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Optical boffins cut the cost of quantum cryptography

Time shift dances the light fantastic

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Boffins at the National Institute of Standards and Technology are trialling advanced optical techniques aimed at reducing the price of quantum cryptography systems.

The new quantum key distribution approach reduces the required number of single photon detectors, which can cost anything between $5K-$20K and are the most costly component of quantum cryptography systems.

Quantum cryptography allows two users on an optical fibre network to exchange secret keys. Each bit of the key is encoded upon a single light particle (or 'photon'). Intercepting this data randomly changes the polarization of the light, irreversibly altering the data.

Because of this quantum mechanics effect any attempt by an eavesdropper to determine a key corrupts the same key with noise. Quantum cryptography systems discard these corrupt keys and only use codes that are known to be secure. These quantum keys, once exchanged, can be used in a one-time pad.

Conventional cryptography setups involve at least two photon detectors, and more commonly four. By adding an optical component that delays the travel of photons to the detector, NIST researchers have been able to halve the number of detectors needed.

The most common polarization-based protocol, BB84, uses four single-photon detectors. One pair of detectors looks for photons with either vertical or horizontal polarization, which signify either 0 or 1. The other set of detectors record diagonally polarized photons.

NIST researchers have reduced the number of detectors required by using an optical component to make these diagonally polarized photons rotate by a further 45 degrees and arrive at the horizontal/vertical detector, but slightly later than photons that started off with this orientation. As such the approach is a type of time division multiplexing.

The same technique was used to number of detectors needed to implement the B92 protocol from two to one.

The approach cuts transmission rates by half but the advantage of lower costs far outweighs this minor disadvantage. The NIST team, led by researcher Xiao Tang, reckon the approach avoids introducing noise and might actually be more secure than conventional approaches.

NIST researchers intend to explain their detection time bin shift (DTBS) scheme in a paper due to be published by the IEEE Communications Letters next month.

In additional research carried out after the publication of this paper, the NIST team has further developed its approach so that the popular BB84 protocol requires only one detector instead of four.

A summary of the research, including diagrams, can be found on the NIST website. ®

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