Pursuing elusive photons: the challenges and triumphs of microwave detection

That is the challenge that researchers at the Institut quantique have taken up, one that deals with photons, the fundamental building blocks of light, on a quantum level rather than a verbal one. This technical achievement, which was published in Physical Review X, paves the way for the development of a microwave photon counter, a useful instrument for a variety of fields, including astrophysics, quantum computing, and the hunt for dark matter.  

The ability to transmit, manipulate, and detect photons is essential to research on quantum information processing. These technologies require the detection of photons across a broad spectrum, ranging from microwave photons, which are detectable by superconducting quantum devices, to infrared and visible light, which are used in optical communication systems. While we have a variety of effective detectors for photons in the optical spectrum, the challenge is greater for microwave photons. Their low energy level makes detection more difficult.  

A light particle megaphone  

The group led by Prof. Hofheinz has created something akin to a megaphone for these light particles. Conventional photon “hearing” devices are frequently affected by ambient noise, like a crackling sound that ruins a phone call. In contrast, the method developed by the group can detect large numbers of particles quickly and without being overwhelmed by background noise. However, because the photons in question have such a low energy, the amplifier must operate at very low temperatures. Atomic vibrations slow down at these cryogenic temperatures, which get close to absolute zero, making detection simpler. Attempting to capture them at room temperature would be like “trying to detect individual photons on the surface of the sun”, says Prof. Hofheinz.  

At the heart of the device: Josephson junctions in action 

A key component of this process are Josephson junctions, which are quantum devices that connect two superconductors divided by an extremely thin barrier that allows electron pairs to tunnel through. The incoming photon and the voltage of the Josephson junction provide the energy to generate three outgoing photons. The team has already described this process theoretically and has just demonstrated its validity through delicate measurements and very precise calibration steps, ensuring that the observed amplification is indeed the result of the photonic interaction and not of other sources of error. 

The promise of amplification: outlook 

Nicolas Bourlet, postdoctoral fellow in the research group and co-author of the study, highlights the potential of this technology. Although quantum computing is the main field of application targeted, “applications in other spheres of physics are also envisaged: cosmology, particle research, astrophysics. To date, there are no truly effective detectors for these applications in this frequency range. By laying the groundwork for a novel type of microwave photon counter, the Hofheinz team is not only expanding the boundaries of quantum detection. It’s inviting us to glimpse at a future where the most elusive phenomena in space could be revealed, sparking entirely new discoveries that will transform our knowledge of the universe.