New experiments finally prove a long-forgotten theory about how quantum particles spin

What makes something quantum? This question has kept a small but dedicated fraction of the world’s population – most of them quantum physicists – up at night for decades.

At very small scales, we know the universe is made up of waves and energy fields ruled by the laws of quantum mechanics, but at the scale of the everyday world around us we mostly see solid objects following the older rules of classical mechanics. When we ask what makes something quantum, we are asking where the line is between these two realms and how it can be drawn.

In a new study published in Newton, we answer this question in a previously undiscovered way. We show that a single spinning particle can show indubitable evidence of quantum behaviour.

The discovery of spin

One hundred years ago, Dutch physicists Samuel Goudsmit and George Uhlenbeck proposed the idea that most tiny particles never really stand still. Instead, they suggested, electrons – elementary particles that form the outer shell of atoms – behave like minuscule spinning tops.

The spin can be either clockwise or anticlockwise, or what physicists call “spin up” and “spin down”. This binary nature of spinning electrons means that they can be used as building blocks for quantum computers.

However, in 1925 Goudsmit and Uhlenbeck’s spinning electron proposal caused an uproar in the physics establishment. At this time, physics was shaped by illustrious names such as Albert Einstein, Max Planck and Paul Ehrenfest, who laid the groundwork for the grand theories of relativity and quantum mechanics that transformed our understanding of the universe.

After eminent physicist and Nobel laureate Hendrik Lorentz criticised the spin theory, Uhlenbeck got cold feet and wanted to retract the paper. Uhlenbeck and Goudsmit’s mentor Ehrenfest told them to persist, writing: “You are both young enough to be able to afford a stupidity!”

Old ideas still remain

This kind of resistance to new ideas is not unusual in physics. As Planck put it, science progresses one funeral at a time.

Much like the scepticism about the discovery of spinning electrons, today many physicists are educated with a misconception about how spin works. Conventional wisdom, still taught in standard textbooks, tells us that spin is a quantum property that is essential to understanding the behaviour of electrons and nuclei. But at the same time, the textbooks say the rotation of the particle is still somehow perfectly described by classical physics.

Tsirelson’s forgotten protocol

A similar consideration applies to another textbook system, the harmonic oscillator (e.g. a pendulum). According to a 1927 theorem by Paul Ehrenfest, the way a quantum pendulum swings is indistinguishable from a swing in the park.

Strikingly, almost 80 years later the Russian-Israeli physicist Boris Tsirelson had an idea showing that it is possible to discern a quantum pendulum from a swing in the park, provided the quantum system is prepared in a truly quantum state. At the time, Tsirelson’s paper attracted little notice.

Another 15 years later, the research team of Valerio Scarani in Singapore resurfaced Tsirelson’s paper from the depths of the internet. Scarani’s student Zaw Lin Htoo extended Tsirelson’s idea, proving theoretically that it actually was possible to detect quantumness in the rotation of a spin.

Bigger particles and Schrödinger’s cat

Our team at the University of New South Wales decided to take on the challenge and prove the quantumness of a spin in a real experiment. However, we couldn’t do it with a simple spin like an electron. Because an electron is so small, it only has two possible spin states: up and down. Again defying widespread intuition, it turns out that an electron spin can only be prepared in quasi-classical states, which obey the old textbook predictions.

Instead we used a much larger particle, the nucleus of an antimony atom. The spin of this particle can point in eight different directions, instead of just two.

We were able to place the atom in a so-called “Schrödinger’s cat” state, in which it is in a superposition of two widely different spin directions at once.

We then performed the Tsirelson-Scarani protocol, which involves measuring not just the average orientation of the spin, but the positivity of it – a very different kind of measurement to what is done in standard spin resonance setups. This experiment showed unquestionable evidence for the quantumness of the antimony’s spin.

What’s next?

Our study is important for discovering fundamental truths about the universe, and for providing clarity on what it means to “be quantum”. However, it may also have real-life applications.

The states that we demonstrated to be quantum with the Tsirelson-Scarani protocol are exactly the kind of thing that give quantum computation and quantum sensing an advantage over classical counterparts. In the future we will focus making the most of these systems for use in technological applications.