# Source code for pennylane._qutrit_device

```
# Copyright 2018-2022 Xanadu Quantum Technologies Inc.
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
# http://www.apache.org/licenses/LICENSE-2.0
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""
This module contains the :class:`QutritDevice` abstract base class.
"""
# For now, arguments may be different from the signatures provided in QubitDevice to minimize size of pull request
# e.g. instead of expval(self, observable, wires, par) have expval(self, observable)
# pylint: disable=arguments-differ, abstract-method, no-value-for-parameter,too-many-instance-attributes,too-many-branches, no-member, bad-option-value, arguments-renamed
import itertools
import numpy as np
import pennylane as qml
from pennylane import QubitDevice
from pennylane.measurements import MeasurementProcess
from pennylane.wires import Wires
[docs]class QutritDevice(QubitDevice): # pylint: disable=too-many-public-methods
"""Abstract base class for PennyLane qutrit devices.
The following abstract method **must** be defined:
* :meth:`~.apply`: append circuit operations, compile the circuit (if applicable),
and perform the quantum computation.
Devices that generate their own samples (such as hardware) may optionally
overwrite :meth:`~.probability`. This method otherwise automatically
computes the probabilities from the generated samples, and **must**
overwrite the following method:
* :meth:`~.generate_samples`: Generate samples from the device from the
exact or approximate probability distribution.
Analytic devices **must** overwrite the following method:
* :meth:`~.analytic_probability`: returns the probability or marginal probability from the
device after circuit execution. :meth:`~.marginal_prob` may be used here.
This device contains common utility methods for qutrit-based devices. These
do not need to be overwritten. Utility methods include:
* :meth:`~.expval`, :meth:`~.var`, :meth:`~.sample`: return expectation values,
variances, and samples of observables after the circuit has been rotated
into the observable eigenbasis.
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']``). Default 1 if not specified.
shots (None, int, list[int]): Number of circuit evaluations/random samples used to estimate
expectation values of observables. If ``None``, the device calculates probability, expectation values,
and variances analytically. If an integer, it specifies the number of samples to estimate these quantities.
If a list of integers is passed, the circuit evaluations are batched over the list of shots.
r_dtype: Real floating point precision type.
c_dtype: Complex floating point precision type.
"""
# TODO: Update set of supported observables as new observables are added
observables = {
"Identity",
"THermitian",
}
[docs] @classmethod
def capabilities(cls):
capabilities = super().capabilities().copy()
capabilities.update(model="qutrit")
return capabilities
[docs] def generate_samples(self):
r"""Returns the computational basis samples generated for all wires.
Note that PennyLane uses the convention :math:`|q_0,q_1,\dots,q_{N-1}\rangle` where
:math:`q_0` is the most significant trit.
.. warning::
This method should be overwritten on devices that
generate their own computational basis samples, with the resulting
computational basis samples stored as ``self._samples``.
Returns:
array[complex]: array of samples in the shape ``(dev.shots, dev.num_wires)``
"""
number_of_states = 3**self.num_wires
rotated_prob = self.analytic_probability()
samples = self.sample_basis_states(number_of_states, rotated_prob)
return self.states_to_ternary(samples, self.num_wires)
[docs] def generate_basis_states(self, num_wires, dtype=np.uint32):
"""Generates basis states in ternary representation according to the number
of wires specified.
Args:
num_wires (int): the number of wires
dtype=np.uint32 (type): the data type of the arrays to use
Returns:
array[int]: the sampled basis states
"""
basis_states_generator = itertools.product((0, 1, 2), repeat=num_wires)
return np.fromiter(itertools.chain(*basis_states_generator), dtype=dtype).reshape(
-1, num_wires
)
[docs] @staticmethod
def states_to_ternary(samples, num_wires, dtype=np.int64):
"""Convert basis states from base 10 to ternary representation.
This is an auxiliary method to the generate_samples method.
Args:
samples (array[int]): samples of basis states in base 10 representation
num_wires (int): the number of qutrits
dtype (type): Type of the internal integer array to be used. Can be
important to specify for large systems for memory allocation
purposes.
Returns:
array[int]: basis states in ternary representation
"""
ternary_arr = []
for sample in samples:
num = []
for _ in range(num_wires):
sample, r = divmod(sample, 3)
num.append(r)
ternary_arr.append(num[::-1])
return np.array(ternary_arr, dtype=dtype)
[docs] def density_matrix(self, wires):
"""Returns the reduced density matrix prior to measurement.
Args:
wires (Wires): wires of the reduced system
Raises:
QuantumFunctionError: density matrix is currently unsupported on :class:`~.QutritDevice`
"""
# TODO: Add support for DensityMatrix return type. Currently, qml.math is hard coded to calculate this for qubit
# states (see `qml.math.reduced_dm()`), so it needs to be updated before DensityMatrix can be supported for qutrits.
# For now, if a user tries to request this return type, an error will be raised.
raise qml.QuantumFunctionError(
"Unsupported return type specified for observable density matrix"
)
[docs] def vn_entropy(self, wires, log_base):
r"""Returns the Von Neumann entropy prior to measurement.
.. math::
S( \rho ) = -\text{Tr}( \rho \log ( \rho ))
Args:
wires (Wires): Wires of the considered subsystem.
log_base (float): Base for the logarithm, default is None the natural logarithm is used in this case.
Raises:
QuantumFunctionError: Von Neumann entropy is currently unsupported on :class:`~.QutritDevice`
"""
# TODO: Add support for VnEntropy return type. Currently, qml.math is hard coded to calculate this for qubit
# states (see `qml.math.vn_entropy()`), so it needs to be updated before VnEntropy can be supported for qutrits.
# For now, if a user tries to request this return type, an error will be raised.
raise qml.QuantumFunctionError(
"Unsupported return type specified for observable Von Neumann entropy"
)
[docs] def mutual_info(self, wires0, wires1, log_base):
r"""Returns the mutual information prior to measurement:
.. math::
I(A, B) = S(\rho^A) + S(\rho^B) - S(\rho^{AB})
where :math:`S` is the von Neumann entropy.
Args:
wires0 (Wires): wires of the first subsystem
wires1 (Wires): wires of the second subsystem
log_base (float): base to use in the logarithm
Raises:
QuantumFunctionError: Mutual information is currently unsupported on :class:`~.QutritDevice`
"""
# TODO: Add support for MutualInfo return type. Currently, qml.math is hard coded to calculate this for qubit
# states (see `qml.math.mutual_info()`), so it needs to be updated before MutualInfo can be supported for qutrits.
# For now, if a user tries to request this return type, an error will be raised.
raise qml.QuantumFunctionError(
"Unsupported return type specified for observable mutual information"
)
[docs] def estimate_probability(self, wires=None, shot_range=None, bin_size=None):
"""Return the estimated probability of each computational basis state
using the generated samples.
Args:
wires (Iterable[Number, str], Number, str, Wires): wires to calculate
marginal probabilities for. Wires not provided are traced out of the system.
shot_range (tuple[int]): 2-tuple of integers specifying the range of samples
to use. If not specified, all samples are used.
bin_size (int): Divides the shot range into bins of size ``bin_size``, and
returns the measurement statistic separately over each bin. If not
provided, the entire shot range is treated as a single bin.
Returns:
array[float]: list of the probabilities
"""
wires = wires or self.wires
# convert to a wires object
wires = Wires(wires)
# translate to wire labels used by device
device_wires = self.map_wires(wires)
sample_slice = Ellipsis if shot_range is None else slice(*shot_range)
samples = self._samples[sample_slice, device_wires]
# convert samples from a list of 0, 1, 2 integers, to base 10 representation
powers_of_three = 3 ** np.arange(len(device_wires))[::-1]
indices = samples @ powers_of_three
# count the basis state occurrences, and construct the probability vector
if bin_size is not None:
bins = len(samples) // bin_size
indices = indices.reshape((bins, -1))
prob = np.zeros([3 ** len(device_wires), bins], dtype=np.float64)
for b, idx in enumerate(indices):
basis_states, counts = np.unique(idx, return_counts=True)
prob[basis_states, b] = counts / bin_size
else:
basis_states, counts = np.unique(indices, return_counts=True)
prob = np.zeros([3 ** len(device_wires)], dtype=np.float64)
prob[basis_states] = counts / len(samples)
return self._asarray(prob, dtype=self.R_DTYPE)
[docs] def marginal_prob(self, prob, wires=None):
r"""Return the marginal probability of the computational basis
states by summing the probabiliites on the non-specified wires.
If no wires are specified, then all the basis states representable by
the device are considered and no marginalization takes place.
.. note::
If the provided wires are not in the order as they appear on the device,
the returned marginal probabilities take this permutation into account.
For example, if the addressable wires on this device are ``Wires([0, 1, 2])`` and
this function gets passed ``wires=[2, 0]``, then the returned marginal
probability vector will take this 'reversal' of the two wires
into account:
.. math::
\mathbb{P}^{(2, 0)}
= \left[
|00\rangle, |10\rangle, |20\rangle, |01\rangle, |11\rangle,
|21\rangle, |02\rangle, |12\rangle, |22\rangle
\right]
Args:
prob: The probabilities to return the marginal probabilities
for
wires (Iterable[Number, str], Number, str, Wires): wires to return
marginal probabilities for. Wires not provided
are traced out of the system.
Returns:
array[float]: array of the resulting marginal probabilities.
"""
if wires is None:
# no need to marginalize
return prob
wires = Wires(wires)
# determine which subsystems are to be summed over
inactive_wires = Wires.unique_wires([self.wires, wires])
# translate to wire labels used by device
device_wires = self.map_wires(wires)
inactive_device_wires = self.map_wires(inactive_wires)
# reshape the probability so that each axis corresponds to a wire
prob = self._reshape(prob, [3] * self.num_wires)
# sum over all inactive wires
# hotfix to catch when default.qutrit uses this method
# since then device_wires is a list
if isinstance(inactive_device_wires, Wires):
wires = inactive_device_wires.labels
else:
wires = inactive_device_wires
prob = self._flatten(self._reduce_sum(prob, wires))
# The wires provided might not be in consecutive order (i.e., wires might be [2, 0]).
# If this is the case, we must permute the marginalized probability so that
# it corresponds to the orders of the wires passed.
num_wires = len(device_wires)
basis_states = self.generate_basis_states(num_wires)
basis_states = basis_states[:, np.argsort(np.argsort(device_wires))]
powers_of_three = 3 ** np.arange(len(device_wires))[::-1]
perm = basis_states @ powers_of_three
return self._gather(prob, perm)
[docs] def sample(self, observable, shot_range=None, bin_size=None, counts=False):
def _samples_to_counts(samples, no_observable_provided):
"""Group the obtained samples into a dictionary.
**Example**
>>> samples
tensor([[0, 0, 1],
[0, 0, 1],
[1, 1, 1]], requires_grad=True)
>>> self._samples_to_counts(samples)
{'111':1, '001':2}
"""
if no_observable_provided:
# If we describe a state vector, we need to convert its list representation
# into string (it's hashable and good-looking).
# Before converting to str, we need to extract elements from arrays
# to satisfy the case of jax interface, as jax arrays do not support str.
samples = ["".join([str(s.item()) for s in sample]) for sample in samples]
states, counts = np.unique(samples, return_counts=True)
return dict(zip(states, counts))
# TODO: Add special cases for any observables that require them once list of
# observables is updated.
# translate to wire labels used by device
device_wires = self.map_wires(observable.wires)
name = observable.name # pylint: disable=unused-variable
sample_slice = Ellipsis if shot_range is None else slice(*shot_range)
no_observable_provided = isinstance(observable, MeasurementProcess)
if no_observable_provided: # if no observable was provided then return the raw samples
if (
len(observable.wires) != 0
): # if wires are provided, then we only return samples from those wires
samples = self._samples[sample_slice, np.array(device_wires)]
else:
samples = self._samples[sample_slice]
else:
# Replace the basis state in the computational basis with the correct eigenvalue.
# Extract only the columns of the basis samples required based on ``wires``.
samples = self._samples[
sample_slice, np.array(device_wires)
] # Add np.array here for Jax support.
powers_of_three = 3 ** np.arange(samples.shape[-1])[::-1]
indices = samples @ powers_of_three
indices = np.array(indices) # Add np.array here for Jax support.
try:
samples = observable.eigvals()[indices]
except qml.operation.EigvalsUndefinedError as e:
# if observable has no info on eigenvalues, we cannot return this measurement
raise qml.operation.EigvalsUndefinedError(
f"Cannot compute samples of {observable.name}."
) from e
if bin_size is None:
if counts:
return _samples_to_counts(samples, no_observable_provided)
return samples
num_wires = len(device_wires) if len(device_wires) > 0 else self.num_wires
if counts:
shape = (-1, bin_size, num_wires) if no_observable_provided else (-1, bin_size)
return [
_samples_to_counts(bin_sample, no_observable_provided)
for bin_sample in samples.reshape(shape)
]
return (
samples.reshape((num_wires, bin_size, -1))
if no_observable_provided
else samples.reshape((bin_size, -1))
)
# TODO: Implement function. Currently unimplemented due to lack of decompositions available
# for existing operations and lack of non-parametrized observables.
[docs] def adjoint_jacobian(
self, tape, starting_state=None, use_device_state=False
): # pylint: disable=missing-function-docstring
raise NotImplementedError
```

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