Let’S Step Back

Valerio Scarani

Published Jan 5, 2006• 0 citations

Abstract

Abstract Leaving Saint-Michel, I take my leave of Jean-Paul who is going home, and I turn left to walk down the College Steps, a covered lane that descends into the heart of Fribourg. There, in front of the cathedral, three busy roads meet to form a roundabout. I continue my walk along the south side of the cathedral. Cars continue to stream down the road. Now that the roundabout is no longer in view, I cannot know by which route each car has entered it. In order to arrive alongside me, as it does now, this car could have taken three possible paths. I have put myself, in relation to the cars, in a situation of indistinguishability. Well then, why is the road network not a quantum interferometer? The answer is a matter of common sense – the indistinguishability of cars is only apparent, it is due to an ignorance that can easily be overcome. In order to obtain the information about the path by which each car entered the roundabout, it would suffice for me to go back a few paces or to ask the driver – neither of these steps would modify what the destination of the car will be.

DOI: 10.1093/oso/9780198570479.003.0002

Related papers

Indistinguishability At A Distance

Valerio Scarani

Abstract During my first lecture at Saint-Michel College, I had sidestepped the question about randomness that the student had asked. As the reader will recall, it was the experiment of Fig. 1.1, as simple as it was, in which the particles arrived one after the other at a beam splitter. I had said that we do not know how to describe the behaviour of each particle, we do not know why a given particle is reﬂected rather than transmitted. Is it really uncertain, ran- dom (which is the current opinion among physicists) or is there a mechanism yet to be discovered? I had deliberately turned attention from this problem, in order to concentrate on the experiment of Fig. 1.3, the Mach–Zehnder interferometer. We have seen, then, that quantum randomness is a curious randomness – if we assemble two ‘randomness generators’ (beam splitters) in a certain way, we rediscover the certainty! I brieﬂy remind the assembled students of this, a week later, in the old granary of the College.

Jan 5, 20060 citations

On The Origin Of Correlations

Valerio Scarani

Abstract 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. The arguments in Chapter 2, as well as the failure of the ‘Heisenberg mechanism’ discussed in Chapter 4, have taught us that quantum indistinguishability is not a kind of insurmountable ignorance. Quantum indistinguishability is therefore a new idea – to affirm that two paths for a car are indistinguishable is not the same as affirming that two alternatives for a particle are indistinguishable. We are going to see that it is possible to argue in a similar way on the subject of correlations – in the quantum world, the notion of correlations appears in a new, unexpected light.

Jan 5, 20060 citations

Attempts At Explanation

Valerio Scarani

Abstract During the lectures to the students in Fribourg, as with other people foreign to the world of physics, I have stated that my audience was driven to raise the same questions as the physicists: can we accept randomness in physical phenomena? Is interference determined by the mechanisms, and correlations at a distance by an exchange of information? In the presence of non-separability of physical properties, can we still talk about distinct entities? We feel the need to interpret quantum phenomena because we are not at ease with the indistinguishability principle and with some of its consequences. The unease comes from the fact that this principle, which appears to be omnipresent in the microscopic world, is unknown in everyday life and even seems to contradict our normal perceptions – the reader must recall the game with the sets of cars in Chapter 2. Seen from this point of view, the interpretations can be divided into three broad categories:

Jan 5, 20060 citations

At The Heart Of The Problem

Valerio Scarani

Abstract ‘When all is said and done, physics is only a description, not an explanation’. I have uttered similar statements many times myself in informal discussions, when my interlocutor happened to slide into an excessive praise of experimental science and an excessive contempt of other forms of human knowledge. In the mouth of Jean-Paul Fragnière, a friend and philosophy teacher who lives in the small Swiss town of Fribourg, this sentence sounds, however, the expression of a different and distant line of thought of a rivalry between human and natural sciences that is still latent in the French-speaking world, a relic of the arrogance of Positivism. We are in the massive building of Saint-Michel College – a sixteenth century Jesuit establishment, a public school for several decades; and though Jean-Paul’s classroom is a modernization of what was the granary, history weighs heavily here. For the first generations of young people educated in this school, heliocentrism was one of the hot topics; now Jean-Paul has invited me to speak to his students about quantum physics.

Jan 5, 20060 citations

Roads to the past: how to go and not to go backward in time in quantum theories

Cristian López

European Journal for Philosophy of ScienceMar 18, 201914 citations

Purpose and Plan

A. Pais

Abstract It must have been around 1950. I was accompanying Einstein on a walk from The Institute for Advanced Study to his home, when he suddenly stopped, turned to me, and asked me if I really believed that the moon exists only if I look at it. The nature of our conversation was not particularly metaphysical. Rather, we were discussing the quantum theory, in particular what is doable and knowable in the sense of physical observation. The twentieth century physicist does not, of course, claim to have the definitive answer to this question. He does know, however, that the answer given by his nineteenth century ancestors will no longer do. They were almost exactly right, to be sure, as far as conditions of everyday life are concerned, but their answer cannot be extrapolated to things moving nearly as fast as light, or to things that are as small as atoms, or—in some respects—to things that are as heavy as stars.

Aug 25, 20050 citations