Artificial vacuums unleash speed of quantum computing

Lynn Savage, lynn.savage@laurin.com

Instilling photonic crystals with the occasional quantum dot triggers a vacuumlike effect that alters light in such a way that may make ultrafast optical computing possible.

Sajeev John of the University of Toronto and his student Xun Ma were investigating the mechanisms of optical switching using photonic crystals, part of an effort to develop an all-optical transistor that operates within a photonic chip as an electronic transistor acts in a computer chip.

According to Ma, he and John sculpted an artificial vacuum inside a three-dimensional photonic bandgap material, or photonic crystal, and embedded it with artificial atoms (quantum dots) inside the vacuum. Firing picosecond laser pulses on the quantum dots enabled them to control the electronic state of the artificial atoms. Each pulse raised the quantum dots out of their ground state, and a train of pulses kept them there as long as the pulses were tuned just below the atomic resonance. A stream of laser pulses just above the resonance dropped the quantum dots back to their ground state.

“We designed a vacuum in which light passes through circuit paths that are one one-hundredth of the thickness of a human hair, and whose character changes drastically and abruptly with the wavelength of the light,” John said. “A vacuum experienced by light is not completely empty and can be made even emptier. It’s not the traditional understanding of a vacuum.”

Ma added that the color-coded laser pulses sequentially excite and de-excite the quantum dots in trillionths of a second. The particles, in turn, can control other streams of optical pulses, enabling optical information processing and computing.

The ultrafast laser pulses also carry and impart very little energy – on the order of femtojoules – meaning that any optical switches made with the technique would not be subject to the overheating experienced by common electronics.

“This new mechanism enables micrometer-scale integrated all-optical transistors to perform logic operations over multiple frequency channels in trillionths of a second at microwatt power levels, which are about one-millionth of the power required by a household lightbulb,” John said. “That this mechanism allows for computing over many wavelengths, as opposed to electronic circuits which use only one channel, would significantly surpass the performance of current-day electronic transistors.”

The research is reported in the Dec. 4, 2009, issue of Physical Review Letters.