qml.shadows¶
Overview¶
Measurements¶

The classical shadow measurement protocol. 

Compute expectation values using classical shadows in a differentiable manner. 
Shadow class for classical postprocessing¶

Class for classical shadow postprocessing expectation values, approximate states, and entropies. 
QNode transforms¶

Transform a circuit returning a classical shadow into one that returns the approximate expectation values in a differentiable manner. 

Transform a circuit returning a classical shadow into one that returns the reconstructed state in a differentiable manner. 
Classical Shadows formalism¶
Note
As per arXiv:2103.07510, when computing multiple expectation values it is advisable to directly estimate the desired observables by simultaneously measuring
qubitwisecommuting terms. One way of doing this in PennyLane is via Hamiltonian
and setting grouping_type="qwc"
. For more details on this topic, see the PennyLane demo
on estimating expectation values with classical shadows.
A ClassicalShadow
is a classical description of a quantum state that is capable of reproducing expectation values of local Pauli observables, see arXiv:2002.08953.
The idea is to capture \(T\) local snapshots (given by the shots
set in the device) of the state by performing measurements in random Pauli bases at each qubit.
The measurement outcomes, denoted bits
, as well as the choices of measurement bases, recipes
, are recorded in two (T, len(wires))
integer tensors, respectively.
From the \(t\)th measurement, we can reconstruct the local_snapshots
(see ClassicalShadow
methods)
where \(U_i\) is the rotation corresponding to the measurement (e.g. \(U_i=H\) for measurement in \(X\)) of qubit \(i\) at snapshot \(t\) and \(b_i\rangle = (1  b_i, b_i)\) the corresponding computational basis state given the output bit \(b_i\).
From these local snapshots, one can compute expectation values of local Pauli strings, where locality refers to the number of nonIdentity operators.
The accuracy of the procedure is determined by the number of measurements \(T\) (shots
).
To target an error \(\epsilon\), one needs of order \(T = \mathcal{O}\left( \log(M) 4^\ell/\epsilon^2 \right)\) measurements to determine \(M\) different,
\(\ell\)local observables.
One can in principle also reconstruct the global state \(\sum_t \rho^{(t)}/T\), though it is not advisable nor practical for larger systems due to its exponential scaling.
Basic usage¶
The easiest way of computing expectation values with classical shadows in PennyLane is to return shadow_expval()
directly from the qnode.
H = qml.Hamiltonian([1., 1.], [qml.Z(0) @ qml.Z(1), qml.X(0) @ qml.Z(1)])
dev = qml.device("default.qubit", shots=10000)
# shadow_expval + midcircuit measurements require to defer measurements
@qml.defer_measurements
@qml.qnode(dev)
def qnode(x):
qml.Hadamard(0)
qml.CNOT((0,1))
qml.RX(x, wires=0)
qml.measure(1)
return qml.shadow_expval(H)
x = np.array(0.5, requires_grad=True)
The big advantage of this way of computing expectation values is that it is differentiable.
>>> qnode(x)
array(0.8406)
>>> qml.grad(qnode)(x)
0.49680000000000013
There are more options for postprocessing classical shadows in ClassicalShadow
.