&ball; Physics 14, 94
The researchers made two successive measurements of the same photon as it traveled through an optical fiber.
In a step towards tracking photons as they move, a team of physicists performed non-destructive detections of a photon at two different locations as it traveled along an optical fiber. . In the past, multiple non-destructive detections were only performed on stationary photons that existed inside microwave cavities. The new technique could lead to systems for tracking photons in a quantum communication network.
A typical photon detector absorbs the particle to register its presence. This measurement technique destroys the photon, which is problematic for quantum computing, because the photon can contain information used in a calculation. To avoid such problems, researchers have developed ways to detect a photon without destroying it, usually by observing its interaction with another quantum system. For example, a team in Switzerland recently demonstrated the non-destructive 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).
The researchers also performed several sequential detections of the same photon, but so far these repeated measurements have only been performed on stationary photons, existing as oscillating fields inside microwave cavities. Stephan Welte of the Max Planck Institute for Quantum Optics (MPQ) in Germany and his colleagues have now demonstrated two non-destructive measurements of a single moving photon.
The team’s non-destructive detector consisted of a single rubidium atom trapped inside an optical cavity. The cavity was designed to reflect any incoming optical photon and to change the quantum state of the atom due to the photon-cavity interaction. The team was able to monitor the state of the atom by observing its effect on a subsequent laser pulse.
Welte and his colleagues placed two of their cavity detectors at locations 60 m apart along an optical fiber. Using small lengths of extra fiber, the team connected the detectors to the long fiber and placed devices called circulators at the T-shaped fiber intersections to direct photon traffic. A photon entering a circulator would be directed to a detector and then, after reflection from the detector, would be redirected along the main fiber in its original direction. To conduct the experiment, the researchers initialized the atoms in a known quantum state, then sent laser photons along the fiber.
They observed correlated changes in the states of atoms that indicated that the same photon was interacting sequentially with each of the detectors. No one has previously performed multiple non-destructive measurements on moving photons of any wavelength, Welte says. “It is exciting to see that the path of a photon flying through a fiberglass can be followed in this way.”
In theory, a large number of cavity detectors could be connected to a fiber, allowing researchers to accurately track a photon, explains Emanuele Distante, member of the MPQ team. In reality, however, every time a photon interacts with one of these detectors, there is about a 1/3 chance that the photon will disappear from the fiber. Thus, although the number of detectors could be increased well beyond two, these losses would force designers to carefully choose the locations of detectors in a quantum network and to use as little as possible.
Welte says he and his colleagues plan to improve the temporal resolution of the detection process, which will allow them to more accurately determine when each photon interacts with each detector. This improved temporal resolution could be useful for quantum technologies, where a photon interacting with a detector could trigger the release of another photon elsewhere in the system.
This experiment is “a very fundamental demonstration of quantum mechanics which was only possible in gedanken [thought] experiments before, ”explains quantum physicist Yasunobu Nakamura of the University of Tokyo. He agrees with Welte that the technique could be used to monitor photons carrying quantum information along a fiber.
Non-destructive measurements of photons are extremely difficult to perform, especially when the photons are moving, explains quantum physicist Jeff Thompson of Princeton University. Here the researchers demonstrate a way to do it and do it over and over. “This work will have a significant impact on the development of quantum communication networks,” he says.
– Katherine Wright
Katherine Wright is Associate Editor-in-Chief of Physics.
- E. Distant et al., “Detect a traveling optical photon twice without destroying it”, Phys. Rev. Lett. 126, 253603 (2021).