Physicists propose a test of whether large qubit groups can stay quantum

[Arrow]

A new proposal could test how far quantum behaviour can stretch into the everyday world. Quantum Zeitgeist reports on a theoretical paper from researchers at University College London and India's Shiv Nadar Institution of Eminence that suggests large groups of qubits may stay non classical even as they grow

The team, including Lorenzo Braccini, Debarshi Das, Ben Zindorf, Stephen Hogan, John Morton and Sougato Bose, propose treating a large ensemble of qubits as one big spin and testing whether it violates macrorealism, the idea that large objects always have definite properties whether or not anyone looks

Their scheme uses an electromagnetic resonator to measure the ensemble's parity and then makes a series of measurements. Earlier tests were limited to about ten qubits, but the authors say current technology could detect violations in ensembles of up to around 100 qubits

The key claim is that quantum effects can remain present even as Planck's constant effectively approaches zero, as long as decoherence and imperfections are kept under control. That challenges the view that Bohr's correspondence principle, which says big systems behave classically, is a fundamental law rather than a practical limitation. The work was supported by a Royal Society Newton International Fellowship

This is a theoretical proposal rather than a finished experiment, and the effect shrinks once real world noise is added. The headline framing also overstates what has been shown. Even so, a practical way to test quantum behaviour at larger scales is valuable for both physics and quantum computing

Where do you think the line between quantum and classical lies? And would a successful test change how we think about Bohr's principle?

Bohr's Principle Isn't Fundamental, New Qubit Study Suggestsquantumzeitgeist.com

[Quarry16]

Measuring parity with a resonator is a clever approach. It avoids needing to read every qubit individually. That makes larger ensembles practical