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 brings Quantum Measure Theory (QMT)—a framework rooted in quantum foundations and the search for quantum gravity—into laboratory practice with potential applications in quantum measurement and computing.
The experiment involved constructing an "event-filter" that selected a specific collection of photon paths 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 while preserving interference effects. Researchers measured input and output laser powers to infer photon-detection probability and used the event-filter's calibration to determine the quantum measure.
The team measured a quantum measure of approximately 1.17, which agreed within experimental uncertainty with the predicted value of about 1.18 after accounting for apparatus imperfections. This value demonstrates that quantum measure behaves differently from classical probability, which cannot exceed one. The phenomenon occurs because contributions from different photon paths interfere like waves, allowing the quantum measure incorporating such interference to exceed unity, while the actual detector probability remains an ordinary probability below one.
The study, published in the journal Quantum (DOI: 10.22331/q-2026-09-24-2215), represents a significant advancement in connecting theoretical quantum concepts to experimental measurement. The research suggests that future implementations could use such event-filtering as a new tool for quantum operations, though the authors clarify this does not resolve the quantum measurement problem. The work also holds personal significance as co-author Rafael Sorkin, who developed QMT partly in search of quantum gravity, passed away shortly after the paper's acceptance on September 12, 2026.