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It seems that Quantum Information has been seriously considered as something very important for fundamental physics, and in particular, to quantum gravity.

For example, this is the topic of this year's prospects in theoretical physics. So we see that it seems some quite big names on the community, which are mainly focused on purely theoretical and fundamental problems, like Witten and Susskind, are taking Quantum Information quite seriously on this. The program description of this event says:

PiTP 2018 is titled "From Qubits to Spacetime," and will cover topics ranging from the connections between quantum information and the strucuture of spacetime, to how information is manipulated by the dynamics and how quantum effects can be included in black hole thermodynamics.

Now, on the other hand, every time I begin taking a look on it, it appears to be the opposite. It seems the deal with Quantum Information is all about how to produce a quantum computer. So it seems in the end, that the deal with Quantum Information is not exactly on "how does nature behave fundamentaly", but rather, "how can we harness nature's quantum behavior to create technology".

This can been for instance in quantum algorithms, quantum error correction, quantum criptography, and so forth. So, as in computer science, this seems to have the clear intention not on understanding nature very deeply, but rather on harnessing it to create technology. In particular here one is trying to use the properties of quantum systems to create better computers.

This is all nice in itself, but I can't seem how all of this can be connected to fundamental physics to the point of being related to quantum gravity, string theory, and so forth. I mean, what nature cares about algorithms, error correction and cryptography? These are things that are of interest for us, humans, which want better technology. I'm failing to imagine why a fundamental force of nature as gravity, in its most fundamental form, would even be related to these things.

So the question here is: if on the one hand, quantum information seems extremely tied to technology development in computer science, how can it be so important as it seem to be, in the understanding of fundamental physics and in particular, quantum gravity and string theory?

Edit: Perhaps the OP wasn't clear. I'm not asking, what is the intent of quantum information. I'm saying the following: if we search on the web about QI, we mostly find applied stuff aimed at quantum computers. Algorithms, codes and so forth. Nature doesn't care whether or not we build computers. So how can these things (algorithms, codes, etc) have any relation whatsoever with nature's fundamental workings, and for example the nature of spacetime (see citation above)? That is the objective question. I'm not saying a field can't have an applied and a theoretical part. Obviously it can (as far as I know, GR has been used in the development of the GPS). What I'm failing to see is where the theoretical part is on QI. Is how a field that seems to be pure computer science have anything to do with fundamental physics.

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  • $\begingroup$ Mathematics is also extremely tied to technology development in computer science, while also being important in the understanding of fundamental physics. It's actually quite common for fields to have both "pure" and "applied" components. $\endgroup$ – probably_someone May 31 '18 at 15:28
  • $\begingroup$ @probably_someone I agree with this. The point of the question is exactly trying to understand the "pure" component as you say. Because in math, the "pure" component is self evident. We have things like Differential Geometry, point-set and algebraic Topology, we have Functional Analysis, we have Lie-group and Lie-algebra theories and the study of the associated representations, and all of those are totally dettached from applications, and their importance in physics is usually clear. Actually, it seems some of these fields developed motivated by fundamental physics needs. $\endgroup$ – user1620696 May 31 '18 at 15:41
  • $\begingroup$ First of all, if something in math is important in physics, that counts as an "application" in most definitions, so I don't think the "pure" component is "totally detached from applications," unless you mean something different than most people when you use the words "detached" and "applications." Second, why is it any less intuitive that quantum information could have developed in the same way that you describe here? $\endgroup$ – probably_someone May 31 '18 at 15:47
  • $\begingroup$ I think we are meaning two different things. It is not one application intended to build any sort of technology at first, but purely for describing facts about nature. Example: Lie groups describing continuous symmetry properties of systems. If you study the representation theory of the Poincare group in order to understand the possible one-particle states in QFT, sure, this counts as application of Lie group theory, but we can agree this surely has nothing to do with technology at first (technology isn't the intent behind it). By applied I mean: developed from start aiming at technology. $\endgroup$ – user1620696 May 31 '18 at 15:51
  • $\begingroup$ I see it less intuitive because these branches of math weren't developed aiming at technology. The general theory of relativity wasn't developed aiming at technology, QFT wasn't developed aiming at technology. But it does seem, that QI has as its main objective, the development of technology. Thus, although a valid field (technology is certainly important), I fail to grasp how in the end a field focused on how to build technology is important for fundamental physics, which is certainly independent of technology or human needs whatsoever. $\endgroup$ – user1620696 May 31 '18 at 15:53
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(My background: I'm not a fundamental physicist nor a quantum information theorist, although I work with a few of the latter.)

Information theory is a framework for studying and characterising randomness. In particular, information theory is good at answering questions about random variables such as "how random" or how strongly correlated random variables are. It turns out that such properties are often characterised by non-linear functionals of the probability distribution, such as the entropy, mutual information, etc.

Quantum information theory is a framework for studying and characterising randomness in quantum mechanics. It provides tools for answering questions such as how strongly correlated two parts of a quantum system are. Usually such properties are well characterised by non-linear functions of the density operator, such as the (entanglement) entropy, the mutual information etc. This is rather different from the usual agenda of physicists (fundamental or otherwise) who are typically interested in observables/correlation functions, which are linear functions of the density operator.

In my (necessarily limited) experience, quantum information theorists are often interested in questions of a kinematical nature, e.g. how to classify quantum states according to their entanglement properties, or what kinds of quantum states can be transformed into each other under very general constraints like unitarity, locality, energy conservation etc., without worrying about the detailed dynamics of such a transformation. On the other hand, many physicists are deeply concerned with writing down the actual Lagrangian describing the fundamental dynamics of the Universe. In one (debatable) sense, quantum information theory is "even more fundamental" than this noble endeavour, since many of its theorems will continue to be applicable even if one day we discover a much better Lagrangian than the Standard Model. However, I suspect that the apparent interest in quantum information theory shown by certain people working on fundamental dynamics is more related to questions such as the black-hole thermodynamics, entanglement entropy of quantum-field vacua, holographic tensor network states etc.

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(My background: researcher in quantum info theory, but not really in "fundamental" work like QI in field theories.)

Well, my first answer is that it's important to understand where science comes from when you survey a field. Science is produced by working scientists, and naturally there are more working scientists in fields that are closer to the "applied" fields because someone has to pay them. Companies, defense agencies, and even "pure research" funding agencies are going to favor research that is oriented towards technological goals. So don't be surprised that that's the focus you're seeing in the field.

On the other hand, the question is, why would quantum information yield any insight on fundamental questions? I think the main reason is that quantum information often has a lot to say about how microscopic dynamics can be translated into large-scale properties of a system. For instance, you bring up error-correcting. Error correcting is, of course, a very technologically interesting field. But it's also, fundamentally, about trying to learn how a quantum system can avoid losing its quantum nature when exposed to outside noise or interference. That's a very fundamental question about how robust the quantum nature of reality is, and can teach us about what microscopic effects can and will survive in the system at large.

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  • $\begingroup$ Thanks for the answer! So your point is: the fact that when I search information about QI I end up finding much more applied things than theoretical ones, is just because the number of researches on the applied field is bigger, not because QI itself is just meant for applications? $\endgroup$ – user1620696 May 31 '18 at 15:43
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    $\begingroup$ I think you might need to define what specifically you mean by "theoretical" and "applied" before we proceed further. $\endgroup$ – probably_someone May 31 '18 at 15:47
  • $\begingroup$ @probably_someone. I think the distinction is clear from the question. I just edited it to emphasize the claim actually made that quantum information may explain the nature of spacetime. Seems theoretical enough for me. $\endgroup$ – Stéphane Rollandin May 31 '18 at 17:38
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    $\begingroup$ @user1620696 I mean, much of the field is developed with applications of some kind in mind (although research can be "theoretical"/"basic"/"curiosity-driven" without being "fundamental" as in being about space-time and gravity). But when you read the "Motivation" section of papers, etc, you're getting the researcher's perspective on why they did that work, not an exhaustive exploration of how else that work might be applied. $\endgroup$ – zeldredge May 31 '18 at 17:40
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    $\begingroup$ I also think these categories probably don't stand up to serious scrutiny. Lots of fundamental science turns out to have applications; lots of science done for application purposes yields fundamental insights. $\endgroup$ – zeldredge May 31 '18 at 17:42

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