The ability to detect single photons (the smallest energy packets constituting electromagnetic radiation) in the infrared range has become a pressing need across numerous fields, from medical imaging and astrophysics to emerging quantum technologies. In observational astronomy, for example, the light from distant celestial objects can be extremely faint and require exceptional sensitivity in the mid-infrared. Similarly, in free-space quantum communication—where single photons need to travel across vast distances—operating in the mid-infrared can provide key advantages in signal clarity.
The widespread use of single photon detectors in this range is limited by the need for large, costly, and energy-intensive cryogenic systems to keep the temperature below 1 Kelvin. This also hinders the integration of the resulting detectors into modern photonic circuits, the backbone of today's information technologies.
An international team of researchers led by ICFO has now shown one way to overcome this limitation. They have used two-dimensional materials (which are only one-atom thick) to detect long-wavelength single photons (up to the mid-infrared) at relatively high temperatures (around 25 degrees Kelvin). This milestone has caught the attention of the European Space Agency (ESA), which is seeking the use of detectors with these type of properties for space exploration. Today, ICFO researchers, Dr. Krystian Nowakowski, Dr. Hitesh Agarwal, Dr. Julien Barrier, Dr. David Barcons Ruiz, Dr. Geng Li, Riccardo Bertini, Matteo Ceccanti, Dr. Iacopo Torre, Dr. Antoine Reserbat-Plantey, led by Dr. Roshan Krishna Kumar and ICREA Prof. at ICFO Frank Koppens, in collaboration with Prof. Pablo Jarillo-Herrero, researcher and professor at Massachusetts Institute of Technology (MIT) and Distinguished Invited Professor at ICFO, as well as researchers from the University of Manchester, University of Antwerp, among others, have reported their results in Science.
Bistability: a novel mechanism for single photon detection
"In our group, we combine different 2D materials. We stack them, twist them, and then observe what happens. And, sometimes, surprises come out," comments ICREA Prof. at ICFO Frank Koppens, senior author of the study and long-time expert in 2D materials.
A slight twist between layers induces an interference pattern known as the moiré pattern, which modifies the properties of the electrons in the material. Several exotic properties, including superconductivity or orbital magnetism, have been observed in moiré lattices. In this work, ICFO together with the international team have added another exotic property to the list, a phenomenon known as bistability. Bistability allows a system to rest in two distinct states under the same external conditions, like a light switch that can remain stable in either the "on" or "off" positions.
The team has shown that bistability can serve as a new mechanism for single-photon detection—one of those surprises Koppens describes. "We noticed that the material was not behaving as we expected," he recalls. "So, we thought, 'Let's shine some light on it and see what happens.' That's when we suddenly observed an extreme sensitivity to illumination." And the deeper they looked, the clearer it became that the material was responding to individual photons.
As for the results obtained in the experiment, Prof. Jarillo-Herrero highlights, "This experiment showcases the great potential of moiré quantum devices not only in terms of fundamental science but also for novel applications in quantum technologies".
Single photons: the straw that broke the camel's back
The detector itself is structurally simple. It consists of bilayer graphene (a one-atom-thick layer of carbon atoms, which shows relevant physical properties) sandwiched between layers of hexagonal boron nitride (hBN), another 2D material which acts as a protective shield. However, "building the device was tricky," explains Dr. Hitesh Agarwal, first co-author of the study, primarily because achieving the precise alignment between bilayer graphene and hBN had only a 50% success rate. "In the end, we managed to solve it through careful design and lessons from earlier experiments," he adds.
So how does this device detect single photons? At an intuitive level, the answer can be grasped with a metaphor. Imagine a huge, empty box on a table, and put a handful of straws (or grains of rice) inside. Nothing happens. But what if you keep putting in more and more straws or rice? Eventually, the weight will become too much, and the table will collapse.
In the laboratory, the researchers engineered a system at the edge of collapse. "Instead of straws, we have electrical current that is flowing", says Dr. Krystian Nowakowski, first co-author of the paper. "And when we reach the critical point, the device doesn't break but suddenly switches from one stable state to another. When a single photon is absorbed, it's like that final straw—it triggers the transition, and that's what we detect."
But how exactly does a lone, single photon tip the system? "This is something we would all love to know," admits Dr. Nowakowski. "We have some hypotheses at the moment, but we need to do more experiments to be able to discern between them." For now, we'll have to live with the mystery.
What is clear, however, is that this mechanism differs fundamentally from conventional superconducting or semiconductor-based processes. It was precisely this underlying mechanism that allowed the device to detect long wavelength photons (up to the mid-infrared) at relatively high temperatures (around 25 degrees Kelvin) . "The unique physical mechanism at the heart of our detector architecture allows us to break the fundamental limits that held previous technologies back," shares Dr. Krishna Kumar, who co-supervised the work.
The team is now focused on making the system more compact and pushing the operating temperature even higher, since this is usually the deciding factor that determines whether a certain detector will be used at all. However, many more factors come into play when determining whether a given technology will become practical. Perhaps this new method for detecting single photons won't prove useful for studying distant galaxies, medically relevant molecules, or quantum information carriers —or perhaps it will become a turning point.
LINKS
Link to Reference: Single-photon detection enabled by negative differential conductivity in moiré superlattices, K. Nowakowski, H. Agarwal, S. Slizovskiy , R.Smeyers , X. Wang , Z. Zheng , J. Barrier , D. Barcons Ruiz, G. Li , R. Bertini, M. Ceccanti, I. Torre, B. Jorissen, A. Reserbat-Plantey, K. Watanabe, T. Taniguchi, L. Covaci, M. V. Milošević , V. Fal'ko , P. Jarillo-Herrero, R. Krishna Kumar, F. H. L. Koppens, 2025, Science.
