许晏to沈惠川教授的信：
Sakurai（樱井）的《Modern Quantum Mechanics》一书中已谈到“量子信息传输”和超光速通讯是根本不可能的

沈老师，您好！
 
             Sakurai的《现代量子力学》(Modern Quantum Mechanics)修订版在2005年由世图影印，科大的书店里就有。该书观点中立，书中第三章第四节讲系综。第三章第九节讲了自旋关联和Bell不等式，在此节末尾（世图影印版第231~232页）谈到了“量子信息传输”，原文如下：
 
        "... we cannot use spin-correlation measurements to transmit any useful information between two macroscopically separated points. In particular, superluminal (faster than light) communications are impossible". (231页倒数第二段)
 
        中文大意：“... 自旋相关的测量实验，无法让我们在两个宏观上分离的点之间传输任何有用的信息。特别地，超光速通讯是根本不可能的。”
       
        紧接着，作者详细说明了无法用量子自旋关联传递有用信息的原因：
 
        "Suppose A and B both agree in advance to measure Sz; then without asking A, B knows precisely what A is getting. But this does not mean that A and B are communicating; B just observes a random sequence of positive and negative signs. There is obviously no useful information contained in it. B verifies the remarkable correlations predicted by quantum mechanics only after he or she gets together withA and compares the notes (or computer sheets)." （231~232页）
 
         中文大意：“设想A和B事先约定好去测量Sz；那么不用去问A，B 可以确切地知道A得到的结果。然而，这并非意味着A与B之间在通信；B观测到的只是一个随机出现的无规正负符号的序列。这里面显然不含有任何有用的信息。只有当事后B与A一同对比各自观测得到的记录时，B才可以证实由量子力学所预测的明显的自旋关联。”
 
         "It might be thought that A and B can communicate if one of them suddenly changes the orientation of his or her analyzing apparatus. Let us suppose that A agrees initially to measure Sz, and B, Sx. The results of A's measurements are completely uncorrelated with the results of B's measurements, so there is no information transferred. But then,suppose A suddenly breaks his or her promise and without telling B starts measuring Sx.There are now complete correlations between A's results and B's results. However, B has no way of inferring that A has changed the orientation of his or her analyzer. B continues to see just a random sequence of +'s and -'s by looking at his or her own notebook only.So again there is no information transferred." （232页第二段）
 
          中文大意：“或许有下面的想法存在：若是A和B中的任一方突然改变他（她）的测量装置的取向，他们之间就可能通讯。让我们来设想A事前同意去测量Sz，而B同意去测量Sx。这样的话，A的观测结果就与B的测量结果完全没有任何相关，当然在它们之间就不会有任何信息传递。然而，假设A没有通知B就突然违背了先前的约定而改去测量Sx。这时A与B的观测结果之间就有了完全的关联。但是，B无法从任何途径推测得知A改变了他（她）的测量装置的取向。B通过审视己方的观测记录，无非只是继续观测到一个随机的正负观测值的序列。所以同样不会有信息的传递。”
         
               以上是从Sakurai书中摘录的作者对“量子通信”的驳斥，和我根据英文原文的直译。
 
              
 
                                                  许晏