Can you prove whether a large quantum system truly behaves according to the weird and wonderful rules of quantum mechanics - or if it just looks like it does? In a groundbreaking study, physicists from Leiden, Beijing en Hangzhou found the answer to this question.
You could call it a 'quantum lie detector': Bell's test designed by famous physicist John Bell. This test shows whether a machine, like a quantum computer, is truly using quantum effects or just mimics them.
As quantum technologies become more mature, ever more stringent tests of quantumness become necessary. In this new study, the researchers took things to the next level, testing Bell correlations in systems with up to 73 qubits - the basic building blocks of a quantum computer.
The study involved a global team: theoretical physicists Jordi Tura, Patrick Emonts, PhD candidate Mengyao Hu from Leiden University, together with colleagues from Tsinghua University (Beijing) and experimental physicists from Zhejiang University (Hangzhou).
The world of quantum physics
Quantum mechanics is the science that explains how the tiniest particles in the universe - like atoms and electrons - behave. It's a world full of strange and counterintuitive ideas.
One of those is quantum nonlocality, where particles appear to instantly affect each other, even when far apart. Although it sounds strange, it's a real effect, and it won the Nobel Prize in Physics in 2022. This research is focused on proving the occurrence of nonlocal correlation, also known as Bell correlations.
Clever experimenting
It was an extremely ambitious plan, but the team's well-optimized strategy made all the difference. Instead of trying to directly measure the complex Bell correlations, they focused on something quantum devices are already good at: minimizing energy.
And it paid off. The team created a special quantum state using 73 qubits in a superconducting quantum processor and measured energies far below what would be possible in a classical system. The difference was striking - 48 standard deviations - making it almost impossible that the result was due to chance.
But the team didn't stop there. They went on to certify a rare and more demanding type of nonlocality - known as genuine multipartite Bell correlations. In this kind of quantum correlation, all qubits in the system must be involved, making it much harder to generate - and even harder to verify. Remarkably, the researchers succeeded in preparing a whole series of low-energy states that passed this test up to 24 qubits, confirming these special correlations efficiently.
This result shows that quantum computers are not just getting bigger - they are also becoming better at displaying and proving truly quantum behaviour.
Why this matters
This study proves that it's possible to certify deep quantum behaviour in large, complex systems - something never done at this scale before. It's a big step toward making sure quantum computers are truly quantum.
These insights are more than just theoretical. Understanding and controlling Bell correlations could improve quantum communication, make cryptography more secure, and help develop new quantum algorithms.
The paper 'Probing many-body Bell correlation depth with superconducting qubits' was published in PRX.
