ACM TechNews

Enigma Variations

Economist (07/10/08) Vol. 388, No. 8588, P. 88

The development of a photon detector by Andrew Shields and colleagues at Toshiba's research laboratory in Cambridge, England, is viewed as an important step in the enablement of practical quantum cryptography, which promises unbreakable codes for messages. The device can count single photons at room temperature, and represents a simple tweaking of the design for avalanche photodiodes that are being used to detect multiple photons, which should ease implementation. In an avalanche photodiode, the striking of a semiconductor by photons can be read by detecting positively charged "holes" in the crystal lattice left by the displacement of electrons caused by the photonic impact, but determining the number of photons that have arrived requires analysis of the signal just after it has been formed. Shields has tackled this challenge through a technique that filters out noise and allows the signal to be extracted. Without a practical photon counter, photon repeaters that do not destroy quantum states cannot be constructed. Shields' device allows cryptographers to harness the phenomenon of quantum entanglement, in which photons share quantum states, to support this breakthrough.

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