Reflections on the Discord Bubble

The following is a guest post by Marco Piani.
A couple of months ago Steve wrote a post on “the discord bubble“. Let me try to provide a summary of his post and of his thoughts.
There have been too many works, too often of insufficient quality either technically or conceptually (“pointless” works in Steve’s words) about a property of quantum systems and correlations—the so-called quantum discord—that has not been proven yet to be key in our understanding of nature or of the `inner workings’ of quantum information processing. That is, such flurry of activity does not appear to be justified and is rather due to “hype”; most importantly, the product of such activity is way too often of questionable quality. Steve goes on to suggest that the bubble needs to be put under control, so that we would go down to a reasonable rate of publications on the subject, hopefully of higher average quality. In order to assess such quality, Steve proposes some rules of thumb—check them out in Steve’s post—to be applied hopefully by the authors themselves. In his words, Steve’s intention was

“not to trash the subject as intrinsically uninteresting; rather, [he wanted] to highlight the epidemic of pointless papers that constitute the discord bubble. [He hoped] that thinning the herd will increase the quality of the results in the field and decrease the hype surrounding it, because it has really gotten completely out of control.”

While I found Steve’s post essentially to the point and I furthermore highly appreciated his invitation to highlight in the comments good research on quantum discord—because there is good research on the topic—I feel that there is room for more reflection and discussion, in particular within—but of course not limited to—the community working on discord-related problems. To start it, I would like to express some of my own opinions.

Is there a bubble? What is it at its origin?

Yes, there is a bubble. Yes, there is too much hype. Yes, there are too many papers on the subject that are of questionable quality.
On one hand, the subfield of the `general quantumness of correlations’—as I like to think of discord and related concepts—still lacks a killer application to justify the present level of activity. The explosion of interest in the subfield was in a good part due to hints that such quantumness could help explain the quantum advantage in some noisy models of (limited) quantum computation. Such hints, to my knowledge, remain just hints; that is, too little to justify so many papers. We are still searching for a task/protocol that convincingly highlights discord or similar related properties as `resources’  or `the key’ in a general enough scenario—see this related post by Valerio Scarani. On the other hand, as Steve pointed out, many papers on discord are characterized by a low scientific merit, technically and/or conceptually.
I believe that the “discord bubble” is due to a combination of several factors, which have all come together to create the `perfect bubble’. Some are:
1) we live in a “publish or perish” (academic) world: if hype starts to be generated regarding a topic, many researchers will flock to that topic in the hope to score well in the number and kind of publications—high-impact journals—and the corresponding citations they are bound to get;
2) when there is a `new’ topic (like `discord’ with respect to the good ol’ `entanglement’) the technical and conceptual threshold to contribute new results is lower; on the other hand, quantum information is by now a mature field and new exciting results in `traditional’ theoretical topics—quantum Shannon theory, quantum error correction, quantum computation, … —require a high level of expertise and skills;
3) some form of `confirmation bias’: positive statements on the value and significance of new and old results on the topic are easily accepted and perpetuated, especially in light of 1) and 2).
Let me stress that these factors are mostly independent of the topic itself—discord. I am pretty sure that other fields of science have their own bubbles.

Is it worth working on discord and related issues?

Quantum information processing aims at exploiting quantum features to provide us with new, powerful means to manipulate information. Reaching this goal requires the best possible understanding of such features.
More specifically for our case, there are quantum features that 1) are proper of bi- and multi-partite systems (i.e., they do not have a real correspondent for single systems)  and 2) do not reduce to entanglement, e.g, they can be present also in the absence of entanglement.  For example, one of these properties is related to the celebrated no-cloning theorem; another one is the unavoidable disturbance introduced by local measurements. In this sense there is a `general quantumness of correlations’, which, for the above reasons, I believe is worth investigating.
Of course, an argument like “[PROPERTY] could be useful, so we should study it” is not  enough, if not substantiated. There should be some concrete evidence and some convincing perspective of such usefulness in order to motivate the related investment of resources. I think that what we know about the general quantumness of correlations, although not enough to justify the number of papers dealing with it, satisfies these requirements, at least partially. More clearly:
We have concrete evidence that the general quantumness of correlations is a useful concept to consider.
Such usefulness goes from foundations—e.g., addressing the measurement problem and the emergence of classicality—to the alternative take on the no-cloning theorem mentioned above, to the study of quantum effects in the `locking’ of classical correlations in quantum information theory.
On the other hand:
We have not yet found a way to think about and exploit the general quantumness of correlations that makes such quantumness worthy of the central stage of quantum information processing, or of the title of `resource’.
As mentioned above, the protocols so far designed that pinpoint discord as the relevant `resource’ at play can be considered contrived. More generally, I doubt that discord will ever achieve a status of resource similar to entanglement. I rather believe that, for example, discord is—with the risk of sounding like I am trying to be witty—something that makes it possible to make something else impossible. For example, discord makes it impossible to access `in a classical way’  part of the information content of correlations.
All in all, I do believe that the general quantumness of correlations will further prove its usefulness, both as a conceptual tool and as concrete property present in distributed systems. What we have to remember is that such usefulness is not well established and that most of the work on the subject should still go in the direction of clarifying the value and applicability of the concept rather than, e.g., calculating discord in all sorts of physical systems.
Let me add that I consider the study of the general quantumness of correlations also as an attempt to think about the quantum from a different/larger perspective—different/larger with respect to what was previously done. In my case, thinking in terms of the `general quantumness of correlations’ has helped me, for example, to better understand entanglement itself.

What should people who work or consider working on discord do? What about the rest of the quantum information community?

The effort worth investing—as individual researchers, as a scientific (in particular, quantum information) community, and as a society (e.g., in terms of funding)—in the study of the general quantumness of correlations is not easy to determine. This actually holds in general, for essentially any research subject. What is worrisome about a bubble, is that it can reduce the efficiency of the procedures in place—in particular the peer-review process—to shape the activity of the community, to reallocate its resources, and to induce researchers to adopt a good practice. [We actually know that the peer-review process, in particular in the standard form associated to publication in journals, does not work perfectly in general.]  As personal experience goes, I have often rejected low-quality papers on discord only to see them published in another journal. So it is important to recognize the presence of a bubble and bring the problem to the attention of the community, so that extra care can be taken in assessing the value of papers, both in terms of correctness and relevance. While I endorse Steve’s rules of thumb to assess the quality of papers related to discord, I would like to address the problem of the discord bubble with a list of suggestions mostly comprising general good practices.
So, to the people working/interested in working on discord:
a) re-evaluate why you are working on the topic. A useful exercise it that of imagining explaining to someone who is NOT already working on discord why he should be interested in investigating the subject. If whatever reason you provide is based on previous results in the field, make sure that you have checked the source at a sufficient level of detail to be sure that those results—and in particular any related strong claim—would be convincing for him, and, most importantly, that they are convincing for you;
b) be aware of the large body of work that already exists in quantum information processing: what you think is new and exciting may well be already known (either published, or easy to see and part of the `folklore’);
c) analyze critically your work and that of the others, both as author and as referee: cite only literature that is significant and relevant to your paper and reject papers that do not provide substantial advance in neither understanding nor applications;
d) focus on providing more—and possibly conclusive—evidence that the study of the quantumness of correlations is justified: we need `killer applications’ and `killer concepts’;
e) entanglement theory has been a very fruitful field of study because entanglement is a fundamental concept and it can be understood/analyzed as a resource in a reasonable, operationally justified framework—that of distant labs, where the quantum operations allowed are only local, at most coordinated by classical communication. Trying to mimic successful stories might be a good idea, but there are dangers involved. Let us avoid creating the Bizzarro version of entanglement.
To the rest of the quantum information community:
Be critical and open-minded at the same time.
While complaining about the existence of a discord bubble is more than reasonable and, from my point of view, quite welcome, attacking the study of discord per se is unjustified. Please challenge whoever makes claims that are too strong, question observations and calculations that you judge irrelevant, push people who work on discord to meet the highest standards in international research, reject papers when they do not meet such standards. But please do not dismiss a talk or a paper just because it deals with discord; evaluate it only on the basis of its specific scientific merit.  Furthermore, if you feel like it, you can ponder for some minutes on questions like “Is there anything quantum about a distributed quantum state that is not solely due to entanglement?”, and “Can we make use of it?”.

On the fractal nature of bubbles

The best results of recognizing that there is a `discord bubble’ and of taking corresponding action—maybe on the lines suggested above—would be, on one hand, to improve the research activity on discord, and, on the other hand, to avoid the risk that the subject and the people working on it acquire a bad reputation.
Let me mention that there are scientists who think that research on quantum computation is itself a bubble—see this post by Scott Aaronson. Is this further proof that bubbles can have a fractal structure? Of course the `accusation’ of being bubbles is at different levels for discord and quantum information, and not just in the sense of the `fractal level’. The criticism towards quantum computing is mostly about its realizability, while discord, although a useful conceptual tool, has not convincingly been proven to be a `resource’, even in theory.  Nonetheless, there is something in common about the two accusations of being a bubble: hype—again. Indeed, Scott writes about the accusations of the quantum computing skeptic M. I. Dyakonov:

“Dyakonov fumes about how popular articles, funding agency reports, and so forth have overhyped progress in quantum computing, leaving the conditions out of theorems and presenting incremental advances as breakthroughs.  Here I sadly agree.”

So, let me add a point to the above list of suggestions:
f) try to reduce—or at least do not contribute to—the hype.
This will have the effect of making the topic of discord less attractive for people whose work will not actually improve the standing of the topic, and improve its reputation within the quantum information community, potentially attracting good researchers.
The above-mentioned post by Scott is also notable for having started a high-quality discussion in the comments section on whether quantum computing deserves the accusation of being a bubble. It would be great to have a similar discussion on discord in the comments below. I expressed already my personal opinion: we are in front of a discord bubble, but there is merit in studying the general quantumness of correlations. I would be particularly happy to have a discussion on whether—and why—my opinion is too harsh or too mild, and to receive, as Steve already asked in his post, motivated suggestions about works that should survive a `pop’ of the discord bubble.

4 Replies to “Reflections on the Discord Bubble”

  1. Actually, one can see many papers in high-impact journals (e.g. Nature, Science, PRL) to report the entangled states in all sorts of physical systems.
    For example, three particle GHZ state are reported in photon, ion trap, cavity QED,
    superconducting junctions, etc. All these works are publised in Nature, Science etc.
    However, are there any difference of the entanglement properties except the underlying physical systems?
    Is this a bubble of entanglment?

  2. I think the difference is that each of these represents some new experimental feat. They don’t represent surprises in that of course we expect Schrodinger’s equation to always hold, but success in entangling ions doesn’t mean that it becomes easy to sustain entanglement in a superconductor.

  3. @Tao Tu,
    We’ve already had an entanglement bubble… it was in the early 2000’s. At the time, many people believed (not necessarily unjustly) that studying entanglement monotones would lead to a better understanding of all aspects of quantum information theory. There were untold numbers of papers published about monotones for 3 qubit mixed states that didn’t generalize and/or were completely impossible to compute. Eventually, people realized that the “resource-based” view of entanglement was more fruitful. They were able to identify a small handful of operationally meaningful monotones and focus their study on those. I hope that discord can make a similar transition into a post-bubble phase, where discord has some kind of operational meaning and people use it as a tool to understand interesting problems, rather than just calculating it for its own sake.

  4. @Tao Tu: Entanglement and discord have at the moment a different status, both conceptually and practically. I think we all agree that entanglement is a central feature of quantum mechanics; also, it plays a central role in quantum information processing. While discord is also related to some basic quantum concepts — like superposition — and could conceivably be considered a very relevant quantum feature itself, we do not know how to exploit it effectively, nor we were able to construct a reasonable “resource theory” around it that would elevate it to “resource”. On the other hand, as Aram pointed out, a number of experiments aim at proving the level of control scientists have reached in manipulating different physical systems, rather than testing “new physics”. Each physical system has pro and con when it comes to quantum information processing, so it is worth checking that each can be made to encode the desired — entangled — quantum state.

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