Past exam of the mathematics course of the University of Cambridge / 2014 / iii / Paper 63 / 1 / b

For an arbitrary vector $|\Psi ⟩={\sum }_{t=0}^T|{\eta }_t⟩|t⟩$, the preceding positive quadratic forms show that zero energy requiresThus all its clock components are generated by one correctly initialized input. Ifthen $\ker Q$ consists of $|\zeta ⟩\otimes |a⟩$, with unrestricted quantum witness $\zeta$. Normalization gives the computational history stateConversely, every such computational history state annihilates every propagation term and the input term, hence has zero energy. These states are exactly the ground space, not merely examples of its vectors. In particular, histories of computational basis quantum witnesses span the ground space; their coherent superpositions are also zero-energy states.

Another way to see the propagation constraint is to conjugate by $W={\sum }_t{V}_t\otimes |t⟩⟨t|$. Then $W^{†}{H}_{\mathrm{prop}}W=I\otimes E$, whereThe spectrum of a path propagation Hamiltonian has a unique uniform clock zero mode. This is the path with $T+1$ vertices. The upper summation index in the printed auxiliary formula must be $T-1$, rather than $T$, to agree with its stated dimension and spectrum; using $T$ would introduce an extra vertex.