Block-encoding structured matrices for data input in quantum computing

Quantum 8, 1226 (2024).

https://doi.org/10.22331/q-2024-01-11-1226

The cost of data input can dominate the run-time of quantum algorithms. Here, we consider data input of arithmetically structured matrices via $textit{block encoding}$ circuits, the input model for the quantum singular value transform and related algorithms. We demonstrate how to construct block encoding circuits based on an arithmetic description of the sparsity and pattern of repeated values of a matrix. We present schemes yielding different subnormalisations of the block encoding; a comparison shows that the best choice depends on the specific matrix. The resulting circuits reduce flag qubit number according to sparsity, and data loading cost according to repeated values, leading to an exponential improvement for certain matrices. We give examples of applying our block encoding schemes to a few families of matrices, including Toeplitz and tridiagonal matrices.