qml.sample

sample(op=None, wires=None)

Sample from the supplied observable, with the number of shots determined from the dev.shots attribute of the corresponding device, returning raw samples. If no observable is provided then basis state samples are returned directly from the device.

Note that the output shape of this measurement process depends on the shots specified on the device.

Parameters

• op (Observable or MeasurementValue) – a quantum observable object. To get samples for mid-circuit measurements, op should be a MeasurementValue.

• wires (Sequence[int] or int or None) – the wires we wish to sample from; ONLY set wires if op is None.

Returns

Measurement process instance

Return type

SampleMP

Raises

ValueError – Cannot set wires if an observable is provided

The samples are drawn from the eigenvalues \(\{\lambda_i\}\) of the observable. The probability of drawing eigenvalue \(\lambda_i\) is given by \(p(\lambda_i) = |\langle \xi_i | \psi \rangle|^2\), where \(| \xi_i \rangle\) is the corresponding basis state from the observable’s eigenbasis.

Note

QNodes that return samples cannot, in general, be differentiated, since the derivative with respect to a sample — a stochastic process — is ill-defined. An alternative approach would be to use single-shot expectation values. For example, instead of this:

dev = qml.device("default.qubit", shots=10)

@qml.qnode(dev, diff_method="parameter-shift")
def circuit(angle):
    qml.RX(angle, wires=0)
    return qml.sample(qml.PauliX(0))

angle = qml.numpy.array(0.1)
res = qml.jacobian(circuit)(angle)

Consider using expval() and a sequence of single shots, like this:

dev = qml.device("default.qubit", shots=[(1, 10)])

@qml.qnode(dev, diff_method="parameter-shift")
def circuit(angle):
    qml.RX(angle, wires=0)
    return qml.expval(qml.PauliX(0))

def cost(angle):
    return qml.math.hstack(circuit(angle))

angle = qml.numpy.array(0.1)
res = qml.jacobian(cost)(angle)

Example

dev = qml.device("default.qubit", wires=2, shots=4)

@qml.qnode(dev)
def circuit(x):
    qml.RX(x, wires=0)
    qml.Hadamard(wires=1)
    qml.CNOT(wires=[0, 1])
    return qml.sample(qml.Y(0))

Executing this QNode:

>>> circuit(0.5)
array([ 1.,  1.,  1., -1.])

If no observable is provided, then the raw basis state samples obtained from device are returned (e.g., for a qubit device, samples from the computational device are returned). In this case, wires can be specified so that sample results only include measurement results of the qubits of interest.

dev = qml.device("default.qubit", wires=2, shots=4)

@qml.qnode(dev)
def circuit(x):
    qml.RX(x, wires=0)
    qml.Hadamard(wires=1)
    qml.CNOT(wires=[0, 1])
    return qml.sample()

Executing this QNode:

>>> circuit(0.5)
array([[0, 1],
       [0, 0],
       [1, 1],
       [0, 0]])

code/api/pennylane.sample

 

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