Quantum Computing: Researchers Create Optical Device That Can Absorb Just One Photon

The ability to create, manipulate and modify the quantum properties of individual photons is key to creation of viable optical quantum computers. The problem is, devices currently capable of generating single photons are extremely unreliable.

Now, in a step that may soon help scientists create high-fidelity single photon emitters, a team of researchers from the University of Stuttgart in Germany have developed a device that does the exact opposite — absorb just one photon from a beam of light containing billions of them.

According to a study published last week in the journal Physical Review Letters, the single-photon absorber uses a micron-sized cloud of rubidium atoms cooled to near absolute zero (which is -459.67 degrees Fahrenheit). When this cloud is illuminated with a highly-precise beam of laser light, one of the rubidium atoms is converted into what’s known as a Rydberg atom.

An atom can be converted to a Rydberg atom by exciting its outermost electron to a very high energy level. In this particular case, the Rydberg atom so created was able to interact with its neighboring rubidium atoms located roughly 10 microns away.

“The presence of the first Rydberg atom has such a strong influence that it changes the resonance conditions for all the other atoms,” Sebastian Hofferberth from the University of Stuttgart told Physics World.

As a result, once a rubidium atoms absorbs a photon, it swells up to such a size that no other atom in the diffuse cloud can accept any more photons.

So what can this device be used for? As the researchers explain in the study, a key application would be as a quantum gate, where the number of qubits — the quantum computing equivalent of the bits used in conventional computers — needs to be stringently controlled. Another use would be as a precise photon counter — a key component of a devices used in a wide array of fields, including quantum encryption, astrophysics and fiber-optic communication.

“We have now built the most primitive version of such a tool for manipulating light,” Hofferberth told Physics World. “We can subtract exactly one photon.”

Ultimately, though, the researchers hope to create a device that emits exactly one photon using the same technique.