Monthly Archives: December 2017

Podcast on quantum mechanics

I have been interviewed by Vincent Debierre (in French) for Liberté Académique. 

Dans ce podcast nous parlons principalement de mécanique quantique et un peu de sa popularisation. Si je suis peut-être un peu provocateur dans ma critique des effets de manche de certains physicien, je crois ne pas avoir dit trop de bêtises. Je me suis trompé sur une date : l’axiomatisation de la mécanique quantique due à Von Neumann date de 1932 et non de 1926 (qui correspond à la “première vague”, avec Heisenberg, Born et Jordan, suivie de la formulation de Dirac en 1930).

Grâce à Thomas Leblé, j’ai retrouvé un brouillon que j’avais écrit il y a 3 ans sur ce que je comprenais des difficultés de la mécanique quantique et qui fait echo à ce que je raconte dans ce podcast. En le relisant, je suis étonné d’être assez d’accord avec ce que j’avais écrit à l’époque. Naturellement, il y a pas mal de formulations un peu malheureuses (notamment je pense qu’on peut supprimer “réaliste” dans “réaliste local”), ainsi que pas mal de fautes d’orthographes. Ceci dit je préfère laisser le texte momentanément dans son jus, avec ses problèmes, plutôt que de passer un temps indéterminé à le retravailler sans que jamais personne ne puisse le lire. Bref, vous pouvez regarder si ça vous intéresse, en gardant à l’esprit que c’est un vieux brouillon.

Status of the article pipeline

Ideas have a long way to go before they become articles. Usually for me the first step is to write down the computations and main arguments on paper. Then I write down a better typeset draft and iterate on it until it looks like a decent preprint. I then put it on arXiv, hoping to gather some feedback. Then I further iterate on the draft, rewrite things, correct mistakes, and send it to a first journal. The first journal typically rejects it but hopefully I get constructive reports. So I go to a second journal or a third and an article usually quite different from the original idea gets published. In the process, my ego gets shattered but the article becomes, I think, substantially better. This process is very long and so many articles are at different stages in the “pipeline” at the same time. This week and the last, I have made a little bit of progress emptying the pipeline.

1) Exact signal correlators in continuous quantum measurement

This is a new preprint I am quite happy about. In continuous quantum measurement, the objective is usually to reconstruct a continuous quantum trajectory \rho_t from a noisy continuous measurement readout I_t. People often make the confusing remark that the  quantum trajectory \rho_t is not directly “measurable”, it is just reconstructed from a model. This is misleading. One can do projective measurements every time the state reaches a given value \rho_t = \sigma and then check that the statistics obtained do match the theoretical prediction from continuous measurement theory. Nonetheless there is a valid point which is that this is inconvenient and that to validate the model or measure its parameters, it would be more convenient to talk only about the statistics of the continuous measurement readout. So instead of using the theory to reconstruct the state \rho_t, we can use the theory to compute the statistics of the signal I_t. This would allow read the free parameters of the model from “directly” obtained quantities. This point is not from me and has been made recently notably in a recent preprint by Atalaya et al. . In this article, the authors compute the n-point correlation function of the signal for qubits with a method that is (or at least seems) ad hoc. Reading this preprint, I remembered that I knew how to compute the n-point functions in full generality. I just had never understood that it could be useful. My only fear is that the result is known but buried in the Russian literature of the nineties. In that case, it would be the end of the journey for this preprint in the pipeline.

2) Binding quantum matter and spacetime, without romanticism

This is an essay I had written for the “Beyond spacetime” essay contest. In the absence of empirical evidence, I am defending semiclassical gravity as a sober and metaphysically sounder alternative to quantum gravity. While most people advocating semiclassical gravity criticize the cheap rebuttals made against it, I have tried to also be constructive, pushing the explicit examples we have introduced with Lajos Diosi. The essay is voluntarily provocative and certainly not optimally constructed. Perhaps I have tried a bit too hard to stretch my arguments to fit into the subject (and actually take its counterpoint). Anyway, while it was apparently shortlisted, it didn’t win the prize. So I am left with a rather specific essay I do not know what to do with.

I am not sure what will happen to it. The organizers of the contests kindly gave me the referee reports along with a suggestion to submit the essay (after corrections) to a philosophy of science journal. But I think I would need to substantially modify it to transform it into a proper article and I am a bit overwhelmed by the task. On the other hand, although imperfect, I think this essay made a few points that are insufficiently known. I am also tempted by the sunk cost fallacy: I have already spent quite some time on this and I would hate for it to be totally wasted. Before I make up my mind, you’re welcome to read the present version of the essay.

3) Ghirardi Rimini Weber model with massive flashes

This is an article I had already talked about here. It started as a simple toy model to explain the basic idea of new approaches to semiclassical gravity. Stimulated by the perspective that such non-bullshity foundational work might be acceptable in Physical Review Letters, I spent a bit of time polishing the sentences and shortening the article. The objective was to make it as easy to read as possible (thanks to Dustin Lazarovici who helped me a great deal). This is a decently good letter I think, in that it is really self contained and readable by a general audience interested in foundations (and not the grandiosely oversold summary of a 20 page Supplementary Material that letters tend to be). Well, PRL eventually refused but it was accepted as is in Phys. Rev. D Rapid Communications which is probably the next best I could hope for. It will soon be published but you can check the latest version already here.