Seeing a Photon Without Absorbing It

[Daedalus]

Physicists at the Max-Planck-Institute of Quantum Optics have developed a device that "leaves the photon untouched upon detection".

All current methods of detecting light share a common property: absorption and thus destruction of a photon. It has been a long-standing dream to be able to watch individual photons fly by without absorbing them. A team of scientists in the Quantum Dynamics Division of Prof. Gerhard Rempe at the Max-Planck-Institute of Quantum Optics has now for the first time realized a device which leaves the photon untouched upon detection (Science Express, 14 November 2013).

After reviewing the link to the original article, I found the following that explains that "an incoming photon is reflected off an optical resonator containing a single atom prepared in a superposition of two states". So, I wouldn't say that the photons are untouched. However, the claim is that the detector allows the photons to continue on their way after being detected:

All optical detectors to date annihilate photons upon detection, thus excluding repeated measurements. Here, we demonstrate a robust photon detection scheme which does not rely on absorption. Instead, an incoming photon is reflected off an optical resonator containing a single atom prepared in a superposition of two states. The reflection toggles the superposition phase which is then measured to trace the photon. Characterizing the device with faint laser pulses, a single-photon detection efficiency of 74% and a survival probability of 66% is achieved. The efficiency can be further increased by observing the photon repeatedly. The large single-photon nonlinearity of the experiment should enable the development of photonic quantum gates and the preparation of novel quantum states of light.

This non-destructive technique of observing the photon may lead to new advances in quantum computing, communication, and information processing:

The ability to observe single photons without destroying them or changing any of their degrees of freedom opens the perspective for a number of new experiments. A single photon can be detected repeatedly by combining several nondestructive devices. This also provides new possibilities for using single photons in quantum communication and quantum information processing. The successful transfer of a photon in a quantum network could be detected without destroying the fragile quantum information encoded in it. Based on the mechanism used for single-photon detection, it should also be possible to realize a deterministic, universal quantum gate between a reflected single photon and the single atom and even between two photons. Because quantum gates are the functional building blocks of a quantum computer, this is a long-standing dream in optical quantum computing.

 

Pretty cool!