Einstein, Podolski, Rosen

Demystifying quantum mechanics VI

When one says that one wants to demystify quantum mechanics, then it may create the false impression that there is nothing strange about quantum mechanics. Well, that would be a misleading notion. Quantum mechanics does have a counterintuitive aspect (perhaps even more than one). However, that does not mean that quantum mechanics need to be mysterious. We can still understand this aspect, and accept its counterintuitive aspect as part of nature, even though we don’t experience it in everyday life.

The counterintuitive aspect of quantum mechanics is perhaps best revealed by the phenomenon of quantum entanglement. But before I discuss quantum entanglement, it may be helpful to discuss some of the historical development of this concept. Therefore, I’ll focus on an apparent paradox that Einstein, Podolski and Rosen (EPR) presented.

They proposed a simple experiment to challenge the idea that one cannot measure position and momentum of a particle with arbitrary accuracy, due to the Heisenberg uncertainty. In the experiment, an unstable particle would be allowed to decay into two particles. Then, one would measure the momentum of one of the particles and the position of the other particle. Due to the conservation momentum, one can then relate the momentum of the one particle to that of the other. The idea is now that one should be able to make the respective measurements as accurately as possible so that the combined information would then give one the position and momentum of one particle more accurately than what Heisenberg uncertainty should allow.

Previously, I explained that the Heisenberg uncertainty principle has a perfectly understandable foundation, which has nothing to do with quantum mechanics apart from the de Broglie relationship, which links momentum to the wave number. However, what the EPR trio revealed in their hypothetical experiment is a concept which, at the time, was quite shocking, even for those people that thought they understood quantum mechanics. This concept eventually led to the notion of quantum entanglement. But, I’m getting ahead of myself.

John Bell

The next development came from John Bell, who also did not quite buy into all this quantum mechanics. So, to try and understand what would happen in the EPR experiment, he made a derivation of the statistics that one can expect to observe in such an experiment. The result was an inequality, which shows that, under some apparently innocuous assumptions, the measurement results when combine in a particular way must always give a value smaller than a certain maximum value. These “innocuous” assumptions were: (a) that there is a unique reality, (b) that there are no nonlocal interactions (“spooky action at a distance”) .

It took a while before an actual experiment that tested the EPR paradox could be perform. However, eventually such experiments were performed, notably by Alain Aspect in 1982. He used polarization of light instead of position and momentum, but the same principle applies. And guess what? When he combined the measurement result as proposed for the Bell inequality, he found that it violated the Bell inequality!

So, what does this imply? It means that at least one of the assumption made by Bell must be wrong. Either, the physical universe does not have a unique reality, or there are nonlocal interactions allowed. The problem with the latter is that it would then also contradict special relativity. So, then we have to conclude that there is no unique reality.

It is this lack of a unique reality that lies at the heart of an understand of the concept of quantum entanglement. More about that later.

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Transcending the impasse, part II

Classical vs quantum

It is a strange thing. Why the obsession with something that in the end comes down to a rather artificial distinction. Nature is the way it is. There is no dualism in nature. The distinction we make between classical and quantum is just an artifact of the theoretical model we build to understand nature. Or is it?

Well there is a history. It started with Einstein’s skepticism about quantum mechanics. Together with some co-workers, he eventually came up with a very good argument to justify the idea that quantum mechanics must be incomplete. At least, it seemed like a good argument until it was eventually shown to be wrong. It was found that the idea that quantum mechanics is incomplete and needs some extra hidden variables does not agree with experimental observations. The obsession with the distinction between what is classical and what is quantum is a remnant of this debate that originated with Einstein.

Today, we have a very successful formalism, which is simply called quantum mechanics, and can be used to model quantum phenomena. Strictly speaking, there are different versions of the quantum mechanics formalism, but they are all equivalent. The choice of specific formalism is usually based on convenience and personal taste.

Though Einstein’s issues with quantum mechanics may have been resolved, the mystery of what it really means remains. Therefore, many people are trying to probe deeper to find out why quantum mechanics works the way it does. However, despite all the probing, nothing seems to be discovered that disagrees with the quantum mechanics formalism, which is by now almost a hundred years old. The strange concepts, such as entanglement, discord, and contextuality, that have been distilled from quantum physics, turn out to be aspects that are already built into the quantum mechanics formalism. So, in effect all the probing merely comes down to an attempt to understand the implications of the formalism. We do not uncover any new physics.

But now a new understanding is rearing it ugly head. It turns out that the quantum mechanics formalism is not only successful for situation where we are clearly dealing with quantum physics. It is equally successful in situations where the physical phenomena are clearly classical. The consequence is that many of the so-called quintessential quantum properties, are actually properties of the formalism and are for that reason also present in cases where one can apply the formalism to classical scenarios.

I’ll give two examples. The one is the celebrated concept of entanglement. It has been shown now that the non-separability, which signals entanglement, is also present in classical optical fields. The difference is, in classical field it is restricted to local properties and cannot be separated over a distance as in the quantum case. This classical non-separability display many of the features that were traditionally associated with quantum entanglement. Many people now impose a dogmatic restriction on the use of the term entanglement, reserving it for those cases where it is clearly associated with quantum phenomena.

It does not serve the scientific community well to be dogmatic. It reminds us of the dogmatism that prevailed shortly after the advent of quantum mechanics. For a long while, any questioning of this dogma was simply not tolerated. It has led to a stagnation in progress in the understanding of quantum physics. Eventually, through the work of dissidents such as J. S. Bell, this stagnation was overthrown.

The other example is where certain properties of quasi-probability distributions are used as an indication of the quantum nature of a state. For instance, in the case of the Wigner distribution, any presence of negative values in the function is used as such an indication of it quantum nature. Nothing prevents one from using the Wigner distribution for classical fields. One can for instance consider the mode profiles of classical optical beams. Some of these mode profiles produce Wigner distributions that take on negative values at certain points. Obviously, it would be misleading to use this as a indication of a quantum nature. So, to avoid this situation, one needs to impose the dogmatic restriction that one can only used this indication in those cases where the Wigner distribution is computed for quantum state. But then the indication becomes somewhat circular, doesn’t it?

It occurs to me that the fact that we can use the quantum mechanics formalism in classical scenarios provides us with an opportunity to question our understanding of what it truly means to be quantum. What are the fundamental properties of nature that indicates scenarios that can be unambiguously identified as quantum phenomena? Through a process of elimination we may be able to arrive at such unambiguous properties. That may help us to see that the difference between the quantum nature of things and the classical nature of things is perhaps not as big as we thought.

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