Rolando Somma: Quantum Algorithms for Systems of Linear Equations
A talk by Rolando Somma at the Quantum Machine Learning Workshop, hosted September 24-28, 2018 by the Joint Center for Quantum Information and Computer Science at the University of Maryland (QuICS).
Abstract: Harrow, Hassidim, and Lloyd (HHL) proved that, for a matrix A of dimension NxN and a vector b of dimension N, there exists a quantum algorithm that prepares a quantum state proportional to the solution of the linear system A.x=b. When the matrix A is sparse and well-conditioned, the HHL algorithm has complexity that is polynomial in 1/ε and log(N), where ε is greater than 0 is the precision. This result represents a quantum speedup with respect to classical algorithms. In this talk I will describe improved quantum algorithms for the linear system’s problem with a significant reduction in complexity and other resources. One such quantum algorithm has complexity that depends logarithmically in 1/ε, exponentially improving the complexity dependence on precision while keeping essentially the same dependence on other parameters. The other quantum algorithm is based on quantum adiabatic evolutions and does not use complex subroutines like phase estimation or variable time amplitude amplification that require several ancillary qubits.