The Cost of Garbage in Quantum Computing: Why You Must Clean Up Junk Bits
Malcolm Low explains why junk bits are fatal in quantum computing. In an ideal circuit, a qubit passing through two Hadamard gates cancels to |0> with 100% probability. However, if an ancilla qubit becomes entangled via a CNOT gate, it records which path the qubit took, destroying interference. The output becomes a 50/50 mix of |0> and |1>. This demonstrates the necessity of uncomputation or error mitigation to remove junk bits before measurement. The article, originally published on malcolmlow.net, uses simple examples to illustrate how any 'witness' qubit that learns intermediate state information collapses quantum superpositions, preventing algorithms like Grover's or Shor's from functioning correctly.
Developers must manage ancilla qubits to prevent junk bits from destroying quantum interference.