Experimental Proof of Quantum Spin: Redefining the Boundary Between Classical and Quantum Physics

Insider Brief:

• Researchers from CQT and UNSW experimentally demonstrated that nuclear spin precession is fundamentally quantum, overturning the long-held assumption that it behaves classically.

• By measuring a single antimony nucleus in a precisely prepared quantum state, they observed deviations from classical physics, providing direct evidence that nuclear spin is a quantum resource.

• The findings introduce a new method to verify quantum states, which could benefit quantum information processing, sensing, and error correction, making quantum verification simpler and more practical.

• This study challenges classical assumptions in quantum mechanics and highlights how much remains to be uncovered, reinforcing the significance of quantum research as we advance toward scalable quantum technologies.

• Image Credit: University of New South Wales

For decades, the behavior of atomic spins has been unknowingly elusive. When observed under magnetic resonance, their precession—the slow, rhythmic rotation of spin in a magnetic field—has long been considered indistinguishable from classical motion. The equations that describe this motion resemble those of a gyroscope, leading scientists to believe that in this specific domain, quantum and classical physics blur into one. But a new study challenges this assumption, proving that, under the right conditions, the spinning nucleus of a single atom is inherently quantum—and that its behavior cannot be explained by classical physics alone.

A team of researchers from the Centre for Quantum Technologies in Singapore and the University of New South Wales Sydney has recently demonstrated that spin precession at the nuclear level is not just an illusion of classical physics but a uniquely quantum resource. By precisely measuring the spin of a single antimony nucleus implanted in silicon, they observed deviations that defy classical explanation. The work, led by Professor Valerio Scarani at CQT and Professor Andrea Morello at UNSW, provides a fundamental answer to the question: what makes spin truly quantum?

The Quantum Nature of Spin

Spin, in its simplest form, is an intrinsic property of fundamental particles, akin to angular momentum but without a tangible axis of rotation. In classical physics, a spinning object, like a Wheel of Fortune or a toy top, rotates predictably, its orientation at any given moment following deterministic rules. Quantum mechanics, however, introduces a strange new dimension: particles seemingly exist in multiple states at once, and their spin orientations are not fixed but probabilistic.

Despite this, in common scenarios—such as those used in medical MRI machines—the quantum mechanical nature of spin is not apparent. The spins of protons in the human body align with a magnetic field, precessing like a classical object and producing signals that medical devices can detect. This led to the long-held belief that spin, while fundamentally quantum, behaves classically in practice.

However, the new research demonstrates that this classical approximation is not universal. When placed in carefully prepared quantum states, known as Schrödinger cat states, a nuclear spin exhibits behavior that is fundamentally impossible under classical physics.

Beyond Classical

The key insight from this study is the experimental verification of quantum behavior through precession itself. Previous experiments testing the quantumness of a system relied on indirect methods, such as Bell’s inequalities, which require measuring correlations between multiple particles. But Scarani and Morello’s approach works with a single particle, a single nuclear spin.

Their method focused on measuring positivity, which is the likelihood that a spin points in a given direction at different moments in time. Classically, this value follows strict probability limits. In a classical system, if a spinning wheel were measured at random points in its cycle, the maximum frequency at which it could point in a given direction would be 4 out of 7 times. Any result exceeding this classical threshold would imply a violation of classical physics.

When the UNSW team set the nucleus of antimony into motion and measured its spin precession, they found such a violation. In a specially prepared quantum state, the nucleus pointed in the expected direction more frequently than the classical limit allows. The deviation was small but statistically significant, providing direct evidence that nuclear spin, in its most fundamental form, is a quantum mechanical entity.

Implications for Quantum Science

This discovery is not just a theoretical curiosity, but rather, it has real implications for quantum technologies. Quantum information processing, quantum sensing, and even quantum error correction rely on creating and manipulating non-classical states. The study provides a new way to certify that a system is genuinely quantum by observing its precession alone, a method that is simpler and more practical than previous approaches.

Furthermore, the finding reaffirms a broader truth in physics: we are still coming to terms with the nature of quantum mechanics. A century after its foundations were laid, quantum theory continues to reveal surprises, challenging our classical intuitions. The fact that an experiment in 2025 can overturn a long-held assumption about spin precession underscores how much there is left to uncover.

As Morello put it in a recent release from the Centre for Quantum Technologies, “Quantum mechanics has been around for 100 years, so you might think we’d have figured it all out, right? And yet, in 2025, we can still come up with a really simple but clever idea, which we can test in a real experiment, that makes you rethink what it means to be quantum.”

For quantum computing, this study provides a new perspective on how quantum information can be stored and manipulated. High-dimensional quantum states, such as those in nuclear spins, are promising candidates for robust quantum memories and processors. The ability to verify the quantum nature of these states with simple measurements could accelerate their practical use in scalable quantum technologies.

Toward Deeper Understanding of Quantum Mechanics

The research by CQT and UNSW represents a fundamental step in understanding what makes a system truly quantum. By proving that nuclear spin precession, long assumed to be classical, can exhibit genuine quantum behavior, the study rewrites an important chapter in quantum mechanics.

As we enter the International Year of Quantum Science and Technology, this work stands as a testament to the ever-evolving nature of quantum physics. Even as we push toward practical quantum computing, the fundamental questions of what it means to be quantum remain open.

Contributing authors on the study include Arjen Vaartjes, Martin Nurizzo, Lin Htoo Zaw, Benjamin Wilhelm, Xi Yu, Danielle Holmes, Daniel Schwienbacher, Anders Kringhøj, Mark R. van Blankenstein, Alexander M. Jakob, Fay E. Hudson, Kohei M. Itoh, Riley J. Murray, Robin Blume-Kohout, Namit Anand, Andrew S. Dzurak, David N. Jamieson, Valerio Scarani, and Andrea Morello.