Past exam of the mathematics course of the University of Cambridge 2014 iii Paper 61 4 a ii Solution Created 2026-10-03 Updated 2026-10-06
An explicit measurement-based quantum computation pattern uses six vertices . Prepare a graph state with every vertex in and apply a Controlled-Z gate for each edgeThe first two links on each wire permit graph-state preparation of a computational-basis input followed by the logical J gate. Use the following single-qubit measurements:
All entangling edges can be made at preparation time because Controlled-Z gates commute. A future edge that does not touch a currently measured vertex can equivalently be deferred, which allows the one-bit teleportation identities to be applied in their logical order.
The two initial measurements implement with Pauli frames on the logical inputs. The first adaptive J gate then has output frame on . Propagating through gives framesup to branchwise global phase. The second adaptive J gate converts the latter intoA correction does not alter a computational-basis measurement, while an correction flips its bit. Consequently the deterministic classical postprocessing isThis reproduces the output-bit distribution of the original quantum circuit, including its known byproduct corrections.
Six-vertex graph state, adaptive equatorial measurements and classical parity correction for the two-wire circuit
. There is an additional simplification for these particular zero inputs. Since , , and , the exact final state is , independently of the angles. The requested bit is therefore fair. A single isolated graph-state vertex measured in already simulates that bit distribution; the six-vertex pattern also explicitly realizes the circuit and its corrections.
