Quantum Computing Hardware
Why can't quantum error correction directly use the classical approach of making three copies of the data and taking a majority vote?
- AAn unknown quantum state cannot be copied exactly (the no-cloning theorem), and measuring it to check its value destroys the superposition
- BA qubit takes only the two values 0 and 1, so a majority vote is meaningless
- CQubit errors are only bit flips, so they can be corrected by adding a single parity bit, without making copies
Show answer and explanation
Answer: A. An unknown quantum state cannot be copied exactly (the no-cloning theorem), and measuring it to check its value destroys the superposition
By the no-cloning theorem, there is no operation that copies an unknown quantum state α|0⟩ + β|1⟩ exactly onto another qubit. Measuring the qubits that hold the data directly would also destroy the superposition. Quantum error correction therefore spreads the information of one logical qubit over an entangled state of several physical qubits and uses auxiliary qubits to measure only clues to the errors (the syndrome), such as the parity of neighboring qubits. Qubit errors include phase flips as well as bit flips, and both must be detected.
- A. Correct. The impossibility of copying and the destruction of the state by measurement are why the classical approach cannot be used as is.
- B. A qubit can be in a superposition, so it is wrong to say it takes only two values.
- C. Qubits also suffer phase-flip errors, which cannot be corrected by looking at bit flips alone.