A little bit more than magic: The secret to quantum computing may lie in negativity

Quantum computers have the potential to revolutionize fields like drug discovery and cybersecurity, but scientists are still unsure what gives them their edge over classical computers. A new study from the Cavendish Laboratory shows that quantum computers are more complex than previously thought and require a deeper understanding of what makes them work.

The research, published in Physical Review Letters, identifies the specific quantum states, or "magic states," that are truly useful for quantum computation and differentiates them from those that are not. Quantum computers use qubits, which can exist in multiple states at once, and require special starting configurations to run algorithms that outperform classical computers.

However, not all "magic states" are equal, and many previously thought to be useful turn out to offer no quantum advantage. By clarifying which quantum states have "useful" magic and which do not, the study provides a clearer picture of what is needed for quantum computers to demonstrate an advantage over classical machines. The research uses a mathematical framework developed by Paul Dirac in 1945, which involves "negative probabilities" and has been shown to be a meaningful signal in quantum computing.

The team's findings could help guide the design of quantum software and the production of magic states, which are essential for building large-scale quantum computers.