The counter-intuitive behaviour of photons
We usually think of photons as particles that can pass through each other without interacting. Yet, when several photons meet in the right conditions, they can behave as if they prefer to travel together, a phenomenon known as bosonic bunching.
Researchers at INL and Universidade Federal Fluminense, in collaboration with Sapienza Università di Roma, Consiglio Nazionale delle Ricerche (IFN-CNR), and Politecnico di Milano, have observed something that challenges this intuition: under specific conditions, making photons more similar to one another, can reduce their probability of bunching.
Leonardo Novo, research group leader at INL, explains: “In experiments with two photons this bunching phenomenon is simple to explain: the more similar the photons are when they arrive at different inputs of an interferometer (in their frequency, polarization, etc.) the more likely it becomes that they leave the interferometer together in the same output. However, when three or more photons interfere together, a much more nuanced picture arises.”
The researchers observed this behaviour of photonic bunching in a three photon interferometry experiment. Leonardo Novo, Carlos Fernandes and Ernesto F. Galvão, former INL group leader, provided theoretical support for this experiment performed by the research team in Rome. By carefully controlling how similar the photons were in properties such as their arrival time and polarisation, the team could explore how these subtle differences influence where the photons end up.
Leonardo Novo adds, “The researchers observed that, for certain types of interferometers, indistinguishable photons can actually minimise the probability that two or more photons bunch together at the output of the device; in this case, increasing distinguishability between the photons can only make bunching events more likely, which goes against the usual intuition.”
“In this work, we also observed a second more subtle counter-intuitive effect due to quantum mechanics. If we call our photons A, B, and C it is possible to manipulate their quantum state such that, when we look at each pair, B became more similar to A and C also became more similar to A and B. One would naturally expect that now the three particles are closer to a state of three identical photons. It turns out this is not always the case. Surprisingly, it is possible to make photons more indistinguishable pairwise but, at the same time, reduce the probability that all the three photons are in the same quantum state. We observed this effect in the lab, which is reflected in a decrease in the probability that all photons end up together in the same output even though each pair of particles became more similar.”
These results reveal the complex relationship between photon similarity and bunching. Understanding this behaviour could help researchers better control how photons are routed and manipulated given the current technological limitations of photon sources which are unable to generate ideal, perfectly indistinguishable photons. This is an important step for the development of photonic quantum technologies such as photonic quantum computers.
The study, published in the journal Light: Science & Applications, was supported by the FET project PHOQUSING and the European Union’s Horizon Europe project EPIQUE.
Reference Experimental observation of counter-intuitive features of photonic bunching
Giovanni Rodari, Carlos Fernandes, Eugenio Caruccio, Alessia Suprano, Francesco Hoch, Taira Giordani, Gonzalo Carvacho, Riccardo Albiero, Niki Di Giano, Giacomo Corrielli, Francesco Ceccarelli, Roberto Osellame, Daniel J. Brod, Leonardo Novo, Nicolò Spagnolo, Ernesto F. Galvão, and Fabio Sciarrino
