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arXiv quant-ph/0208189quant-ph
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量子绝热计算中大自旋隧穿势垒的全抑制

Total suppression of a large spin tunneling barrier in quantum adiabatic computation

Alex Bulatov, Vadim Smelyanskiy

摘要:

我们应用量子绝热演化算法的组合优化问题的成本函数完全取决于在一个n位字符串(汉明重量)的单位位的数量。优化问题的解被编码为问题哈密顿量H_p的基态,用于总自旋n/2的z投影。我们表明,隧道势垒的总自旋可以完全抑制在算法过程中,如果初始的哈密顿量有其基态扩展的空间中的z-投影的自旋。即使成本函数具有深度和良好分离的局部最小值,也会发生这种抑制。我们提供了一个直观的图片,这种效果,并表明,它保证了多项式复杂性的算法在一个非常广泛的成本函数类。我们提出一个绝热演化哈密顿量的简单例子:H(tau)=(1-tau)hat S_{x} ^{2} + tau H_p,其中参数tau随时间在0和1之间缓慢变化。我们使用WKB分析的大自旋估计的最小能隙之间的两个最低的绝热本征值的H(τ)。

英文摘要:

We apply a quantum adiabatic evolution algorithm to a combinatorial optimization problem where the cost function depends entirely on the of the number of unit bits in a n-bit string (Hamming weight). The solution of the optimization problem is encoded as a ground state of the problem Hamiltonian H_p for the z-projection of a total spin-n/2. We show that tunneling barriers for the total spin can be completely suppressed during the algorithm if the initial Hamiltonian has its ground state extended in the space of the z-projections of the spin. This suppression takes place even if the cost function has deep and well separated local minima. We provide an intuitive picture for this effect and show that it guarantees the polynomial complexity of the algorithm in a very broad class of cost functions. We suggest a simple example of the Hamiltonian for the adiabatic evolution: H(tau) = (1-tau) hat S_{x}^{2} + tau H_p, with parameter tau slowly varying in time between 0 and 1. We use WKB analysis for the large spin to estimate the minimum energy gap between the two lowest adiabatic eigenvalues of H(tau).

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