From Thought Experiments to Quantum Information

If you have been following Metanexus for the last few weeks or so, you know that there was a symposium in honor of the 90th year of John Archibald Wheeler—physicist and thinker extraordinaire. In continuation of that theme, we will look today at quantum superposition and quantum weirdness ... of the feline variety.



The peculiar rules of quantum mechanics were thrashed out in the 1930s with the help of many famous 'thought experiments.' Perhaps the most famous is that of Schrodinger's cat. Imagine a cat incarcerated in a box together with a radioactive substance, a flask of cyanide gas and a trigger device such that if a nucleus of the radioactive substance decays, the flask is smashed and the cat dies. The problem, so it seemed to Schrodinger, is how to interpret the state of the cat if the nucleus is in a superposition of decayed and undecayed states. Is the cat then in a ghostly hybrid live-dead state? Does its health depend on whether an external observer sneaks a look in the box?

The thought experiment was designed to amplify quantum weirdness to everyday feline dimensions in order to startle us with its weirdness. It was not intended to solve the problem of quantum observation, although it has been interpreted as a reductio ad absurdum of quantum mechanics in some quarters. I doubt that Schrodinger, Bohr, Einstein or any of the other physicists who debated the subject in the 1930s ever supposed that this type of experiment would become a practical proposition. Remarkably, however, technology has advanced to the point where quantum superpositions can be created, if not of cats, then at least of enough atoms at once to be seen by the unaided eye. This is the world of 'mesoscopic physics,' which lies closer to the scale of cats and boxes than of atoms. For the first time it is feasible to suggest that we could follow quantum weirdness as far as everyday dimensions, and find out whether anything new intrudes. This is the field of Serge Harocheof the College de France and Ecole Normale Superieure, Paris, whose paper is featured in this posting.

A key factor in the development of the ideas of Haroche and others is to identify the appropriate parameter for the transition to the 'everyday world.' Is physical size the relevant quantity? Is it the mass that counts? Or could it be more subtle? After all, the concept of 'cat' and 'alive' pertain not so much to the physical dimensions of the system, but to its complexity. Can we be sure that no new physics—physics associated with the onset of classicality—arises in a system that is sufficiently complex? Blobs of atoms are one thing, cats (or even bacteria) quite another. A quantum superposition of a bacterium might, I venture to suggest, provide a few surprises.

Let me close with a warning. I once tried re-enacting Schrodinger's cat experiment for BBC television (symbolically, cat lovers note), using a suitably docile studio moggie. After half a dozen takes, shoving the creature into a box and closing the lid, the indignant animal meowed so loudly that the wave function kept collapsing before I could deliver my lines. This proved to me the television adage: never work with children or animals.

—Paul Davies



One of the reasons why the study of the quantum-classical boundary has recently become a hot topic in physics is the remarkable development of experiments which manipulate and study isolated quantum objects. We can now trap single atoms or single photons in a box, entangle their states according to a well defined program, observe directly their quantum jumps and realize in this way some of the thought experiments imagined by the founding fathers of quantum physics. Schrodinger, who believed that manipulating and observing an atom so to speak in vivo would remain forever impossible, would have been amazed if he could have seen what experimentalists can now achieve by manipulating and probing atoms with laser beams.

These experiments are not just textbook illustrations of quantum concepts. If they are so popular today, it is because they are considered as first baby steps toward harnessing the quantum world and achieving tasks impossible to perform classically. A quantum computer, for example, would be a machine using quantum interference effects at a macroscopic scale in order to perform massive parallelism in computation, achieving an exponential speed-up to solve some classes of problems such as the factoring of large numbers. The formidable enemy which will have to be defeated to build such a device is decoherence, which destroys with a remarkable efficiency quantum superpositions, transforming them into mundane classical mixtures of states. Experimentalists are thus given the daunting task to fight decoherence, to find the way to isolate quantum systems from their environment or to correct efficiently decoherence effects in complex entangled systems. Whether they will succeed to build a real computer is highly debatable, but it is clear that, by probing into entangled systems of increasing complexity, we will learn more about the quantum. As it has been so often the case in the past, applications are bound to emerge even if they are not the ones we expect. The catchy word quantum computer should be considered at this stage as a metaphor for a very ambitious field of research in which theorists in quantum information and experimental physicists emulate with each other to probe the frontier between the microscopic and the macroscopic, the quantum and the classical worlds.

They are two ways a quantum superposition can be considered as macroscopic. If two particles are in an entangled state, separated by a large distance, quantum effects manifest themselves —in a sense—at amacroscopic scale. This is the well-known non-locality problem first discussed by Einstein, Podolsky, and Rosen, then by Bell, and tested in beautiful experiments over the last twenty years. A quantum superposition can also be considered as macroscopic—in a deeper sense—if it involves alarge number of particles or quanta. Such situations are usually referred to as Schrodinger cats since they recall the fate of the mythical feline that Schrodinger had imagined to be suspended in a superposition of dead and alive states. The experimental investigation of these states has developed very fast over the last few years, in quantum optics as well as in mesoscopic solid state physics. We can now even consider states which would combine the two kinds of macroscopic features: Schrodinger cat states which at the same time would contain many particles and would be delocalized at two different places in space. It would be—so to speak—a cat simultaneously dead in one box and alive in another, combining two weirdnesses at once. In this chapter, I will describe various ways of preparing and studying such strange systems, which illustrate—I believe—some of the deep questions that Archibald Wheeler has asked about the quantum.

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