## Metadata - Author: [[Valerio Scarani]] - Full Title: Quantum Physics - Category: #books ## Highlights - We have just derived here one of the main reasons for our surprise–the physical properties of quantum systems, contrary to the physical properties of cars or other everyday objects, are not bound to each other according to the rules of set theory. ([Location 476](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=476)) - Let’s summarise: mechanics, thermodynamics, fluid mechanics, electricity and magnetism. This list corresponds to the physics programme in high school and the first years of university–the reader probably knows from experience how these disciplines can appear without any link between one and the others. In this context, the hypothesis of atomism is presented as a unifying vision. Atomism proposes to reduce all physical phenomena of matter (thermal, electrical, magnetic, turbulence phenomena) to mechanical phenomena–the manner of the motion of atoms. For example, an electrical current will be described as the displacement of particles carrying an ‘electrical charge’, the temperature of a gas will be associated with the average speed of the particles that form the gas. On the scale of particles, we would like there to be only intrinsic properties (such as mass and electrical charge) and motion. ([Location 516](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=516)) - Contrary to what we might think, the word wave in physics refers to a rather abstract idea. The reader should make a list of the different types of waves of which they have heard: waves of water, sound waves, radio waves, microwaves … Sound waves and radio waves are very different from each other, that is well known, and moreover, if radio waves could be heard by the human ear, our daily lives would take place amid an unbearable din. In physics courses, we give the name ‘wave’ as much to the vibration of the string of a violin as to the sound that this vibration produces. In summary, the concept of the ‘wave’ is a concept that allows the description of a class of phenomena. It is therefore even more abstract than that of the ‘particle’, which initially evokes an object. ([Location 532](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=532)) - We have seen that if we determine by which path (or for the Young experiment, through which slit) the particle travels, the interference disappears. This is consistent with the indistinguishability principle–in making a measurement, we remove all possibilities but one, therefore the particle behaves in a classical manner. ([Location 797](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=797)) - According to our principle, interference disappears as soon as the two paths are distinguishable, whatever the cause. Heisenberg proposes one very precise cause–the crudeness of our measurements, which prevents us from having access to all of the significant parameters. If we were capable of making more precise measurements, we would not lose the interference. Unhappily, suggests the German physicist, we are not capable. The challenge is thrown down: will we be capable of it one day, or is nature (of which, let’s not forget, we are a part, as are our measurement devices) designed to keep this issue from being resolved? ([Location 824](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=824)) - The word complementarity was forged by Niels Bohr. The concept that it conveys is closely linked with the indistinguishability principle that has been discussed in this book. In order to clarify the idea, let’s go back to the apparatus of Fig. 1.3. Our description was this: if we do not know by which path the particles travel in the interferometer (two indistinguishable paths), all of the particles take a certain output (interference); if we detect the particles in the interferometer (distinguishability), the output will be random. Bohr would say instead that the path and the output are two pieces of complementary information–we cannot arrange it so that all of the particles take the same path and the same output. At the risk of missing something very profound, we will remember that Bohr’s complementarity principle says the same thing as the indistinguishability principle, from a different angle. The future will tell us if one of these concepts will disappear to the benefit of the other, if they are destined to survive together, or if both will be erased by new, more precise notions. ([Location 878](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=878)) - In effect, since the beginnings of quantum physics, the majority of physicists have presented the surprising aspects of the behaviour of particles as limitations–we cannot know by which path a particle travels, we cannot make a measurement without disrupting the results of successive measurements … Quantum cryptography does not repudiate that, but it benefits from it–since any measurement disrupts the results, the eavesdropper is going to be detected as soon as they try to make a measurement, to get information. It is a radical change of perspective–quantum physics is not an imperfect physics, but a new physics, which allows the achievement of tasks that are otherwise impossible37. ([Location 1028](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=1028)) - Before examining the implications of this prediction, I would like to mention to the reader that the Franson interferometer does not have an analogy for classical waves, whereas, as we have seen in Chapter 2, the Mach–Zehnder and Young interferometers were conceived initially in the framework of classical optics. The reason is that classical waves do no obey a principle of indistinguishability. This comment shows how the notion of ‘wave-particle duality’, forged at the beginnings of quantum physics, has dated, to the benefit of more general notions such as indistinguishability or Bohr’s complementarity. ([Location 1106](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=1106)) - In Chapter 2, we saw that indistinguishability deriving from cars in relation to the knowledge of a pedestrian does not lead to any surprising modification in the properties of those cars. Indistinguishability in everyday life is linked to the ignorance of the observer, which can be overcome, while indistinguishability at the particle level cannot be removed without leading to qualitative change of the physical properties. ([Location 1188](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=1188)) - the failure of the ‘Heisenberg mechanism’ ([Location 1191](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=1191)) - Note: i dont see how he has shown that the heisenberg principle has failed. How can we know that we have exhausted all the ways of measuring a particle? - The Bell theorem can be described with the tools of elementary mathematics–the following paragraphs are devoted to this approach. This is the most difficult part of the book. The reader can skip it and keep in mind simply that the Bell theorem furnishes a criterion to experimentally exclude the hypothesis that quantum correlations are established at the source (that is, to rule out an alternative description of quantum phenomena based on local variables). I find, however, that the effort of understanding the Bell theorem is worth it, because there are few other profound results in modern physics that can be explained with elementary concepts. ([Location 1212](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=1212)) - If every action, even human, is pre–determined, then the Bell theorem is not valid–but in fact, if everything is pre–determined, there is no further need to look for an explanation for quantum correlations or for anything else! ([Location 1234](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=1234)) - Note: Is this strictly true? - When two quantum systems are correlated, it becomes impossible to describe them separately (Schrödinger), whatever the distance that separates them (EPR). ([Location 1367](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=1367)) - At one time or another, every one of us obtains an experimental result that disagrees with the theoretical prediction. We have looked for the error, and if we failed to find it, we have written in our report a loose statement like, ‘the instruments are too imprecise’. We have cited the uncertainty of the measurements to explain the disagreement between the experiment and theoretical calculation. The detection loophole is perhaps the first example in the history of physics where the imprecision of the measurements is cited to explain the perfect agreement between theory and experiment! ([Location 1510](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=1510)) - The second challenge consists of trying to experimentally test the difference between Bohr and Everett’s visions: is there a definite boundary between the quantum and classical worlds (the so-called Heisenberg cut), or not? ([Location 1697](https://readwise.io/to_kindle?action=open&asin=B005X3S9LQ&location=1697))