[BUG] Different ground states when batching with vmap or calling the function directly

[erikrecio]

[BUG] Different ground states when batching with vmap or calling the function directly

[albi3ro]

Thanks for opening this issue.

When trying it out locally, I get True for both master and v0.36.0, the latest release. But I do get False for version v0.35.0. We changed our system for representing arithmetic observables in v0.36.0.

Would you mind posting type(hamiltonian) and it's contents?

def ground_state(j1, j2):
    
    hamiltonian = 0
    
    for i in range(nqubits):
        hamiltonian += Z(i)
        hamiltonian -= j1 * X(i) @ X((i+1)%nqubits)
        hamiltonian -= j2 * X((i-1)%nqubits) @ Z(i) @ X((i+1)%nqubits)

    print(type(hamiltonian))
    print(hamiltonian)
    ham_matrix = qml.matrix(hamiltonian)
    _, eigvecs = jnp.linalg.eigh(ham_matrix)
    
    return eigvecs[:,0]

[erikrecio]

Seems like the new version fixes it! The types of hamiltonians before were different (from vmap to direct calling). Now they are both <class 'pennylane.ops.op_math.sum.Sum'>. The issues I was having seem to be fixed. Thanks for the quick answer!

[albi3ro]

Also, with the latest release, you can directly create qml.X ,qml.Y, qml.Z and qml.I.

[albi3ro]

Sorry. one last thing to mention, you may also want to do ham_matrix = qml.matrix(hamiltonian, wire_order=(0,1,2,3)). The new Sum operator gets it's wires from order of occurance.

[erikrecio]

Great, I'll take a look. Thanks!