In recent years, scientists have succeeded in developing photodetectors capable of detecting individual quanta of light. In the infrared region of the electromagnetic spectrum, such detectors can be used in optical communications in deep space and for distributing “quantum keys” via fibre networks. At terahertz frequencies, they allow astronomers to study how galaxies form by detecting photons from the cosmic infrared background. Microwave single photon detectors and so-called photon number resolvers could, for their part, be employed in a number of potential future quantum technologies, such as remote entanglement of superconducting qubits and high-fidelity quantum measurements.

Detecting low-frequency photons is no easy task, however. Existing techniques rely on recording the temperature increase when the detector material absorbs single photons. For example, transition edge sensors and superconducting nanowire single photon detectors can register infrared photons as they go about breaking Cooper pairs in the superconductors, and high-sensitivity calorimeters can detect single photons by monitoring the temperature rise produced by absorbed photons. The problem with these techniques is that they become less sensitive as the energy of the photons decreases.

Electrons in graphene heat up

The new device, developed by a team led by Dirk Englund of the Massachusetts Institute of Technology and Kin Chung Fong of Raytheon BBN Technologies, also in Massachusetts, consists of a sheet of graphene contacted on two ends by superconductors. This configuration, in which the two superconductors are separated by a weak link (graphene in this case) is called a Josephson junction.

“In the absence of photons, a supercurrent can pass through the graphene and we detect no voltage difference between the superconducting contacts,” explains Englund. “When a photon is absorbed, however, the electrons in graphene heat up, blocking the supercurrent and leading to a voltage spike that we can measure.”

More sensitive to low-frequency light

The device is more sensitive to low-frequency light than previous single photon detectors thanks to graphene’s unique properties, he tells nanotechweb.org. “First, graphene can absorb light at nearly any wavelength in the electromagnetic spectrum. Second, because graphene is two-dimensional, it can be easily integrated into structures that can further enhance its light absorption.” Such structures include microwave cavities for microwave photons or photonic crystal cavities for infrared photons.

“Lastly, and most importantly however, graphene has a special pseudo-relativistic electronic band structure that leaves it with very small electron state density compared to other materials,” explains Englund. “Because it has so few electron states, it has an exceptionally low heat capacity, which means that it takes very little energy to raise the average temperature of the electrons.”

Just one microwave photon doubles graphene’s temperature

In their work, the researchers show that it takes just one microwave photon to double graphene’s temperature (when it starts out at cryogenic temperatures). “This is like sitting at room temperature, absorbing a photon and all of a sudden being at 300°C,” says Englund. “According to our calculations, the device can detect individual infrared photons at a rate of up to a billion times per second and microwave photons at a rate of one million times per second.”

The new single photon detector could be used in astronomy as well as in quantum information processing, say the researchers. “The latter is especially promising since the best quantum computing technology to date relies on Josephson junctions, so it would be somewhat natural to couple our detector to these systems,” adds Englund.

The research is detailed in Physical Review Applied DOI:https://doi.org/10.1103/PhysRevApplied.8.024022.