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NICT, BBN, LSU Establish a Fundamental Rate-loss Tradeoff for Optical Quantum Cryptography

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October 29, 2014

The National Institute of Information and Communications Technology (NICT, President: Dr. Masao Sakauchi), in collaboration with Raytheon BBN Technologies (BBN) and Louisiana State University (LSU), have proven a fundamental tradeoff between the secret key generation rate and the channel loss for point-to-point optical quantum key distribution (QKD). QKD is a new emerging technology of cryptography which is provably secure against any possible deciphering attacks (the security does not rely on the eavesdropper’s computational powers and the protocol is even secure against any future technologies such as quantum computers). The key generation rate limit proved here suggests that there is still room to improve the key rate of currently existing QKD systems at least ten times while the fundamental rate-loss scaling limit will severely limit the future performance of the point-to-point QKD systems. The result provides key aspects on further developments of current QKD technologies as well as future technologies beyond point-to-point QKD such as quantum repeaters.
The result was published in Nature Communications on October 24, 2014.


Quantum cryptography, consisting of QKD and one-time-pad, is a new emerging technology of cryptography based on quantum mechanics which is proven to be secure against any possible deciphering attacks (the security does not rely on the eavesdropper’s computational powers and even secure against any future technologies such as quantum computers). Some of the QKD systems are now turning from science into practical technologies.
The secret key generation rate of the QKD system is determined by the photon pulse generation rate per second [pulses/second] and the key rate per pulse [keys/pulse], where the former depends on the device speed and the number of wavelength multiplexing while the latter depends on the protocol of QKD. Since the invention of the first QKD protocol, called BB84, various different QKD protocols have been proposed so far. However, it is recognized that the key rates of all known protocols decay exponentially with fiber distance.

[Click picture to enlarge]

The research team of NICT, BBN, and LSU has proven a new theorem in quantum information theory which states that the key rate attainable by any point-to-point QKD protocol through a lossy optical channel is fundamentally limited by a simple upper bound that decays exponentially with distance and is purely a function of the channel loss.
Also, it was demonstrated that there exists a gap between this limit and the key rate achieved by the current devices and protocol (See the right figure).

Future outlook

The result suggests that on one hand, the key rate of the current QKD system could be further improved (up to ten times). This could be done by pursuing a better protocol. Also another factor, the photon pulse generation rate, should be improved by the device technologies, such as developing better photon detectors or implementing wavelength division multiplexing.
On the other hand, in the future we will meet this fundamental limit in the secret generation rate of any point-to-point QKD systems. One possibility to overcome this limit is to install quantum repeaters instead of a point-to-point QKD. Although an operational demonstration of a quantum repeater is challenging, it should be conducted in a strategic way as a long-term research.


Journal:  Nature Communications (Nature Publishing Group), DOI: 10.1038/ncomms6235
Title:  Fundamental rate-loss tradeoff for optical quantum key distribution
Authors:  Masahiro Takeoka, Saikat Guha, and Mark M. Wilde

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Masahiro Takeoka
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Advanced ICT Research Institute
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