Hosten Onur, Rakher Matthew T, Barreiro Julio T, Peters Nicholas A, Kwiat Paul G
Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
Nature. 2006 Feb 23;439(7079):949-52. doi: 10.1038/nature04523.
The logic underlying the coherent nature of quantum information processing often deviates from intuitive reasoning, leading to surprising effects. Counterfactual computation constitutes a striking example: the potential outcome of a quantum computation can be inferred, even if the computer is not run. Relying on similar arguments to interaction-free measurements (or quantum interrogation), counterfactual computation is accomplished by putting the computer in a superposition of 'running' and 'not running' states, and then interfering the two histories. Conditional on the as-yet-unknown outcome of the computation, it is sometimes possible to counterfactually infer information about the solution. Here we demonstrate counterfactual computation, implementing Grover's search algorithm with an all-optical approach. It was believed that the overall probability of such counterfactual inference is intrinsically limited, so that it could not perform better on average than random guesses. However, using a novel 'chained' version of the quantum Zeno effect, we show how to boost the counterfactual inference probability to unity, thereby beating the random guessing limit. Our methods are general and apply to any physical system, as illustrated by a discussion of trapped-ion systems. Finally, we briefly show that, in certain circumstances, counterfactual computation can eliminate errors induced by decoherence.
量子信息处理的相干性质背后的逻辑常常偏离直观推理，从而导致令人惊讶的效应。反事实计算就是一个显著的例子：即使量子计算机未运行，也能推断出量子计算的潜在结果。基于与无相互作用测量（或量子询问）类似的原理，反事实计算通过将计算机置于“运行”和“未运行”状态的叠加态，然后干涉这两种历史来实现。根据计算尚未知晓的结果，有时可以反事实地推断出关于解的信息。在此，我们展示反事实计算，采用全光学方法实现格罗弗搜索算法。人们曾认为这种反事实推断的总体概率本质上是有限的，以至于其平均表现不会优于随机猜测。然而，通过使用量子芝诺效应的一种新颖“链式”版本，我们展示了如何将反事实推断概率提高到 unity，从而超越随机猜测的极限。我们的方法具有通用性，适用于任何物理系统，对囚禁离子系统的讨论就说明了这一点。最后，我们简要表明，在某些情况下，反事实计算可以消除退相干引起的误差。
