Optical logic on a chip

Researchers at Stanford and the University of California at Santa Barbara have demonstrated an optical logic gate on a semiconductor chip, possibly leading the way towards a practical device for use in quantum computing.

The logic gate provides an optical output whose condition is determined by the state of two inputs, which in this case are two photons of light. One photon arrives from a signal beam and the other from a control beam, interacting with each other in a quantum dot.

A quantum dot is a small semiconductor, typically a nanometre in size, which traps the charged particles produced by the absorption of a photon, in three dimensions. The small cavity size results in some very interesting optical effects, which are not seen in the bulk material.

The logic gate is fabricated on a semiconductor chip, comprising a quantum dot formed by a ball of indium arsenide molecules, inside a microcavity formed from a photonic crystal. The crystal is made up of a periodic array of small holes and serves the purpose of trapping the photons to ensure they interact with the quantum dot.

With the control beam turned off, the signal photon will be re-emitted by the quantum dot unchanged. By turning on the control beam, the interaction of the signal and control photons in the quantum dot causes the signal photon to remain in the cavity for longer, which results in a re-emitted photon with a phase delay. This optical effect can be detected by measuring the polarisation of the photon and provides the output of the optical gate. Much like the ubiquitous semiconductor chip in our everyday gadgets, in which electronic ones and zeros do the calculations, optical gates like this could perform information processing in the very same way.

Although this is not the first optical logic device, it may be the first demonstration of a practical one, because it uses semiconductor fabrication technology that could be integrated into existing manufacturing processes. It may also lead the way towards a working quantum computer, in which photons would interact and make calculations using quantum mechanical phenomena. Such a machine would herald a new age in information processing, with speeds far in excess of today's most powerful computers.

References: "Controlled Phase Shifts With a Single Quantum Dot", Ilya Fushman, Dirk Englund, Andrei Faraon, Nick Stoltz, Pierre Petroff, Jelena Vuckovic, Science 9 May 2008: Vol. 320. no. 5877, pp. 769 - 772. DOI: 10.1126/science.1154643