Quantum communication: making two from one

An electron involved in quantum tunnelling generates two photons much more frequently than theoretically predicted

In the future, quantum physics could become the guarantor of secure information technology. To achieve this, individual particles of light – photons – are used for secure transmission of data. Findings by physicists from the Max Planck Institute for Solid State Research could play a key role. The researchers accidentally came across a light source that generates a photon pair from the energy of an electron. One of these particles of light has the potential to serve as a carrier of the fragile quantum information, the other, as a messenger to provide prior notification of its twin.

In contrast to quantum communication, a cook has the luxury of being able to look if all the ingredients he or she needs for a recipe are in the cupboard. After all, flour doesn’t go bad the moment you glance at it. A physicist trying to test whether a procedure to transmit quantum information has worked as planned is in a much trickier position. Quantum objects change their state when they are observed, i.e. measured. In quantum communication, this makes it difficult to control the information transmitted by photons. But that’s the critically important point. Every contact with the environment can destroy the quantum information transported by photons, and in addition, sources of individual light particles often generate single photons only very irregularly. Just how do you guarantee a photon is on its way without measuring it? Pairs of photons are the solution. One photon might be able to serve as a messenger for its twin.

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