qml.devices.default_qubit.DefaultQubit¶

class
DefaultQubit
(wires, *, shots=None, cache=0, analytic=None)[source]¶ Bases:
pennylane._qubit_device.QubitDevice
Default qubit device for PennyLane.
 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) – How many times the circuit should be evaluated (or sampled) to estimate the expectation values. Defaults to
None
if not specified, which means that the device returns analytical results.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.
Attributes
Whether shots is None or not.
Number of device executions to store in a cache to speed up subsequent executions.
The hash of the circuit upon the last execution.
Number of times this device is executed by the evaluation of QNodes running on this device
The observables to be measured and returned.
The operation queue to be applied.
Mapping from free parameter index to the list of
Operations
in the device queue that depend on it.Returns the shot vector, a sparse representation of the shot sequence used by the device when evaluating QNodes.
Number of circuit evaluations/random samples used to estimate expectation values of observables
Returns the state vector of the circuit prior to measurement.
Ordered dictionary that defines the map from userprovided wire labels to the wire labels used on this device
All wires that can be addressed on this device

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

cache
¶ Number of device executions to store in a cache to speed up subsequent executions. If set to zero, no caching occurs.
 Type
int

circuit_hash
¶ The hash of the circuit upon the last execution.
This can be used by devices in
apply()
for parametric compilation.

name
= 'Default qubit PennyLane 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
= {'Hadamard', 'Hamiltonian', 'Hermitian', 'Identity', 'PauliX', 'PauliY', 'PauliZ', 'Projector', 'SparseHamiltonian'}¶

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
= {'BasisState', 'CNOT', 'CPhase', 'CRX', 'CRY', 'CRZ', 'CRot', 'CSWAP', 'CY', 'CZ', 'ControlledPhaseShift', 'ControlledQubitUnitary', 'DiagonalQubitUnitary', 'DoubleExcitation', 'DoubleExcitationMinus', 'DoubleExcitationPlus', 'Hadamard', 'ISWAP', 'IsingXX', 'IsingYY', 'IsingZZ', 'MultiControlledX', 'MultiRZ', 'OrbitalRotation', 'PauliX', 'PauliY', 'PauliZ', 'PhaseShift', 'QubitCarry', 'QubitStateVector', 'QubitSum', 'QubitUnitary', 'RX', 'RY', 'RZ', 'Rot', 'S', 'SISWAP', 'SQISW', 'SWAP', 'SX', 'SingleExcitation', 'SingleExcitationMinus', 'SingleExcitationPlus', 'T', 'Toffoli'}¶

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
= '0.19.0dev'¶

short_name
= 'default.qubit'¶

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

state
¶

version
= '0.19.0dev'¶

wire_map
¶ Ordered dictionary that defines the map from userprovided wire labels to the wire labels used on this device

wires
¶ All wires that can be addressed on this device
Methods
access_state
([wires])Check that the device has access to an internal state and return it if available.
active_wires
(operators)Returns the wires acted on by a set of operators.
adjoint_jacobian
(tape[, starting_state, …])Implements the adjoint method outlined in Jones and Gacon to differentiate an input tape.
analytic_probability
([wires])Return the (marginal) probability of each computational basis state from the last run of the device.
apply
(operations[, rotations])Apply quantum operations, rotate the circuit into the measurement basis, and compile and execute the quantum circuit.
batch_execute
(circuits)Execute a batch of quantum circuits on the device.
batch_transform
(circuit)Apply a differentiable batch transform for preprocessing a circuit prior to execution.
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 userprovided wire labels to the wire labels used by the device.
density_matrix
(wires)Returns the reduced density matrix of a given set of wires.
estimate_probability
([wires, shot_range, …])Return the estimated probability of each computational basis state using the generated samples.
execute
(circuit, **kwargs)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()
.expand_fn
(circuit[, max_expansion])Method for expanding or decomposing an input circuit.
expval
(observable[, shot_range, bin_size])Returns the expectation value of a Hamiltonian observable. When the observable is a
generate_basis_states
(num_wires[, dtype])Generates basis states in binary representation according to the number of wires specified.
Returns the computational basis samples generated for all 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.
marginal_prob
(prob[, wires])Return the marginal probability of the computational basis states by summing the probabiliites on the nonspecified wires.
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, shot_range, bin_size])Return either the analytic probability or estimated probability of each computational basis state.
reset
()Reset the device
sample
(observable[, shot_range, bin_size])Return a sample of an observable.
sample_basis_states
(number_of_states, …)Sample from the computational basis states based on the state probability.
states_to_binary
(samples, num_wires[, dtype])Convert basis states from base 10 to binary representation.
statistics
(observables[, shot_range, bin_size])Process measurement results from circuit execution and return statistics.
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[, shot_range, bin_size])Returns the variance of observable on specified wires.

access_state
(wires=None)¶ Check that the device has access to an internal state and return it if available.
 Parameters
wires (Wires) – wires of the reduced system
 Raises
QuantumFunctionError – if the device is not capable of returning the state
 Returns
the state or the density matrix of the device
 Return type
array or tensor

static
active_wires
(operators)¶ Returns the wires acted on by a set of operators.
 Parameters
operators (list[Operation]) – operators for which we are gathering the active wires
 Returns
wires activated by the specified operators
 Return type

adjoint_jacobian
(tape, starting_state=None, use_device_state=False)¶ Implements the adjoint method outlined in Jones and Gacon to differentiate an input tape.
After a forward pass, the circuit is reversed by iteratively applying inverse (adjoint) gates to scan backwards through the circuit. This method is similar to the reversible method, but has a lower time overhead and a similar memory overhead.
Note
The adjoint differentiation method has the following restrictions:
As it requires knowledge of the statevector, only statevector simulator devices can be used.
Only expectation values are supported as measurements.
Does not work for Hamiltonian observables.
 Parameters
tape (QuantumTape) – circuit that the function takes the gradient of
 Keyword Arguments
starting_state (tensor_like) – postforward pass state to start execution with. It should be complexvalued. Takes precedence over
use_device_state
.use_device_state (bool) – use current device state to initialize. A forward pass of the same circuit should be the last thing the device has executed. If a
starting_state
is provided, that takes precedence.
 Returns
the derivative of the tape with respect to trainable parameters. Dimensions are
(len(observables), len(trainable_params))
. Return type
array
 Raises
QuantumFunctionError – if the input tape has measurements that are not expectation values or contains a multiparameter operation aside from
Rot

analytic_probability
(wires=None)[source]¶ Return the (marginal) probability of each computational basis state from the last run of the device.
PennyLane uses the convention \(q_0,q_1,\dots,q_{N1}\rangle\) where \(q_0\) is the most significant bit.
If no wires are specified, then all the basis states representable by the device are considered and no marginalization takes place.
Note
marginal_prob()
may be used as a utility method to calculate the marginal probability distribution. Parameters
wires (Iterable[Number, str], Number, str, Wires) – wires to return marginal probabilities for. Wires not provided are traced out of the system.
 Returns
list of the probabilities
 Return type
array[float]

apply
(operations, rotations=None, **kwargs)[source]¶ Apply quantum operations, rotate the circuit into the measurement basis, and compile and execute the quantum circuit.
This method receives a list of quantum operations queued by the QNode, and should be responsible for:
Constructing the quantum program
(Optional) Rotating the quantum circuit using the rotation operations provided. This diagonalizes the circuit so that arbitrary observables can be measured in the computational basis.
Compile the circuit
Execute the quantum circuit
Both arguments are provided as lists of PennyLane
Operation
instances. Useful properties includename
,wires
, andparameters
, andinverse
:>>> op = qml.RX(0.2, wires=[0]) >>> op.name # returns the operation name "RX" >>> op.wires # returns a Wires object representing the wires that the operation acts on <Wires = [0]> >>> op.parameters # returns a list of parameters [0.2] >>> op.inverse # check if the operation should be inverted False >>> op = qml.RX(0.2, wires=[0]).inv >>> op.inverse True
 Parameters
operations (list[Operation]) – operations to apply to the device
 Keyword Arguments
rotations (list[Operation]) – operations that rotate the circuit premeasurement into the eigenbasis of the observables.
hash (int) – the hash value of the circuit constructed by CircuitGraph.hash

batch_execute
(circuits)¶ Execute a batch of quantum circuits on the device.
The circuits are represented by tapes, and they are executed onebyone 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[tapes.QuantumTape]) – circuits to execute on the device
 Returns
list of measured value(s)
 Return type
list[array[float]]

batch_transform
(circuit)¶ Apply a differentiable batch transform for preprocessing a circuit prior to execution. This method is called directly by the QNode, and should be overwritten if the device requires a transform that generates multiple circuits prior to execution.
By default, this method contains logic for generating multiple circuits, one per term, of a circuit that terminates in
expval(H)
, if the underlying device does not support Hamiltonian expectation values, or if the device requires finite shots.Warning
This method will be tracked by autodifferentiation libraries, such as Autograd, JAX, TensorFlow, and Torch. Please make sure to use
qml.math
for autodiffagnostic tensor processing if required. Parameters
circuit (QuantumTape) – the circuit to preprocess
 Returns
Returns a tuple containing the sequence of circuits to be executed, and a postprocessing function to be applied to the list of evaluated circuit results.
 Return type
tuple[Sequence[QuantumTape], callable]

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)[source]¶ Create the map from userprovided 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, nonconsecutive and/or noninteger wire labels.
 Parameters
wires (Wires) – userprovided 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]>)])

density_matrix
(wires)[source]¶ Returns the reduced density matrix of a given set of wires.
 Parameters
wires (Wires) – wires of the reduced system.
 Returns
complex tensor of shape
(2 ** len(wires), 2 ** len(wires))
representing the reduced density matrix. Return type
array[complex]

estimate_probability
(wires=None, shot_range=None, bin_size=None)¶ Return the estimated probability of each computational basis state using the generated samples.
 Parameters
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]) – 2tuple 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
list of the probabilities
 Return type
array[float]

execute
(circuit, **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
Additional keyword arguments may be passed to the this method that can be utilised by
apply()
. An example would be passing theQNode
hash that can be used later for parametric compilation. Parameters
circuit (CircuitGraph) – circuit to execute on the device
 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 onebyone 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()
andexpval()
) are then evaluated within the context of this context manager (see the source ofDevice.execute()
for more details).

expand_fn
(circuit, max_expansion=10)¶ Method for expanding or decomposing an input circuit. This method should be overwritten if custom expansion logic is required.
By default, this method expands the tape if:
nested tapes are present,
any operations are not supported on the device, or
multiple observables are measured on the same wire.
 Parameters
circuit (QuantumTape) – the circuit to expand.
max_expansion (int) – The number of times the circuit should be expanded. Expansion occurs when an operation or measurement is not supported, and results in a gate decomposition. If any operations in the decomposition remain unsupported by the device, another expansion occurs.
 Returns
The expanded/decomposed circuit, such that the device will natively support all operations.
 Return type

expval
(observable, shot_range=None, bin_size=None)[source]¶  Returns the expectation value of a Hamiltonian observable. When the observable is a
SparseHamiltonian
object, the expectation value is computed directly for the full Hamiltonian, which leads to faster execution.
 Parameters
observable (Observable) – a PennyLane observable
shot_range (tuple[int]) – 2tuple 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
returns the expectation value of the observable
 Return type
float

static
generate_basis_states
(num_wires, dtype=<class 'numpy.uint32'>)¶ Generates basis states in binary representation according to the number of wires specified.
The states_to_binary method creates basis states faster (for larger systems at times over x25 times faster) than the approach using
itertools.product
, at the expense of using slightly more memory.Due to the large size of the integer arrays for more than 32 bits, memory allocation errors may arise in the states_to_binary method. Hence we constraint the dtype of the array to represent unsigned integers on 32 bits. Due to this constraint, an overflow occurs for 32 or more wires, therefore this approach is used only for fewer wires.
For smaller number of wires speed is comparable to the next approach (using
itertools.product
), hence we resort to that one for testing purposes. Parameters
num_wires (int) – the number wires
dtype=np.uint32 (type) – the data type of the arrays to use
 Returns
the sampled basis states
 Return type
array[int]

generate_samples
()¶ Returns the computational basis samples generated for all wires.
Note that PennyLane uses the convention \(q_0,q_1,\dots,q_{N1}\rangle\) where \(q_0\) is the most significant bit.
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 of samples in the shape
(dev.shots, dev.num_wires)
 Return type
array[complex]

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]]

marginal_prob
(prob, wires=None)¶ Return the marginal probability of the computational basis states by summing the probabiliites on the nonspecified 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 passedwires=[2, 0]
, then the returned marginal probability vector will take this ‘reversal’ of the two wires into account:\[\mathbb{P}^{(2, 0)} = \left[ 00\rangle, 10\rangle, 01\rangle, 11\rangle \right]\] Parameters
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 of the resulting marginal probabilities.
 Return type
array[float]

probability
(wires=None, shot_range=None, bin_size=None)¶ Return either the analytic probability or estimated probability of each computational basis state.
Devices that require a finite number of shots always return the estimated probability.
 Parameters
wires (Iterable[Number, str], Number, str, Wires) – wires to return marginal probabilities for. Wires not provided are traced out of the system.
 Returns
list of the probabilities
 Return type
array[float]

sample
(observable, shot_range=None, bin_size=None)¶ 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]

sample_basis_states
(number_of_states, state_probability)¶ Sample from the computational basis states based on the state probability.
This is an auxiliary method to the generate_samples method.
 Parameters
number_of_states (int) – the number of basis states to sample from
state_probability (array[float]) – the computational basis probability vector
 Returns
the sampled basis states
 Return type
array[int]

static
states_to_binary
(samples, num_wires, dtype=<class 'numpy.int64'>)¶ Convert basis states from base 10 to binary representation.
This is an auxiliary method to the generate_samples method.
 Parameters
samples (array[int]) – samples of basis states in base 10 representation
num_wires (int) – the number of qubits
dtype (type) – Type of the internal integer array to be used. Can be important to specify for large systems for memory allocation purposes.
 Returns
basis states in binary representation
 Return type
array[int]

statistics
(observables, shot_range=None, bin_size=None)¶ Process measurement results from circuit execution and return statistics.
This includes returning expectation values, variance, samples, probabilities, states, and density matrices.
 Parameters
observables (List[Observable]) – the observables to be measured
shot_range (tuple[int]) – 2tuple 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.
 Raises
QuantumFunctionError – if the value of
return_type
is not supported Returns
the corresponding statistics
 Return type
Union[float, List[float]]
Usage Details
The
shot_range
andbin_size
arguments allow for the statistics to be performed on only a subset of device samples. This finer level of control is accessible from the main UI by instantiating a device with a batch of shots.For example, consider the following device:
>>> dev = qml.device("my_device", shots=[5, (10, 3), 100])
This device will execute QNodes using 135 shots, however measurement statistics will be course grained across these 135 shots:
All measurement statistics will first be computed using the first 5 shots — that is,
shots_range=[0, 5]
,bin_size=5
.Next, the tuple
(10, 3)
indicates 10 shots, repeated 3 times. We will want to useshot_range=[5, 35]
, performing the expectation value in bins of size 10 (bin_size=10
).Finally, we repeat the measurement statistics for the final 100 shots,
shot_range=[35, 135]
,bin_size=100
.

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, shot_range=None, bin_size=None)¶ 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
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