The Cost of Garbage in Quantum Computing: Why You Must Clean Up Junk Bits
Malcolm Low
Originally published on malcolmlow.net In quantum computing, anything that "knows" what a qubit is doing acts as a Witness . Leftover data ( Junk Bits ) on an ancilla qubit act as witnesses, destroying the interference your algorithm needs to work. 1 . The Observer Effect Consider a simple circuit where a qubit passes through two Hadamard gates. Classically, two inversions cancel. Quantum mechanically, the same is true — only if no information leaks out between them . Case A: Ideal — No Junk q0: