Quantum Zeno tomography
Quantum Zeno tomography
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量子芝诺断层扫描
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
J. Řeháček
中科院分区：
文献类型：
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作者：
P. Facchi;Z. Hradil;G. Krenn;S. Pascazio;J. Řeháček
我们证明，只要样品中不同的灰度级不是均匀分布的，标准吸收层析成像的每吸收颗粒的分辨率可以通过简单的干涉装置来获得。这项技术依赖于量子Zeno效应，并已在数值模拟中进行了测试。我们提出的方案可以在紫外光、中子或X射线的实验中实施。吸收层析成像是一种揭示物质内部结构的重要实验技术。通过测量通过样品的粒子束的衰减，可以推断出吸收系数~密度！在梁部分的样品。区分两种密度略有不同的材料的可能性通常是至关重要的。在其他理想情况下，与照明光束的离散特性相关的散粒噪声对吸收断层扫描的分辨率设置了上限：例如，脑肿瘤投射的阴影可能会完全消失在噪声数据中。克服这种波动的一种可能性是增加光束的强度。然而，在许多情况下，例如在医学中，由于吸收的辐射引起的损害，照明光束的强度不能任意地高。迈向“无吸收断层摄影术”的重要一步来自量子理论。理论上，1，2#和实验上，3#都证明了用干涉装置可以在不吸收任何粒子的情况下区分完全的透射体和吸收体。这个想法实际上是量子Zeno效应的巧妙实现@4#，并建立在“无相互作用”测量@5#的概念上。一种经典的测量仪器~这里的黑色样品！放在干涉仪的一只手臂上，将照明粒子投射到另一只手臂上，破坏干扰，冻结进化，迫使粒子通过不同的通道出射，而如果双臂都是透明的~白色样品！然而，在实际应用中，样品通常不是黑色也不是白色：它们是灰色的。在本文中，我们致力于了解量子Zeno效应的应用是否也有利于更实际的任务，即区分两个具有不同传输系数的灰体。更具体地说，我们问：有没有可能通过量子Zeno效应来减少吸收的粒子数量，同时保持分辨率？我们表明，这确实是可能的。最近，其他作者调查了密切相关的问题@6，7#。我们的结论有些乐观：我们表明，如果样品中不同水平的灰色出现的频率不一致，标准吸收断层扫描的性能可以被Zeno设置得更好。此外，与标准设置不同，Zeno设置有两个检测通道：正如我们将看到的，如果正确利用这一功能，在Zeno案例中会产生更好的性能。
We show that the resolution 'per absorbed particle' of standard absorption tomography can be outper- formed by a simple interferometric setup, provided that the different levels of 'gray' in the sample are not uniformly distributed. The technique hinges upon the quantum Zeno effect and has been tested in numerical simulations. The scheme we propose could be implemented in experiments with uv light, neutrons, or x rays. Absorption tomography is an important experimental technique revealing the internal structure of material bodies. By measuring the attenuation of a beam of particles passing through a sample one infers the absorption coefficient ~den- sity! of the sample in the beam section. The possibility of distinguishing two slightly different densities of the material is often of vital importance. Under otherwise ideal conditions the shot noise associated with the discrete character of the illuminating beam sets an upper limit to the resolution of absorption tomography: for instance, the shadow cast by a brain tumor might become totally lost in the noisy data. One possibility to overcome the fluctuations is to increase the intensity of the beam. However, in many situations, as in medicine for example, the intensity of the illuminating beam cannot be made arbitrarily high due to the damage provoked by the absorbed radiation. A significant step toward an 'absorption-free tomogra- phy' came from quantum theory. It was demonstrated, both theoretically @1,2# and experimentally @3#, that totally trans- mitting and absorbing bodies can be distinguished without absorbing any particles, by using an interferometric setup. This idea is in fact a clever implementation of the quantum Zeno effect @4# and hinges upon the notion of 'interaction- free' measurement @5#. A classical measuring apparatus ~here the black sample!, placed in one arm of the interferom- eter, projects the illuminating particle into the other arm, destroying interference, freezing the evolution, and forcing the particle to exit through a different channel from that it would have chosen had both arms been transparent ~white sample!. In practical applications, however, samples are normally neither black nor white: they are gray. In this paper we en- deavor to understand whether application of the quantum Zeno effect, which turns out to be ideal for discriminating black and white, might be advantageous also for the more practical task of discriminating two gray bodies with differ- ent transmission coefficients. More specifically, we ask: Is it possible by quantum Zeno effect to reduce the number of absorbed particles while preserving the resolution? We show that this is indeed possible. Closely related questions have recently been investigated by other authors @6,7#. Our con- clusions are somewhat optimistic: we show that standard ab- sorption tomography can be outperformed by a Zeno setup, provided that the frequency of occurrences of the different levels of 'gray' in the sample is not uniform. In addition, the Zeno setup, unlike the standard one, is endowed with two detection channels: as we shall see, this feature, if properly exploited, leads to even better performances in the Zeno case.