FOR the first time, quantum cryptography can create unbreakable encryption fast enough to protect applications as data-hungry as video streaming. The 鈥渜uantum keys鈥� can be generated at the rate of 1 million bits per second 鈥� 100 times faster than anything achieved before.
Quantum cryptography uses the laws of quantum mechanics to provide a digital key, a series of 0s and 1s, that can be used to make another data stream unrecognisable. Only someone with access to the same key can 鈥渦nlock鈥� the data.
Researchers at the National Institute for Standards and Technology in Boulder, Colorado (NIST) created their quantum key using an infrared laser. This generates photons that encode a 0 or a 1 by giving each photon a particular polarisation. The photons can be polarised at one of four angles: +45掳, 鈭�45掳, 0掳 or 90掳. For example, a photon polarised at 0掳 might represent 1, while a photon polarised at 90掳 would represent a 0.
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To transmit the key, the sender, Alice, fires the polarised photons at the receiver, Bob, who has two kinds of polarisation detectors. He measures the polarisation of the photons he receives by selecting his detectors at random (see Diagram). The result he gets depends on the polarisation of the incoming photon, and the polarisation of his detector.
To work out which bits to accept or ignore, Bob then talks to Alice across an open channel, such as a phone line, and they agree on which photons they can use to construct their key. The method is very secure because any eavesdropper will betray their presence by affecting the statistics of the match between sent and received photons.
The NIST researchers fired their photons 730 metres through the air across the NIST site, with the laser and detector both linked to a high-precision clock. This lets them put a 鈥渢ime-stamp鈥� on each photon they send, allowing the detector to ignore all photons other than the ones arriving at a pre-determined time. It is this time-stamping that gives NIST the edge, because the ability to look for the photons at precisely specified times maximises the rate at which they can successfully detect the photons that form part of their key.
Another advance is the group鈥檚 use of high-speed data-processing equipment (1 billion bits per second). Almost every component of the system is commercially available, and relatively cheap. 鈥淥ur equipment is off-the-shelf,鈥� says Joshua Bienfang, a physicist on the NIST team. 鈥淭he detectors, which are about $3000, are the most expensive thing.鈥�
He believes that integrating proven technology will help speed the development of quantum technology. 鈥淲e will see more and more of this,鈥� he says.
It may be that quantum video streaming never takes off because it might be considered overkill for many purposes. But the US Defense Advanced Research Projects Agency, which co-sponsored the work, could be interested in secure video images from a reconnaissance drone flying over a battlefield, for example. But for now the NIST researchers see their system as the first step towards establishing standard benchmarks for the speed and security of quantum cryptography systems.
