人类真的能够制造出构想中的量子计算机吗？
作者：落基山脉（Rocky Mountains)


1.   序言


      关于制造量子计算机(Quantum Computer )的报道，近年来在世界各地的许多学术会议、各种语言的媒体及网络上有着大量的报道和讨论。从这些文章及报道来看，主要是偏重于报道量子计算机（某些硬件及软件）工程研究及制造的进展。鲜有文章从工程和量子力学基础理论相结合的角度进行量子计算机研究制造的可行性分析。


       这篇文章的目的就是试图从工程和量子力学基础理论相结合的角度来探讨一下目前构想的量子计算机制造的可行性，即人类能否真正最后成功地制造出可以投入到实用的量子计算机，从而把人类社会文明向前推进一步。因为如果在人类投入了大量的金钱和时间以后，到最后反而制造不出来量子计算机，那么，量子计算机将会成为人类历史上的第二个“永动机”，将是人类的又一巨大损失。


       在此首先声明，


       第一，作者对多年来世界各国从事量子计算机理论研究和制造的科学家和工程师们充满了敬意，决无冒犯之意；


       第二，本文的观点仅仅是作者的一家之言。只是本着实事求是的科学态度表达一下作者自己的看法及观点；


       第三，作者尽量用非专业语言和词汇来描述量子计算机及量子力学，但有些地方仍难以避免使用某些专业术语，望广大读者予以谅解为盼。


2.   量子力学学派的分组


       在开始讨论量子计算机之前，我们首先把量子力学理论做一个简单的划分。这种划分主要是根据不同的物理学家对量子力学的观点而加以划分，别无其他。


       基于这个标准，量子力学理论大略的可划分成二个组：


       第一组主要是由玻尔、海森堡和玻恩为代表的哥本哈根学派。他们对早期的量子实验进行了理论分析和总结，叫作哥本哈根量子力学的诠释或解释，英文叫做Interpretation。在中文里面，“诠释”和“解释” 是略有不同的。前者的地位高于后者，有“正宗” 或者“主流”的意思在里面。


       第二组主要是以爱因斯坦、薛定谔和德布罗意为代表的另一部分物理学家对早期的量子实验进行的理论分析和总结。


       这六人均为诺贝尔物理学奖获得者，都对量子力学的发展做出过巨大的贡献。因此，他们均在同一个档次上。


       第一组的理论和第二组的理论异多同少，而且在许多基本方面是相矛盾的。


       例如，著名的薛定谔猫就是第二组的成员薛定谔发展了爱因斯坦的思想而提出的一个思想实验来反对第一组的理论的。


       再例如，今天很热的量子纠缠这个概念就是来自于第二组的成员们。实际上，量子纠缠思想实验， 或叫EPR思想实验，是第二组的著名物理学家爱因斯坦及二位助手首先提出来, 后又经薛定谔发展后反对第一组理论的。


       关于第一组和第二组理论的差别异同，本文不想作进一步的讨论。有兴趣的读者可以直接阅读有关文章书籍，包括中文版本或者英文版本。如果可能的话，建议直接阅读德文版本。


       根据这种划分方法，整个量子力学的理论实际上是由二部分组成：第一组的理论（哥本哈根学说）和第二组的理论（不妨暂把它称之为爱因斯坦和薛定谔学说）。


      如果用公式表示，那么可以写成如下的一个公式：


      量子力学理论=哥本哈根的理论+ 爱因斯坦/薛定谔学的理论


      基于上述公式，哥本哈根学派的理论只是量子力学学派中的一个学派，不能代表整个量子力学的理论。用现在网络语言描述的话，它只是一个“之一”。


     特别应该指出的是，由于不严格地对上述二组的理论加以区分，对量子力学的科学教学和普及造成了很大的困扰。


     例如，当人们谈论量子力学时，总是想当然的认为哥本哈根学说就代表了整个量子力学，哥本哈根学派的理论是量子力学的正宗。目前的量子力学教科书里（中文或英文），在讲解量子理论时，多数都遵循哥本哈根学说并且把它当作量子力学的正宗解释。但是，这个所谓的正宗解释在量子力学及科学中的地位越来越像一个宗教，不仅不容质疑，甚至还到了对学生们进行强制洗脑的地步。例如，在量子力学教学中经常出现面对学生们提出的哥本哈根学派的问题时，教授们常常避免正面回答，甚至告诫学生“别多问，只管算！”。即便哥本哈根学派不能“解释”它为什么是这样的，只能“告诉”你它是这样的。例如，面对著名的双缝实验和波粒二象性，只能用既是、又是来表达。


     因此，当谈及量子力学时，最好应该对此加以区别。例如，当有些文章谈到量子力学的理论指引工程发展取得了巨大进展的成功时，由于不具体指出是那个学派的理论，很多人想当然地认为是哥本哈根学派理论的成功。可实际上却不是。


量子计算机概念的由来
       1981年, 第一届国际计算物理会议在著名的大学MIT 举行。在会议期间，美国著名理论物理物学家、诺贝尔奖获得者理查德·费曼提出利用量子体系来实现通用计算的新奇想法。他认为，如果用量子系统所构成的计算机来模拟量子现象则运算时间可大幅度减少。在第二年，即1982年，国际理论物理杂志，《International Journal of Theoretical Physics》，请参看参考文献（1），出版了费曼在该次会议上作的题为“Simulating physics with computers” 的报告。因此，量子计算机的概念诞生了。


       在这里应该着重说明的是，费曼是基于哥本哈根学派的学说提出量子计算机这个概念的。即基于哥本哈根的叠加态理论上提出量子计算机概念的。在上一部分，我们已经说明了哥本哈根学派只是量子力学的一个学派，并且这个学派的基础是受到爱因斯斯坦和薛定谔学说挑战的。特别是，著名的薛定谔猫就是挑战这个理论的。因此，量子计算机的基础理论依据是不牢固的，并不是完全建立在无懈可击的实验基础之上的，只是建立在某个学派的假设之上的。


 量子计算机的发展 


        从费曼提出量子计算机概念之后，某些国家、大学或实验室, 特别是近年来,许多大的国际跨国公司都在量子计算机领域进行了投资。当然，大家是用不同的量子物理体系来构造量子计算机。如光子的两个正交的偏振方向；磁场中电子的自旋方向；核自旋的两个方向；原子中量子处在的两个不同能级等。总之，各种途径都在探索。就目前来说，哪一种物理体系更加适合于做量子计算机，还没有一个确定的答案。但是不论采用何种途径，最基本的一点是相同的：这些构想中的量子计算机最本质的特征都是使用了量子的叠加性。


        根据到目前为止所发表的论文及报告来看，人们构造中的量子计算机，从原理上来讲类似于经典计算机：它也是一种要进行逻辑运算、存储和处理信息的物理装置。不同的是这个物理装置处理和计算的是量子信息，运行的是量子算法。


量子计算机原理的简单概括


        为了了解量子计算机的工作原理，现以经典计算机为基础加以简单地工程解释。


         经典计算机：经典计算机是二进制数系统 并且是以计算机中的基本单元（比特）来进行运算。 每个“0”或“1”就是一个基本单元。基本单元是数据存储的最小单位。换句话说，一个经典基本单元在一个确定的时间里只能有一个确定的”0”或“1”的状态。注意这里的“或”字。


        量子计算机：根据目前人们的构想，量子计算机也是二进制数系统 ，并且是以计算机的基本单元（量子比特）来进行运算。在每一个量子基本单元中，可以同时保存“0”和“1”的状态。 注意这里的“同时”和 “和”字。


        经典计算机和量子计算机的差别就在于，经典计算机的基本单元在一确定的时间仅能存储“0”或“1”其中一个；而量子计算机的基本单元能同时存储“0”和“1”。两种计算机的差距就是“或”和“和”的差别：
经典计算机：“0” 或 “1”；
量子计算机：“0” 和 ”1”。


        那么可以简单的计算一下，在经典计算机中，如果有K个经典基本单元，它只可以同时保存K个单位信息；在量子计算机中，对于K个量子基本单元，它们却可以同时保存2^K个单位信息。随着量子基本单元数量的增加，其信息数量便可以迅速增大。


       要用量子计算机实现量子计算，也必须像经典计算机一样，用三个步骤完成其计算：


第一步是必须构造出一个量子基本单元；


第二步是对量子基本单元进行操控，让 “0” 和 “1” 状态同时存在；


第三步，需要把K个相同的量子基本单元组合起来，构成一个量子计算系统, 从而实现量子计算机的超级计算能力。


所以，量子计算机不是只含有一个量子基本单元，它是含有多个量子基本单元结合在一起的物理系统。


         构建量子计算机最难的是第一步，即必须构造出一个量子基本单元。这个单元必须让“0” 和“1” 状态同时存在。注意，这里要求必须 “同时” 存在！如果不是“同时”存在，那么，这就不是量子基本单元，而是经典基本单元。


         在第二步，人们构想用 量子处理器来操作以实现对每个量子基本单元的构造和操控。它要求在量子基本单元中，“0”和“1”必须同时存在，即人工形成量子叠加态而且要保持稳定。如果 “0” 和 “1” 不能同时存在，那么，这种量子计算机的精度再高，它本质上还是经典计算机。


         读者们可能有时会从网络上看到某些报道，称已经做出了几千量子基本单元的量子计算机等等。这给人们产生了一种好像马上就造出来量子计算机的印象，让人倍感鼓舞。实际上，这是构造量子计算机的第三步，它在构建量子计算机的三步曲中是最容易实现的。即不论造出来的量子处理器是否符合的标准，先大量生产再说！


        现在我们用量子力学理论的术语来进行表述量子计算机的原理：


经典计算机的基本单元中（经典比特）没有叠加态，经典比特只能为 “0” 或者“ 1”。量子计算机的基本单元（量子比特）是一种量子叠加态，它是同时具有“0” 和 “1” 的量子叠加态的一个基本单元，即一个量子比特能够同时包含 “0”和 “1” 的信息。对叠加的量子比特进行操作，便能够同时完成对 “0” 和 “1” 的操作。这样随着量子比特在量子计算机中数量的增长，量子计算机能处理的信息将呈指数增长。


       总结上述的工程描述和理论表述可以看出，量子计算机能否实现，最关键的一点就是能否构造出一个合格的、稳定的量子基本单元。再进一步说，就是能否制造出一个保持稳定叠加态的量子处理器。这样，量子叠加态就成了决定量子计算机能否成功的关键因素。


       因此，制造量子计算机这个工程上的宏观问题就转化成了一个量子力学基础理论上的一个微观问题。


量子叠加态的基础理论问题


       根据上面的讨论，量子计算机的制造问题从工程问题变成了基础理论问题，从宏观问题变成了微观问题。


       现在，在构造量子计算机前，我们必须回答一个最基础的问题，那就是，量子的叠加态是否真的存在。如果存在，这意味着无论多久，人类一定能造出量子计算机。即便造出的第一台原型量子计算机是多么的粗糙，计算过程和结果是多么的不稳定，但我们相信，通过不断的改进，最后一定能造出让人类满意的量子计算机。可是，如果量子叠加态不存在，那么量子计算机将会成为第二个永动机，永远不可能成功。


       为了深刻理解量子比特的叠加态，必须回到量子力学的最基本的理论中去理解它的来龙去脉。


      前面已经说了，量子计算机的概念是基于量子力学中的哥本哈根学派的解释。这个解释主要是由玻尔、海森堡和玻恩提出并总结。哥本哈根解释包含了至少如下几个重要的观点。


一个量子系统的量子叠加态可以用薛定谔方程中的波函数来表述，波函数的描述是概率性函数。一个粒子的位置和动量无法同时被确定。物质具有波粒二象性。一个实验可以展示出物质的粒子行为，或波动行为；但不能同时展示出两种行为，这是互补原理。


      由于没有足够的实验支持，而且其理论的表述内容令人疑惑，特别是根据其理论而推出的某些推论更是让人怀疑，因此，有众多的量子物理学家对于哥本哈根学派充满了怀疑和不信任。这些人物中有很多杰出的人物。这些杰出的人物是以爱因斯坦、薛定谔、德布罗意等一些著名的科学家们为代表的。例如，薛定谔提出了薛定谔方程，但是，他本人坚决反对哥本哈根学派对其方程的概率解释。


       虽然某些数学家们帮助哥本哈根解释建立了在数学意义上相对严密的体系，但是由于缺乏足够的物理实验支持，这种所谓的严密数学体系并不能代表着完美的物理解释。


       其实，哥本哈根学派的解释真正惹人争议的地方之一就是在于量子叠加态的存在。


     下面举二个例子来说明量子叠加态存在是否的问题。


例子 1


     很多读者都知道，在量子力学发展史上，薛定谔尝试着用一个猫来说明哥本哈根解释中量子叠加态的不确定之处。


     薛定谔的猫（Schrödinger's Cat）是关于微观世界中量子叠加态的一个思想实验。关于这个实验虽然已经有很多科普文章讲解了，可是作者建议有兴趣的读者自己去读一下并作出自己的判断。


     在这个思想实验中，按照哥本哈根的解释，容器中的猫处于 “死－活” 叠加态。也就是说，这个猫既是死的也是活着的！要等到打开容器看猫一眼才能决定其生死。请注意，不是发现而是决定。杀死这个猫不用任何的武器，仅仅看一眼就足以杀死。


     这个思想实验使微观叠加态变成了宏观上的不确定。这个既活又死的猫的叠加态违背了客观规律、逻辑思维和人类日常生活的感觉。因此，薛定谔用这个思想实验说明了微观中的量子叠加态并不存在。历史上，这个思想实验得到了爱因斯坦的背书。


     从逻辑上来说，如果接受了薛定谔猫的实验，那么就承认了哥本哈根学派中的量子叠加态不存在。如果接受了哥本哈本的解释，就是承认了在我们日常生活中至少有一个既死又活的猫。这完全违反了我们的日常生活的观察结论。


     哥本哈根学派的创始人及拥护者曾经辩解说，微观世界和宏观世界是不同的。那么问题来了，如果说微观世界和宏观世界是不同的，人类就应该有二个不同的逻辑思维的基本原则：一个是微观世界的逻辑思维原则，一个是宏观世界的逻辑思维原则。可是，又一个问题出现了：区别微观世界和宏观世界的标准是是什么？这个区别的标准是微观的还是宏观的？难道是既是微观的又是宏观的叠加态？


     虽然有些实验号称做出了薛定谔猫，即产生了量子叠加态。但是，当认真阅读这些论文的时候会发现，这些实验均存在漏洞或者不严密，因为叠加态的存在要求二种状态同时存在，无时间间隔的问题。


例子 2


      在2019年6月，几个研究量子力学的科学家们在2019年6月发表了一篇论文。中文媒体称之为耶鲁大学的实验。他们报告了他们所进行的实验。论文首先发表在2019年6月3日的在线版《自然》期刊上。后来这项新研究发表在《物理评论研究》2020 年9月29号，请看参考文献（2）。


     虽然对这个实验的结果有不同的理解和解释，但是实验的结果严重地动摇了哥本哈根学派的基柱性教条之一，即量子行为的概率性，支持了爱因斯坦和薛定谔学说，在一定程度上支持了爱因斯坦的明言“ 上帝不会掷骰子”。


　 在这个实验中，研究人员首次揭示了量子跃迁的微妙行为，表明在量子系统中，部分行为是可以预测的，不完全是随机性的。有兴趣的读者可以直接阅读论文的原文，见参考文章2）。


      这个结果和哥本哈根学派的解释是有矛盾的，因为哥本哈根学派认为量子系统的的行为完全是随机的，不可预知的。这就好像哥本哈根学派说一张纸全是白色的，没有黑色。可是这个实验表明，这个纸并不全是白色的，有一部分是黑色的。


      另外，量子从一种状态跃迁到另一种状态是需要时间的，实验的二种状态不能同时存在。也就是说，至少在这个实验中，量子的叠加态并没有被发现。




7 ） 结论和讨论


a. 这篇文章是从工程和基础理论的角度来考虑量子计算机的制造问题。通过讨论量子计算机的基本单元，把一个宏观工程问题（量子计算机的工程制造问题）转化成一个基础理论的微观问题（量子系统的叠加态问题）。通过分析认为，现在人类所构想的量子计算机，即以量子力学中哥本哈根学派中的叠加态为根据的量子计算机，是不可能造出来的，这种量子计算机将会是人类历史上的第二个“永动机”。


a.  现在在制造量子计算机过程中遇到了量子的严重不稳定的问题。为什么会有这个问题？原因其实很简单，因为在微观世界里根本就没有叠加态存在，所以在工程中才遇到了一个在量子计算机的基本单元（量子比特）不稳定的问题，即量子相关性。试想一下，如果叠加态存在的话，其稳定性会是一个严重的问题吗？正是因为不存在，所以才有稳定性的问题。


b.  量子计算机的制造虽然是一个工程问题，但它也是一个实验。这个实验是对哥本哈根理论的一个巨大的、史无前例的实验和考验，即能否在这个学派的理论指引下实现量子计算机的制造。如果最后失败了，那么这也只是哥本哈根学派的失败，但决不是量子力学这个伟大学科的失败，因为哥本哈根学派只是量子力学的一个学派而已。它将会是科学的胜利，再次证明了科学的进步必须以实验为基础。
c. 经典计算机是以基本单元是 “0” 或者“1” 存在为基础的，量子计算机是基本单元“0” 和“1”二种状态同时存在为基础的。这种“或者”和“同时”是区别经典计算机和量子计算机的唯一标准。如果一种量子计算机的基本单元中的“0” 和“1”状态不能同时存在，那么，它还将会是经典计算机。


d. 我们应该严格地区分量子力学理论和哥本哈根学派的理论。哥本哈根学派的理论只是量子力学这门学科中理论中的的一部分，不是它的全部。




参考文献


1）Feynman, Richard P. "Simulating physics with computers." International Journal of Theoretical Physics 21.6-7(1982):467-488.
2） Kyrylo Snizhko ,1Parveen Kumar ,and Alessandro Romito，“Quantum Zeno effect appears in stages”，Received 25 March 2020; accepted 13 August 2020; published 29 September 2020，PHYSICAL REVIEW RESEARCH 2, 033512 (2020)。 6park.com
English Version 6park.com

Can humanity really make the the imaged quantum computer？


Author: Rocky Mountains


1. Introduction


      Regarding the reports on the making of Quantum Computer, there have been a large number of reports, papers and discussions in many academic conferences around the world, media in various languages, and the Internet in recent years. Judging from these articles and reports, the main focus is to report on the progress of quantum computer (hardware and software) engineering research and manufacturing. Few articles analyze the feasibility of quantum computer research and manufacturing from the perspective of combining engineering and the basic theories of quantum mechanics.


       The purpose of this article is to try to explore the feasibility of the currently conceived quantum computer manufacturing from the perspective of combining engineering and the basic theories of quantum mechanics, that is, whether human beings can finally successfully manufacture a quantum computer that can be put into practical use, and thus Push the civilization of human society one step forward. Because if mankind has invested a lot of money and time, it is impossible to produce a quantum computer in the end, then quantum computer will become the second "perpetual motion machine" in human history, and it will be another huge loss for mankind.


       First of all,


       First, the author is full of respect for the scientists and engineers who have been engaged in the theoretical research and manufacture of quantum computers from all over the world for many years, without any offense;


       Secondly, the views in this article are only the author’s personal statement, but to express the author’s own views and opinions in a scientific manner that seeks truth from facts;


       Third, the author tries his best to use non-professional language and vocabulary to describe quantum computers and quantum mechanics. However, in some places, it is still difficult to avoid the use of certain professional terms. I hope readers will understand and hope.


2. Grouping of the Schools of Quantum Mechanics


       Before starting to discuss quantum computers, we first make a simple division of quantum mechanics theory. This division is mainly based on different physicists' views on quantum mechanics, and nothing else.


       Based on this standard, the theory of quantum mechanics can be roughly divided into two groups:


       The first group is mainly the Copenhagen School represented by Bohr, Heisenberg and Born. They conducted a theoretical analysis and summary of the early quantum experiments, called the interpretation or interpretation of Copenhagen quantum mechanics, which is called Interpretation in English. In Chinese, "Interpretation" and "Explanation" are slightly different. The former has a higher status than the latter, and it means "authentic" or "mainstream" in it.


       The second group is mainly based on the theoretical analysis and summary of early quantum experiments by another part of physicists represented by Einstein, Schrödinger and De Broglie.


       These six are all Nobel Prize winners in physics and have all made great contributions to the development of quantum mechanics. Therefore, they are all on the same level.


       The theories of the first group and the second group are more similar and contradictory in many basic aspects.


       For example, the famous Schrödinger cat is a thought experiment proposed by Schrödinger, a member of the second group, who developed Einstein's ideas to oppose the theory of the first group.


       For another example, the concept of quantum entanglement, which is very hot today, comes from the members of the second group. In fact, the quantum entanglement thought experiment, or EPR thought experiment, was first proposed by the famous physicist Einstein of the second group and his two assistants, and later developed by Schrödinger against the first group of theories.


       Regarding the differences and similarities between the first group and the second group theories, this article does not want to discuss further. Interested readers can directly read related articles and books, including Chinese or English versions. If possible, it is recommended to read the German version directly.


       According to this division method, the whole theory of quantum mechanics is actually composed of two parts: the theory of the first group (the Copenhagen theory) and the theory of the second group (may be called the theory of Einstein and Schrödinger for the time being).


      If expressed by a formula, it can be written as a formula as follows:


      Quantum mechanics theory = Copenhagen's theory + Einstein/Schrodinger's theory


      Based on the above formula, the Copenhagen School's theory is only one school of the quantum mechanics school, and it cannot represent the entire quantum mechanics theory. It is just a "one" if it is described in current online language.


     In particular, it should be pointed out that because the two groups of theories are not strictly distinguished, it has caused great troubles to the scientific teaching and popularization of quantum mechanics.


     For example, when people talk about quantum mechanics, they always take it for granted that the Copenhagen Doctrine represents the entire quantum mechanics, and the Copenhagen School’s theory is the authenticity of quantum mechanics. In the current textbooks of quantum mechanics (Chinese or English), when explaining quantum theory, most of them follow the Copenhagen Doctrine and regard it as the authentic interpretation of quantum mechanics. However, the status of this so-called authentic explanation in quantum mechanics and science is becoming more and more like a religion, not only can not be questioned, but even to the point of compulsory brainwashing of students. For example, in the teaching of quantum mechanics, when facing the Copenhagen School questions raised by students, professors often avoid answering positively, and even warn students "Don't ask too much, just count!". Even if the Copenhagen School cannot "explain" why it is like this, it can only "tell you" that it is like this. For example, in the face of the famous double-slit experiment and wave-particle duality, it can only be expressed in terms of both being and being.


     Therefore, when talking about quantum mechanics, it is better to distinguish this. For example, when some articles talk about the success of quantum mechanics in guiding the development of engineering with great progress, many people take it for granted that it is the success of the Copenhagen school because they do not specify the theory of that school. But in fact it is not.


3. The origin of the concept of quantum computer


       In 1981, the first International Conference on Computational Physics was held at the famous university MIT. During the conference, Richard Feynman, a well-known American theoretical physicist and Nobel laureate, proposed a novel idea of ​​using quantum systems to realize general-purpose computing. He believes that if a computer composed of subsystems is used to simulate quantum phenomena, the computing time can be greatly reduced. In the second year, in 1982, the International Journal of Theoretical Physics, "International Journal of Theoretical Physics", please refer to Reference (1), published Feynman’s "Simulating physics with computers" at the conference report. Therefore, the concept of quantum computers was born.


       It should be emphasized here that Feynman proposed the concept of quantum computer based on the Copenhagen School. That is, the concept of quantum computer is proposed based on Copenhagen's superposition state theory. In the last part, we have explained that the Copenhagen School is only a school of quantum mechanics, and the foundation of this school is challenged by the theories of Einstein and Schrödinger. In particular, the famous Schrödinger cat challenged this theory. Therefore, the basic theoretical basis of quantum computers is not solid, and it is not entirely based on impeccable experiments, but based on the assumptions of a certain school.


4. The development of quantum computers


        After Feynman put forward the concept of quantum computer, some countries, universities or laboratories, especially in recent years, many large international multinational companies have invested in the field of quantum computers. Of course, everyone uses different quantum physics systems to construct quantum computers. For example, the two orthogonal polarization directions of photons; the spin direction of electrons in the magnetic field; the two nuclear spin directions; the two different energy levels of the quantum in the atom. In short, various ways are being explored. For now, there is no definite answer as to which physical system is more suitable for quantum computers. But no matter what approach is adopted, the most basic point is the same: the most essential feature of quantum computers in these ideas is the use of quantum superposition.


        According to the papers and reports published so far, the quantum computer being constructed by people is similar to a classical computer in principle: it is also a physical device for logical operations, storage and processing of information. The difference is that this physical device processes and calculates quantum information and runs quantum algorithms.


5. A brief summary of the principles of quantum computers


        In order to understand the working principle of quantum computers, a simple engineering explanation is now based on classical computers.


         Classical computer: Classical computer is a binary number system and is based on the basic unit (bit) in the computer to perform operations. Each "0" or "1" is a basic unit. The basic unit is the smallest unit of data storage. In other words, a classic basic unit can only have a certain "0" or "1" state in a certain time. Pay attention to the word "or" here.


        Quantum computer: According to people's current thinking, quantum computer is also a binary number system, and is based on the basic unit (qubit) of the computer to perform operations. In each quantum basic unit, the states of "0" and "1" can be stored at the same time. Note the words "simultaneous" and "and" here.


        The difference between a classical computer and a quantum computer is that the basic unit of a classical computer can only store one of "0" or "1" at a certain time; while the basic unit of a quantum computer can store both "0" and "1" at the same time. The difference between the two computers is the difference between "or" and "and":
Classic computer: "0" or "1";
Quantum computer: "0" and "1".


        Then you can simply calculate. In a classical computer, if there are K classical basic units, it can only store K units of information at the same time; in a quantum computer, for K quantum basic units, they can store 2^K at the same time. Unit information. As the number of quantum basic units increases, the amount of information can increase rapidly.


       To realize quantum computing with a quantum computer, it must also complete its calculations in three steps like a classical computer:


The first step is to construct a quantum basic unit;


The second step is to manipulate the quantum basic unit so that the "0" and "1" states exist at the same time;


The third step is to combine K identical quantum basic units to form a quantum computing system, so as to realize the supercomputing capability of a quantum computer.


Therefore, a quantum computer does not only contain one quantum basic unit, it is a physical system that contains multiple quantum basic units combined together.


         The most difficult part of building a quantum computer is the first step, that is, a quantum basic unit must be constructed. This unit must allow the "0" and "1" states to exist at the same time. Note that this requirement must exist "at the same time"! If it does not exist "simultaneously", then this is not a quantum basic unit, but a classical basic unit.


         In the second step, people conceive of using a quantum processor to operate to realize the construction and manipulation of each quantum basic unit. It requires that in the quantum basic unit, "0" and "1" must exist at the same time, that is, artificially forming a quantum superposition state and maintaining stability. If "0" and "1" cannot exist at the same time, then no matter how high the accuracy of this quantum computer is, it is essentially a classical computer.


         Readers may sometimes see certain reports on the Internet that quantum computers with thousands of quantum basic units have been built, and so on. This gives people the impression that a quantum computer will be built right away, which is very encouraging. In fact, this is the third step in the construction of a quantum computer, and it is the easiest to implement in the three steps of building a quantum computer. That is, regardless of whether the manufactured quantum processor meets the standards, mass production will take place first!


        Now we use the terms of quantum mechanics theory to describe the principles of quantum computers:


There is no superposition state in the basic unit of a classic computer (classic bit), and the classic bit can only be "0" or "1". The basic unit (qubit) of a quantum computer is a quantum superposition state, which is a basic unit of a quantum superposition state with both "0" and "1", that is, a qubit can contain both "0" and "1". Information. By operating on the superimposed qubits, the operations on "0" and "1" can be completed at the same time. In this way, as the number of qubits in quantum computers increases, the information that quantum computers can process will increase exponentially.


       Summarizing the above-mentioned engineering descriptions and theoretical expressions, we can see that whether a quantum computer can be realized, the most critical point is whether a qualified and stable quantum basic unit can be constructed. Furthermore, it is whether a quantum processor that maintains a stable superposition state can be manufactured. In this way, the quantum superposition state has become a key factor in determining the success of a quantum computer.


       Therefore, the engineering macro-problem of manufacturing quantum computers has been transformed into a micro-problem based on the basic theory of quantum mechanics.


6. Basic theoretical issues of quantum superposition state


       According to the above discussion, the manufacturing problem of quantum computers has changed from an engineering problem to a basic theoretical problem, and from a macro problem to a micro problem.


       Now, before constructing a quantum computer, we must answer the most basic question, that is, whether the superposition state of quantum really exists. If it exists, it means that no matter how long it takes, humans will be able to build quantum computers. Even if the first prototype quantum computer is so rough and the calculation process and results are so unstable, we believe that through continuous improvement, we will be able to create a quantum computer that satisfies human beings. However, if the quantum superposition state does not exist, then the quantum computer will become the second perpetual motion machine and will never succeed.


       In order to deeply understand the superposition state of qubits, one must return to the most basic theory of quantum mechanics to understand its ins and outs.


      As mentioned earlier, the concept of quantum computers is based on the Copenhagen School's interpretation of quantum mechanics. This explanation is mainly put forward and summarized by Bohr, Heisenberg and Born. The Copenhagen interpretation contains at least the following important points.


1. The quantum superposition state of a quantum system can be expressed by the wave function in the Schrodinger equation. The description of the wave function is a probabilistic function.
2. The position and momentum of a particle cannot be determined at the same time.
3. Matter has wave-particle duality.
5. An experiment can show the particle behavior or wave behavior of matter; but it cannot show both behaviors at the same time. This is the principle of complementarity.


Because there is not enough experimental support, and the content of its theory is puzzling, especially some inferences derived from its theory are even more suspicious. Therefore, many quantum physicists are full of doubts and doubts about the Copenhagen School. Distrust. There are many outstanding figures among these characters. These outstanding figures are represented by some famous scientists such as Einstein, Schrodinger, and De Broglie. For example, Schrödinger proposed the Schrödinger equation, but he himself firmly opposed the Copenhagen School’s probabilistic interpretation of his equation.


       Although some mathematicians helped Copenhagen explain to establish a relatively rigorous system in the mathematical sense, due to the lack of sufficient physical experiment support, this so-called rigorous mathematical system does not represent a perfect physical explanation.


       In fact, one of the truly controversial aspects of the Copenhagen School’s explanation is the existence of quantum superposition states.


     Here are two examples to illustrate whether the quantum superposition state exists or not.


Example 1


     Many readers know that in the history of quantum mechanics, Schrödinger tried to use a cat to illustrate the uncertainty of the quantum superposition state in the Copenhagen interpretation.


     Schrödinger's Cat (Schrödinger's Cat) is a thought experiment about quantum superposition states in the microscopic world. Although there have been many popular science articles on this experiment, the author recommends that interested readers read it and make their own judgments.


     In this thought experiment, according to Copenhagen's explanation, the cat in the container is in a "dead-alive" superposition state. In other words, this cat is both dead and alive! You have to wait until you open the container and take a look at the cat to determine its life or death. Please note that it is not a discovery but a decision. You don't need any weapons to kill this cat, just a glance is enough to kill.


     This thought experiment turned the microscopic superposition state into a macroscopic uncertainty. The superposition of this cat, both alive and dead, violates objective laws, logical thinking, and the feelings of human daily life. Therefore, Schrödinger used this thought experiment to show that there is no quantum superposition state in the microcosm. Historically, this thought experiment was endorsed by Einstein.


     Logically speaking, if you accept Schrödinger’s cat experiment, then it is admitted that the quantum superposition state in the Copenhagen school does not exist. If we accept Copenhagen’s explanation, we will admit that there is at least one dead and alive cat in our daily lives. This completely violates the observation conclusion of our daily life.


     The founders and supporters of the Copenhagen School have argued that the micro world is different from the macro world. So here comes the problem. If we say that the micro world and the macro world are different, humans should have two different basic principles of logical thinking: one is the logical thinking principle of the micro world, and the other is the logical thinking principle of the macro world. However, another question arises: What is the standard for distinguishing the micro-world from the macro-world? Is the standard of this distinction microscopic or macroscopic? Is it a superposition state that is both microscopic and macroscopic?


     Although some experiments claim to have made Schrödinger’s cat, a quantum superposition state has been produced. However, when you read these papers carefully, you will find that these experiments have loopholes or are not rigorous, because the existence of superposition requires two states to exist at the same time, without the problem of time interval.


Example 2


      In June 2019, several scientists studying quantum mechanics published a paper in June 2019. The Chinese media call it the Yale University experiment. They reported on the experiments they conducted. The paper was first published in the online edition of the journal Nature on June 3, 2019. Later, this new research was published in Physical Review Research on September 29, 2020. Please see Reference (2).


     Although there are different understandings and interpretations of the results of this experiment, the results of the experiment severely shaken one of the fundamental dogmas of the Copenhagen School, that is, the probabilistic nature of quantum behavior, and supports the theories of Einstein and Schrödinger to a certain extent. The board supported Einstein's clear statement "God does not roll the dice".


　 In this experiment, the researchers revealed the subtle behavior of quantum transitions for the first time, showing that in a quantum system, part of the behavior is predictable, not entirely random. Interested readers can directly read the original text of the paper, see reference article 2).


      This result is contradictory to the Copenhagen School’s interpretation, because the Copenhagen School believes that the behavior of quantum systems is completely random and unpredictable. It's like the Copenhagen School said that a piece of paper is all white, not black. However, this experiment showed that the paper is not all white, and part of it is black.


      In addition, it takes time for quantum to transition from one state to another, and the two experimental states cannot exist at the same time. In other words, at least in this experiment, the superposition state of quantum has not been discovered.




7) Conclusion and discussion


a. This article considers the manufacturing of quantum computers from the perspective of engineering and basic theory. By discussing the basic units of quantum computers, a macro-engineering problem (the engineering manufacturing problem of quantum computers) is transformed into a basic theoretical micro-problem (the problem of superposition states of quantum systems). Through analysis, it is believed that the quantum computer currently conceived by mankind, that is, the quantum computer based on the superposition state of the Copenhagen school of quantum mechanics, cannot be built. This kind of quantum computer will be the second in human history. "Perpetual motion machine."


a. Now in the process of manufacturing quantum computers, we have encountered serious quantum instability problems. Why is there such a problem? The reason is actually very simple. Because there is no superposition state at all in the microscopic world, engineering has encountered a problem of instability in the basic unit (qubit) of a quantum computer, that is, quantum correlation. Just imagine, if the superposition state exists, will its stability be a serious problem? It is precisely because it does not exist that there is a problem of stability.


b. Although the manufacture of quantum computers is an engineering problem, it is also an experiment. This experiment is a huge, unprecedented experiment and test of the Copenhagen theory, that is, whether the manufacturing of quantum computers can be realized under the guidance of the theory of this school. If it fails in the end, then this is just a failure of the Copenhagen School, but it is by no means a failure of the great discipline of quantum mechanics, because the Copenhagen School is only a school of quantum mechanics. It will be a victory for science and proves once again that scientific progress must be based on experiments.
c. Classical computers are based on the existence of the basic unit "0" or "1", and quantum computers are based on the simultaneous existence of the basic unit "0" and "1". This "or" and "simultaneous" is the only standard that distinguishes a classical computer from a quantum computer. If the "0" and "1" states in the basic unit of a quantum computer cannot exist at the same time, then it will still be a classical computer.


d. We should strictly distinguish between quantum mechanics theory and Copenhagen school theory. The rationalism of the Copenhagen School is only a part of the idealism in the discipline of quantum mechanics, not the whole of it.


References:


1）Feynman, Richard P. "Simulating physics with computers." International Journal of Theoretical Physics 21.6-7(1982):467-488.

2） Kyrylo Snizhko ,1Parveen Kumar ,and Alessandro Romito，“Quantum Zeno effect appears in stages”，Received 25 March 2020; accepted 13 August 2020; published 29 September 2020，PHYSICAL REVIEW RESEARCH 2, 033512 (2020)。





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