anton_zeilinger's picture
Nobel laureate (2022 - Physics); Physicist, University of Vienna; Scientific Director, Institute of Quantum Optics and Quantum Information; President, Austrian Academy of Sciences; Author, Dance of the Photons: From Einstein to Quantum Teleportation

Yes, I have learned, like many others,

— to write short e-mails, because people don’t want to read beyond line 10.

— to write single-issue e-mails, because any second or third issues get lost.

— to check my e-mails on the i-phone or blackberry every five minutes, because the important message could be arriving at any moment.

— to expect that our brain function will significantly be reduced in the coming decades to very simple decision-making, and so on and so on.

Well, seriously, I find it utterly impressive how the notion of information is becoming more and more important in our society. Or rather, of what we think what information is. What is information? From a very pragmatic operational point of view, one could argue that information is the truth value of a proposition. Is it raining now? Yes/no. Are airplanes flying because they are lighter than air? Yes/no. Does she love me? Yes/no.

Evidently, there are questions which are easier to answer, and others which are very difficult, or maybe even impossible to answer in a reliable way like the last one. While for the first two questions, we can devise scientific procedures how to decide them, even including borderline cases, for the last question, such an algorithm seems impossible, even though some of our biology friends try to convince us that it is just a matter of deterministic procedures in our brains and in our bodies. There are other questions which will forever be beyond any methodical scientific decision procedures, like: Does God exist? Or: Which of the two slits in a double-slit interference experiment does a quantum particle take?

These last two questions are of a very different nature, although both are unanswerable. The question whether God exists is not only beyond any solid scientific argumentation, it must be like that. Any other possibility would be the end of religion. If God were provably existent, then the notion of belief is empty. Any religious behaviour would be mere opportunism. But what about the quantum question? Which of the two paths does a particle take in a double-slit experiment?

We learned from quantum physics that to answer this kind of question, we need to do an experiment which allows us to determine whether the particle takes slit A or slit B. But that, we also learned, significantly modifies the experiment itself. Answering the question implies introducing the specific apparatus which allows us to answer that specific question. Introducing an apparatus which permits to determine which slit a particle takes automatically means that the phenomenon of quantum interference disappears because of the unavoidable interaction with that apparatus. Or, in the picture of the famous Schrödinger cat, asking whether the cat is alive or dead immediately destroys the quantum superposition of the alive and dead states.

Therefore, we here have a completely new situation, not encountered before in science and probably not in philosophy either. Creating a situation where a question can be answered completely modifies the old situation. An experimental quantum setup, or any quantum situation, can only represent a finite amount of information, here either interference or path information. And it is up to the experimentalist to decide which information is actually existing, real, manifest, in a concrete situation. The experimentalist does this by choosing appropriate apparatus. So, information has a very fundamental nature of a new kind not present in classical, non-quantum science.

What does this all have to do with the Internet? Today, we are busy developing quantum communication over large distances. Using quantum communication links, one will connect future quantum computers which work in a completely new complexity class compared to existing computers. To the best of my knowledge, this is the first time that humanity develops a technology which has no parallel at all in the known Universe. There are no quantum computers out there, assuming that the functioning of the brain can, in the end, be explained by non-quantum processes.

What will all that mean for our communication? This is impossible to tell. It is more impossible to tell than the historic fact that it was impossible to predict the applications of inventions like the laser or microchips, just to name two more recent examples. We will be entering a completely new world where information is even more fundamental than today. And it is hoped that, looking back, when the present irritation experienced by many because of the Internet, will appear to have been just an episode in the development of humanity. But maybe I am too optimistic.