Another next step towards an all-optical computer has been taken with the development of a prototype optical memory chip. Previously it was thought that light could be not be trapped for long enough to store data. Now Achim Wixforth and colleagues of the Technical University of Munich and the Lugwig- Maximilians University in Munich have shown that photons can be stored by converting them into electron-hole pairs in a semiconductor. The electron-hole pairs can be trapped in a quantum well. To read the data the electron and hole are allowed to "recombine" and produce a photon (Science 283 1292).
The pairs are created by illuminating a semiconductor quantum well with a pulsed laser diode. A slight voltage difference on the semiconductor keeps the electron and the hole in separate levels inside the quantum well. When the voltage is switched off, the electron-hole pair move together under Coulomb attraction and radiatively recombine – producing a photon. So far the researchers have been able to ‘store’ a photon for 35 microseconds – five times longer than its natural lifetime and long enough for such a memory chip to work in an optical computer.
