Quantum key distribution with single photons

Lasers create plenty of useful photons, but for
maximum security, what is needed is exactly one
German researchers have improved a method to make secure codes called quantum key distribution (QKD) by using the smallest possible packets of light.

The approach's security exploits the fact that quantum systems, once observed, are irrevocably changed.

QKD is not a means of creating codes, or ciphers, but rather a way to share them securely.

But to exploit the fullest security allowed by the peculiar rules of quantum mechanics, each "bit" of information in the key should be encoded onto exactly one photon, sent through air or down a fibre.

Yet it is surprisingly difficult to reliably produce exactly one photon. Many systems that squeeze out tiny amounts of light do so sometimes with one photon, and other times with two or more.

Most often, researchers have used laser sources, filtering down their trillions of output photons in a bid to allow just one to pass.

Each extra photon represents a security risk, as it can be intercepted en route and the receiver would never know.

The solution, according to Sven Hoefling of the University of Wuerzburg, Germany, and colleagues is to use what are called quantum dots to create single photons.

Quantum dots are tiny structures made of semiconductors whose size determines, again through rules set by quantum mechanics, exactly what colour and amount of light they produce - and how often they produce it.

This combination of assured single-photon emission at high rates driven by power no different from a battery, Dr Hoefling said, brings QKD much closer to commercial application.

The work is outlined in the New Journal of Physics.

A great deal of work has gone into QKD in recent years, and a number of companies even offer off-the-shelf systems that promise quantum security - providing you can install a fibre-optic link between the sender and receiver.

It has even spawned a group of quantum hackers at the Norwegian University of Science and Technology, dedicated to finding ways to beat such systems.

In 2007, the Swiss government even secured its election results with QKD technology.



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