A superconductor-graphene-superconductor Josephson junction has shown a noise-equivalent power of 7 x 10-19W/√Hz, which corresponds to an energy resolution of a single 32GHz photon, according to an international research team.

graphene Joephson bolometer US Army

The bolometer is embedded in a 7.9GHz microwave resonator with >99% coupling efficiency.

“Our results establish that two-dimensional materials could enable the development of bolometers with the highest sensitivity allowed by the laws of thermodynamics,” according to to ‘Graphene-based Josephson junction microwave bolometer’, a paper published in Nature.

Bolometers work by converting incident radiation into heat, then measuring temperature rise in the sensor to indicate radiation energy. As such, they need to couple well with the radiation and be small and light for a fast response.

In this case, the sensing structure is built into resonant antenna for best coupling, mono-layer graphene is about as light as a solid can be, and using the graphene as the insulator in a Josephson junction sandwhich turns the structure into its own thermometer as Josephson switching current is dependent on temperature.

The project was sponsored by the US Army and the team included scientists from Harvard University, the Barcelona Institute of Science and Technology, MIT, Pohang University of Science and Technology Korea, Raytheon BBN Technologies and the National Institute for Materials Science, Japan.

“The microwave bolometer developed under this project is so sensitive that it is capable of detecting a single microwave photon, which is the smallest amount of energy in nature,” said Army programme manager Joe Qiu. “This technology will potentially enable new capabilities for applications such as quantum sensing and radar.”

In addition to being thin, graphene has valence and conduction bands that meet only one point – the Dirac point. “The density of states vanishes there so that when the electrons receive the photon energy, the temperature rise is high while the heat leakage is small,” said Raytheon scientist Kin Chung Fong.

As well as radar, other potential applications, according to the Army, are  night vision, thermal imaging, lidar, communication, quantum information science and the search for dark matter.

The Nature paper, ‘Graphene-based Josephson junction microwave bolometer’ is available here

The image is an artistic impression of the device operating, from the US Army