# qml.beta.devices.DefaultTensor¶

class DefaultTensor(wires, shots=None, representation='exact', contraction_method='auto')[source]

Bases: pennylane._device.Device

Experimental Tensor Network simulator device for PennyLane.

Short name: default.tensor

This experimental device uses the TensorNetwork library to provide a basic tensor-network-based simulator backend for PennyLane. Tensor network simulators can faster or more efficient for certain types of circuit structures.

To use this device, you will need to install TensorNetwork version 0.3:

pip install tensornetwork==0.3


The default.tensor device supports two types of tensor networks: "exact" and "mps".

The (default) "exact" representation does not make any approximations, using exact dense tensors for the simulator’s quantum states and for the matrices of quantum gates and observables.

The "mps" representation (standing for “matrix product state”) approximates the quantum state using a one-dimensional grid of qubits with nearest-neighbour connectivity. As such, it does not support multi-qubit gates/observables that do not act on nearest-neighbour qubits.

The preferred contraction method can also be specified when using the "exact" representation. Available options are “auto”, “greedy”, “branch”, or “optimal”. See the TensorNetwork documentation for more details.

Example

>>> exact_tensornet = qml.device("default.tensor", wires=2, contraction_method="greedy")
>>> mps_tensornet = qml.device("default.tensor", wires=2, representation="mps")

Parameters
• 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) – Number of circuit evaluations/random samples to return when sampling from the device. Defaults to None if not specified, which means that the device returns analytical results.

• representation (str) – Underlying representation used for the tensor network simulation. Valid options are “exact” (no approximations made) or “mps” (simulated quantum state is approximated as a Matrix Product State).

• contraction_method (str) – Method used to perform tensor network contractions. Only applicable for the “exact” representation. Valid options are “auto”, “greedy”, “branch”, or “optimal”. See documentation of the TensorNetwork library for more information about contraction methods.

 analytic Whether shots is None or not. author backend contraction_method The contraction method used by the tensor network. name num_executions Number of times this device is executed by the evaluation of QNodes running on this device obs_queue The observables to be measured and returned. observables Get the supported set of observables. op_queue The operation queue to be applied. operations Get the supported set of operations. parameters Mapping from free parameter index to the list of Operations in the device queue that depend on it. pennylane_requires short_name shot_vector Returns the shot vector, a sparse representation of the shot sequence used by the device when evaluating QNodes. shots Number of circuit evaluations/random samples used to estimate expectation values of observables version wire_map Ordered dictionary that defines the map from user-provided wire labels to the wire labels used on this device wires All wires that can be addressed on this device
analytic

Whether shots is None or not. Kept for backwards compatability.

author = 'Xanadu Inc.'
backend = 'numpy'
contraction_method

The contraction method used by the tensor network. Available options are “auto”, “greedy”, “branch”, or “optimal”. See TensorNetwork library documentation for more details.

name = 'PennyLane TensorNetwork simulator plugin'
num_executions

Number of times this device is executed by the evaluation of QNodes running on this device

Returns

number of executions

Return type

int

obs_queue

The observables to be measured and returned.

Note that this property can only be accessed within the execution context of execute().

Raises

ValueError – if outside of the execution context

Returns

list[~.operation.Observable]

observables
op_queue

The operation queue to be applied.

Note that this property can only be accessed within the execution context of execute().

Raises

ValueError – if outside of the execution context

Returns

list[~.operation.Operation]

operations
parameters

Mapping from free parameter index to the list of Operations in the device queue that depend on it.

Note that this property can only be accessed within the execution context of execute().

Raises

ValueError – if outside of the execution context

Returns

the mapping

Return type

dict[int->list[ParameterDependency]]

pennylane_requires = '0.18.0'
short_name = 'default.tensor'
shot_vector

Returns the shot vector, a sparse representation of the shot sequence used by the device when evaluating QNodes.

Example

>>> dev = qml.device("default.qubit", wires=2, shots=[3, 1, 2, 2, 2, 2, 6, 1, 1, 5, 12, 10, 10])
>>> dev.shots
57
>>> dev.shot_vector
[ShotTuple(shots=3, copies=1),
ShotTuple(shots=1, copies=1),
ShotTuple(shots=2, copies=4),
ShotTuple(shots=6, copies=1),
ShotTuple(shots=1, copies=2),
ShotTuple(shots=5, copies=1),
ShotTuple(shots=12, copies=1),
ShotTuple(shots=10, copies=2)]


The sparse representation of the shot sequence is returned, where tuples indicate the number of times a shot integer is repeated.

Type

list[ShotTuple[int, int]]

shots

Number of circuit evaluations/random samples used to estimate expectation values of observables

version = '0.18.0'
wire_map

Ordered dictionary that defines the map from user-provided wire labels to the wire labels used on this device

wires

All wires that can be addressed on this device

 apply(operation, wires, par) Apply a quantum operation. batch_execute(circuits) Execute a batch of quantum circuits on the device. Get the capabilities of this device class. check_validity(queue, observables) Checks whether the operations and observables in queue are all supported by the device. define_wire_map(wires) Create the map from user-provided wire labels to the wire labels used by the device. ev(obs_nodes, obs_wires) Expectation value of observables on specified wires. execute(queue, observables[, parameters]) Execute a queue of quantum operations on the device and then measure the given observables. execute_and_gradients(circuits[, method]) Execute a batch of quantum circuits on the device, and return both the results and the gradients. The device execution context used during calls to execute(). expval(observable, wires, par) Returns the expectation value of observable on specified wires. gradients(circuits[, method]) Return the gradients of a batch of quantum circuits on the device. map_wires(wires) Map the wire labels of wires using this device’s wire map. Called during execute() after the individual operations have been executed. Called during execute() after the individual observables have been measured. Called during execute() before the individual operations are executed. Called during execute() before the individual observables are measured. probability([wires]) Return the (marginal) probability of each computational basis state from the last run of the device. Reset the device. sample(observable, wires, par) Return a sample of an observable. supports_observable(observable) Checks if an observable is supported by this device. Raises a ValueError, supports_operation(operation) Checks if an operation is supported by this device. var(observable, wires, par) Returns the variance of observable on specified wires.
apply(operation, wires, par)[source]

Apply a quantum operation.

For plugin developers: this function should apply the operation on the device.

Parameters
• operation (str) – name of the operation

• wires (Wires) – wires that the operation is applied to

• par (tuple) – parameters for the operation

batch_execute(circuits)

Execute a batch of quantum circuits on the device.

The circuits are represented by tapes, and they are executed one-by-one using the device’s execute method. The results are collected in a list.

For plugin developers: This function should be overwritten if the device can efficiently run multiple circuits on a backend, for example using parallel and/or asynchronous executions.

Parameters

circuits (list[tape.QuantumTape]) – circuits to execute on the device

Returns

list of measured value(s)

Return type

list[array[float]]

classmethod capabilities()[source]

Get the capabilities of this device class.

Inheriting classes that change or add capabilities must override this method, for example via

@classmethod
def capabilities(cls):
capabilities = super().capabilities().copy()
capabilities.update(
supports_inverse_operations=False,
supports_a_new_capability=True,
)
return capabilities

Returns

results

Return type

dict[str->*]

check_validity(queue, observables)

Checks whether the operations and observables in queue are all supported by the device. Includes checks for inverse operations.

Parameters
• queue (Iterable[Operation]) – quantum operation objects which are intended to be applied on the device

• observables (Iterable[Observable]) – observables which are intended to be evaluated on the device

Raises

DeviceError – if there are operations in the queue or observables that the device does not support

define_wire_map(wires)

Create the map from user-provided wire labels to the wire labels used by the device.

The default wire map maps the user wire labels to wire labels that are consecutive integers.

However, by overwriting this function, devices can specify their preferred, non-consecutive and/or non-integer wire labels.

Parameters

wires (Wires) – user-provided wires for this device

Returns

dictionary specifying the wire map

Return type

OrderedDict

Example

>>> dev = device('my.device', wires=['b', 'a'])
>>> dev.wire_map()
OrderedDict( [(<Wires = ['a']>, <Wires = [0]>), (<Wires = ['b']>, <Wires = [1]>)])

ev(obs_nodes, obs_wires)[source]

Expectation value of observables on specified wires.

Parameters
• obs_nodes (Sequence[tn.Node]) – the observables as TensorNetwork Nodes

• obs_wires (Sequence[Wires]) – measured wires for each observable

Returns

expectation value $$\expect{A} = \bra{\psi}A\ket{\psi}$$

Return type

float

execute(queue, observables, parameters={}, **kwargs)

Execute a queue of quantum operations on the device and then measure the given observables.

For plugin developers: Instead of overwriting this, consider implementing a suitable subset of pre_apply(), apply(), post_apply(), pre_measure(), expval(), var(), sample(), post_measure(), and execution_context().

Parameters
• queue (Iterable[Operation]) – operations to execute on the device

• observables (Iterable[Observable]) – observables to measure and return

• parameters (dict[int, list[ParameterDependency]]) – Mapping from free parameter index to the list of Operations (in the queue) that depend on it.

Keyword Arguments

return_native_type (bool) – If True, return the result in whatever type the device uses internally, otherwise convert it into array[float]. Default: False.

Raises

QuantumFunctionError – if the value of return_type is not supported

Returns

measured value(s)

Return type

array[float]

execute_and_gradients(circuits, method='jacobian', **kwargs)

Execute a batch of quantum circuits on the device, and return both the results and the gradients.

The circuits are represented by tapes, and they are executed one-by-one using the device’s execute method. The results and the corresponding Jacobians are collected in a list.

For plugin developers: This method should be overwritten if the device can efficiently run multiple circuits on a backend, for example using parallel and/or asynchronous executions, and return both the results and the Jacobians.

Parameters
• circuits (list[tape.QuantumTape]) – circuits to execute on the device

• method (str) – the device method to call to compute the Jacobian of a single circuit

• **kwargs – keyword argument to pass when calling method

Returns

Tuple containing list of measured value(s) and list of Jacobians. Returned Jacobians should be of shape (output_shape, num_params).

Return type

tuple[list[array[float]], list[array[float]]]

execution_context()

The device execution context used during calls to execute().

You can overwrite this function to return a context manager in case your quantum library requires that; all operations and method calls (including apply() and expval()) are then evaluated within the context of this context manager (see the source of Device.execute() for more details).

expval(observable, wires, par)[source]

Returns the expectation value of observable on specified wires.

Note: all arguments accept _lists_, which indicate a tensor product of observables.

Parameters
• observable (str or list[str]) – name of the observable(s)

• wires (Wires) – wires the observable(s) are to be measured on

• par (tuple or list[tuple]]) – parameters for the observable(s)

Returns

expectation value $$\expect{A} = \bra{\psi}A\ket{\psi}$$

Return type

float

gradients(circuits, method='jacobian', **kwargs)

Return the gradients of a batch of quantum circuits on the device.

The gradient method method is called sequentially for each circuit, and the corresponding Jacobians are collected in a list.

For plugin developers: This method should be overwritten if the device can efficiently compute the gradient of multiple circuits on a backend, for example using parallel and/or asynchronous executions.

Parameters
• circuits (list[tape.QuantumTape]) – circuits to execute on the device

• method (str) – the device method to call to compute the Jacobian of a single circuit

• **kwargs – keyword argument to pass when calling method

Returns

List of Jacobians. Returned Jacobians should be of shape (output_shape, num_params).

Return type

list[array[float]]

map_wires(wires)

Map the wire labels of wires using this device’s wire map.

Parameters

wires (Wires) – wires whose labels we want to map to the device’s internal labelling scheme

Returns

wires with new labels

Return type

Wires

post_apply()

Called during execute() after the individual operations have been executed.

post_measure()

Called during execute() after the individual observables have been measured.

pre_apply()

Called during execute() before the individual operations are executed.

pre_measure()

Called during execute() before the individual observables are measured.

probability(wires=None)

Return the (marginal) probability of each computational basis state from the last run of the device.

Parameters

wires (Sequence[int]) – Sequence of wires to return marginal probabilities for. Wires not provided are traced out of the system.

Returns

Dictionary mapping a tuple representing the state to the resulting probability. The dictionary should be sorted such that the state tuples are in lexicographical order.

Return type

OrderedDict[tuple, float]

reset()[source]

Reset the device.

sample(observable, wires, par)[source]

Return a sample of an observable.

The number of samples is determined by the value of Device.shots, which can be directly modified.

Note: all arguments support _lists_, which indicate a tensor product of observables.

Parameters
• observable (str or list[str]) – name of the observable(s)

• wires (Wires) – wires the observable(s) is to be measured on

• par (tuple or list[tuple]]) – parameters for the observable(s)

Raises

NotImplementedError – if the device does not support sampling

Returns

samples in an array of dimension (shots,)

Return type

array[float]

supports_observable(observable)
Checks if an observable is supported by this device. Raises a ValueError,

if not a subclass or string of an Observable was passed.

Parameters

observable (type or str) – observable to be checked

Raises

ValueError – if observable is not a Observable class or string

Returns

True iff supplied observable is supported

Return type

bool

supports_operation(operation)

Checks if an operation is supported by this device.

Parameters

operation (type or str) – operation to be checked

Raises

ValueError – if operation is not a Operation class or string

Returns

True iff supplied operation is supported

Return type

bool

var(observable, wires, par)[source]

Returns the variance of observable on specified wires.

Note: all arguments support _lists_, which indicate a tensor product of observables.

Parameters
• observable (str or list[str]) – name of the observable(s)

• wires (Wires) – wires the observable(s) is to be measured on

• par (tuple or list[tuple]]) – parameters for the observable(s)

Raises

NotImplementedError – if the device does not support variance computation

Returns

variance $$\mathrm{var}(A) = \bra{\psi}A^2\ket{\psi} - \bra{\psi}A\ket{\psi}^2$$

Return type

float