Bad Qubits — Learn Quantum by Doing

Multiple choice

In the continuous-time quantum walk (CTQW) model of Farhi and Gutmann, the walker evolves on a graph G under the Schrodinger equation. What plays the role of the Hamiltonian?

The identity matrix, so the state never changes in time
A separate coin operator applied to an extra coin qubit at each step
The conditional shift operator that moves the walker left or right
The adjacency matrix A of the graph (H = -gamma*A), up to a sign convention and rate constant gamma