The Hidden Witness

Why You Must Clean Up “Junk Bits” with Uncomputation

1. The “Observer” Effect

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.

2. Mathematical Working

3. The Solution: Uncomputation

To restore interference, we follow the Compute-Copy-Uncompute pattern. This resets our ancilla to |0> and removes the “witness.”

4. Qiskit Implementation

from qiskit import QuantumCircuit, transpile
from qiskit_aer import AerSimulator

qc = QuantumCircuit(3)
qc.h(0) 
qc.cx(0, 1) # COMPUTE: Create Junk on q1
qc.cx(1, 2) # COPY Result to q2
qc.cx(0, 1) # UNCOMPUTE: Clean Junk back to |0>
qc.h(0)     # Interference Restored!

qc.measure_all()
counts = AerSimulator().run(transpile(qc, AerSimulator())).result().get_counts()
print(f"Resulting state: {counts}")