New Microwave Device Produces Single, Tunable Photons

As we dive headlong into the era of quantum computing, one of the most pressing technical problems in the development of scalable quantum computers is figuring out how to produce and manipulate individual photons on demand.

In many quantum information technologies, such as quantum computers or quantum enigma machines, photons are used to encode information as qubits, the quantum analog of the classical bit (rather than being a one or a zero, a qubit is a superposition of both values at the same time). Generally speaking, quantum computers make use of photons by coupling them with a resonance circuit, which allows photons operating at a certain frequency to pass through the circuit. This means that each circuit must be constructed to operate only on a certain photon frequency, which limits the amount of information the circuit is capable of transmitting.

To overcome this limitation, researchers at the National Physical Laboratory (NPL) have created a device which is not only capable of generating single photons, but is able to manipulate the frequencies of these photons on-demand, obviating the need to reconstruct entire circuits when a different photon frequency is needed.

The researchers involved in the project say the development has big potential for quantum computing applications, as well as more fundamental research on the way light interacts with matter.

"Among the advantages of our circuit is its simplicity," the researchers wrote in a paper describing the device, published earlier this week in Nature Communications. "It consists of a single element."

This single element is an artificial atom connected to a pair of transmission lines in a circuit, where the atom serves as a qubit. According to the team, the artificial atom is a macroscale device that contains billions of atoms and leverages the superconducting properties of the material comprising the device to mimic the behavior of a natural atom.

To make the device work, an input microwave pulse is used to briefly excite the artificial atom into a higher energy state. The return of the artificial atom to its previous energy level after the excitement produces a single photon, the frequency of which is determined by the energy of the input pulse which can be fine-tuned to the demands of the researchers.

According to the researchers, the device produces photons with at least 65 percent efficiency over a wide range of frequencies, which they cite as comparable to other more cumbersome methods of producing photons.

Now that its device has had a successful proof-of-concept run, the team says the next goal is getting a single photon source and single photon detector on the same chip. Although single-photon detectors are more well established than single photon source devices, the NPL team's device will help test detectors and improve upon already existing designs.

Increasing control over the production and detection of photons will allow physicists to harness microwave photons as vessels capable of carrying quantum information through a circuit, a central technical aspect of a

scalable, programmable quantum computer

.