Kevin J. Sung

I've upgraded [ffsim](https://github.com/qiskit-community/ffsim).

@abbycross I marked this issue as "on hold" for now since @javabster and I decided to wait until https://github.com/Qiskit/documentation/issues/524 is finished before doing this. This issue will require a fair...

> @kevinsung Thanks! I left some comments via the ReviewNB app [here](https://app.reviewnb.com/Qiskit/documentation/blob/kjs/skqd%2Fdocs%2Ftutorials%2Fimproving-energy-estimation-of-a-fermionic-lattice-model-with-sqd.ipynb%2F/discussion). @jenglick I just updated the PR to use the new `diagonalize_fermionic_hamiltonian` funtion and then went to revisit your...

> This looks great! Just a couple comments > > * I think we should also refer to the [KQD paper ](https://arxiv.org/abs/2407.14431)(and [tutorial](https://quantum.cloud.ibm.com/docs/en/tutorials/krylov-quantum-diagonalization)) as the convergence guarantee of the unitary...