Physicists in the UK and India have set out to test “large” objects for bizarre quantum behaviour such as the capacity to exist in a Schrodinger cat-like state, neither dead nor alive.

The UK Engineering and Physical Sciences Research Council has approved funding for tabletop experiments proposed by the physicists in London, Calcutta, and Southampton to test quantum mechanics on scales never attempted before.

Quantum mechanics, a branch of physics that emerged in the early 20th century, is the best and only way to understand nature at its tiniest scales such as the behaviour of light or subatomic particles. But quantum effects have not been observed in a lab on any objects much heavier than atoms or molecules.

Now, Sougato Bose and Debarshi Das at the University College London, Dipankar Home at Calcutta’s Bose Institute, and Hendrik Ulbricht at the University of Southampton have proposed an experiment to look for quantum effects in objects, regardless of their mass.

“If we do see quantum effects in large objects, reconciling quantum theory with everyday experiences, with reality as we understand it, will become an even bigger challenge than it is at present,” said Bose, professor of physics and astronomy at the UCL.

For, quantum mechanics comes with conditions and consequences that appear to challenge traditional concepts of reality.

One seemingly absurd concept is a premise that the act of observing something can alter it. “We know this is true at the subatomic level — our experiment will test whether this is also valid at much bigger scales,” said Das. A positive result, Das said,
if extrapolated into the everyday realm, would imply that a crowd watching a football match could influence its outcome by staring at the match.

The Schrodinger cat is another such hard-to-digest quantum concept. The European physicist Erwin Schrodinger — one of the founders of quantum mechanics — came up with an imaginary experiment in the 1930s, arguing that the laws of quantum mechanics would allow a cat under certain conditions to be neither dead nor alive until it is observed. Physicists call this a superposition of two states.

In April 2023, physicist Yiwen Chu and her colleagues at ETH Zurich, Switzerland, announced the results of an experiment in which they demonstrated such a superposition of two states in a crystal particle weighing 16 micrograms.

“We have effectively created a Schrodinger cat weighing 16 micrograms,” Chu had said then.

The experiment that the UK-India physics team has proposed effectively bypasses the need to directly create large Schrodinger cats. Instead, Bose said, “our experiment will test the quantum effect that the act of just observing a large object can disturb it”.

The proposed experiment, published in the journal Physical Review Letters on Tuesday, can potentially be implemented with tiny crystals or even using 10kg mirrors.

Home, who has studied the foundations of quantum mechanics for over 40 years and has collaborated with Bose and Das in the past, said the demonstration of quantum effects in large objects has been a longstanding, unachieved goal in physics.

The experiment, to be performed by Ulbricht’s group at Southampton, Home said, is expected to provide “the most emphatic demonstration of quantum behaviour at a scale never achieved before”.

The experiment will involve observing tiny silica particles levitating in an electromagnetic field, while they oscillate like a pendulum. By shining laser beams on the silica particles at different times, the experiment will seek to verify their quantum behaviour.