Warn/raise in case shot noise and caching are enabled at the same time?

Feature details

When default.qubit with shots != None but also cache=1 is used then the same (noisy) result is returned again and again when a QNode is called. It is hard to imagine a case where this is what a user wants.

Either they want a QNode to return a noisy result that is different each time it is called (they can get this by setting cache=0), or they want only a single noisy result, in which case caching doesn’t provide any benefit.

I think it would be nice to raise an exception or at least a warning when a user makes a device with shot noise and caching enabled.

Alternatively the the device could go back to state caching, as it used to be in ancient times… This would give a speed benefit because the circuit would not need to be re-evaluated when the same shot-noisy QNode is called several times.

Implementation

To demonstrate the “problem” consider this sample program:

import pennylane as qml
from pennylane import numpy as np

shots=76

dev = qml.device(
    'default.qubit',
    wires=2,
    shots=shots,
    cache=1,
)

np.random.seed(87)
params = np.random.randn(2, 2, 3)
observable_as_array = np.array([[ 1. +0.j,  0. +0.j,  0. +0.j,  0. +0.j],
                                [ 0. +0.j,  0.4+0.j, 21.6+0.j,  0. +0.j],
                                [ 0. +0.j, 21.6+0.j, -0.4+0.j,  0. +0.j],
                                [ 0. +0.j,  0. +0.j,  0. +0.j, -1. +0.j]])

@qml.qnode(dev, diff_method='parameter-shift')
def circuit(params):
    qml.templates.layers.StronglyEntanglingLayers(params, wires=range(dev.num_wires))
    return qml.expval(qml.Hermitian(observable_as_array, wires=range(dev.num_wires)))

for _ in range(10):
    print(circuit(params))

How important would you say this feature is?

1: Not important. Would be nice to have.

Additional information

No response

[cvjjm]

Yes, that is entirely correct.

[josh146]

Good points @trbromley, this is a good reason to support the same tape multiple times 🤔

but I don’t think qml.execute supports a differerent shot value for each input tape?

It doesn’t atm, but this is something I would like to add - allow shots to be metadata on tape objects.