Bengaluru scientists measured a number that beats 100 per cent. It's no mistake

Scientists at Bengaluru's Raman Research Institute have measured a quantum score of about 1.17, a number that sails past the ceiling of one that ordinary chance can never cross. Professor Urbasi Sinha explains why this is not a broken rule but a new way to measure the hidden journey of light.

In Short

• Bengaluru scientists measure a quantum score of 1.17, beating one
• The figure is not a probability, so no rule breaks
• Light interference lets quantum measure climb past 100 per cent

A particle of light slips out of a laser. It bounces off a mirror, passes through a special sheet of glass, and finally lands on a detector with a soft click. We know exactly where it finished its journey. The far trickier question, the one that has teased physicists for a century, is this: how on earth did it get there?

A team at the Raman Research Institute (RRI) in Bengaluru has taken that question head on. In a study published in the journal Quantum, scientists measured something called the quantum measure, a kind of score for all the different routes a single particle of light could have taken.

The number came out to about 1.17, almost exactly matching the value their theory predicted, which was close to 1.18.

Now, 1.17 might not sound dramatic. But look again. It is bigger than one. In everyday life, nothing can be more than 100 per cent certain. You cannot be 117 per cent sure your train will arrive. So how can a serious scientific measurement sail past the ceiling that ordinary chance can never cross?

That puzzle is the entire story, and the answer is far stranger and more beautiful than a broken calculator.

A FILM, NOT A PHOTOGRAPH

Most quantum experiments behave like a snapshot. They freeze a single moment, usually the last one, and ask a simple question: where did the particle end up?

The quantum measure works more like a film. It follows the whole journey from start to finish. Think of driving from Bengaluru to Mysuru. One question is whether you arrived. A completely different question is whether you travelled through Mandya or through Ramanagara. The quantum measure is built to answer the second kind of question, the one about the route rather than the destination.

In the language of the theory, a chosen group of possible routes is called an event. The quantum measure is the score handed to that event. Crucially, it captures something ordinary chance ignores, which is the way those routes mingle and influence one another along the way.

WHY THE NUMBER CAN CLIMB ABOVE ONE

Everyday chances behave politely. If one route has a 50 per cent chance and another has 30 per cent, the chance of taking either is simply 80 per cent. You add them up, and the total can never pass 100 per cent.

Light does not play by these gentle rules. It travels as a wave, and waves can team up. When two waves rise and fall in step, like ripples meeting on a pond, they merge into a single, bigger wave. Physicists call this interference. The quantum measure counts that extra boost as a bonus on top of the individual scores. When the boost is large enough, the total creeps past one.

Professor Urbasi Sinha, who led the research, is firm about what the figure does and does not mean. She told India Today Digital that 1.17 does not tell us how likely the event was. It tells us how the histories that make up that event combine in a quantum way.

In other words, think of it as an interference-sensitive weight attached to a whole bundle of journeys, not as a frequency with which something happens. Add one more route to the bundle and the total does not always rise. It can climb if the waves reinforce, or sink if they cancel. Through all of this, the plain, old-fashioned chance of the detector actually clicking stayed exactly where it should, comfortably below one.

COLOURED STICKERS FOR BEAMS OF LIGHT

Here lies the real headache. A group of histories does not match any single button you can press on a laboratory instrument. So the team built a clever device called an event filter, designed to pick out only the routes they cared about.

They sent laser light down several possible paths. Each path was then tagged using polarisation, which is simply the direction in which a light wave wobbles as it travels. It works rather like sticking coloured labels on envelopes so you can sort them later. Physicists call such a helper tag an ancilla.

The filter kept the chosen paths, binned the rest, and then peeled off the labels so the surviving paths could once again blend and interfere. The researchers carefully measured how much light power went in and how much came out. Because the device had been precisely calibrated, that ratio revealed the quantum measure, and out popped the figure of 1.17.

The experiment was carried out by Sanchari Chakraborti, then a PhD student at RRI, under Professor Sinha. Their co-author was Rafael Sorkin, the theoretical physicist who dreamt up the whole framework back in 1994.

BUT THEY USED A LASER, SO IS IT TRULY QUANTUM?

It is a fair challenge. Laser light behaves like an ordinary wave, and ordinary waves interfere all the time. Professor Sinha happily agrees that the interference itself is nothing a classical physicist would faint over.

In their setup, light passes only through mirrors and glass that respond in simple proportion to it, an arrangement known as linear optics. In that gentle world, she explained, the ratios of laser power behave exactly like the probabilities you would record if you fired one particle of light at a time. Sorkin worked through the sums himself, and his proof sits quietly in an appendix to the paper.

The genuinely quantum twist, she said, is not the interference but what the number is given meaning for. It is the score of an entire set of journeys, a quantity that obeys none of the usual rules of probability. Firing single particles would not make the result more quantum. It would simply open a different door, towards building a filter gentle enough to leave the particles intact.

A NUMBER THAT LEAVES THE PAGE

So why does any of this matter? Until now, the quantum measure was something physicists could scribble on paper but never actually catch in the act.

There is a world of difference, Professor Sinha said, between being able to write a quantity down and knowing how to connect it to a real experiment on a real table. She is careful about the limits.

Standard quantum mechanics already predicts this optical result, so nothing here rewrites the textbooks. It does not crack the famous measurement problem, the deep riddle of how a world fizzing with many possibilities settles into one definite answer the moment we look. Nor does it test quantum gravity, the long-sought theory that would marry quantum physics with Einstein's gravity, even though Sorkin chased that dream for much of his life.

What has shifted, she said, is that a quantity describing an entire quantum process, rather than just its final outcome, has moved from something we can calculate to something we can measure.

THE MAN BEHIND THE EQUATIONS

Rafael Sorkin was a researcher emeritus at Canada’s Perimeter Institute and a distinguished visiting faculty member at RRI from 2011. He is best known for the causal set idea, the striking notion that spacetime is ultimately built from tiny discrete grains linked by cause and effect. He died in Ontario on September 12, shortly after the paper was accepted.

Professor Sinha recalls no single flash of insight, only one question that kept returning between them. He would push on what it all meant, while she pushed on what could actually be built and measured on the optical table. The laser appendix was born from exactly that tug of war.

On a bench in Bengaluru, light is still being split, labelled and recombined. And a number that once lived only in Sorkin's equations can finally be read straight off the light itself.

- Ends