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.