Trying to find the boundary between classical and quantum Physics to unravel quantum limits

The question of where the boundary between classical and quantum physics lies is one of the longest-standing pursuits of modern scientific research, and in new research published recently, scientists demonstrate a novel platform that could help us find an answer.

The laws of quantum physics govern the behavior of particles at miniscule scales, leading to phenomena such as quantum entanglement, where the properties of entangled particles become inextricably linked in ways that cannot be explained by classical physics.

Research in quantum physics helps us to fill gaps in our knowledge of physics and can give us a more complete picture of reality, but the tiny scales at which quantum systems operate can make them difficult to observe and study.

Over the past century, physicists have successfully observed quantum phenomena in increasingly larger objects, all the way from subatomic particles like electrons to molecules which contain thousands of atoms.

More recently, the field of levitated optomechanics, which deals with the control of high-mass micron-scale objects in vacuum, aims to push the envelope further by testing the validity of quantum phenomena in objects that are several orders of magnitude heavier than atoms and molecules. However, as the mass and size of an object increase, the interactions that result in delicate quantum features such as entanglement get lost to the environment, resulting in the classical behaviour we observe.

But now physicists have established a new approach to overcome this problem in an experiment carried out at ETH Zurich, published in the journal Nature Physics.

To observe quantum phenomena at larger scales and shed light on the classical-quantum transition, quantum features need to be preserved in the presence of noise from the environment. As you can imagine, there are two ways to do this; one is to suppress the noise, and the second is to boost the quantum features.

This new work demonstrates a way to tackle the challenge by taking the second approach. Scientists showed that the interactions needed for entanglement between two optically trapped 0.1-micron-sized glass particles can be amplified by several orders of magnitude to overcome losses to the environment.

The scientists placed the particles between two highly reflective mirrors which form an optical cavity. This way, the photons scattered by each particle bounce between the mirrors several thousand times before leaving the cavity, leading to a significantly higher chance of interacting with the other particle.

 Because the optical interactions are mediated by the cavity, its strength does not decay with distance, meaning physicists could couple micron-scale particles over several millimeters.

The researchers also demonstrate the remarkable ability to finely adjust or control the interaction strength by varying the laser frequencies and position of the particles within the cavity.

The findings represent a significant leap towards understanding fundamental physics, but also hold promise for practical applications, particularly in sensor technology that could be used towards environmental monitoring and offline navigation.

The key strength of levitated mechanical sensors is their high mass relative to other quantum systems using sensing. The high mass makes them well-suited for detecting gravitational forces and accelerations, resulting in better sensitivity. As such, quantum sensors can be used in many different applications in various fields, such as monitoring polar ice for climate research and measuring accelerations for navigation purposes.

Now, the team of researchers will combine the new capabilities with well-established quantum cooling techniques in a stride towards validating quantum entanglement. If successful, achieving entanglement of levitated nano- and micro-particles could narrow the gap between the quantum world and everyday classical mechanics.

Cavity-mediated long-range interactions in levitated optomechanics, Nature Physics (2024). DOI: 10.1038/s41567-024-02405-3. www.nature.com/articles/s41567-024-02405-3