What does T-depth measure?

Question: A circuit has 200 T gates, but many can be executed in parallel after scheduling. What does T-depth measure? A) The total number of physical qubits used by all T gates B) The number of sequential layers of T gates after accounting for parallel execution constraints C) The probability that magic-state distillation succeeds D) The number of Clifford gates between the first and last T gate Correct: B Explanation: T-count counts all T gates, while T-depth counts how many T-gate layers must occur sequentially. A circuit can have high T-count but lower T-depth if many T gates can be parallelized. Topic: advanced quantum computing / T-depth / circuit resources