In the last part of this series, we saw that any classical cryptography approach can, with enough computing power, be broken.   Quantum key distribution (QKD), on the other hand, can guarantee unconditionally secure information -- try as she mi ght, eavesdropper Eve  Bob without disturbing it and giving away her evil intentions. In this installment, we will look at some current implementation of quantum key distribution. 

Probably the most common form of quantum key distribution is BB84, which is poorly encrypted code for Brassard and Bennet who invented the scheme in 1984. At that time, the idea of quantum cryptography seemed outlandish and highly abstract.  Apple had just introduced its Macintosh computer; but thinking back, its slogan "see why 1984 won't be like 1984" describes BB84 even more aptly. Indeed, BB84 may be the first time that people saw that quantum mechanics could actually be used to do something 'useful' (this was before the discovery of high-temperature superconductivity).

Quantum cryptography has come a long way since, as the list of companies now offering QKD systems shows.  The first pioneering companies were little startups; now, big companies such as NEC and IBM are joining the fray. There are three main drivers for the rapid growth of QKD suppliers.

One contributor is that classical cryptography is increasingly viewed as insecure as computers are becoming powerful enough to crack messages.  High-value data links, such as between financial institutions or military and government buildings, are therefore being upgraded to QKD systems.  In the 2007 Swiss elections, the data link to Geneva's ballot data-entry was a quantum cryptography system.  The system was provided by the company id Quantique, a spin-off from the University of Geneva. Critics will say that RSA is practically extremely secure if a large enough key is used -- for example, 2048-bit-long keys should remain untouchable for quite a number of years.  Unless, that is, a useful quantum computer is developed. Some companies such as D-wave of Canada say will happen soon, and this thought would no doubt worry a great number of people and organizations. It would also make the QKD providers instant celebrities.

A second driver of the surge in QKD suppliers is certainly technology.  Any new technology takes a while to mature. QKD has in fact matured remarkably fast, going from an abstract idea in the early 90s to real products around the turn of the millennium.

One of the main difficulties was extending communication distance.  A photon can only fly so far before it encounters enough matter to be absorbed.  Probably the best way to send a photon a long distance is through an optical fiber. Silica glass, which has been perfected with high purity for telecommunications, has a minimum of absorption near a wavelength of 1500 nanometers, in the infrared part of the spectrum. Under these conditions, photons can travel about 100-200 km. Unfortunately, the sources and especially single-photon detectors that are required in QKD are more challenging than for higher-energy photons.  Detectors in particular suffer from a high rate of false photon detection events.

Recently, a group of researchers led by Yoshihisa Yamamoto, Professor at Stanoford and the National Institute of Informatics in Tokyo, achieved a record-long QKD implementation over 200km of optical fiber [1]. Key to their success were superconducting single-photon detectors based on NbN nanowires.  These have high timing resolution (60 ps).  Their false detection rate is very low, below that of the InGaAs avalanche photodetectors used by id Quantique and others. The data rate was 12 bits/s at 200km -- not exactly broad-band yet, but maybe good enough for crucial messages. Both approaches use attenuated laser beams; however, single photons would be more secure[2,3].

To reach greater range and faster data transmission, a way to amplify the quantum signal is needed. In classical optical networks, such amplification is commonplace.  However, quantum mechanics prohibits duplication of quantum states, and standard amplification is not possible. Instead, a quantum repeater is needed. Such a device would boost the strength of the quantum state without measuring or duplicating it.  Many researchers are currently pursuing a myriad of approaches in this direction. In the next installment of this series, we will consider some of the most promising ones. 

[1] Takesue et al. Nature Photonics 1, 343 - 348 (2007)

[2] Waks et al. , Nature 420, 762 (2002)

[3] Controlling the Spontaneous Emission Rate of Single Quantum Dots in a 2D Photonic Crystal, Dirk Englund, David Fattal, Edo Waks, Glenn Solomon, Bingyang Zhang, Toshihiro Nakaoka, Yasuhiko Arakawa, Yoshihisa Yamamoto, and Jelena Vuckovic, Physical Review Letters vol. 95, article 013904 (2005)

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