Vol. 332 no. 6035 p. 1243
For certain critical transactions and communications, you want to be secure in the knowledge that your message cannot be stolen or compromised by a malicious hacker. Encrypting messages with keys distributed beforehand to all interested parties is the usual method to ensure security. For ultimate or unconditional security, however, one key per message or transaction is allowed, after which the key is discarded. This “one time pad” requirement can place a hefty overhead on distributing the keys and would not be particularly practical for everyday use. In quantum key distribution (QKD), the encryption keys are made up of a series of quantum bits, single photons of light, for instance, with the orthogonal polarization states encoding a logical 1 or 0. Because the bits are quantum mechanical in nature, any attempt by an eaves-dropper to measure them would compromise that effort by a telltale sign. Sasaki et al. have now demonstrated the feasibility of quantum key distribution over an optical network in and around the metropolitan Tokyo area. Meshing together six separate QKD systems, they achieve secure video conferencing, encrypted with quantum keys, over a distance of 45 km. Stable operation and interfacing to the mobile telephone network widens the possible applications of quantum security.