Source code for pennylane.devices.default_mixed

# Copyright 2018-2021 Xanadu Quantum Technologies Inc.

# Licensed under the Apache License, Version 2.0 (the "License");
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The default.mixed device is PennyLane's standard qubit simulator for mixed-state computations.

It implements the necessary :class:`~pennylane.Device` methods as well as some built-in
qubit :doc:`operations </introduction/operations>`, providing a simple mixed-state simulation of
qubit-based quantum circuits.

import functools
import itertools
from string import ascii_letters as ABC

import numpy as np
from pennylane import QubitDevice, QubitStateVector, BasisState, DeviceError
from pennylane.operation import DiagonalOperation, Channel
from pennylane.wires import Wires
from .._version import __version__

ABC_ARRAY = np.array(list(ABC))
tolerance = 1e-10

[docs]class DefaultMixed(QubitDevice): """Default qubit device for performing mixed-state computations in PennyLane. Args: wires (int, Iterable[Number, str]): Number of subsystems represented by the device, or iterable that contains unique labels for the subsystems as numbers (i.e., ``[-1, 0, 2]``) or strings (``['ancilla', 'q1', 'q2']``). shots (None, int): Number of times the circuit should be evaluated (or sampled) to estimate the expectation values. Defaults to ``None`` if not specified, which means that outputs are computed exactly. cache (int): Number of device executions to store in a cache to speed up subsequent executions. A value of ``0`` indicates that no caching will take place. Once filled, older elements of the cache are removed and replaced with the most recent device executions to keep the cache up to date. """ name = "Default mixed-state qubit PennyLane plugin" short_name = "default.mixed" pennylane_requires = __version__ version = __version__ author = "Xanadu Inc." operations = { "BasisState", "QubitStateVector", "QubitUnitary", "ControlledQubitUnitary", "MultiControlledX", "DiagonalQubitUnitary", "PauliX", "PauliY", "PauliZ", "MultiRZ", "Hadamard", "S", "T", "SX", "CNOT", "SWAP", "CSWAP", "Toffoli", "CZ", "PhaseShift", "ControlledPhaseShift", "RX", "RY", "RZ", "Rot", "CRX", "CRY", "CRZ", "CRot", "AmplitudeDamping", "GeneralizedAmplitudeDamping", "PhaseDamping", "DepolarizingChannel", "BitFlip", "PhaseFlip", "QubitChannel", "QFT", "SingleExcitation", "SingleExcitationPlus", "SingleExcitationMinus", "DoubleExcitation", "DoubleExcitationPlus", "DoubleExcitationMinus", "QubitCarry", "QubitSum", } def __init__(self, wires, *, shots=None, cache=0, analytic=None): if isinstance(wires, int) and wires > 23: raise ValueError( "This device does not currently support computations on more than 23 wires" ) # call QubitDevice init super().__init__(wires, shots, cache=cache, analytic=analytic) # Create the initial state. self._state = self._create_basis_state(0) self._pre_rotated_state = self._state def _create_basis_state(self, index): """Return the density matrix representing a computational basis state over all wires. Args: index (int): integer representing the computational basis state. Returns: array[complex]: complex array of shape ``[2] * (2 * num_wires)`` representing the density matrix of the basis state. """ rho = np.zeros((2 ** self.num_wires, 2 ** self.num_wires), dtype=np.complex128) rho[index, index] = 1 rho = self._asarray(rho, dtype=self.C_DTYPE) return self._reshape(rho, [2] * (2 * self.num_wires))
[docs] @classmethod def capabilities(cls): capabilities = super().capabilities().copy() capabilities.update( returns_state=True, ) return capabilities
@property def state(self): """Returns the state density matrix of the circuit prior to measurement""" dim = 2 ** self.num_wires # User obtains state as a matrix return self._reshape(self._pre_rotated_state, (dim, dim))
[docs] def density_matrix(self, wires): """Returns the reduced density matrix over the given wires. Args: wires (Wires): wires of the reduced system Returns: array[complex]: complex array of shape ``(2 ** len(wires), 2 ** len(wires))`` representing the reduced density matrix of the state prior to measurement. """ # Return the full density matrix if all the wires are given if wires == self.wires: return self.state traced_wires = [x for x in self.wires if x not in wires] # Trace first subsystem by applying kraus operators of the partial trace tr_op = self._cast(np.eye(2), dtype=self.C_DTYPE) tr_op = self._reshape(tr_op, (2, 1, 2)) self._apply_channel(tr_op, Wires(traced_wires[0])) # Trace next subsystem by applying kraus operators of the partial trace for traced_wire in traced_wires[1:]: self._apply_channel(tr_op, Wires(traced_wire)) return self._reshape(self._state, (2 ** len(wires), 2 ** len(wires)))
[docs] def reset(self): """Resets the device""" super().reset() self._state = self._create_basis_state(0) self._pre_rotated_state = self._state
[docs] def analytic_probability(self, wires=None): if self._state is None: return None # convert rho from tensor to matrix rho = self._reshape(self._state, (2 ** self.num_wires, 2 ** self.num_wires)) # probs are diagonal elements probs = self.marginal_prob(self._diag(rho), wires) # take the real part so probabilities are not shown as complex numbers return self._real(probs)
def _get_kraus(self, operation): # pylint: disable=no-self-use """Return the Kraus operators representing the operation. Args: operation (.Operation): a PennyLane operation Returns: list[array[complex]]: Returns a list of 2D matrices representing the Kraus operators. If the operation is unitary, returns a single Kraus operator. In the case of a diagonal unitary, returns a 1D array representing the matrix diagonal. """ if isinstance(operation, DiagonalOperation): return operation.eigvals if isinstance(operation, Channel): return operation.kraus_matrices return [operation.matrix] def _apply_channel(self, kraus, wires): r"""Apply a quantum channel specified by a list of Kraus operators to subsystems of the quantum state. For a unitary gate, there is a single Kraus operator. Args: kraus (list[array]): Kraus operators wires (Wires): target wires """ channel_wires = self.map_wires(wires) rho_dim = 2 * self.num_wires num_ch_wires = len(channel_wires) # Computes K^\dagger, needed for the transformation K \rho K^\dagger kraus_dagger = [self._conj(self._transpose(k)) for k in kraus] # Changes tensor shape if kraus[0].shape[0] == kraus[0].shape[1]: kraus_shape = [len(kraus)] + [2] * num_ch_wires * 2 kraus = self._cast(self._reshape(kraus, kraus_shape), dtype=self.C_DTYPE) kraus_dagger = self._cast(self._reshape(kraus_dagger, kraus_shape), dtype=self.C_DTYPE) # Add the possibility to give a (1,2) shape Kraus operator elif (kraus[0].shape == (1, 2)) and (num_ch_wires == 1): kraus_shape = [len(kraus)] + list(kraus[0].shape) kraus = self._cast(self._reshape(kraus, kraus_shape), dtype=self.C_DTYPE) kraus_dagger_shape = [len(kraus)] + list(kraus[0].shape)[::-1] kraus_dagger = self._cast( self._reshape(kraus_dagger, kraus_dagger_shape), dtype=self.C_DTYPE ) # Tensor indices of the state. For each qubit, need an index for rows *and* columns state_indices = ABC[:rho_dim] # row indices of the quantum state affected by this operation row_wires_list = channel_wires.tolist() row_indices = "".join(ABC_ARRAY[row_wires_list].tolist()) # column indices are shifted by the number of wires col_wires_list = [w + self.num_wires for w in row_wires_list] col_indices = "".join(ABC_ARRAY[col_wires_list].tolist()) # indices in einsum must be replaced with new ones new_row_indices = ABC[rho_dim : rho_dim + num_ch_wires] new_col_indices = ABC[rho_dim + num_ch_wires : rho_dim + 2 * num_ch_wires] # index for summation over Kraus operators kraus_index = ABC[rho_dim + 2 * num_ch_wires : rho_dim + 2 * num_ch_wires + 1] # new state indices replace row and column indices with new ones new_state_indices = functools.reduce( lambda old_string, idx_pair: old_string.replace(idx_pair[0], idx_pair[1]), zip(col_indices + row_indices, new_col_indices + new_row_indices), state_indices, ) # index mapping for einsum, e.g., 'iga,abcdef,idh->gbchef' einsum_indices = ( "{kraus_index}{new_row_indices}{row_indices}, {state_indices}," "{kraus_index}{col_indices}{new_col_indices}->{new_state_indices}".format( kraus_index=kraus_index, new_col_indices=new_col_indices, col_indices=col_indices, state_indices=state_indices, row_indices=row_indices, new_row_indices=new_row_indices, new_state_indices=new_state_indices, ) ) self._state = self._einsum(einsum_indices, kraus, self._state, kraus_dagger) def _apply_diagonal_unitary(self, eigvals, wires): r"""Apply a diagonal unitary gate specified by a list of eigenvalues. This method uses the fact that the unitary is diagonal for a more efficient implementation. Args: eigvals (array): eigenvalues (phases) of the diagonal unitary wires (Wires): target wires """ channel_wires = self.map_wires(wires) # reshape vectors eigvals = self._cast(self._reshape(eigvals, [2] * len(channel_wires)), dtype=self.C_DTYPE) # Tensor indices of the state. For each qubit, need an index for rows *and* columns state_indices = ABC[: 2 * self.num_wires] # row indices of the quantum state affected by this operation row_wires_list = channel_wires.tolist() row_indices = "".join(ABC_ARRAY[row_wires_list].tolist()) # column indices are shifted by the number of wires col_wires_list = [w + self.num_wires for w in row_wires_list] col_indices = "".join(ABC_ARRAY[col_wires_list].tolist()) einsum_indices = "{row_indices},{state_indices},{col_indices}->{state_indices}".format( col_indices=col_indices, state_indices=state_indices, row_indices=row_indices ) self._state = self._einsum(einsum_indices, eigvals, self._state, self._conj(eigvals)) def _apply_basis_state(self, state, wires): """Initialize the device in a specified computational basis state. Args: state (array[int]): computational basis state of shape ``(wires,)`` consisting of 0s and 1s. wires (Wires): wires that the provided computational state should be initialized on """ # translate to wire labels used by device device_wires = self.map_wires(wires) # length of basis state parameter n_basis_state = len(state) if not set(state).issubset({0, 1}): raise ValueError("BasisState parameter must consist of 0 or 1 integers.") if n_basis_state != len(device_wires): raise ValueError("BasisState parameter and wires must be of equal length.") # get computational basis state number basis_states = 2 ** (self.num_wires - 1 - device_wires.toarray()) num = int(, basis_states)) self._state = self._create_basis_state(num) def _apply_state_vector(self, state, device_wires): """Initialize the internal state in a specified pure state. Args: state (array[complex]): normalized input state of length ``2**len(wires)`` device_wires (Wires): wires that get initialized in the state """ # translate to wire labels used by device device_wires = self.map_wires(device_wires) state = self._asarray(state, dtype=self.C_DTYPE) n_state_vector = state.shape[0] if state.ndim != 1 or n_state_vector != 2 ** len(device_wires): raise ValueError("State vector must be of length 2**wires.") if not np.allclose(np.linalg.norm(state, ord=2), 1.0, atol=tolerance): raise ValueError("Sum of amplitudes-squared does not equal one.") if len(device_wires) == self.num_wires and sorted(device_wires.labels) == list( device_wires.labels ): # Initialize the entire wires with the state rho = self._outer(state, self._conj(state)) self._state = self._reshape(rho, [2] * 2 * self.num_wires) else: # generate basis states on subset of qubits via the cartesian product basis_states = np.array(list(itertools.product([0, 1], repeat=len(device_wires)))) # get basis states to alter on full set of qubits unravelled_indices = np.zeros((2 ** len(device_wires), self.num_wires), dtype=int) unravelled_indices[:, device_wires] = basis_states # get indices for which the state is changed to input state vector elements ravelled_indices = np.ravel_multi_index(unravelled_indices.T, [2] * self.num_wires) state = self._scatter(ravelled_indices, state, [2 ** self.num_wires]) rho = self._outer(state, self._conj(state)) rho = self._reshape(rho, [2] * 2 * self.num_wires) self._state = self._asarray(rho, dtype=self.C_DTYPE) def _apply_operation(self, operation): """Applies operations to the internal device state. Args: operation (.Operation): operation to apply on the device """ wires = operation.wires if isinstance(operation, QubitStateVector): self._apply_state_vector(operation.parameters[0], wires) return if isinstance(operation, BasisState): self._apply_basis_state(operation.parameters[0], wires) return matrices = self._get_kraus(operation) if isinstance(operation, DiagonalOperation): self._apply_diagonal_unitary(matrices, wires) else: self._apply_channel(matrices, wires) # pylint: disable=arguments-differ
[docs] def apply(self, operations, rotations=None, **kwargs): rotations = rotations or [] # apply the circuit operations for i, operation in enumerate(operations): if i > 0 and isinstance(operation, (QubitStateVector, BasisState)): raise DeviceError( "Operation {} cannot be used after other Operations have already been applied " "on a {} device.".format(, self.short_name) ) for operation in operations: self._apply_operation(operation) # store the pre-rotated state self._pre_rotated_state = self._state # apply the circuit rotations for operation in rotations: self._apply_operation(operation)