RRI experiment measures quantum value beyond classical probability limit

Researchers at the Raman Research Institute (RRI), an autonomous institute under the Department of Science and Technology, have carried out the first experimental measurement of a quantum measure that exceeds one — a value impossible under ordinary probability. The result, published in the journal Quantum, brings Quantum Measure Theory (QMT) from the realm of foundational theory into the laboratory.

Quantum mechanics is typically framed in terms of a system's state at a given moment and the outcomes of measurements on it. An alternative approach describes a system through its possible histories — the various ways it can evolve between preparation and detection, such as the different routes a photon might take from a laser to a detector. QMT assigns a generalized measure to sets of such histories, factoring in the interference between them. Unlike ordinary probability, this quantum measure can exceed one.

The challenge has been experimental access: a general set of histories does not necessarily map onto a sequence of ordinary measurements, making its quantum measure hard to observe directly. To bridge that gap, the RRI team built an "event-filter" designed to select a chosen collection of photon routes, referred to as an event. Sanchari Chakraborti, then a PhD student at RRI, conducted the experiment under the supervision of Urbasi Sinha, a co-author of the paper and a senior professor at the institute.

In the setup, photons in a laser beam were allowed to travel along different paths. Polarisation — the orientation in which light waves oscillate — was used to distinguish the routes and isolate the desired set, after which the distinguishing information was erased so the routes could interfere. The researchers inferred the photon-detection probability from input and output laser power measurements, then used the event-filter's calibration to arrive at the quantum measure.

The measured value was about 1.17, consistent within experimental uncertainty with a prediction of roughly 1.18 once apparatus imperfections were taken into account. The figure above one confirms that the quantum measure behaves differently from classical probability, which cannot exceed unity. The excess arises because contributions from different photon routes can interfere like waves, with overlapping crests producing a larger combined amplitude. Crucially, the experiment did not record a probability greater than one: the detector probability remains an ordinary probability between zero and one, and it is only the quantum measure that crosses the classical bound.

"There is something particularly satisfying about seeing an idea that grew out of fundamental questions about quantum mechanics become an optical experiment on a laboratory table," Sinha said. "We're not claiming that this resolves the quantum measurement problem, but we've enlarged the class of questions about quantum processes that can be connected to an experimental measurement."

The quantum measure is central to QMT, which co-author Rafael Sorkin developed partly in the search for quantum gravity. Its emphasis on complete histories makes it a natural framework for describing quantum processes across spacetime. The experiment brings that framework into the laboratory, though it does not test a theory of quantum gravity. Sorkin, a researcher emeritus at Canada's Perimeter Institute for Theoretical Physics and a Distinguished Visiting Faculty member at RRI, died on 12 September 2026, shortly after the paper was accepted. "This paper means a great deal to me personally as well as scientifically," Sinha said.

Looking ahead, the team suggests that a filter capable of selecting a chosen collection of photon paths while leaving the photons available for further quantum operations could turn such event-filtering into a new tool for quantum measurement and quantum computing.