Researchers at the Raman Research Institute (RRI), an autonomous institute of the Department of Science and Technology (DST), have experimentally measured a quantum measure exceeding one for the first time, bringing an important concept from Quantum Measure Theory (QMT) into the laboratory.
The researchers measured a quantum measure of approximately 1.17, compared with a theoretical prediction of about 1.18 after accounting for imperfections in the experimental apparatus. The finding demonstrates that quantum measure can exceed the upper limit of ordinary probability, potentially opening new avenues in quantum measurement and quantum computing.
The study, published in the journal Quantum, focuses on the behaviour of possible photon paths between a laser source and a detector.
Measuring quantum histories
Quantum mechanics is commonly described in terms of the state of a system at a particular time and the outcomes obtained when it is measured. Quantum Measure Theory offers a complementary approach by describing a quantum system through its possible histories – the different ways a system can evolve between its preparation and detection.
In an optical experiment, for example, these histories can be understood as the different routes a photon may take between a laser and a detector.
QMT assigns a generalized measure to a collection of such histories while taking into account the interference between them. Unlike an ordinary probability, this quantum measure is not restricted to values between zero and one.
However, experimentally accessing the quantum measure is challenging because a general collection of histories does not necessarily correspond to a sequence of conventional measurements.
The RRI experiment addressed this challenge by constructing an “event-filter” capable of selecting a chosen collection of possible photon paths.
How the experiment worked
Sanchari Chakraborti, then a PhD student at RRI, carried out the experiment under the supervision of Urbasi Sinha, senior professor at RRI and co-author of the study.
The researchers allowed photons from a laser beam to travel through different routes in an optical setup. They used polarisation, which describes the orientation in which light waves oscillate, to distinguish between different routes and select the particular collection of routes forming the event.
The distinguishing information was subsequently erased, allowing the different routes to interfere with one another.
The researchers measured the input and output laser powers to infer the photon-detection probability. They then used the calibration of the event-filter to determine the corresponding quantum measure.
The measured value was approximately 1.17, in agreement within experimental uncertainty with the predicted value of around 1.18.
Why a value above one is possible
The result does not mean that the researchers measured a probability greater than one.
The probability of photon detection remains an ordinary probability and therefore stays below one. What exceeded one was the quantum measure, which also incorporates interference between different possible histories.
In quantum systems, different photon routes can interfere like waves. When contributions reinforce one another, their combined effect can become larger than what would be allowed by an ordinary classical probability measure.
The experiment therefore demonstrates a distinction between conventional probability and the generalized measure used in Quantum Measure Theory.
From theoretical concept to laboratory measurement
The study provides the first experimental measurement of a quantum measure exceeding one, showing that the quantity is not merely a theoretical construct but can be accessed experimentally.
Sinha said the experiment does not resolve the quantum measurement problem, but it expands the range of questions about quantum processes that can be connected to experimental measurements.
The researchers also point to a possible future application of the event-filtering approach. A future implementation could use a filter to select a chosen collection of photon paths while keeping the photons available for subsequent quantum operations.
Such an arrangement could potentially make event-filtering a new tool for quantum measurement and quantum computing.
Connection with Quantum Measure Theory and quantum gravity
Quantum Measure Theory is rooted in the study of quantum foundations and the description of quantum processes through complete histories. The framework was developed by co-author Rafael Sorkin, partly in the context of the search for a theory of quantum gravity.
Because QMT focuses on complete histories, it provides a framework for describing quantum processes across spacetime.
The RRI experiment brings this theoretical framework into an optical laboratory setting. However, the researchers clarify that the experiment does not test a theory of quantum gravity.
Sorkin, researcher emeritus at Canada’s Perimeter Institute for Theoretical Physics and a Distinguished Visiting Faculty member at RRI, died on September 12, 2026, shortly after the paper was accepted.
Reflecting on the work, Sinha described the paper as significant to her both personally and scientifically.
