[BUG] Wrong expectation value of Hamiltonian with return qml.expval(H)

[chichun-chen]

Expected behavior

Expected to get the same expectation value of a qml.Hamiltonian no matter

1. how the operators and the corresponding coefficient are ordered when it was constructed.
2. applied the grouping method ('QWC') or not.

Actual behavior

When the below conditions are satisfied, the expectation value will not take the value (coefficient) of qml.Identity into account.

1. There is a "grouping_type" argument when qml.Hamiltonian is constructed.
2. The operator list is not in the order of qubit number.

Example H_disordered = qml.Hamiltonian([1.0, 1.0, 1.0], [qml.PauliZ(1), qml.PauliZ(0), qml.Identity(0)], grouping_type='QWC') The list of operator is disordered in terms of qubit number. However its expectation value differs with the value of qml.Identity from an ordered Hamiltonian
H_ordered = qml.Hamiltonian([1.0, 1.0, 1.0], [qml.Identity(0), qml.PauliZ(0), qml.PauliZ(1)], grouping_type='QWC')

Additional information

No response

Source code

import pennylane as qml

dev = qml.device("default.qubit", wires=2, shots=1000)
@qml.qnode(dev)
def circuit():
    return qml.expval(H)

H = qml.Hamiltonian([1.0, 1.0, 1.0], [qml.PauliZ(0), qml.PauliZ(1), qml.Identity(0)], grouping_type='QWC')
print(circuit())

H = qml.Hamiltonian([1.0, 1.0, 1.0], [qml.PauliZ(1), qml.PauliZ(0), qml.Identity(0)], grouping_type='QWC')
print(circuit())

H = qml.Hamiltonian([1.0, 1.0, 1.0], [qml.PauliZ(1), qml.PauliZ(0), qml.Identity(0)], grouping_type='commuting')
print(circuit())

H = qml.Hamiltonian([1.0, 1.0, 1.0], [qml.PauliZ(1), qml.PauliZ(0), qml.Identity(0)])
print(circuit())

Tracebacks

No response

System information

Platform info:           macOS-13.0.1-arm64-arm-64bit
Python version:          3.9.12
Numpy version:           1.23.3
Scipy version:           1.9.3
Installed devices:
- default.gaussian (PennyLane-0.30.0)
- default.mixed (PennyLane-0.30.0)
- default.qubit (PennyLane-0.30.0)
- default.qubit.autograd (PennyLane-0.30.0)
- default.qubit.jax (PennyLane-0.30.0)
- default.qubit.tf (PennyLane-0.30.0)
- default.qubit.torch (PennyLane-0.30.0)
- default.qutrit (PennyLane-0.30.0)
- null.qubit (PennyLane-0.30.0)
- lightning.qubit (PennyLane-Lightning-0.30.0)

Existing GitHub issues

• [X] I have searched existing GitHub issues to make sure the issue does not already exist.

[albi3ro]

Thanks for the bug report @chichun-chen .

We now have a bug fix up for review. The fix should be merged to master within a few days, and will be out in our next release on June 27.