Source code for qiskit.quantum_info.operators.operator
# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2019.
#
# 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.
"""
Matrix Operator class.
"""
import copy
import re
from numbers import Number
import numpy as np
from qiskit.circuit.quantumcircuit import QuantumCircuit
from qiskit.circuit.instruction import Instruction
from qiskit.circuit.library.standard_gates import IGate, XGate, YGate, ZGate, HGate, SGate, TGate
from qiskit.exceptions import QiskitError
from qiskit.quantum_info.operators.predicates import is_unitary_matrix, matrix_equal
from qiskit.quantum_info.operators.linear_op import LinearOp
from qiskit.quantum_info.operators.mixins import generate_apidocs
[docs]class Operator(LinearOp):
r"""Matrix operator class
This represents a matrix operator :math:`M` that will
:meth:`~Statevector.evolve` a :class:`Statevector` :math:`|\psi\rangle`
by matrix-vector multiplication
.. math::
|\psi\rangle \mapsto M|\psi\rangle,
and will :meth:`~DensityMatrix.evolve` a :class:`DensityMatrix` :math:`\rho`
by left and right multiplication
.. math::
\rho \mapsto M \rho M^\dagger.
"""
def __init__(self, data, input_dims=None, output_dims=None):
"""Initialize an operator object.
Args:
data (QuantumCircuit or
Instruction or
BaseOperator or
matrix): data to initialize operator.
input_dims (tuple): the input subsystem dimensions.
[Default: None]
output_dims (tuple): the output subsystem dimensions.
[Default: None]
Raises:
QiskitError: if input data cannot be initialized as an operator.
Additional Information:
If the input or output dimensions are None, they will be
automatically determined from the input data. If the input data is
a Numpy array of shape (2**N, 2**N) qubit systems will be used. If
the input operator is not an N-qubit operator, it will assign a
single subsystem with dimension specified by the shape of the input.
"""
op_shape = None
if isinstance(data, (list, np.ndarray)):
# Default initialization from list or numpy array matrix
self._data = np.asarray(data, dtype=complex)
elif isinstance(data, (QuantumCircuit, Instruction)):
# If the input is a Terra QuantumCircuit or Instruction we
# perform a simulation to construct the unitary operator.
# This will only work if the circuit or instruction can be
# defined in terms of unitary gate instructions which have a
# 'to_matrix' method defined. Any other instructions such as
# conditional gates, measure, or reset will cause an
# exception to be raised.
self._data = self._init_instruction(data).data
elif hasattr(data, "to_operator"):
# If the data object has a 'to_operator' attribute this is given
# higher preference than the 'to_matrix' method for initializing
# an Operator object.
data = data.to_operator()
self._data = data.data
op_shape = data._op_shape
elif hasattr(data, "to_matrix"):
# If no 'to_operator' attribute exists we next look for a
# 'to_matrix' attribute to a matrix that will be cast into
# a complex numpy matrix.
self._data = np.asarray(data.to_matrix(), dtype=complex)
else:
raise QiskitError("Invalid input data format for Operator")
super().__init__(
op_shape=op_shape,
input_dims=input_dims,
output_dims=output_dims,
shape=self._data.shape,
)
def __array__(self, dtype=None):
if dtype:
return np.asarray(self.data, dtype=dtype)
return self.data
def __repr__(self):
prefix = "Operator("
pad = len(prefix) * " "
return "{}{},\n{}input_dims={}, output_dims={})".format(
prefix,
np.array2string(self.data, separator=", ", prefix=prefix),
pad,
self.input_dims(),
self.output_dims(),
)
def __eq__(self, other):
"""Test if two Operators are equal."""
if not super().__eq__(other):
return False
return np.allclose(self.data, other.data, rtol=self.rtol, atol=self.atol)
@property
def data(self):
"""Return data."""
return self._data
@property
def settings(self):
"""Return operator settings."""
return {
"data": self._data,
"input_dims": self.input_dims(),
"output_dims": self.output_dims(),
}
[docs] @classmethod
def from_label(cls, label):
"""Return a tensor product of single-qubit operators.
Args:
label (string): single-qubit operator string.
Returns:
Operator: The N-qubit operator.
Raises:
QiskitError: if the label contains invalid characters, or the
length of the label is larger than an explicitly
specified num_qubits.
Additional Information:
The labels correspond to the single-qubit matrices:
'I': [[1, 0], [0, 1]]
'X': [[0, 1], [1, 0]]
'Y': [[0, -1j], [1j, 0]]
'Z': [[1, 0], [0, -1]]
'H': [[1, 1], [1, -1]] / sqrt(2)
'S': [[1, 0], [0 , 1j]]
'T': [[1, 0], [0, (1+1j) / sqrt(2)]]
'0': [[1, 0], [0, 0]]
'1': [[0, 0], [0, 1]]
'+': [[0.5, 0.5], [0.5 , 0.5]]
'-': [[0.5, -0.5], [-0.5 , 0.5]]
'r': [[0.5, -0.5j], [0.5j , 0.5]]
'l': [[0.5, 0.5j], [-0.5j , 0.5]]
"""
# Check label is valid
label_mats = {
"I": IGate().to_matrix(),
"X": XGate().to_matrix(),
"Y": YGate().to_matrix(),
"Z": ZGate().to_matrix(),
"H": HGate().to_matrix(),
"S": SGate().to_matrix(),
"T": TGate().to_matrix(),
"0": np.array([[1, 0], [0, 0]], dtype=complex),
"1": np.array([[0, 0], [0, 1]], dtype=complex),
"+": np.array([[0.5, 0.5], [0.5, 0.5]], dtype=complex),
"-": np.array([[0.5, -0.5], [-0.5, 0.5]], dtype=complex),
"r": np.array([[0.5, -0.5j], [0.5j, 0.5]], dtype=complex),
"l": np.array([[0.5, 0.5j], [-0.5j, 0.5]], dtype=complex),
}
if re.match(r"^[IXYZHST01rl\-+]+$", label) is None:
raise QiskitError("Label contains invalid characters.")
# Initialize an identity matrix and apply each gate
num_qubits = len(label)
op = Operator(np.eye(2 ** num_qubits, dtype=complex))
for qubit, char in enumerate(reversed(label)):
if char != "I":
op = op.compose(label_mats[char], qargs=[qubit])
return op
[docs] def is_unitary(self, atol=None, rtol=None):
"""Return True if operator is a unitary matrix."""
if atol is None:
atol = self.atol
if rtol is None:
rtol = self.rtol
return is_unitary_matrix(self._data, rtol=rtol, atol=atol)
[docs] def to_instruction(self):
"""Convert to a UnitaryGate instruction."""
# pylint: disable=cyclic-import
from qiskit.extensions.unitary import UnitaryGate
return UnitaryGate(self.data)
[docs] def conjugate(self):
# Make a shallow copy and update array
ret = copy.copy(self)
ret._data = np.conj(self._data)
return ret
[docs] def transpose(self):
# Make a shallow copy and update array
ret = copy.copy(self)
ret._data = np.transpose(self._data)
ret._op_shape = self._op_shape.transpose()
return ret
[docs] def compose(self, other, qargs=None, front=False):
if qargs is None:
qargs = getattr(other, "qargs", None)
if not isinstance(other, Operator):
other = Operator(other)
# Validate dimensions are compatible and return the composed
# operator dimensions
new_shape = self._op_shape.compose(other._op_shape, qargs, front)
input_dims = new_shape.dims_r()
output_dims = new_shape.dims_l()
# Full composition of operators
if qargs is None:
if front:
# Composition self * other
data = np.dot(self._data, other.data)
else:
# Composition other * self
data = np.dot(other.data, self._data)
ret = Operator(data, input_dims, output_dims)
ret._op_shape = new_shape
return ret
# Compose with other on subsystem
num_qargs_l, num_qargs_r = self._op_shape.num_qargs
if front:
num_indices = num_qargs_r
shift = num_qargs_l
right_mul = True
else:
num_indices = num_qargs_l
shift = 0
right_mul = False
# Reshape current matrix
# Note that we must reverse the subsystem dimension order as
# qubit 0 corresponds to the right-most position in the tensor
# product, which is the last tensor wire index.
tensor = np.reshape(self.data, self._op_shape.tensor_shape)
mat = np.reshape(other.data, other._op_shape.tensor_shape)
indices = [num_indices - 1 - qubit for qubit in qargs]
final_shape = [np.product(output_dims), np.product(input_dims)]
data = np.reshape(
Operator._einsum_matmul(tensor, mat, indices, shift, right_mul), final_shape
)
ret = Operator(data, input_dims, output_dims)
ret._op_shape = new_shape
return ret
[docs] def power(self, n):
"""Return the matrix power of the operator.
Args:
n (float): the power to raise the matrix to.
Returns:
Operator: the resulting operator ``O ** n``.
Raises:
QiskitError: if the input and output dimensions of the operator
are not equal.
"""
if self.input_dims() != self.output_dims():
raise QiskitError("Can only power with input_dims = output_dims.")
ret = copy.copy(self)
ret._data = np.linalg.matrix_power(self.data, n)
return ret
[docs] def tensor(self, other):
if not isinstance(other, Operator):
other = Operator(other)
return self._tensor(self, other)
[docs] def expand(self, other):
if not isinstance(other, Operator):
other = Operator(other)
return self._tensor(other, self)
@classmethod
def _tensor(cls, a, b):
ret = copy.copy(a)
ret._op_shape = a._op_shape.tensor(b._op_shape)
ret._data = np.kron(a.data, b.data)
return ret
def _add(self, other, qargs=None):
"""Return the operator self + other.
If ``qargs`` are specified the other operator will be added
assuming it is identity on all other subsystems.
Args:
other (Operator): an operator object.
qargs (None or list): optional subsystems to add on
(Default: None)
Returns:
Operator: the operator self + other.
Raises:
QiskitError: if other is not an operator, or has incompatible
dimensions.
"""
# pylint: disable=cyclic-import
from qiskit.quantum_info.operators.scalar_op import ScalarOp
if qargs is None:
qargs = getattr(other, "qargs", None)
if not isinstance(other, Operator):
other = Operator(other)
self._op_shape._validate_add(other._op_shape, qargs)
other = ScalarOp._pad_with_identity(self, other, qargs)
ret = copy.copy(self)
ret._data = self.data + other.data
return ret
def _multiply(self, other):
"""Return the operator self * other.
Args:
other (complex): a complex number.
Returns:
Operator: the operator other * self.
Raises:
QiskitError: if other is not a valid complex number.
"""
if not isinstance(other, Number):
raise QiskitError("other is not a number")
ret = copy.copy(self)
ret._data = other * self._data
return ret
[docs] def equiv(self, other, rtol=None, atol=None):
"""Return True if operators are equivalent up to global phase.
Args:
other (Operator): an operator object.
rtol (float): relative tolerance value for comparison.
atol (float): absolute tolerance value for comparison.
Returns:
bool: True if operators are equivalent up to global phase.
"""
if not isinstance(other, Operator):
try:
other = Operator(other)
except QiskitError:
return False
if self.dim != other.dim:
return False
if atol is None:
atol = self.atol
if rtol is None:
rtol = self.rtol
return matrix_equal(self.data, other.data, ignore_phase=True, rtol=rtol, atol=atol)
[docs] def reverse_qargs(self):
r"""Return an Operator with reversed subsystem ordering.
For a tensor product operator this is equivalent to reversing
the order of tensor product subsystems. For an operator
:math:`A = A_{n-1} \otimes ... \otimes A_0`
the returned operator will be
:math:`A_0 \otimes ... \otimes A_{n-1}`.
Returns:
Operator: the operator with reversed subsystem order.
"""
ret = copy.copy(self)
axes = tuple(range(self._op_shape._num_qargs_l - 1, -1, -1))
axes = axes + tuple(len(axes) + i for i in axes)
ret._data = np.reshape(
np.transpose(np.reshape(self.data, self._op_shape.tensor_shape), axes),
self._op_shape.shape,
)
ret._op_shape = self._op_shape.reverse()
return ret
@classmethod
def _einsum_matmul(cls, tensor, mat, indices, shift=0, right_mul=False):
"""Perform a contraction using Numpy.einsum
Args:
tensor (np.array): a vector or matrix reshaped to a rank-N tensor.
mat (np.array): a matrix reshaped to a rank-2M tensor.
indices (list): tensor indices to contract with mat.
shift (int): shift for indices of tensor to contract [Default: 0].
right_mul (bool): if True right multiply tensor by mat
(else left multiply) [Default: False].
Returns:
Numpy.ndarray: the matrix multiplied rank-N tensor.
Raises:
QiskitError: if mat is not an even rank tensor.
"""
rank = tensor.ndim
rank_mat = mat.ndim
if rank_mat % 2 != 0:
raise QiskitError("Contracted matrix must have an even number of indices.")
# Get einsum indices for tensor
indices_tensor = list(range(rank))
for j, index in enumerate(indices):
indices_tensor[index + shift] = rank + j
# Get einsum indices for mat
mat_contract = list(reversed(range(rank, rank + len(indices))))
mat_free = [index + shift for index in reversed(indices)]
if right_mul:
indices_mat = mat_contract + mat_free
else:
indices_mat = mat_free + mat_contract
return np.einsum(tensor, indices_tensor, mat, indices_mat)
@classmethod
def _init_instruction(cls, instruction):
"""Convert a QuantumCircuit or Instruction to an Operator."""
# Initialize an identity operator of the correct size of the circuit
if hasattr(instruction, "__array__"):
return Operator(np.array(instruction, dtype=complex))
dimension = 2 ** instruction.num_qubits
op = Operator(np.eye(dimension))
# Convert circuit to an instruction
if isinstance(instruction, QuantumCircuit):
instruction = instruction.to_instruction()
op._append_instruction(instruction)
return op
@classmethod
def _instruction_to_matrix(cls, obj):
"""Return Operator for instruction if defined or None otherwise."""
if not isinstance(obj, Instruction):
raise QiskitError("Input is not an instruction.")
mat = None
if hasattr(obj, "to_matrix"):
# If instruction is a gate first we see if it has a
# `to_matrix` definition and if so use that.
try:
mat = obj.to_matrix()
except QiskitError:
pass
return mat
def _append_instruction(self, obj, qargs=None):
"""Update the current Operator by apply an instruction."""
from qiskit.circuit.barrier import Barrier
from .scalar_op import ScalarOp
mat = self._instruction_to_matrix(obj)
if mat is not None:
# Perform the composition and inplace update the current state
# of the operator
op = self.compose(mat, qargs=qargs)
self._data = op.data
elif isinstance(obj, Barrier):
return
else:
# If the instruction doesn't have a matrix defined we use its
# circuit decomposition definition if it exists, otherwise we
# cannot compose this gate and raise an error.
if obj.definition is None:
raise QiskitError(f"Cannot apply Instruction: {obj.name}")
if not isinstance(obj.definition, QuantumCircuit):
raise QiskitError(
'Instruction "{}" '
"definition is {} but expected QuantumCircuit.".format(
obj.name, type(obj.definition)
)
)
if obj.definition.global_phase:
dimension = 2 ** obj.num_qubits
op = self.compose(
ScalarOp(dimension, np.exp(1j * float(obj.definition.global_phase))),
qargs=qargs,
)
self._data = op.data
flat_instr = obj.definition
bit_indices = {
bit: index
for bits in [flat_instr.qubits, flat_instr.clbits]
for index, bit in enumerate(bits)
}
for instr, qregs, cregs in flat_instr:
if cregs:
raise QiskitError(
f"Cannot apply instruction with classical registers: {instr.name}"
)
# Get the integer position of the flat register
if qargs is None:
new_qargs = [bit_indices[tup] for tup in qregs]
else:
new_qargs = [qargs[bit_indices[tup]] for tup in qregs]
self._append_instruction(instr, qargs=new_qargs)
# Update docstrings for API docs
generate_apidocs(Operator)