How do you measure magic in a quantum computer?

Quantum computers could one day outpace ordinary computers, but only if they can correct their errors and hold a fragile ingredient called ‘magic’.

Our new research links the two, and for some error-correcting codes, can measure how much ‘magic’ is left after each round of correction.

Contents:

1. What is a quantum computer?
2. What is ‘magic’ in quantum computing?
3. How does quantum error correction work?
4. Why is quantum error correction hard?
5. Our work: linking magic and error correction
6. Why does this matter for quantum computing?
7. Looking ahead

What is a quantum computer?

Quantum computers are a new type of computing technology that harnesses the unusual behaviour of particles at the quantum scale.

Rather than processing information as simple 0s and 1s, they use quantum bits, or qubits, which can exist in multiple states simultaneously. This allows quantum machines to tackle some complex problems in ways that conventional computers cannot.

Unfortunately, however, when we measure, or 'read out', our qubits, we can only see one of these possible states, so a lot of the difficulty in quantum computing is making sure the state we see is one which is useful.

What is ‘magic’ in quantum computing?

Another issue for quantum computing, alongside finding algorithms which give the right “read-out”, is that a large family of quantum operations, called stabiliser operations, can be simulated efficiently on an ordinary computer, regardless how many qubits are involved.