Quantum Constant Propagation

A quantum circuit is often executed on the initial state where each qubit is in the zero state. Therefore, we propose to perform a symbolic execution of the circuit. Our approach simulates groups of entangled qubits exactly up to a given complexity. Here, the complexity corresponds to the number of basis states expressing the quantum state of one entanglement group. By doing that, the groups need neither be determined upfront nor be bound by the number of involved qubits. Still, we ensure that the simulation runs in polynomial time - opposed to exponential time as required for the simulation of the entire circuit. The information made available at gates is exploited to remove superfluous controls and gates. We implemented our approach in the tool quantum constant propagation (QCP) and evaluated it on the circuits in the benchmark suite MQTBench. By applying our tool, only the work that cannot be carried out efficiently on a classical computer is left for the quantum computer, hence exploiting the strengths of both worlds.