By Matthew Broersma
03 June 2008
Researchers at the US National Institute of Standards and Technology
(NIST) have demonstrated a technique that could make quantum
cryptography significantly cheaper to implement, moving it nearer to
possible commercial acceptance.
The technique, outlined in a paper to be published this month in the
journal IEEE Communications Letters, is aimed at cutting the cost of
equipment needed for quantum key distribution (QKD), designed to
distribute cryptographic keys using a secure system based on the
principles of quantum mechanics.
Such systems are typically based on the distribution of photons through
a fibre-optic network, with information encoded in the polarisation of
They are designed to allow an absolute level of security, wth any
attempt to monitor the system by a third party, for instance,
necessarily disrupting the system.
However, such systems are expensive to implement, with the most common
polarisation-based protocol, known as BB84, requiring four single-photon
detectors, costing $5,000 (2,500) to $20,000 (10,000) each, the NIST
The new method, called detector-time-bin-shift (DTBS), shifts photons
into two distinct "time bins".
This means a set of two detectors can be used to sequentially record the
two sets of photons, rather than requiring four detectors to
simultaneously record all the photons, the NIST said.
In their work on a protocol called B92, the researchers reduced the
required number of detectors from two to one, the NIST said.
Further work carried out since the completion of this month's paper
further reduced the number of photon detectors needed for the BB84
protocol from four to one, the NIST said.
The arrangement detailed in the paper cuts transmission rates by half,
but the NIST system still works at broadband speeds, it said. The
organisation said its experimental network can encrypt and decrypt
webcam-quality video streams in real time using DTBS.
In 2006 the NIST managed to shift quantum-encrypted information at a
"raw" throughput of 4 million bits per second across a 1 km-long fiber
This was at least twice NIST's previous record, which has been rising
since the agency announced it had broken the 1 million bits per second
barrier in May 2004. At such transfer rates, it becomes practical to use
QKD cryptography to secure a video stream.
In 2005 Toshiba Europe reached the demonstration phase of what it
claimed was the first system to use quantum cryptography to secure a
real-time video and voice data stream, developed by a 30-person team of
scientists working at the company's Cambridge Research Laboratory.
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