors is precisely calibrated to the wavelength of the light, Vuletić explains, “Basically, a very large field builds up inside the cavity that cancels the field coming back and goes in the forward direction.” In other words, the mirrors become transparent to light of the right wavelength.