One of the highlights in the Physics World calendar is the announcement of our Breakthrough of the Year, which will be made this year on Tuesday 14 December.
Today, we are revealing the 10 finalists for 2021, which serves as a shortlist from which we will pick the winner.
This year’s Top 10 Breakthroughs were selected by a crack team of five Physics World editors, who have sifted through hundreds of research updates published on the website this year. In addition to having been reported in Physics World in 2021, selections must meet the following criteria:
- Significant advance in knowledge or understanding
- Importance of work for scientific progress and/or development of real-world applications
- Of general interest to Physics World readers
To Tai Hyun Yoon and Minhaeng Cho of the Institute for Basic Science, South Korea; Xiaofeng Qian of the Stevens Institute of Technology, US; and Girish Agarwal of Texas A&M University, US for experimental and theoretical work quantifying the “wave-ness” and “particle-ness” of a photon and demonstrating that both properties are related to the purity of the photon source. In their experiment, Yoon and Cho tightly controlled the quantum state of pairs of photons – a “signal” and an “idler” – emitted by two crystals of lithium niobate. By independently altering the chances that each crystal would emit photons, they showed that this so-called source purity is related to the visibility of interference fringes (a wave-like property) and path distinguishability (a particle-like property) by a simple mathematical expression first articulated by Qian and Agarwal in 2020. The result has applications in quantum information and puts a new twist on interpretations of complementarity – the idea, originating from the 20th-century quantum pioneer Niels Bohr, that quantum objects sometimes behave like waves, and sometimes like particles.