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Atom cavity sees the same photon twice

Second sound detection
A view into one of the two vacuum chambers used in the experiments. Credit: Max-Planck-Institute of Quantum Optics

For the first time, physicists have succeeded in measuring the same photon at two different locations within an optical fibre – all without destroying the photon. The new non-destructive technique, which was developed by researchers at the Max Planck Institute of Quantum Optics (MPQ) in Germany, is based on the principles of cavity quantum electrodynamics and could aid the development of quantum communications networks that rely on information-carrying photons.

Although researchers are generally able to detect itinerant photons, the detectors they use invariably destroy the photons being measured. Alternative, non-destructive quantum measurements have important applications in many areas of physics, including quantum sensing, quantum computing and quantum communications.

Quantum non-demolition detector

A team led by Stephan Welte and Emanuele Distante has now developed a “quantum non-demolition” (QND) detector to address this problem. This QND detector consists of a single rubidium atom that has been prepared in a known quantum state that is coupled to a reflective optical cavity. The researchers placed two of these detectors 60 metres apart in an optical fibre. They then used small lengths of additional fibre to connect the detectors to the main fibre, placing “circulators” at the fibre intersections to direct the flow of a beam of laser photons that they sent into the fibre. As a photon enters a circulator, it gets directed towards a detector before being reflected from it and guided back along the main fibre in its original direction.

Improved time resolution

The MPQ team now plan to improve the time resolution of their detection process. This will allow them to more precisely determine the direction in which the measured photon is travelling – information that is only accessible with QND detectors.

They also hope to improve their system so that fewer photons are lost between the two detectors. “Such a non-destructive system could be employed to herald photon loss in a glass fibre,” Welte says. “Once the photon loss has been detected, a given protocol could be stopped and restarted immediately by sending in another new photon,” he tells Physics World. “This way, the rate of the protocol could be increased.”

The research is detailed in Physical Review Letters.

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