Fig. 2
From: Interfering trajectories in experimental quantum-enhanced stochastic simulation
Conceptual representation of the multiple-step quantum simulation of a perturbed coin. The memory system of the simulator is initialised in a qubit state |Si〉, where i ∈ {0, 1} depends on the past of the process. The ancillary qubits are all initialised in a fixed known state |0〉—the logical zero state—and thus contain no information about the past of the process. At each time step tk, the simulator interacts with the kth ancilla through the same unitary operator U. The inset shows how we implement the relevant unitary operator. The gates are a controlled-X, a single-qubit rotation R such that R|0〉 = |S0〉, and a controlled-V such that VR|1〉 = |S1〉. This sequence of interactions results in an entangled internal state that includes all the ancillary qubits and the memory state of the simulator. Measuring the ancillas samples the statistical distribution of the process, and at the same time the internal state of the simulator collapses into the correct memory state required for further simulation steps
