Encryption roundupQuantum keys sent over 200-km fiber-optic link

The latest rage in encryption technology is called “quantum encryption.” In an experiment conducted at a Stanford lab, particles of light serving as “quantum keys” have been sent over a record-setting 200-kilometer fiber-optic link. Researchers from the National Institute of Standards and Technology (NIST), NTT Corp. in Japan, and Stanford University used mostly standard components and transmitting at telecommunications frequencies. The experiment opens the way for making practical inter-city terrestrial quantum communications networks as well as long-range wireless systems using communication satellites.

The demonstration is described in the current issue of Nature Photonics (sub. req.). It was conducted with optical fiber wrapped around a spool. In addition to setting a distance record for quantum key distribution (QKD), it also is the first gigabit-rate experiment — transmitting at 10 billion light pulses per second — to produce secure keys. The rate of processed key production, that is, the keys corrected for errors and enhanced for privacy, was much lower owing to the long distance involved, and the key was not used to encrypt a digital message as it would be in a complete QKD system. QKD systems transmit a stream of single photons with their electric fields in different orientations to represent 1s and 0s, which are used to make quantum keys to encrypt and decrypt messages. In theory, and if properly executed, quantum encryption is unbreakable because eavesdropping changes the state of the photons.

For the technically inclined: An important aspect of the experiment was the use of ultrafast superconducting single-photon detectors developed in Russia, with packaging and cooling technology custom-made at NIST labs in Boulder, Colorado. Photons are the smallest particles of light, and counting single photons rapidly and reliably has always been a major challenge which, so far, has limited the development of practical QKD systems. The Russian detectors help in this regard: They have very low false count rates because of their low-noise cryogenic operation, and they also have a superior timing resolution owing to the physics of superconductors, in which electrons can switch from excited to relaxed states in trillionths of a second. Each detector consists of a superconducting niobium nitride nanowire operating just below the critical current at which it conducts electricity without resistance. Now, when a single photon hits the wire, a hot spot is formed and the current density increases until it exceeds the critical current. It is at this point that a non-superconducting barrier forms across the wire and a voltage pulse is created, and voilà: The starting edge of the voltage pulse pinpoints the photon’s arrival time.

-read about the Stanford Lab experiment in Hiroki Takesue et al., “Quantum Key Distribution over a 40-dB Channel Loss Using Superconducting Single-photon Detectors,” Nature Photonics 1 (1 June 2007): 343-48 (sub. req.); see also P. Shankar Rao and J. Aditya, “Quantum Cryptography” (Stanford University, 11 January 2007)