Picking one photon out of the flow

This work has recently been published and selected as an Editor's Suggestion in Physical Review Letters.

This method paves the way towards future quantum communication and computation using the subtle aspects of quantum mechanics for technological applications.

Light is made up of little indivisible packets of energy, or particles, known as photons. A defining property of photons is that they are non-interacting, they simply pass through each other totally unaffected.

In the context of quantum communication, this is a very useful feature, as it ultimately enables low-loss transmission of optically encoded data over very large distances. However, many emerging ideas for quantum information processing would greatly benefit from the ability to make two photons interact, whereby one can affect the propagation or state of another.

Over the past years, ultracold atomic gases have proven to provide an ideal medium in which to manipulate light. For example, using a technique known as electromagnetically induced transparency, one can drastically modify the velocity of light propagation and slow light down to astonishingly slow speeds of just a few meters per second.

Perhaps even more remarkably, one can bring light to a full halt by converting the photons into atomic excitations within the medium. By reversing this process, and mapping the excitations back onto photons, this procedure realises a photonic quantum memory, where photons can be temporarily stored and retrieved on demand.

Together with the team from Aarhus University and collaborators from the Joint Quantum Institute at the University of Maryland, the experimental team in Odense has implemented such a photonic memory, but with a special form of atomic gas in which the constituent atoms feature strong interactions.

This effectively makes the photons feel each other's presence in the quantum memory, allowing one to manipulate light on a nonlinear level. Using this idea, the groups have devised and demonstrated a novel way to subtract a single photon from an optical beam by using another beam of light.

The general idea is to first store an optical field, and then send another one through the medium. Photons in the second beam take notice of the stored photons and interact with them in such a way that exactly one is tagged and later on discarded on retrieval. Being robbed of a single photon, the original light beam is left in a peculiar quantum state that in itself has numerous scientific and technological applications.

Indeed, the underlying idea of manipulating photons using such a nonlinear quantum memory, holds promise for many different applications in quantum information science. Whilst much remains to be understood before such capabilities become fully operational, the demonstrated prototype photon subtractor is an important milestone to this grand vision of quantum technologies based on interacting photons.