def get_expectation_value(qubit_op, wavefunction, reverse_operator=True):
"""Get the expectation value of a qubit operator with respect to a wavefunction.
Args:
qubit_op (openfermion.ops.QubitOperator): the operator
wavefunction (pyquil.wavefunction.Wavefunction): the wavefunction
reverse_operator (boolean): whether to reverse order of qubit operator
before computing expectation value. This should be True if the convention
of the basis states used for the wavefunction is the opposite of the one in
the qubit operator. This is the case, e.g. when the wavefunction comes from
Pyquil.
Returns:
complex: the expectation value
"""
n_qubits = wavefunction.amplitudes.shape[0].bit_length() - 1
# Convert the qubit operator to a sparse matrix. Note that the qubit indices
# must be reversed because OpenFermion and pyquil use different conventions
# for how to order the computational basis states!
if reverse_operator:
qubit_op = reverse_qubit_order(qubit_op, n_qubits=n_qubits)
sparse_op = get_sparse_operator(qubit_op, n_qubits=n_qubits)
# Computer the expectation value
exp_val = openfermion_expectation(sparse_op, wavefunction.amplitudes)
return exp_val
def get_expectation_value(qubit_op: QubitOperator,
wavefunction: Wavefunction,
reverse_operator: bool = False) -> complex:
"""Get the expectation value of a qubit operator with respect to a wavefunction.
Args:
qubit_op: the operator
wavefunction: the wavefunction
reverse_operator: whether to reverse order of qubit operator
before computing expectation value. This should be True if the convention
of the basis states used for the wavefunction is the opposite of the one in
the qubit operator. This is the case when the wavefunction uses
Rigetti convention (https://arxiv.org/abs/1711.02086) of ordering qubits.
Returns:
the expectation value
"""
n_qubits = wavefunction.amplitudes.shape[0].bit_length() - 1
# Convert the qubit operator to a sparse matrix. Note that the qubit indices
# must be reversed because OpenFermion and our Wavefunction use
# different conventions for how to order the computational basis states!
if reverse_operator:
qubit_op = reverse_qubit_order(qubit_op, n_qubits=n_qubits)
sparse_op = get_sparse_operator(qubit_op, n_qubits=n_qubits)
# Computer the expectation value
exp_val = openfermion_expectation(sparse_op, wavefunction.amplitudes)
return exp_val