  The rotation of subspaces by a chosen angle is a fundamental quantum
computing operation, with applications in error correction and quantum
algorithms such as the Quantum Approximate Optimization Algorithm, the
Variational Quantum Eigensolver and the quantum singular value transformation.
Such rotations are usually implemented at the hardware level via
multiple-controlled-phase gates, which lead to large circuit depth when
decomposed into one- and two-qubit gates. Here, we propose a fast,
high-fidelity way to implement such operations via topological quantum walks,
where a sequence of single-qubit $z$ rotations of an ancilla qubit are
interleaved with the evolution of a system Hamiltonian in which a matrix $A$ is
embedded. The subspace spanned by the left or right singular vectors of $A$
with non-zero singular values is rotated, depending on the state of the
ancilla. This procedure can be implemented in superconducting qubits, ion-traps
and Rydberg atoms with star-type connectivity, significantly reducing the total
gate time required compared to previous proposals.
