Scientists have devised an experiment for testing the domain of validity of quantum theory for objects much more massive than the usual microphysical objects (atoms, molecules etc), beyond which the classical theory has to be necessarily used. This study can also help in developing high precision quantum sensors which are important tools in the cutting- edge quantum technologies.

The principles of Quantum Mechanics replacing that of Newtonian classical mechanics were developed nearly 100 years back. Yet, a number of quantum foundational issues remain problematic. For example, the boundary between the quantum mechanical microworld and the large scale macroscopic classical world of everyday objects obeying Newtonian Laws remains unspecified. The question--up to what level the quantum mechanical principles be valid for macroscopic objects-- continues to be one of the most fundamental open questions in contemporary physics.

This question is also intimately related to another hotly pursued fundamental issue-- testing whether gravity is quantum mechanical or not. 

All the proposed laboratory-based schemes seeking to demonstrate the quantum mechanical nature of gravity crucially rest on assuming applicability of fundamental quantum principles for sufficiently massive objects.

However, the state –of- the- art demonstrations of quantum features have so far reached only up to macromolecules of masses ten thousand times the hydrogen atom. Hence, breakthrough ideas, feasible to be implemented experimentally in the near future, are the need of the hour in order to scale up the tests of macroscopic quantumness to ever more massive objects.

Prof. Dipankar Home from Bose Institute, Kolkata, an autonomous institute of the Department of Science and Technology (DST), in collaboration with D. Das, S. Bose (University College London) and H. Ulbricht (University of Southampton, UK) have addressed this challenge by formulating a novel procedure for demonstrating an observable signature of quantum behaviour for an oscillating object like pendulum having any large mass. 

These scientists have found a novel way for detecting measurement induced disturbance for an arbitrarily massive quantum mechanical pendulum. They have formulated an implementable scheme based on using lasers to suspend a single nanocrystal of silica (a microscopic glass bead) as it oscillates around the focal point of a small parabolic mirror carved out of a block of aluminum housed in a vacuum chamber.

In a typical classical pendulum, the bead would move regularly from point A to point B and back again, unaffected by any observation. However, a quantum pendulum should behave very differently. Its position will change depending on whether or not someone is watching. If we were to detect at any instant where the pendulum bob was, there would be an immediate change of its future behavior. Such a disturbance is an unavoidable consequence of any measurement process involving quantum mechanical system. The scheme proposed by these scientists would enable detecting such measurement induced quantum disturbance for objects much more massive than usual microphysical objects. 

Given the present state- of- the -art technology, this envisaged experiment could be realizable in the coming years for systems ranging from oscillating nano-objects (like that of a grain of dust, about trillion times heavier than hydrogen atom) to oscillating mirrors having effective mass of about 10 kg used for gravitational wave detection.

An experimental study has already been launched by one of the co-authors of this paper, Prof. H.Ulbricht and his group at University of Southampton, UK using optically levitated nano-diamonds about billion times heavier than hydrogen atom.

Thus, this work would pave the way for experiments providing the most emphatic demonstration of large scale quantumness and would open up the possibility for leveraging such macroscopic quantumness for practical applications, such as by developing high precision quantum sensors which are key ingredients in the emerging quantum technologies.

Publication link: D0I:10.1103/PhysRevLett132.030202(2024)