An experiment that disproves Einstein's idea of reality

An unreal world?

Quantum physicists in the city have conducted experiments proving that reality as we think of it may not exist — and in the process have not only conclusively disproved an Einsteinian idea of reality but have also paved the way for more secure information transfer.

That all of this should be achieved by quantum scientists should come as little surprise. Quantum mechanics has already been expanding our concept of what reality is. Previous experiments around the world, for example, have shown that particles can be in more than one place at a time, but a key tenet of quantum theory is that an object only assumes a definite position if it is seen by the observer.

Bothered by this, Albert Einstein famously said, “I like to think that the moon is there even if I am not looking at it.”

His statement reflects the every-day believed notion of realism, which suggests that a system has well-defined properties in any instant, even when not measured. But what if the quirks of quantum physics go beyond mere atoms or particles?

Testing realism

This has prompted a spate of experiments to determine, in the words of New Scientist, if “there is a hard boundary between the quantum and classical worlds.” Central to this is the Leggett-Garg inequality, devised in 1985 by Anthony Leggett and Anupam Garg. “This inequality looks for correlations between measurements to see whether quantum or classical rules are being followed,” the New Scientist states. In essence, it is a means of testing realism.

Professor Urbasi Sinha of Raman Research Institute (RRI) explained that the experimental violation of such inequality would not only falsify realism but also would confirm that quantum mechanics is not limited to the micro-world, but can be applied to bigger objects, such as the moon.

“Leggett and Garg realized they could test the quantumness of big objects in theory. Their inequality could tell us whether realism holds true in the everyday world,” she said.

She added that, “this could also make way for harnessing non-classicality or quantumness of single photons for technological applications such as secure quantum communications and quantum sensing, which are crucial in today’s requirements of secure information transfer.”

In recent years, Leggett-Garg experiments carried out on various quantum systems from superconducting fluids and photons to atomic nuclei and tiny crystals have demonstrated that the microscopic world is non-real. For this they have found ways of measuring particles without disturbing it.

Testing the macroscopic limit of quantum mechanics is an important area of research because it can reveal up to what extent quantum principles dominate — revealing the quantum-classical boundary.

However, these experiments have limitations. Scientists worldwide are trying to come up with better technology and appropriately designed strategies for achieving a fully conclusive experimental test.

Now, a team of scientists from RRI has successfully addressed this challenge.

In the course of a two-year experiment, she showed a significant amount of violation of Leggett Garg inequality by studying single photons.

The experiment was performed at the Quantum Information and Computing laboratory of RRI and was led by Urbasi along with her PhD student Kaushik Joarder. Theoretical contributions from Professor Dipankar Home of the Bose Institute Kolkata and Dr Debashis Saha of the S N Bose Centre for Basic Sciences Kolkata played a significant role in the work.

First experiment

The work, published in PRX Quantum, is the first ambiguity-free experiment to show violation of Leggett Garg inequalities.

The team conducted the experiment with single photons (particles of light) and proved the quantumness of the single photon comprehensively. “This is the first experiment that shows the most decisive refutation of the notion of realism by the closure of what are known as ‘loopholes’ plaguing all relevant experiments to date,” Urbasi said.

“Loopholes are elements, such as equipment limitations or study-related factors which can inadvertently alter the experiment or conspire to deviate results,” she told DH.

She added that the strategies and technologies developed for the closure of all the existing loopholes will prove to be very useful for harnessing such non-classicality/quantumness of single photons for technological applications in secure quantum communications and quantum sensing.

Moreover, the experiment further shows remarkable agreement with quantum physics predictions. “In our analysis, we have been able to show that not only are we violating the Leggett Garg inequality in a loophole-free manner, but that we were also showing remarkable agreement with the predictions of quantum mechanics,” Urbasi said.

This work was partially funded by the Centre of Excellence in quantum technologies grant from the Ministry of Electronics and Information Technology as well as the Quantum Enabled Science and Technology grants from DST.