Quantum Measurements with Photons

测量奥运游泳池的长度并不影响里面的水量！我们通常不期望通过测量来改变事物。在量子世界里，事情是非常不同的。量子力学告诉我们世界是如何在最基本的层面上运行的。它预测了非常奇怪的行为，通常只有在物体非常冷和非常小的情况下才能观察到。它有一个内在的偶然因素，允许两种不同状态的叠加，并且包括在我们日常世界中荒谬的物体之间的超强关联——纠缠。尽管有这种奇怪的行为，量子力学是我们所拥有的最成功的理论——它几乎完美地预测了将要发生的事情！然而，它并没有被完全理解，它的一些含义仍在被发现。量子力学最大的谜团之一——测量问题——试图回答这个问题：为什么我们在日常世界中看不到叠加？（比如活的和死的）。测量在量子力学中起着特殊的作用，自上世纪初该理论发展以来一直是激烈辩论的主题。最近，量子测量已经成为一个重要的实际问题。这是量子信息科学出现的结果，它试图回答这样一个问题：在信息的存储、传输和处理中，具体利用量子力学效应可以获得什么优势？预计未来的技术包括具有巨大计算能力的量子计算机，承诺最精确测量的量子计量学，以及已经在商业通信系统中使用并提供完美安全性的量子密码学。与测量一个池子的长度不同，测量一个量子系统必然会干扰这个系统。例如，对处于两种状态叠加的系统进行标准测量，发现系统有一定概率处于其中一种状态。测量后，系统不再处于叠加态，而是处于测量时确定的状态。原始的叠加态永远无法恢复，信息也就丢失了。更一般的量子测量涉及在获得的信息和系统的干扰之间的回报。量子力学还允许在两个或多个系统上进行纠缠测量，使它们处于纠缠状态。最后，我们可以根据测量告诉我们的反馈来有意地操纵被测量的系统。这些通用的量子测量可以在未来的量子技术中发挥重要作用：量子密码的安全性依赖于通过测量必须引起的干扰来检测窃听者；量子计量需要纠缠测量；一些量子计算方案仅通过测量进行。光的单粒子——光子——是发展新的量子测量的极好系统，因为它们几乎没有噪声。它们在未来的量子技术中也有很大的应用潜力：所有光学量子计算机的方案都是主要的竞争者，光子是量子通信和测量光程长度的量子计量方案的明显选择。该项目将实现纠缠的、扰动量可调的、包括反馈的新量子测量。它将使用一个光学晶体来产生多达6个光子，光学电路来实现光子之间的受控相互作用（带反馈），标准雪崩光电二极管来探测光子。一个特别的重点将是开发实用的方案，有效地从量子测量中提取信息。最后，该项目将设计和实施区分多达4个光子的量子过程的技术。

Measuring the length of an Olympic swimming pool doesn't affect how much water it has in it! We normally don't expect measuring things to change them. In the quantum world, things are very different.Quantum mechanics tells us how the world works at its most fundamental level. It predicts very strange behaviour that can typically only be observed when things are very cold and very small. It has an inbuilt element of chance, allows superpositions of two different states, and includes super-strong correlations between objects that would be nonsensical in our everyday world - entanglement . Despite this strange behaviour, quantum mechanics is the most successful theory that we have ever had - it predicts what will happen almost perfectly! However, it is not completely understood, and some of its implications are still being discovered.One of the great mysteries of quantum mechanics - The Measurement Problem - seeks to answer the question Why don't we see superpositions in the everyday world? ( alive and dead for example). Measurements play a special role in quantum mechanics and have been the subject of intense debate since the theory's development early last century. Recently quantum measurements have emerged to become an important practical issue. This is the result of the advent of quantum information science , which seeks to answer the question What advantage can be gained by specifically harnessing quantum mechanical effects in the storing, transmitting and processing of information? Anticipated future technologies include quantum computers with tremendous computational power, quantum metrology which promises the most precise measurements possible, and quantum cryptography which is already being used in commercial communication systems, and offers perfect security.Unlike measuring the length of a pool, measuring a quantum system necessarily disturbs the system. For example a standard measurement of a system in a superposition of two states finds the system in one of those states with some probability. After the measurement, the system is no longer in a superposition, but is in the state it was measured to be in with certainty. The original superposition state can never be recovered, and that information is lost.More general quantum measurements involve a payoff between the information gained and the disturbance of the system. Quantum mechanics also allows entangling measurements on two or more systems, that leave them in an entangle state. Finally, we can intentionally manipulate the system being measured depending on what the measurement tells us - feedback.These general quantum measurements could play an important role in future quantum technologies: the security of quantum cryptography relies on detecting an eavesdropper by the disturbance their measurements must cause; quantum metrology requires entangled measurements; and some schemes for quantum computation proceed via measurements alone.Single particles of light - photons - are excellent system for developing new quantum measurements, because they suffer from almost no noise. They also have great potential for application in future quantum technologies: schemes for all optical quantum computers are leading contenders, and photons are the obvious choice for both quantum communication and for quantum metrology schemes for measuring optical path lengths. This project will realise new quantum measurements which are entangled, tuneable in the amount of disturbance, and include feedback. It will use an optical crystal to produce up to six photons, optical circuits to realise controlled interactions between them (with feedback), and standard avalanche photodiodes to detect them. A particular focus will be on developing practical schemes for efficiently extracting information from quantum measurements. Finally, the project will design and implement techniques for distinguishing between quantum processes on up to 4 photons.