The Physics World 2021 Breakthrough of the Year goes to two independent teams for entangling two macroscopic vibrating drumheads, thereby advancing understanding of the divide between quantum and classical systems.

The winners are Mika Sillanpää and colleagues at Aalto University, Finland and the University of New South Wales, Australia, together with a team led by John Teufel, Parkland High School graduate, now of Erie Colo. and Shlomi Kotler, both of the US National Institute of Standards and Technology.

Nine other achievements are highly commended in our Top 10 Breakthroughs of 2021.

Quantum technology has made great strides over the past two decades and physicists are now able to construct and manipulate systems that were once in the realm of thought experiments.

One particularly fascinating avenue of inquiry is the fuzzy border between quantum and classical physics.

In the past, a clear delineation could be made in terms of size: tiny objects such as photons and electrons inhabit the quantum world whereas large objects such as billiard balls obey classical physics.

Over the past decade, physicists have been pushing the limits of what is quantum using drumlike mechanical resonators measuring around 10 microns across.

Unlike electrons or photons, these drumheads are macroscopic objects that are manufactured using standard micromachining techniques and appear as solid as billiard balls in electron microscope images.

Yet despite the resonators’ tangible nature, researchers have been able to observe their quantum properties, for example, by putting a device into its quantum ground state as Teufel and colleagues did in 2017.

This year, teams led by Teufel and Kotler and independently by Sillanpää went a step further, becoming the first to quantum-mechanically entangle two such drumheads.

The two groups generated their entanglement in different ways.

While the Aalto/Canberra team used a specially chosen resonant frequency to eliminate noise in the system that could have disturbed the entangled state, the NIST group’s entanglement resembled a two-qubit gate in which the form of the entangled state depends on the initial states of the drumheads.

Both teams overcame significant experimental challenges, and their considerable efforts could open the door for entangled resonators to be used as quantum sensors or as nodes in quantum networks.

As a result, this work deserves its place as the first quantum-related Breakthrough of the Year since 2015.