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

Arrived safe and sound: The team working with Christoph Marquardt and Gerd Leuchs transmitted pulses with an elliptical, horizontal contour (right) through the air. In the illustration, the squeezed form of the quantum state registered at the receiver can be clearly seen. The short transverse and long longitudinal axis here represent the spread of two properties which are linked to each other via Heisenberg’s uncertainty principle. The property which is represented on the short axis assumes values only within a narrow range. The property shown on the long longitudinal axis has a larger spread of measured values. The transitions between the two extremes result in the elliptical shape. The corresponding representation of conventional, non-squeezed states has a circular contour.
Credit: MPI for the Science of Light

Scientists have sent a pulse of bright light in a particularly sensitive quantum state through 1.6 kilometers of air. This quantum state, which they call squeezed, was maintained, which is something many physicists thought to be impossible. Eavesdropping on a message protected by quantum cryptography cannot be done without being noticed.

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-Nürnberg 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-Nürnberg: “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-Nürnberg some 1.6 kilometres away. “This even works in broad daylight,” says Christian Peuntinger.

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