论文标题
因果关系叠加的无关定理
A no-go theorem for superpositions of causal orders
论文作者
论文摘要
事件的因果顺序不必固定：公共汽车是否在某个停靠点之前或之后到达是否取决于其他变量 - 例如流量。因果秩序的相干量子控制也是可能的，也是多个任务的有用资源。但是，量子控制意味着控制系统带有哪个订单信息 - 如果对控件的记录得出，则事件的顺序保留在概率混合物中。两个事件的顺序是否可以纯化，与其他系统无关？在这里，我们表明这是一系列广泛的过程：任何一对马尔可夫，具有相等局部维度和不同因果顺序的单一过程的纯正叠加不是一个有效的过程，即，当用某些操作探测时，它会导致非正态概率。该结果对量子信息处理的新资源和量子重力理论中的可能过程施加了限制。
The causal order of events need not be fixed: whether a bus arrives before or after another at a certain stop can depend on other variables -- like traffic. Coherent quantum control of causal order is possible too and is a useful resource for several tasks. However, quantum control implies that a controlling system carries the which-order information -- if the control is traced out, the order of events remains in a probabilistic mixture. Can the order of two events be in a pure superposition, uncorrelated with any other system? Here we show that this is not possible for a broad class of processes: a pure superposition of any pair of Markovian, unitary processes with equal local dimensions and different causal orders is not a valid process, namely it results in non-normalised probabilities when probed with certain operations. The result imposes constraints on novel resources for quantum information processing and on possible processes in a theory of quantum gravity.
