Physicists from Leiden, Beijing, and Hangzhou have developed a method to verify whether a large quantum system is genuinely operating based on the principles of quantum mechanics. The study, which successfully tested a system with up to 73 qubits, provides a way to certify that quantum computers are truly quantum.
The method is a form of Bell’s test, a concept developed by physicist John Bell that acts as a “quantum lie detector.” The test is designed to prove the existence of quantum nonlocality—a phenomenon where entangled particles appear to affect each other instantly, even when separated by large distances. This effect is a cornerstone of quantum mechanics and was the subject of the 2022 Nobel Prize in Physics.
The experimental approach
Directly measuring Bell correlations in a large and complex quantum system is extremely difficult. To overcome this challenge, the international team of researchers developed a different strategy. Instead of a direct measurement, they focused on a task that quantum devices are already good at: minimizing energy.
The team created a special quantum state using 73 qubits in a superconducting quantum processor. They then measured the energy levels of this state and found them to be far below what would be possible in a classical, non-quantum system. The difference was a statistically significant 48 standard deviations, making it nearly impossible that the result was due to random chance.
Certifying a more complex quantum state
The researchers also conducted a more demanding test to certify a rarer type of nonlocality known as “genuine multipartite Bell correlations.” This type of quantum correlation requires that all qubits in the system are involved, making it much harder to generate and verify. The team succeeded in preparing a series of low-energy states that passed this test for systems of up to 24 qubits.
This study is the first to certify deep quantum behavior in a system of this scale. It represents a critical step in the development of quantum computing, as it provides a reliable method for verifying that large-scale quantum processors are performing as expected.
Beyond theoretical validation, understanding and controlling Bell correlations has practical implications. The insights gained from this research could be used to:
- Improve the security and efficiency of quantum communication networks
- Enhance the security of quantum cryptography.
- Aid in the development of new quantum algorithms.
The results show that quantum computers are not only increasing in size but are also becoming more capable of demonstrating and proving their fundamentally quantum nature.
