# This code is part of Qiskit.
#
# (C) Copyright IBM 2018, 2021.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.
"""
Simulator instruction to save exact operator expectation value.
"""
from numpy import allclose
from qiskit.quantum_info import Pauli, SparsePauliOp, Operator
from qiskit.circuit import QuantumCircuit
from qiskit.extensions.exceptions import ExtensionError
from .save_data import SaveAverageData
[docs]class SaveExpectationValue(SaveAverageData):
"""Save expectation value of an operator."""
def __init__(
self,
operator,
label="expectation_value",
unnormalized=False,
pershot=False,
conditional=False,
):
r"""Instruction to save the expectation value of a Hermitian operator.
The expectation value of a Hermitian operator :math:`H` for a simulator
in quantum state :math`\rho`is given by
:math:`\langle H\rangle = \mbox{Tr}[H.\rho]`.
Args:
operator (Pauli or SparsePauliOp or Operator): a Hermitian operator.
label (str): the key for retrieving saved data from results.
unnormalized (bool): If True return save the unnormalized accumulated
or conditional accumulated expectation value
over all shot [Default: False].
pershot (bool): if True save a list of expectation values for each shot
of the simulation rather than the average over
all shots [Default: False].
conditional (bool): if True save the average or pershot data
conditional on the current classical register
values [Default: False].
Raises:
ExtensionError: if the input operator is invalid or not Hermitian.
.. note::
This instruction can be directly appended to a circuit using the
:func:`save_expectation_value` circuit method.
"""
# Convert O to SparsePauliOp representation
if isinstance(operator, Pauli):
operator = SparsePauliOp(operator)
elif not isinstance(operator, SparsePauliOp):
operator = SparsePauliOp.from_operator(Operator(operator))
if not allclose(operator.coeffs.imag, 0):
raise ExtensionError("Input operator is not Hermitian.")
params = _expval_params(operator, variance=False)
super().__init__(
"save_expval",
operator.num_qubits,
label,
unnormalized=unnormalized,
pershot=pershot,
conditional=conditional,
params=params,
)
[docs]class SaveExpectationValueVariance(SaveAverageData):
"""Save expectation value and variance of an operator."""
def __init__(
self,
operator,
label="expectation_value_variance",
unnormalized=False,
pershot=False,
conditional=False,
):
r"""Instruction to save the expectation value and variance of a Hermitian operator.
The expectation value of a Hermitian operator :math:`H` for a
simulator in quantum state :math`\rho`is given by
:math:`\langle H\rangle = \mbox{Tr}[H.\rho]`. The variance is given by
:math:`\sigma^2 = \langle H^2 \rangle - \langle H \rangle>^2`.
Args:
operator (Pauli or SparsePauliOp or Operator): a Hermitian operator.
label (str): the key for retrieving saved data from results.
unnormalized (bool): If True return save the unnormalized accumulated
or conditional accumulated expectation value
over all shot [Default: False].
pershot (bool): if True save a list of expectation values for each shot
of the simulation rather than the average over
all shots [Default: False].
conditional (bool): if True save the average or pershot data
conditional on the current classical register
values [Default: False].
Raises:
ExtensionError: if the input operator is invalid or not Hermitian.
.. note::
This instruction can be directly appended to a circuit using
the :func:`save_expectation_value` circuit method.
"""
# Convert O to SparsePauliOp representation
if isinstance(operator, Pauli):
operator = SparsePauliOp(operator)
elif not isinstance(operator, SparsePauliOp):
operator = SparsePauliOp.from_operator(Operator(operator))
if not allclose(operator.coeffs.imag, 0):
raise ExtensionError("Input operator is not Hermitian.")
params = _expval_params(operator, variance=True)
super().__init__(
"save_expval_var",
operator.num_qubits,
label,
unnormalized=unnormalized,
pershot=pershot,
conditional=conditional,
params=params,
)
def _expval_params(operator, variance=False):
# Convert O to SparsePauliOp representation
if isinstance(operator, Pauli):
operator = SparsePauliOp(operator)
elif not isinstance(operator, SparsePauliOp):
operator = SparsePauliOp.from_operator(Operator(operator))
if not isinstance(operator, SparsePauliOp):
raise ExtensionError("Invalid input operator")
params = {}
# Add Pauli basis components of O
for pauli, coeff in operator.label_iter():
if pauli in params:
coeff1 = params[pauli][0]
params[pauli] = (coeff1 + coeff.real, 0)
else:
params[pauli] = (coeff.real, 0)
# Add Pauli basis components of O^2
if variance:
for pauli, coeff in operator.dot(operator).label_iter():
if pauli in params:
coeff1, coeff2 = params[pauli]
params[pauli] = (coeff1, coeff2 + coeff.real)
else:
params[pauli] = (0, coeff.real)
# Convert to list
return list(params.items())
[docs]def save_expectation_value(
self,
operator,
qubits,
label="expectation_value",
unnormalized=False,
pershot=False,
conditional=False,
):
r"""Save the expectation value of a Hermitian operator.
Args:
operator (Pauli or SparsePauliOp or Operator): a Hermitian operator.
qubits (list): circuit qubits to apply instruction.
label (str): the key for retrieving saved data from results.
unnormalized (bool): If True return save the unnormalized accumulated
or conditional accumulated expectation value
over all shot [Default: False].
pershot (bool): if True save a list of expectation values for each
shot of the simulation rather than the average over
all shots [Default: False].
conditional (bool): if True save the average or pershot data
conditional on the current classical register
values [Default: False].
Returns:
QuantumCircuit: with attached instruction.
Raises:
ExtensionError: if the input operator is invalid or not Hermitian.
.. note::
This method appends a :class:`SaveExpectationValue` instruction to
the quantum circuit.
"""
instr = SaveExpectationValue(
operator, label=label, unnormalized=unnormalized, pershot=pershot, conditional=conditional
)
return self.append(instr, qubits)
[docs]def save_expectation_value_variance(
self,
operator,
qubits,
label="expectation_value_variance",
unnormalized=False,
pershot=False,
conditional=False,
):
r"""Save the expectation value of a Hermitian operator.
Args:
operator (Pauli or SparsePauliOp or Operator): a Hermitian operator.
qubits (list): circuit qubits to apply instruction.
label (str): the key for retrieving saved data from results.
unnormalized (bool): If True return save the unnormalized accumulated
or conditional accumulated expectation value
and variance over all shot [Default: False].
pershot (bool): if True save a list of expectation values and
variances for each shot of the simulation rather than
the average over all shots [Default: False].
conditional (bool): if True save the data conditional on the current
classical register values [Default: False].
Returns:
QuantumCircuit: with attached instruction.
Raises:
ExtensionError: if the input operator is invalid or not Hermitian.
.. note::
This method appends a :class:`SaveExpectationValueVariance`
instruction to the quantum circuit.
"""
instr = SaveExpectationValueVariance(
operator, label=label, unnormalized=unnormalized, pershot=pershot, conditional=conditional
)
return self.append(instr, qubits)
QuantumCircuit.save_expectation_value = save_expectation_value
QuantumCircuit.save_expectation_value_variance = save_expectation_value_variance