&Bullet; physics 14, 94

Researchers took two consecutive measurements of the same photon as it traveled through an optical fiber.

Max Planck Institute for Quantum Optics

The second time is the charm. Using two detectors, each made of a single rubidium atom trapped in an optical cavity (inside that rectangular copper box), the researchers performed sequential, non-destructive measurements of the same photon at two different locations.The second time is the charm. Using two detectors, each made of a single rubidium atom trapped in an optical cavity (inside that rectangular copper box), the researchers made sequential, non-destructive measurements of the same photon at two different locations … show more

In a step to track photons as they move, a team of physicists non-destructively detected a photon in two different locations as it moved along an optical fiber [1] . In the past, several non-destructive detections were performed only on stationary photons that existed in microwave cavities. The new technology 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 computers because the photon can contain information that is used in a calculation. To avoid such problems, researchers have developed methods 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 observing its effect on the quantum state of a superconducting qubit (see Viewpoint: Single Microwave Photons Spotted on the Rebound).

Researchers have also made several sequential detections of the same photon, but so far these repeated measurements have only been made on photons that were stationary and existed as oscillating fields in microwave cavities. Stephan Welte from 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.

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Watch traffic. The new measurements could allow researchers to track photons as they move through a quantum communication network.

The team’s non-destructive detector consisted of a single rubidium atom trapped in an optical cavity. The cavity was designed to reflect any incident optical photon and change the quantum state of the atom as a result of the interaction between photons and the cavity. 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 a distance of 60 m along a glass fiber. Using small lengths of additional fibers, the team connected the detectors to the long fiber and placed so-called circulators at the T-shaped fiber crossings to direct the 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 and then sent laser photons through the fiber.

They observed correlated changes in the state of the atoms, which indicated that the same photon was interacting with each of the detectors in turn. Nobody has carried out multiple non-destructive measurements on moving photons of every wavelength, says Welte. “It is exciting to see that in this way the path of a flying photon can be traced through an optical fiber.”

In theory, a large number of cavity detectors could be connected to a fiber that would allow researchers to precisely track a photon, says MPQ team member Emanuele Distante. In reality, however, every time a photon interacts with one of these detectors, there is a 1/3 chance that the photon will disappear from the fiber. So while the number of detectors could be increased well beyond two, these losses would require designers to carefully choose the positions of the detectors in a quantum network and use as few as possible.

Welte says he and his colleagues plan to improve the time resolution of the detection process to more accurately determine when each photon will interact with each detector. This improved time 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 basic demonstration of quantum mechanics that was only possible in the mind” [thought] Experiments, ”says quantum physicist Yasunobu Nakamura from the University of Tokyo. He agrees with Welte that the technique could be used to monitor photons that carry quantum information along a fiber.

Non-destructive measurements of photons are extremely difficult to make, even more so when the photons are moving, says quantum physicist Jeff Thompson of Princeton University. Here the researchers show one way to do this, and do it over and over again. “This work will have a significant impact on the development of quantum communication networks,” he says.

–Katherine Wright

Katherine Wright is assistant editor of physics.


  1. E. Distant et al., “Detect a wandering optical photon twice without destroying it”, Phys. Rev. Lett.126, 253603 (2021).

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