论文标题
近期量子设备的指数误差抑制
Exponential Error Suppression for Near-Term Quantum Devices
论文作者
论文摘要
随着量子计算机的成熟,将采用量子错误校正代码(QEC),以抑制任何所需的级别$ e $的错误,以QUBIT-COUNT $ N $中的成本,仅是$ 1/E $的poly-logarithmic。但是,在NISQ时代,即使是最小的QEC,也需要采用的复杂性和规模。相反,已经采用了缓解错误技术;通常,这些不需要增加QUBIT计数,但不能提供指数级误差。在这里,我们表明,对于估计可观察到的期望值(几乎所有NISQ算法的关键)的关键案例,人们确实可以实现有效的指数抑制。我们通过disnangement(ESD)方法介绍了误差抑制:通过将量子计数增加$ n \ geq 2 $,将误差置于指数抑制,为$ q^n $,其中$ q <1 $是抑制因素,取决于错误的熵。 ESD方法采用$ n $独立的电路输出,并应用受控的扰动操作员创建一个状态,其对称性会阻止错误状态对预期值的贡献。因此,该方法是“ NISQ友好型”的,因为它在主计算中是模块化的,并且仅需要一个浅层电路,该电路在测量前立即桥接$ n $副本。我们的毁灭性电路中的缺陷确实会降低性能,因此,由于毁灭的显着特性,我们提出了一种减轻这种效果为任意精度的方法。 a)它们分解成线性数量的基本门 - 限制噪声的影响b)它们对噪声高度韧性,并且对它们的缺陷的影响几乎是(几乎)琐碎的。在验证我们方法的数值模拟中,我们使用不超过$ n = 4 $电路副本的电路确认抑制错误抑制$ 10^{ - 6} $。
As quantum computers mature, quantum error correcting codes (QECs) will be adopted in order to suppress errors to any desired level $E$ at a cost in qubit-count $n$ that is merely poly-logarithmic in $1/E$. However in the NISQ era, the complexity and scale required to adopt even the smallest QEC is prohibitive. Instead, error mitigation techniques have been employed; typically these do not require an increase in qubit-count but cannot provide exponential error suppression. Here we show that, for the crucial case of estimating expectation values of observables (key to almost all NISQ algorithms) one can indeed achieve an effective exponential suppression. We introduce the Error Suppression by Derangement (ESD) approach: by increasing the qubit count by a factor of $n\geq 2$, the error is suppressed exponentially as $Q^n$ where $Q<1$ is a suppression factor that depends on the entropy of the errors. The ESD approach takes $n$ independently-prepared circuit outputs and applies a controlled derangement operator to create a state whose symmetries prevent erroneous states from contributing to expected values. The approach is therefore `NISQ-friendly' as it is modular in the main computation and requires only a shallow circuit that bridges the $n$ copies immediately prior to measurement. Imperfections in our derangement circuit do degrade performance and therefore we propose an approach to mitigate this effect to arbitrary precision due to the remarkable properties of derangements. a) they decompose into a linear number of elementary gates -- limiting the impact of noise b) they are highly resilient to noise and the effect of imperfections on them is (almost) trivial. In numerical simulations validating our approach we confirm error suppression below $10^{-6}$ for circuits consisting of several hundred noisy gates (two-qubit gate error $0.5\%$) using no more than $n=4$ circuit copies.