What additional options are available in QSE?

In addition to the necessary ingredients already discussed in How do I use QSE?, there are a few additional options included in the implementation.

Caching Pauli Strings to Expectation Values

Specifically, in order to save runtime, the QSE implementation supports the use of a cache that maps pauli strings to their expectation values. This is taken as an additional parameter in the execute_with_qse function.

Warning

The cache object is modified in place when passing it to execute_with_qse.

The inclusion of the cache significantly speeds up the runtime and avoids the need for re-computation of already computed values. Furthermore, since the cache is modified in place, it can be reused as long as the noise model remains the same.

Requirements for Check Operators

When specifying the check operators, it is not necessary to specify the full exponential number of operators. As many or as few operators can be specified. The tradeoff is the fidelity of the projected state.