No one has previously made multiple nondestructive measurements on moving photons of any wavelength, Welte says. They observed correlated changes in the states of the atoms that indicated that the same photon interacted sequentially with each of the detectors. To run the experiment, the researchers initialized the atoms in a known quantum state and then sent laser photons down the fiber. A photon entering a circulator would be directed toward a detector and then, after reflection from the detector, would be directed back along the main fiber in its original direction. Using small lengths of additional fiber, the team connected the detectors to the long fiber and put devices called circulators at the T-shaped fiber intersections to direct the photon traffic. Welte and his colleagues placed two of their cavity detectors at locations 60 m apart along an optical fiber. The team was able to monitor the atom’s state by observing its effect on a subsequent laser pulse. The cavity was designed to reflect any incoming optical photon and to change the atom’s quantum state as a result of the photon-cavity interaction. The team’s nondestructive detector consisted of a single rubidium atom trapped inside an optical cavity. The new measurements could enable researchers to track photons as they move through a quantum communications network. Stephan Welte of the Max Planck Institute for Quantum Optics (MPQ) in Germany and his colleagues have now demonstrated two nondestructive measurements of a single moving photon. Researchers have also made multiple sequential detections of the same photon, but until now, those repeated measurements had only been made on photons that were stationary, existing as oscillating fields inside microwave cavities. For example, a team in Switzerland recently demonstrated nondestructive detection of a single microwave photon by monitoring its effect on the quantum state of a superconducting qubit (see Viewpoint: Single Microwave Photons Spotted on the Rebound).
To avoid such problems, researchers have developed ways to detect a photon without destroying it, usually by observing its interaction with some other quantum system. This measurement technique destroys the photon, which is problematic for quantum computing, as the photon might contain information used in a computation. The new technique could lead to systems for tracking photons in a quantum communications network.Ī typical photon detector absorbs the particle to register its presence. In the past, multiple nondestructive detections have been performed only on stationary photons that existed inside microwave cavities.
In a step toward tracking photons as they move, a team of physicists has made nondestructive detections of a photon at two different locations as it traveled along an optical fiber. Using two detectors, each made of a single rubidium atom trapped inside an optical cavity (inside this rectangular copper box), researchers made sequential, nondestructive measurements of the same photon at two different locations.
Max Planck Institute of Quantum Optics Second time’s the charm.
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