Researchers at the Max Planck Institute of Quantum Optics have developed a detection method that can be used to track quantum transmissions. Quantum information is sent over long distances in the form of photons which are quickly lost. Finding out after only a partial distance whether such a photon is still on its way to its destination or has already been lost, can significantly reduce the effort required for information processing.
Because most photons are lost in a transmission over around 100 km, thousands of photons would have to be transmitted in order to directly transmit only a single qubit over this distance. The transmission of quantum information can thus become a lengthy affair, even though light travels very fast and can cover the distance from Munich to Berlin (around 600 km) in only about two milliseconds.
The team has developed a physical protocol that can indicate whether the qubit has gone already lost at intermediate stations of the quantum transmission. The detector detects whether the photon is there or not but does not read the quantum information encoded into it.
In order to detect a photon carrying quantum information without reading the message itself, the physicists work with an atom that they trap in two perpendicular resonators. The two resonators each consist of two mirrors so that the atom is surrounded by four mirrors arranged in a cross. One of the resonators is designed in such a way that the atom recognizes the presence of the photon by an extremely gentle touch: The resonator is located at the end of an optical fiber through which a photon reaches it—or not. When the photon arrives there, it is reflected and changes the state of the atom. What is important here is that the quantum information remains unaffected by this process.
The photon influences the state of the atom. In the process, the atomic spin is changed. In contrast, the quantum information is packed into the oscillation (polarization) plane of the photon.
The job of the second resonator is to detect whether the photon was there and changed the state of the atom or not. If no photon arrives at the detector at the expected time, one can make the atom glow by irradiating it with laser light. This glow can be easily detected via the second pair of mirrors and with a classical photodetector. If a photon is reflected at the other resonator, changing the state of the atom, this does not work, and the atom remains dark.
The paper has been published in Nature.
