Bad Qubits — Learn Quantum by Doing

Multiple choice

BQP is defined with acceptance probability thresholds 2/3 (for x in L) and 1/3 (for x not in L). The lesson explains why this particular gap is not special. What technique shows that any inverse-polynomial gap defines the same class?

Uncomputation: returning ancillas to |0> forces the probability gap to exactly 2/3 vs 1/3
Amplification: run the circuit k independent times and take the majority vote, so the error falls off exponentially by a Chernoff bound
Deferred measurement: pushing all measurements to the end widens the gap automatically
Solovay-Kitaev: approximating gates more precisely guarantees exact acceptance probabilities