论文标题

组合优化的扭曲混合算法

Twisted hybrid algorithms for combinatorial optimization

论文作者

Caha, Libor, Kliesch, Alexander, Koenig, Robert

论文摘要

提出的混合算法将组合成本函数编码为哈密顿量的问题,并通过在一组低电路复杂性的状态上变化来优化其能量。经典处理通常仅用于梯度下降后的变异参数的选择。结果,这些方法受相关状态的描述能力的限制。 我们认为,对于某些组合优化问题,可以进一步杂交此类算法,从而利用有效的非本地经典处理的力量。具体而言,我们考虑将量子变量ANSATZ与$ 3 $等级图上的Maxcut-Problem的贪婪经典后处理程序结合在一起。我们表明,该方法产生的平均切割大小可以根据修改后的问题哈密顿量的能量来量化。这激发了对改进的算法的考虑,该算法可变化优化改良的哈密顿量的能量。我们将其称为扭曲的混合算法,因为将其他经典处理步骤与各种变异参数选择相结合。我们使用量子近似优化算法(QAOA)来说明该方法的生存能力,从而在扭曲的QAOA达到的预期近似值率上给出了分析下限。这些表明,与原始QAOA相比,量子Ansatz的必要非局部性可以减少:我们发现,对于$ p = 2,\ ldots,6 $,可以将级别$ p $降低一个,同时大致维持预期的近似值。这将电路深度降低了$ 4 $,而变分参数的数量则减少了$ 2 $。

Proposed hybrid algorithms encode a combinatorial cost function into a problem Hamiltonian and optimize its energy by varying over a set of states with low circuit complexity. Classical processing is typically only used for the choice of variational parameters following gradient descent. As a consequence, these approaches are limited by the descriptive power of the associated states. We argue that for certain combinatorial optimization problems, such algorithms can be hybridized further, thus harnessing the power of efficient non-local classical processing. Specifically, we consider combining a quantum variational ansatz with a greedy classical post-processing procedure for the MaxCut-problem on $3$-regular graphs. We show that the average cut-size produced by this method can be quantified in terms of the energy of a modified problem Hamiltonian. This motivates the consideration of an improved algorithm which variationally optimizes the energy of the modified Hamiltonian. We call this a twisted hybrid algorithm since the additional classical processing step is combined with a different choice of variational parameters. We exemplify the viability of this method using the quantum approximate optimization algorithm (QAOA), giving analytic lower bounds on the expected approximation ratios achieved by twisted QAOA. These show that the necessary non-locality of the quantum ansatz can be reduced compared to the original QAOA: We find that for levels $p=2,\ldots, 6$, the level $p$ can be reduced by one while roughly maintaining the expected approximation ratio. This reduces the circuit depth by $4$ and the number of variational parameters by $2$.

扫码加入交流群

加入微信交流群

微信交流群二维码

扫码加入学术交流群,获取更多资源