It could be difficult for the NSA to hack encrypted messages in the future -- at least if a technology being investigated by scientists at the Max Planck Institute for the Science of Light in Erlangen and the University Erlangen-Nrnberg is successful: quantum cryptography. The physicists are now laying the foundation to make this technique, which can already be used for the generation of secret keys, available for a wider range of applications. They are the first scientists to send a pulse of bright light in a particularly sensitive quantum state through 1.6 kilometres of air from the Max Planck Institute to a University building. This quantum state, which they call squeezed, was maintained, which is something many physicists thought to be impossible.

Using flashes of bright light for quantum communication through the atmosphere would have several advantages compared to the technique usually used today: it allows the photon packets to be transmitted in sunlight, something that is challenging with individual photons. Moreover, the receivers required for this are already presently in use for optical telecommunication via fibre optics and also via satellite.

Eavesdropping on a message protected by quantum cryptography cannot be done without being noticed. This is because quantum physics prevents a spy from reading a key which is encoded by specific quantum states without influencing these states. This can be exploited in a clever procedure for exchanging the key with which the data is encrypted, so that an unwelcome listener is not only detected, but is also prevented from accessing the information.

The quantum-protected communication is a fragile thing, however, and easily disturbed. All the more remarkable is the work of the Erlangen-based scientists working with Gerd Leuchs, Director at the Max Planck Institute for the Science of Light and professor at the University Erlangen-Nrnberg: "We have now succeeded in transmitting a flash of light, namely a pulse which contains many photons, through the atmosphere in a particularly sensitive quantum state," says Christian Peuntinger, who played an important role in the project. He and his colleagues sent a photon packet in a straight line from the roof of the Max Planck Institute in Nuremberg to the building of the University Erlangen-Nrnberg some 1.6 kilometres away. "This even works in broad daylight," says Christian Peuntinger.

The Sun disturbs quantum communication with individual photons

Quantum communication and quantum cryptography have hitherto relied mainly on individual photons being the information carriers. Physicists have already carried out many experiments in which they have sent them through the air as well. The individual photons are only easy to detect in the dark, however, because they are masked by stray light, which abounds on a sunny day -- just as an individual voice can hardly be heard in a noisy pub, especially if the conversation is to take place from one end of the room to the other. But just how useful is data traffic which has to stop when the Sun is shining?

If intense flashes of light transport the information, on the other hand, the communication is also possible in bright sunlight, as the special receivers used for this are not sensitive to stray light, unlike detectors for individual photons. This is not the only advantage that the receivers for the bright pulses offer, however. They are also much faster than the detectors for individual photons and thus allow higher transmission rates. And what makes them very attractive for the researchers in Erlangen: the devices needed are already widespread in optical communication via fibre-optic cable and even orbit Earth on board telecommunication satellites.

Until now, individual photons did seem to have one advantage over flashes of bright light, however. Individual photons can also be lost as they travel through the atmosphere; but if they reach their destination, they arrive in one piece and are unchanged. The attenuation in the atmosphere reduces only the data rate.

Communication with particularly sensitive, squeezed states

Quantum communication which uses flashes of many photons is thus not without its own difficulties: the flashes are suitable for quantum communication only if they exist in sensitive states. These are easily destroyed if a flash of light passes air turbulence and is deformed or weakened. "This is the reason why quantum physicists have not yet even tried to send such signals through the atmosphere," says Christoph Marquardt, Leader of the Quantum Information Processing Group at the Erlangen-based Max Planck Institute. His team has now proved that these pulses are indeed ideal for quantum communication through the air: "We have even used pulses in squeezed quantum states which are particularly sensitive."

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Squeezed quantum communication: Flashes of light in quantum states transmitted through atmosphere