qml.measurements.StateMeasurement

class StateMeasurement(obs=None, wires=None, eigvals=None, id=None)

Bases: pennylane.measurements.measurements.MeasurementProcess

State-based measurement process.

Any class inheriting from StateMeasurement should define its own process_state method, which should have the following arguments:

• state (Sequence[complex]): quantum state

• wire_order (Wires): wires determining the subspace that state acts on; a matrix of

  dimension \(2^n\) acts on a subspace of \(n\) wires

Example:

Let’s create a measurement that returns the diagonal of the reduced density matrix.

>>> class MyMeasurement(StateMeasurement):
...     def process_state(self, state, wire_order):
...         # use the already defined `qml.density_matrix` measurement to compute the
...         # reduced density matrix from the given state
...         density_matrix = qml.density_matrix(wires=self.wires).process_state(state, wire_order)
...         return qml.math.diagonal(qml.math.real(density_matrix))

We can now execute it in a QNode:

>>> dev = qml.device("default.qubit", wires=2)
>>> @qml.qnode(dev)
... def circuit():
...     qml.Hadamard(0)
...     qml.CNOT([0, 1])
...     return MyMeasurement(wires=[0])
>>> circuit()
tensor([0.5, 0.5], requires_grad=True)

Attributes

has_decomposition	Whether or not the MeasurementProcess returns a defined decomposition when calling expand.
hash	returns an integer hash uniquely representing the measurement process
numeric_type	The Python numeric type of the measurement result.
raw_wires	The wires the measurement process acts on.
return_type	Measurement return type.
samples_computational_basis	Whether or not the MeasurementProcess measures in the computational basis.
wires	The wires the measurement process acts on.

has_decomposition

Whether or not the MeasurementProcess returns a defined decomposition when calling expand.

Type

Bool

hash

returns an integer hash uniquely representing the measurement process

Type

int

numeric_type

The Python numeric type of the measurement result.

Returns

The output numeric type; int, float or complex.

Return type

type

Raises

QuantumFunctionError – the return type of the measurement process is unrecognized and cannot deduce the numeric type

raw_wires

The wires the measurement process acts on.

For measurements involving more than one set of wires (such as mutual information), this is a list of the Wires objects. Otherwise, this is the same as wires()

return_type

Measurement return type.

samples_computational_basis

Whether or not the MeasurementProcess measures in the computational basis.

Type

Bool

wires

The wires the measurement process acts on.

This is the union of all the Wires objects of the measurement.

Methods

diagonalizing_gates()	Returns the gates that diagonalize the measured wires such that they are in the eigenbasis of the circuit observables.
eigvals()	Eigenvalues associated with the measurement process.
expand()	Expand the measurement of an observable to a unitary rotation and a measurement in the computational basis.
map_wires(wire_map)	Returns a copy of the current measurement process with its wires changed according to the given wire map.
process_state(state, wire_order)	Process the given quantum state.
queue([context])	Append the measurement process to an annotated queue.
shape([device])	The expected output shape of the MeasurementProcess.
simplify()	Reduce the depth of the observable to the minimum.

diagonalizing_gates()

Returns the gates that diagonalize the measured wires such that they are in the eigenbasis of the circuit observables.

Returns

the operations that diagonalize the observables

Return type

List[Operation]

eigvals()

Eigenvalues associated with the measurement process.

If the measurement process has an associated observable, the eigenvalues will correspond to this observable. Otherwise, they will be the eigenvalues provided when the measurement process was instantiated.

Note that the eigenvalues are not guaranteed to be in any particular order.

Example:

>>> m = MeasurementProcess(Expectation, obs=qml.PauliX(wires=1))
>>> m.eigvals()
array([1, -1])

Returns

eigvals representation

Return type

array

expand()

Expand the measurement of an observable to a unitary rotation and a measurement in the computational basis.

Returns

a quantum tape containing the operations required to diagonalize the observable

Return type

QuantumTape

Example:

Consider a measurement process consisting of the expectation value of an Hermitian observable:

>>> H = np.array([[1, 2], [2, 4]])
>>> obs = qml.Hermitian(H, wires=['a'])
>>> m = MeasurementProcess(Expectation, obs=obs)

Expanding this out:

>>> tape = m.expand()

We can see that the resulting tape has the qubit unitary applied, and a measurement process with no observable, but the eigenvalues specified:

>>> print(tape.operations)
[QubitUnitary(array([[-0.89442719,  0.4472136 ],
      [ 0.4472136 ,  0.89442719]]), wires=['a'])]
>>> print(tape.measurements[0].eigvals())
[0. 5.]
>>> print(tape.measurements[0].obs)
None

map_wires(wire_map)

Returns a copy of the current measurement process with its wires changed according to the given wire map.

Parameters

wire_map (dict) – dictionary containing the old wires as keys and the new wires as values

Returns

new measurement process

Return type

MeasurementProcess

abstract process_state(state, wire_order)

Process the given quantum state.

Parameters

• state (Sequence[complex]) – quantum state

• wire_order (Wires) – wires determining the subspace that state acts on; a matrix of dimension \(2^n\) acts on a subspace of \(n\) wires

queue(context=<class 'pennylane.queuing.QueuingManager'>)

Append the measurement process to an annotated queue.

shape(device=None)

The expected output shape of the MeasurementProcess.

Note that the output shape is dependent on the device when:

• The measurement type is either ProbabilityMP, StateMP (from state()) or SampleMP;

• The shot vector was defined in the device.

For example, assuming a device with shots=None, expectation values and variances define shape=(1,), whereas probabilities in the qubit model define shape=(1, 2**num_wires) where num_wires is the number of wires the measurement acts on.

Note that the shapes for vector-valued measurements such as ProbabilityMP and StateMP are adjusted to the output of qml.execute and may have an extra first element that is squeezed when using QNodes.

Parameters

device (pennylane.Device) – a PennyLane device to use for determining the shape

Returns

the output shape

Return type

tuple

Raises

QuantumFunctionError – the return type of the measurement process is unrecognized and cannot deduce the numeric type

simplify()

Reduce the depth of the observable to the minimum.

Returns

A measurement process with a simplified observable.

Return type

MeasurementProcess

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