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Source code for qiskit.quantum_info.operators.symplectic.clifford

# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2020
#
# 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.
"""
Clifford operator class.
"""
# pylint: disable=invalid-name, abstract-method
import re
import numpy as np

from qiskit.exceptions import QiskitError
from qiskit.circuit import QuantumCircuit, Instruction
from qiskit.circuit.library.standard_gates import IGate, XGate, YGate, ZGate, HGate, SGate
from qiskit.quantum_info.operators.base_operator import BaseOperator
from qiskit.quantum_info.operators.operator import Operator
from qiskit.quantum_info.operators.scalar_op import ScalarOp
from qiskit.quantum_info.synthesis.clifford_decompose import decompose_clifford
from .stabilizer_table import StabilizerTable
from .clifford_circuits import _append_circuit


[docs]class Clifford(BaseOperator): """An N-qubit unitary operator from the Clifford group. **Representation** An *N*-qubit Clifford operator is stored as a length *2N* :class:`~qiskit.quantum_info.StabilizerTable` using the convention from reference [1]. * Rows 0 to *N-1* are the *destabilizer* group generators * Rows *N* to *2N-1* are the *stabilizer* group generators. The internal :class:`~qiskit.quantum_info.StabilizerTable` for the Clifford can be accessed using the :attr:`table` attribute. The destabilizer or stabilizer rows can each be accessed as a length-N Stabilizer table using :attr:`destabilizer` and :attr:`stabilizer` attributes. A more easily human readible representation of the Clifford operator can be obtained by calling the :meth:`to_dict` method. This representation is also used if a Clifford object is printed as in the following example .. jupyter-execute:: from qiskit import QuantumCircuit from qiskit.quantum_info import Clifford # Bell state generation circuit qc = QuantumCircuit(2) qc.h(0) qc.cx(0, 1) cliff = Clifford(qc) # Print the Clifford print(cliff) # Print the Clifford destabilizer rows print(cliff.destabilizer) # Print the Clifford stabilizer rows print(cliff.stabilizer) **Circuit Conversion** Clifford operators can be initialized from circuits containing *only* the following Clifford gates: :class:`~qiskit.circuit.library.IGate`, :class:`~qiskit.circuit.library.XGate`, :class:`~qiskit.circuit.library.YGate`, :class:`~qiskit.circuit.library.ZGate`, :class:`~qiskit.circuit.library.HGate`, :class:`~qiskit.circuit.library.SGate`, :class:`~qiskit.circuit.library.SdgGate`, :class:`~qiskit.circuit.library.CXGate`, :class:`~qiskit.circuit.library.CZGate`, :class:`~qiskit.circuit.library.SwapGate`. They can be converted back into a :class:`~qiskit.circuit.QuantumCircuit`, or :class:`~qiskit.circuit.Gate` object using the :meth:`~Clifford.to_circuit` or :meth:`~Clifford.to_instruction` methods respectively. Note that this decomposition is not necessarily optimal in terms of number of gates. .. note:: A minimally generating set of gates for Clifford circuits is the :class:`~qiskit.circuit.library.HGate` and :class:`~qiskit.circuit.library.SGate` gate and *either* the :class:`~qiskit.circuit.library.CXGate` or :class:`~qiskit.circuit.library.CZGate` two-qubit gate. Clifford operators can also be converted to :class:`~qiskit.quantum_info.Operator` objects using the :meth:`to_operator` method. This is done via decomposing to a circuit, and then simulating the circuit as a unitary operator. References: 1. S. Aaronson, D. Gottesman, *Improved Simulation of Stabilizer Circuits*, Phys. Rev. A 70, 052328 (2004). `arXiv:quant-ph/0406196 <https://arxiv.org/abs/quant-ph/0406196>`_ """
[docs] def __init__(self, data, validate=True): """Initialize an operator object.""" # Initialize from another Clifford by sharing the underlying # StabilizerTable if isinstance(data, Clifford): self._table = data._table # Initialize from ScalarOp as N-qubit identity discarding any global phase elif isinstance(data, ScalarOp): if not data.is_unitary() or set(data._input_dims) != {2}: raise QiskitError("Can only initalize from N-qubit identity ScalarOp.") self._table = StabilizerTable( np.eye(2 * len(data._input_dims), dtype=bool)) # Initialize from a QuantumCircuit or Instruction object elif isinstance(data, (QuantumCircuit, Instruction)): self._table = Clifford.from_circuit(data)._table # Initialize StabilizerTable directly from the data else: self._table = StabilizerTable(data) # Validate table is a symplectic matrix if validate and not Clifford._is_symplectic(self._table.array): raise QiskitError( 'Invalid Clifford. Input StabilizerTable is not a valid' ' symplectic matrix.') # Initialize BaseOperator dims = self._table.num_qubits * (2,) super().__init__(dims, dims)
def __repr__(self): return 'Clifford({})'.format(repr(self.table)) def __str__(self): return 'Clifford: Stabilizer = {}, Destabilizer = {}'.format( str(self.stabilizer.to_labels()), str(self.destabilizer.to_labels())) def __eq__(self, other): """Check if two Clifford tables are equal""" return super().__eq__(other) and self._table == other._table # --------------------------------------------------------------------- # Attributes # --------------------------------------------------------------------- def __getitem__(self, key): """Return a stabilizer Pauli row""" return self._table.__getitem__(key) def __setitem__(self, key, value): """Set a stabilizer Pauli row""" self._table.__setitem__(key, value) @property def table(self): """Return StabilizerTable""" return self._table @table.setter def table(self, value): """Set the stabilizer table""" # Note this setter cannot change the size of the Clifford # It can only replace the contents of the StabilizerTable with # another StabilizerTable of the same size. if not isinstance(value, StabilizerTable): value = StabilizerTable(value) self._table._array[:, :] = value._table._array self._table._phase[:] = value._table._phase @property def stabilizer(self): """Return the stabilizer block of the StabilizerTable.""" return StabilizerTable(self._table[self.num_qubits:2*self.num_qubits]) @stabilizer.setter def stabilizer(self, value): """Set the value of stabilizer block of the StabilizerTable""" inds = slice(self.num_qubits, 2*self.num_qubits) self._table.__setitem__(inds, value) @property def destabilizer(self): """Return the destabilizer block of the StabilizerTable.""" return StabilizerTable(self._table[0:self.num_qubits]) @destabilizer.setter def destabilizer(self, value): """Set the value of destabilizer block of the StabilizerTable""" inds = slice(0, self.num_qubits) self._table.__setitem__(inds, value) # --------------------------------------------------------------------- # Utility Operator methods # ---------------------------------------------------------------------
[docs] def is_unitary(self): """Return True if the Clifford table is valid.""" # A valid Clifford is always unitary, so this function is really # checking that the underlying Stabilizer table array is a valid # Clifford array. return Clifford._is_symplectic(self.table.array)
# --------------------------------------------------------------------- # BaseOperator Abstract Methods # ---------------------------------------------------------------------
[docs] def conjugate(self): """Return the conjugate of the Clifford.""" return Clifford._conjugate_transpose(self, 'C')
[docs] def adjoint(self): """Return the conjugate transpose of the Clifford""" return Clifford._conjugate_transpose(self, 'A')
[docs] def transpose(self): """Return the transpose of the Clifford.""" return Clifford._conjugate_transpose(self, 'T')
[docs] def compose(self, other, qargs=None, front=False): """Return the composed operator. Args: other (Clifford): an operator object. qargs (list or None): a list of subsystem positions to apply other on. If None apply on all subsystems [default: None]. front (bool): If True compose using right operator multiplication, instead of left multiplication [default: False]. Returns: Clifford: The operator self @ other. Raise: QiskitError: if operators have incompatible dimensions for composition. Additional Information: Composition (``@``) is defined as `left` matrix multiplication for matrix operators. That is that ``A @ B`` is equal to ``B * A``. Setting ``front=True`` returns `right` matrix multiplication ``A * B`` and is equivalent to the :meth:`dot` method. """ if qargs is None: qargs = getattr(other, 'qargs', None) # If other is a QuantumCircuit we can more efficiently compose # using the _append_circuit method to update each gate recursively # to the current Clifford, rather than converting to a Clifford first # and then doing the composition of tables. if not front and isinstance(other, (QuantumCircuit, Instruction)): ret = self.copy() _append_circuit(ret, other, qargs=qargs) return ret if not isinstance(other, Clifford): other = Clifford(other) # Validate dimensions. Note we don't need to get updated input or # output dimensions from `_get_compose_dims` as the dimensions of the # Clifford object can't be changed by composition self._get_compose_dims(other, qargs, front) # Pad other with identities if composeing on subsystem other = self._pad_with_identity(other, qargs) return self._compose_clifford(other, front=front)
[docs] def dot(self, other, qargs=None): """Return the right multiplied operator self * other. Args: other (Clifford): an operator object. qargs (list or None): a list of subsystem positions to apply other on. If None apply on all subsystems [default: None]. Returns: Clifford: The operator self * other. Raises: QiskitError: if operators have incompatible dimensions for composition. """ return super().dot(other, qargs=qargs)
[docs] def tensor(self, other): """Return the tensor product operator self ⊗ other. Args: other (Clifford): a operator subclass object. Returns: Clifford: the tensor product operator self ⊗ other. """ return self._tensor_product(other, reverse=False)
[docs] def expand(self, other): """Return the tensor product operator other ⊗ self. Args: other (Clifford): an operator object. Returns: Clifford: the tensor product operator other ⊗ self. """ return self._tensor_product(other, reverse=True)
# --------------------------------------------------------------------- # Representation conversions # ---------------------------------------------------------------------
[docs] def to_dict(self): """Return dictionary represenation of Clifford object.""" return { "stabilizer": self.stabilizer.to_labels(), "destabilizer": self.destabilizer.to_labels() }
[docs] @staticmethod def from_dict(obj): """Load a Clifford from a dictionary""" destabilizer = StabilizerTable.from_labels(obj.get('destabilizer')) stabilizer = StabilizerTable.from_labels(obj.get('stabilizer')) return Clifford(destabilizer + stabilizer)
[docs] def to_matrix(self): """Convert operator to Numpy matrix.""" return self.to_operator().data
[docs] def to_operator(self): """Convert to an Operator object.""" return Operator(self.to_instruction())
[docs] def to_circuit(self): """Return a QuantumCircuit implementing the Clifford. For N <= 3 qubits this is based on optimal CX cost decomposition from reference [1]. For N > 3 qubits this is done using the general non-optimal compilation routine from reference [2]. Return: QuantumCircuit: a circuit implementation of the Clifford. References: 1. S. Bravyi, D. Maslov, *Hadamard-free circuits expose the structure of the Clifford group*, `arXiv:2003.09412 [quant-ph] <https://arxiv.org/abs/2003.09412>`_ 2. S. Aaronson, D. Gottesman, *Improved Simulation of Stabilizer Circuits*, Phys. Rev. A 70, 052328 (2004). `arXiv:quant-ph/0406196 <https://arxiv.org/abs/quant-ph/0406196>`_ """ return decompose_clifford(self)
[docs] def to_instruction(self): """Return a Gate instruction implementing the Clifford.""" return self.to_circuit().to_gate()
[docs] @staticmethod def from_circuit(circuit): """Initialize from a QuantumCircuit or Instruction. Args: circuit (QuantumCircuit or ~qiskit.circuit.Instruction): instruction to initialize. Returns: Clifford: the Clifford object for the instruction. Raises: QiskitError: if the input instruction is non-Clifford or contains classical register instruction. """ if not isinstance(circuit, (QuantumCircuit, Instruction)): raise QiskitError("Input must be a QuantumCircuit or Instruction") # Convert circuit to an instruction if isinstance(circuit, QuantumCircuit): circuit = circuit.to_instruction() # Initialize an identity Clifford clifford = Clifford(np.eye(2 * circuit.num_qubits), validate=False) _append_circuit(clifford, circuit) return clifford
[docs] @staticmethod def from_label(label): """Return a tensor product of single-qubit Clifford gates. Args: label (string): single-qubit operator string. Returns: Clifford: The N-qubit Clifford operator. Raises: QiskitError: if the label contains invalid characters. Additional Information: The labels correspond to the single-qubit Cliffords are * - Label - Stabilizer - Destabilizer * - ``"I"`` - +Z - +X * - ``"X"`` - -Z - +X * - ``"Y"`` - -Z - -X * - ``"Z"`` - +Z - -X * - ``"H"`` - +X - +Z * - ``"S"`` - +Z - +Y """ # Check label is valid label_gates = { 'I': IGate(), 'X': XGate(), 'Y': YGate(), 'Z': ZGate(), 'H': HGate(), 'S': SGate() } if re.match(r'^[IXYZHS\-+]+$', label) is None: raise QiskitError('Label contains invalid characters.') # Initialize an identity matrix and apply each gate num_qubits = len(label) op = Clifford(np.eye(2 * num_qubits, dtype=bool)) for qubit, char in enumerate(reversed(label)): _append_circuit(op, label_gates[char], qargs=[qubit]) return op
# --------------------------------------------------------------------- # Internal helper functions # --------------------------------------------------------------------- @staticmethod def _is_symplectic(mat): """Return True if input is symplectic matrix.""" # Condition is # table.T * [[0, 1], [1, 0]] * table = [[0, 1], [1, 0]] # where we are block matrix multiplying using symplectic product dim = len(mat) // 2 if mat.shape != (2 * dim, 2 * dim): return False one = np.eye(dim, dtype=int) zero = np.zeros((dim, dim), dtype=int) seye = np.block([[zero, one], [one, zero]]) arr = mat.astype(int) return np.array_equal(np.mod(arr.T.dot(seye).dot(arr), 2), seye) @staticmethod def _conjugate_transpose(clifford, method): """Return the adjoint, conjugate, or transpose of the Clifford. Args: clifford (Clifford): a clifford object. method (str): what function to apply 'A', 'C', or 'T'. Returns: Clifford: the modified clifford. """ ret = clifford.copy() if method in ['A', 'T']: # Apply inverse # Update table tmp = ret.destabilizer.X.copy() ret.destabilizer.X = ret.stabilizer.Z.T ret.destabilizer.Z = ret.destabilizer.Z.T ret.stabilizer.X = ret.stabilizer.X.T ret.stabilizer.Z = tmp.T # Update phase ret.table.phase ^= clifford.dot(ret).table.phase if method in ['C', 'T']: # Apply conjugate ret.table.phase ^= np.mod(np.sum( ret.table.X & ret.table.Z, axis=1), 2).astype(bool) return ret def _tensor_product(self, other, reverse=False): """Return the tensor product operator. Args: other (Clifford): another Clifford operator. reverse (bool): If False return self ⊗ other, if True return if True return (other ⊗ self) [Default: False]. Returns: Clifford: the tensor product operator. Raises: QiskitError: if other cannot be converted into an Clifford. """ if not isinstance(other, Clifford): other = Clifford(other) if reverse: cliff0 = self cliff1 = other else: cliff0 = other cliff1 = self # Pad stabilizers and destabilizers destab = (cliff0.destabilizer.expand(cliff1.num_qubits * 'I') + cliff1.destabilizer.tensor(cliff0.num_qubits * 'I')) stab = (cliff0.stabilizer.expand(cliff1.num_qubits * 'I') + cliff1.stabilizer.tensor(cliff0.num_qubits * 'I')) # Add the padded table return Clifford(destab + stab, validate=False) def _pad_with_identity(self, clifford, qargs): """Pad Clifford with identities on other subsystems.""" if qargs is None: return clifford padded = Clifford(StabilizerTable( np.eye(2 * self.num_qubits, dtype=bool)), validate=False) inds = list(qargs) + [self.num_qubits + i for i in qargs] # Pad Pauli array pauli = clifford.table.array for i, pos in enumerate(qargs): padded.table.array[inds, pos] = pauli[:, i] padded.table.array[inds, self.num_qubits + pos] = pauli[:, clifford.num_qubits + i] # Pad phase padded.table.phase[inds] = clifford.table.phase return padded def _compose_clifford(self, other, front=False): """Return the composition channel assume other is Clifford of same size as self.""" if front: table1 = self.table table2 = other.table else: table1 = other.table table2 = self.table num_qubits = self.num_qubits array1 = table1.array.astype(int) phase1 = table1.phase.astype(int) array2 = table2.array.astype(int) phase2 = table2.phase.astype(int) # Update Pauli table pauli = StabilizerTable(array2.dot(array1) % 2) # Add phases phase = np.mod(array2.dot(phase1) + phase2, 2) # Correcting for phase due to Pauli multiplication ifacts = np.zeros(2 * num_qubits, dtype=int) for k in range(2 * num_qubits): row2 = array2[k] x2 = table2.X[k] z2 = table2.Z[k] # Adding a factor of i for each Y in the image of an operator under the # first operation, since Y=iXZ ifacts[k] += np.sum(x2 & z2) # Adding factors of i due to qubit-wise Pauli multiplication for j in range(num_qubits): x = 0 z = 0 for i in range(2 * num_qubits): if row2[i]: x1 = array1[i, j] z1 = array1[i, j + num_qubits] if (x | z) & (x1 | z1): val = np.mod(np.abs(3 * z1 - x1) - np.abs(3 * z - x) - 1, 3) if val == 0: ifacts[k] += 1 elif val == 1: ifacts[k] -= 1 x = np.mod(x + x1, 2) z = np.mod(z + z1, 2) p = np.mod(ifacts, 4) // 2 phase = np.mod(phase + p, 2) return Clifford(StabilizerTable(pauli, phase), validate=False)

© Copyright 2020, Qiskit Development Team. Last updated on 2021/02/18.

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