QSimSimulator.simulate_expectation_values for identity operators is wrong

[yor-dev]

QSimSimulator.simulate_expectation_values returns wrong results for identity operators in observables.
As the following example shows, identity operators in observables seem to be regarded as zero matrices.

This is a straightforward example.

import cirq
import qsimcirq

circuit = cirq.Circuit()
q = cirq.LineQubit.range(1)
ps = cirq.PauliString(cirq.I(q[0]))

sim = qsimcirq.QSimSimulator()
evs = sim.simulate_expectation_values(
    circuit, ps)
print("qsim", evs)

cirq_sim = cirq.Simulator()
evs = cirq_sim.simulate_expectation_values(
    circuit, ps)
print("cirq", evs)

And the result is here.

qsim [0j]
cirq [(1+0j)]

[95-martin-orion]

Identity operators are ignored because cirq.PauliString does not include identities in its .items() representation. The reason for this is tied to some messy details of Cirq's concept of Paulis; thankfully, a workaround is possible (#566).