Physicists just built a quantum lie detector. It works

Often described as a kind of "quantum lie detector," Bell's test -- developed by physicist John Bell -- reveals whether a machine, such as a quantum computer, genuinely relies on quantum effects or merely imitates them.

As quantum technologies advance, the need for tougher tests of "quantumness" grows. In this new work, the researchers went further than ever before, examining Bell correlations in systems containing up to 73 qubits, the fundamental units that store quantum information.

The collaboration brought together theoretical physicists Jordi Tura and Patrick Emonts, PhD candidate Mengyao Hu from Leiden University, experts 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.

Innovative Experimentation

The project was highly ambitious, but a clever approach made it possible. Instead of directly measuring the intricate Bell correlations -- a technically demanding task -- the team focused on something quantum processors excel at: minimizing energy.

Their approach produced remarkable results. Using a superconducting quantum processor, they created a quantum state involving 73 qubits and recorded energy values far lower than any classical system could achieve. The difference was astonishing -- 48 standard deviations -- making it virtually certain that the outcome was not random.

The researchers then took the challenge even further, verifying a tougher form of nonlocality called genuine multipartite Bell correlations. These correlations require every qubit in the system to participate, making them exceptionally difficult to create and confirm. Yet the team succeeded in generating a range of low-energy states that passed this demanding test with up to 24 qubits.

This achievement demonstrates that quantum computers are not only expanding in size but also improving in their ability to exhibit and validate true quantum behavior.

Why It Matters

The findings mark the first time deep quantum behavior has been certified in such large and complex systems. It represents a major milestone in proving that quantum computers operate according to quantum principles rather than classical approximations.

Beyond fundamental science, this work could have practical benefits. A better understanding of Bell correlations may enhance quantum communication, strengthen cryptographic security, and inspire the design of new quantum algorithms.