Researchers at the Raman Research Institute (RRI), an autonomous institute of the Department of Science and Technology, have achieved the first experimental measurement of a quantum measure exceeding one, beyond the upper limit of ordinary probability. The breakthrough was published in the journal Quantum on September 24, 2026, with DOI 10.22331/q-2026-09-24-2215.
The experiment involved constructing an "event-filter" that selected a specific collection of photon routes between a laser source and detector. PhD student Sanchari Chakraborti performed the experiment under the supervision of Professor Urbasi Sinha, using polarization to distinguish different photon routes and then erasing distinguishing information to allow routes to interfere. The team measured input and output laser powers to infer photon-detection probability and used the event-filter's calibration to determine the quantum measure.
The researchers measured a quantum measure of approximately 1.17, which agreed within experimental uncertainty with the theoretical prediction of about 1.18 after accounting for apparatus imperfections. This value demonstrates that quantum measure behaves differently from ordinary classical probability, which cannot exceed one. The phenomenon occurs because contributions from different possible photon routes can interfere with one another like waves, allowing the quantum measure that includes interference between routes to exceed one, while the actual detector probability remains an ordinary probability below one.
The research brings Quantum Measure Theory (QMT), a framework developed partly by co-author Rafael Sorkin in the search for quantum gravity, into laboratory experimentation. The late Dr. Sorkin, researcher emeritus at Canada's Perimeter Institute for Theoretical Physics and Distinguished Visiting Faculty member at RRI, passed away on September 12, 2026, shortly after the paper's acceptance. The study demonstrates that quantum measure is not merely an abstract concept but can be measured experimentally, potentially enabling new tools for quantum measurement and quantum computing through event-filtering techniques that select chosen collections of photon paths while leaving photons available for further quantum operations.