Science News

Tabletop experiment corrects long-standing optics error

Biplab Das

doi:10.1038/nindia.2014.132 Published online 29 September 2014

A tabletop physics experiment is all set to redefine some basic concepts of quantum physics long overlooked by our physics textbooks1.

The experiment proposed by a team of Indian physicists shows a way to correct a long-standing error that creeps into conventional optical experiments probing the quirky world of elementary particles.

One of the cornerstones of quantum theory is the fact that particles can also behave as waves. In conventional optics experiments, beams of photons passing through two slits on a screen generate waves that spread out and interfere with each other. This interference creates a pattern forming an image on a detector screen.

The physicists now claim that an error occurs at this point since the widely used double-slit experiment calculates results only by considering the photons that take straight paths while passing through the slits and leaves out photons that are thought to follow curvy paths. Computing the curvy paths of photons, the physicists have found that they could have significant effects on the results of such experiments, thereby providing a way to control experimental errors.

“We arrived at theoretical results that aim to find the correction to the commonly used naive application in conventional slit-based interference experiments,” says Urbasi Sinha, one of the physicists from Raman Research Institute, Bangalore. If successfully performed, the proposed tabletop experiment will be able to quantify this correction which may even lead to rewriting textbooks, Sinha told Nature India.

Sinha adds that the results of their study will have applications in interferometer-based quantum computing and analysis of observational radio astronomy data related to the early universe.

Physicist Richard Feynman has long predicted that elementary particles such as electrons can take all possible paths while passing through double slits. However, physicists have neglected the contribution of looped (curvy) paths of elementary particles such as electrons and photons to experimental results in slit-based experiments.  

Purple depicts non-classical paths (curvy) of photons and green the classical or straight path.
In physics, nearly straight paths (green in picture) of photons through slits are known as classical paths; whereas looped paths (purple in picture) are known as non-classical paths. To quantify the contribution of photons taking looped paths, the researchers devised a computer model of triple-slit experiment which could be performed on a table. Their model considered all possible paths of photons passing through triple slits on their way from a source to a detector.

In order to analyse the effect of non-classical paths in interference experiments, the researchers considered the effect of such paths on an experimentally measurable parameter denoted by the Greek symbol kappa. They found that the non-classical paths (each photon passing through two slits) of photons contributed to non-zero values of kappa. “What would have been expected to be zero considering only straight line paths now turns out to be measurably non-zero having taken the non-classical ones into account,” says researcher Aninda Sinha from the Indian Institute of Science, Bangalore.

In addition, the researchers found that kappa is very strongly dependent on certain experimental parameters. They claim that keeping all other experimental parameters fixed, kappa increases with an increase in wavelength. The researchers predict that it is possible to find reasonably high value of kappa for a microwave beam passing through wide slits with wider inter-slit distance. Such an experiment could be performed in a radio astronomy lab, they say.


1. Sawant, R. et al. Non-classical paths in interference experiments. Phys. Rev. Lett.113, 120406 (2014)