Photon Turnstile Improves Cryptography

In pursuit of more reliable quantum cryptography systems, a team at Stanford University in California and NTT Basic Research Laboratories in Atsugi, Japan, has demonstrated a system that incorporates a photon turnstile, which enables it to support 28-dB channel losses. Using an attenuated laser in place of the turnstile device, the system fails at a channel loss of 23 dB.

As reported in the Dec. 19/26 issue of Nature, the turnstile comprised a quantum dot in a micropost cavity optically excited by 3-ps pulses from a Ti:sapphire laser. To implement the BB84 quantum cryptography protocol, the photons from the turnstile passed through an electro-optic modulator with a 4-ns switching time that rotated the polarization into one of four states. Passive polarization and avalanche photodiodes detected the photons at the receiving end, one meter away.