What is Leakage in terms of quantum computing?

[David Long]

I am doing a project on stabilizer code, and I keep running into a term about qubits and leakage. What does leakage mean?

 

[DrClaude]

https://arxiv.org/abs/1308.6642

Many physical systems considered promising qubit candidates are not, in fact, two-level systems. Such systems can leak out of the preferred computational states, leading to errors on any qubits that interact with leaked qubits. Without specific methods of dealing with leakage, long-lived leakage can lead to time-correlated errors. We study the impact of such time-correlated errors on topological quantum error correction codes, which are considered highly practical codes, using the repetition code as a representative case study. We show that, under physically reasonable assumptions, a threshold error rate still exists, however performance is significantly degraded. We then describe simple additional quantum circuitry that, when included in the error detection cycle, restores performance to acceptable levels.

 

[Strilanc]

It means that your qubit, which should only allow $|0\rangle$ and $|1\rangle$ states, has ended up in an extra $|2\rangle$ state (or higher) inherent to the physical system you implemented the qubit in. This tends to be very bad, because it consistently propagates errors into adjacent qubits interacting with the qubit in the $|2\rangle$ state. Also, error correcting codes tend to be designed under the assumption that qubits are not secretly qutrits.