def get_parities_from_measurements(measurements: List[Tuple[int]],
ising_operator: IsingOperator) -> Parities:
"""Get expectation values from bitstrings.
Args:
measurements (list): the measured bitstrings
ising_operator (openfermion.ops.IsingOperator): the operator
Returns:
zquantum.core.measurement.Parities: the parities of each term in the operator
"""
# check input format
if isinstance(ising_operator, IsingOperator) == False:
raise Exception("Input operator not openfermion.IsingOperator")
# Count number of occurrences of bitstrings
bitstring_frequencies = Counter(measurements)
# Count parity occurences
values = []
for _, term in enumerate(ising_operator.terms):
values.append([0, 0])
marked_qubits = [op[0] for op in term]
for bitstring, count in bitstring_frequencies.items():
bitstring_int = convert_bitstring_to_int(bitstring)
if parity_even_p(bitstring_int, marked_qubits):
values[-1][0] += count
else:
values[-1][1] += count
# Count parity occurences for pairwise products of operators
correlations = [
np.zeros((len(ising_operator.terms), len(ising_operator.terms), 2))
]
for term1_index, term1 in enumerate(ising_operator.terms):
for term2_index, term2 in enumerate(ising_operator.terms):
marked_qubits_term1 = [op[0] for op in term1]
marked_qubits_term2 = [op[0] for op in term2]
for bitstring, count in bitstring_frequencies.items():
bitstring_int = convert_bitstring_to_int(bitstring)
parity1 = parity_even_p(bitstring_int, marked_qubits_term1)
parity2 = parity_even_p(bitstring_int, marked_qubits_term2)
if parity1 == parity2:
correlations[0][term1_index, term2_index][0] += count
else:
correlations[0][term1_index, term2_index][1] += count
return Parities(np.array(values), correlations)
def get_expectation_values(
self, ising_operator: IsingOperator) -> ExpectationValues:
"""Get the expectation values of an operator from the measurements.
Args:
ising_operator (openfermion.ops.IsingOperator): the operator
Returns:
zquantum.core.measurement.ExpectationValues: the expectation values of each term in the operator
"""
# We require operator to be IsingOperator because measurements are always performed in the Z basis,
# so we need the operator to be Ising (containing only Z terms).
# A general Qubit Operator could have X or Y terms which don’t get directly measured.
if isinstance(ising_operator, IsingOperator) == False:
raise Exception("Input operator is not openfermion.IsingOperator")
# Count number of occurrences of bitstrings
bitstring_frequencies = self.get_counts()
num_measurements = len(self.bitstrings)
# Perform weighted average
expectation_values = []
for term, coefficient in ising_operator.terms.items():
expectation = 0
marked_qubits = [op[0] for op in term]
for bitstring, count in bitstring_frequencies.items():
bitstring_int = convert_bitstring_to_int(bitstring)
if parity_even_p(bitstring_int, marked_qubits):
value = float(count) / num_measurements
else:
value = -float(count) / num_measurements
expectation += np.real(coefficient) * value
expectation_values.append(np.real(expectation))
return ExpectationValues(np.array(expectation_values))
def test_parity_even_p():
state_index = 11
marked_qubits = [0, 1]
with pytest.raises(AssertionError):
parity_even_p("", marked_qubits)
assert parity_even_p(state_index, marked_qubits)