Efficient classical calculation of the Quantum Natural Gradient

Quantum natural gradient has emerged as a superior minimisation technique in quantum variational algorithms. Classically simulating the algorithm running on near-future quantum hardware is paramount in its study, as it is for all variational algorithms. In this case, state-vector simulation of the P-parameter/gate ansatz circuit does not dominate the runtime; instead, calculation of the Fisher information matrix becomes the bottleneck, involving OP³ gate evaluations, though this can be reduced to OP² gates by using O(P) temporary state-vectors. This is similar to the gradient calculation subroutine dominating the simulation of quantum gradient descent, which has attracted HPC strategies and bespoke simulation algorithms with asymptotic speedups. We here present a novel simulation strategy to precisely calculate the quantum natural gradient in OP² gates and O(1) state-vectors. While more complicated, our strategy is in the same spirit as that presented for gradients in Reference 6, and involves iteratively evaluating recurrent forms of the Fisher information matrix. Our strategy uses only "apply gate", "clone state" and "inner product" operations which are present in practically all quantum computing simulators. It is furthermore compatible with parallelisation schemes, like hardware acceleration and distribution. Since our scheme leverages a form of the Fisher information matrix for strictly unitary ansatz circuits, it cannot be simply extended to density matrix simulation of quantum natural gradient with non-unitary circuits.