Researchers from the National University of Singapore (NUS) and University of New South Wales (UNSW) Sydney have proven fundamentally that a spinning atomic nucleus really is a quantum resource.
The teams were led respectively by Professor Valerio Scarani, from NUS Department of Physics, and Scientia Professor Andrea Morello from UNSW Engineering. The paper was published in the journal Newton on 14 February 2025.
It has long been inferred that tiny particles such as electrons or protons are indeed quantum due to the way they get deflected in a magnetic field. However, when left to spin freely, they appear to behave in exactly the same way as a classical spinning item, such as a Wheel of Fortune turning on its axis.
For more than half a century, experts in spin resonance have taken this fact as a universal truth.
For the same reason, a technician or a doctor operating a Magnetic Resonance Imaging (MRI) machine at the hospital never needed to understand quantum mechanics – the spinning of the protons inside the patient's body produces the same kind of magnetic field that would be created by attaching a fridge magnet to a spinning wheel.
The new work involves a more intricate kind of measurement performed on a single atom to demonstrate clearly the quantum behaviour of spinning nuclei.
Solving the riddle
Prof. Scarani, who is also Deputy Director of Singapore's Centre for Quantum Technologies (CQT), first proposed the theory after coming across a paper in 2021, written by Russian-Israeli mathematician Boris Tsirelson some 15 years earlier. The paper explored the probability of detecting objects in particular locations as they moved back and forth in a rhythmical way.
As a quantum physicist, Prof. Scarani wondered if he could build on that work and utilise it instead in relation to the behaviour of single quantum objects as they spin – and particularly to solve the riddle of why such tiny particles seemed to behave no differently from normal, non-quantum objects.
Having worked with his student Mr Zaw Lin Htoo to solve the problem for some years, Prof. Scarani set out to test their theory that in certain special states it would be possible to definitively show that an atomic nucleus does indeed have quantum properties.
A collaboration with Prof. Morello from the School of Electrical Engineering and Telecommunications at UNSW provided the skills and the ultra-precise measurement equipment to run the experiments which proved it was true.
"For one particle on its own, nobody thought that you could get any deviation between the classical and the quantum behaviour," Prof. Scarani said.
"Andrea's group put into motion the spin of a nucleus of antimony, and for every cycle or rotation took seven measurements to check whether it pointed in the positive direction or not. Using the Wheel of Fortune analogy, one would expect to find an arrow on the wheel pointing to the right either four out seven times, or three out of seven times.
"Classically, the absolute maximum is four out of seven – it is inescapable. But the quantum theory predicted that, in the special state we wanted to create, a higher probability could be observed.
"The deviation between the classical and quantum behaviour is quite small, but statistically significant. To notice such deviation, you have to make sure that the measurement is extremely precise and free from noise, otherwise you will not see that difference."
From the UNSW team, PhD student Mr Arjen Vaartjes and Dr Martin Nurizzo, led the experiment which produced the results to prove the new theory.
Prof. Morello took a big thrill out of being able to challenge the long-established textbooks.
"This is fundamental science. Previously it was thought there was no way to ascertain that a nuclear spin is actually a quantum mechanical object by simply watching its precession in a magnetic field. What we have done here is to contradict that.
"We have shown that actually it is possible. Yes, you need to have some special kinds of nuclear spins, and put them in some very peculiar quantum states, and you need a sophisticated way of observing them. The special quantum state we used is called the 'Schrödinger cat state', and has interesting features of its own.
"But the result is a proof that clarifies what is a quantum resource."
Observation of the truth
Prof. Scarani and Prof. Morello are delighted to have made this breakthrough in the understanding of how quantum phenomena manifest themselves in spin precession, regardless of possible applications.
"Within the community of scientists, this experiment will bring these ideas more to the fore," said Prof. Scarani.
"The key challenge is that you need to create these very specific states in order to see the effect. That said, those special Schrödinger cat states are likely to be important quantum resources in quantum computing, so what we have done here could become a very efficient method to certify their creation."
With 2025 designated by the United Nations as the International Year of Quantum Science and Technology, marking a century since the initial development of quantum mechanics, Prof. Morello is proud to have validated such a fundamental principle.
"Quantum mechanics has been around for 100 years, so you might think we'd have figured it all out, right?" he said. "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.
"This is really more of a work of intellectual beauty and intellectual satisfaction than it is an immediate application. It is an observation of the truth."
Key Facts:
Collaboration between experts at National University of Singapore and UNSW Sydney provides long-sought evidence of atomic nuclei displaying quantum behaviour.
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