calculate_cut in the VQE example?

[marionsilv]

Hello! I have been trying to use your code to compute the MaxCut in the VQE jupyter notebook provided in the example sections. I tried to apply the calculate_cut function on the state as
tlq.calculate_cut(state, qubits1, qubits2, weights)
but I am having the following error
TypeError: only integer tensors of a single element can be converted to an index.

I see that the cut is calculated differently in the MBE example, but I would like to know if there is an analogue way of doing it with the VQE. Or should I just adapt my Hamiltonian to maximize the cut?
Any help is appreciated,
Thanks!

[taylorpatti]

Hi Marion,

The answer depends on how you would like to define the MaxCut of a VQE matrix.

In the Ising model Hamiltonian that we provide in the VQE example, we have both z and x-axis terms. If you remove the x-axis terms, then you indeed recover a classical Hamiltonian that encodes a MaxCut problem. You can use the calculate_cut function with the construct:
tlq.calculate_cut(torch.sign(pauli_terms), vertices1, vertices2, weights)
where torch.sign rounds your pauli_terms (the outcome of your single-qubit measurements) to +/- 1, and vertices are integer tensors in torch. The ground state of the Hamiltonian will encode your MaxCut.

If you leave in both the z and x-axis terms in the VQE example, then that Hamiltonian no longer encodes a MaxCut problem, so the definition of using calculate_cut on the output state is no longer clear. I'd be happy to help, but would need more information.

[marionsilv]

Hi Taylor.

Thank you for the quick reply. I was indeed referring to the Ising model without the transversal field. I was somehow expecting that the MaxCut function would need two operators, like in the MBE case. I will try your implementation and I'll return to you if I have any other questions, I hope that is ok!

Best regards,
Marion.