Harrow, Hassidim, and Lloyd showed that for a suitably specified N×N matrix A and N-dimensional vector b⃗ , there is a quantum algorithm that outputs a quantum state proportional to the solution of the linear system of equations Ax⃗ =b⃗ . If A is sparse and well-conditioned, their algorithm runs in time poly(logN,1/ϵ), where ϵ is the desired precision in the output state. We improve this to an algorithm whose running time is polynomial in log(1/ϵ), exponentially improving the dependence on precision while keeping essentially the same dependence on other parameters. Our algorithm is based on a general technique for implementing any operator with a suitable Fourier or Chebyshev series representation. This allows us to bypass the quantum phase estimation algorithm, whose dependence on ϵ is prohibitive.

VL - 46 U4 - 1920-1950 UR - http://epubs.siam.org/doi/10.1137/16M1087072 CP - 6 U5 - 10.1137/16M1087072 ER - TY - JOUR T1 - Simulating Hamiltonian dynamics with a truncated Taylor series JF - Physical Review Letters Y1 - 2015 A1 - Dominic W. Berry A1 - Andrew M. Childs A1 - Richard Cleve A1 - Robin Kothari A1 - Rolando D. Somma AB - We describe a simple, efficient method for simulating Hamiltonian dynamics on a quantum computer by approximating the truncated Taylor series of the evolution operator. Our method can simulate the time evolution of a wide variety of physical systems. As in another recent algorithm, the cost of our method depends only logarithmically on the inverse of the desired precision, which is optimal. However, we simplify the algorithm and its analysis by using a method for implementing linear combinations of unitary operations to directly apply the truncated Taylor series. VL - 114 U4 - 090502 UR - http://arxiv.org/abs/1412.4687v1 CP - 9 J1 - Phys. Rev. Lett. U5 - 10.1103/PhysRevLett.114.090502 ER - TY - JOUR T1 - Exponential improvement in precision for simulating sparse Hamiltonians JF - Proceedings of the 46th ACM Symposium on Theory of Computing (STOC 2014) Y1 - 2014 A1 - Dominic W. Berry A1 - Andrew M. Childs A1 - Richard Cleve A1 - Robin Kothari A1 - Rolando D. Somma AB - We provide a quantum algorithm for simulating the dynamics of sparse Hamiltonians with complexity sublogarithmic in the inverse error, an exponential improvement over previous methods. Specifically, we show that a $d$-sparse Hamiltonian $H$ acting on $n$ qubits can be simulated for time $t$ with precision $\epsilon$ using $O\big(\tau \frac{\log(\tau/\epsilon)}{\log\log(\tau/\epsilon)}\big)$ queries and $O\big(\tau \frac{\log^2(\tau/\epsilon)}{\log\log(\tau/\epsilon)}n\big)$ additional 2-qubit gates, where $\tau = d^2 \|{H}\|_{\max} t$. Unlike previous approaches based on product formulas, the query complexity is independent of the number of qubits acted on, and for time-varying Hamiltonians, the gate complexity is logarithmic in the norm of the derivative of the Hamiltonian. Our algorithm is based on a significantly improved simulation of the continuous- and fractional-query models using discrete quantum queries, showing that the former models are not much more powerful than the discrete model even for very small error. We also simplify the analysis of this conversion, avoiding the need for a complex fault correction procedure. Our simplification relies on a new form of "oblivious amplitude amplification" that can be applied even though the reflection about the input state is unavailable. Finally, we prove new lower bounds showing that our algorithms are optimal as a function of the error. U4 - 283-292 SN - 978-1-4503-2710-7 UR - http://arxiv.org/abs/1312.1414v2 J1 - Proceedings of the 46th ACM Symposium on Theory of Computing (STOC 2014) U5 - 10.1145/2591796.2591854 ER -