Past exam of the mathematics course of the University of Cambridge 2017 iii Paper 324 4 a i Solution Created 2026-10-03 Updated 2026-10-06
The basic one-bit teleportation primitive uses the input as qubit 1 and a fresh quantum ancilla as qubit 2. Apply the Controlled-Z gate and measure qubit 1 in the equatorial qubit measurement basis with angle . If is the result, the normalized post-measurement state on qubit 2 isBoth results have probability one half. For example, writing , the unnormalized branch is . Thus is applied deterministically as a logical gate with known Pauli frame ; a permitted conditional Pauli X gate would remove this byproduct physically.
The literal resource restriction does not need an unstated direct Pauli X gate. A Pauli Z gate can be enacted by applying to the data and a fixed quantum ancilla. Two consecutive angle-zero one-bit teleportations, with results , map an arbitrary current state to . Apply the available to remove the latter factor, up to an irrelevant global phase. This implements a heralded : if , the input is unchanged; if , the required Pauli X gate has been applied.
If the original result is and a physically corrected output is required, repeat this heralded Pauli X correction using controlled-Z and measurements until . Each attempt succeeds with probability one half independent of the input, so it terminates with probability one, using two attempts on average. Only , fixed ancillary quantum states and single-qubit quantum measurements are used. This exact physical correction has no finite worst-case measurement bound; the standard finite deterministic realization is the logical Pauli frame version, which suffices for the later output-simulation parts.