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Photons take a detour

  • Non-classical paths in a triple slit experiment: Their contribution is potentially measurable

Non-classical paths in a triple slit experiment: Their contribution is potentially measurable; Image by Authors.

Researchers have proposed an experiment to show that, sometimes, photons may choose longer paths over the shorter ones. Though this result may look too simple, it is expected to influence physics over a large scale: from the way quantum mechanics is introduced to students to better understanding of quantum computing applications.

A photon is a sub-atomic particle, simply defined as a packet of light (or any other form of electromagnetic radiation). Quantum mechanics is the branch of physics that deals with sub atomic phenomena that happen at nanoscopic scales (which is smaller then microscopic, and cannot be seen, only observed in delicate experiments).

A team of Indian researchers propose a simple experiment to prove that the photons do take bizarre paths to reach from point A to B. Reporting in the prestigeous international journal Physical Review Letters, researchers propose a simple table-top experiment. This is the first experimental proposal to quantify photon paths in interference setups.

The double slit experiment is a milestone in the history of physics. Through this, Thomas Young in 1803, showed that light is a wave, but not a stream of particles as previously thought. However, it's an irony that, about a century later, in 1905, Albert Einstein suggested that light is indeed made up of particles of zero mass callled photons. Today, it is widely accpeted that light has both wave and particle nature.

The double slit experiment is quite simple, though its implications are quite profound. A wave passes through two holes, and waves overlap on each other upon coming out. This produces an intensity pattern on a screen. Generally, it is assumed that the intensity observed when both the holes are open is equal to the sum of the individual intensities when only one hole is open at a time – called the “superposition” priciple..

However, this is just a good approximation. Scientists say a proper analysis of the experiment must take in to account allpossible paths a photon can take from the holes to the screen. When they say “all”, it includes the predictable “classical” straight paths from the holes to the screen, and many more “non-classical” paths like going out from one hole, coming back, and going out again from the other. American Nobel Laureate, Richard P Feynman's path integral formulation gives a framework for dealing with such bizarre paths.

“Suppose we throw a ball out of an open window. A favourite high schooltopic that is taught to us is to calculate the trajectory of this ball. Inclassical mechanics we get a unique answer. Now imagine making the balland the window smaller and smaller. For very small sizes (which can bemade precise), classical physics is no longer adequate and one has to invoke quantum physics. In the quantum world, this quantum ball no longerfollows a unique trajectory but there is a probability that it can followany trajectory that joins the initial and final point. Feynman's pathintegral formalism gives us a way to compute this probability by summingover all such trajectories with a certain weight”, explains Professor Aninda Sinha, one of the authors of the paper.

Thomas Young used two slits  to prove the wave nature of light in the classical experiment; the Indian researchers have proposed using three slits to prove the existance of non-classical paths. They propose to measure “a mathematically beautiful quantity”, denoted by the Greek letter κ (kappa). In fact, it is a normalised form of what is called the Sorkin parameter.

“Our theory provides the first ever theoretical upper boundon kappa. Till we came up with this theory, kappa was known to be zerowithin Quantum Mechanics. Our theory now establishes that kappa can neverbe zero. It provides a direct way of quantifying the effect due to nonclassical paths”, said Professor Urbasi Sinha, who finds herself “lucky to be involved in thinking about fundamental aspects of physics”.

Apart from the physics it contains, this research is also interesting from a completely different angle. The five person research team has two couples: Aninda and Urbasi Sinha, and Joseph Samuel and Supurna Sinha. Co-author Rahul Sawant is a PhD student.

Though these findings are fundamental to quantum mechanics, their repercussions are expected to be felt in other branches of physics as well. “It is especially expected to play a role in better
understanding of interferometer based quantum computing applications”, said Urbasi Sinha.

Author information:

Joseph Samuel, Supurna Sinha, and Urbasi Sinha are at the Raman Research Institute. Aninda Sinha is at the Indian Institute of Science. Rahul Sawant is a PhD student at the Raman Research Institute. Link to paper: