Glossary
Every technical term this platform uses, with what it means and — the part that is harder to find elsewhere — the mistake it prevents. On this subject the intuitive reading is often the wrong one, so each entry says what a reader would otherwise get wrong.
Resources
- T count
How many T gates a circuit needs. The standard measure of a fault-tolerant circuit's cost.
Easy to get wrong: Each T gate consumes a magic state that must be manufactured, and the factories doing that are usually the majority of the machine — often more than 90% of its physical qubits.
Seen in use on /workbench
- Toffoli
A three-qubit controlled-controlled-NOT gate, common in arithmetic.
Easy to get wrong: Usually costed as 4 T gates, so a circuit's Toffoli count is a large part of its real expense even though it looks like one operation.
- Magic state
A specially prepared state that lets a fault-tolerant computer perform a T gate, which it cannot do directly.
Easy to get wrong: They cannot be prepared reliably, only distilled from many noisy copies — which is why they dominate the cost.
Seen in use on /workbench
- Distillation
Consuming many low-quality magic states to produce fewer better ones.
Easy to get wrong: It only improves a state below a fixed point. Above roughly 1/√35 for the standard protocol, each round makes things worse and no number of rounds helps.
- Error budget
The total probability of failure you are willing to accept across a whole computation.
Easy to get wrong: It is divided among every logical qubit and every cycle, so a modest-sounding budget implies a very small per-operation error rate.
Seen in use on /workbench
- Lattice surgery
Performing operations between logical qubits by merging and splitting their patches on the grid.
Easy to get wrong: It needs free space beside the data to move information through, so a register of n logical qubits occupies more than n patches — Microsoft's estimator uses 2n + ⌈√(8n)⌉ + 1.