More Quantum

A friend of mine, Miguel Navascués, has just started a new science blog, which I’m happy to advertise here: https://sciencecommunicationexplained.blogspot.com/

His inaugural post, written in his signature incendiary style, is about a subject that has personally pissed him off: another bullshit story in Quanta, this time claiming that his well-known paper on experimentally disproving real quantum mechanics had been overturned. Well, I’m not surprised. After they fell for the wormhole nonsense I don’t expect anything from them anymore. On the contrary, I’m pleasantly surprised they didn’t fall for the nonlocality without entanglement bullshit. Instead, this dubious honour goes to New Scientist.

Still, it’s a clear case of journalistic malpractice. They dutifully interviewed independent experts and some of the original authors for the story. But they didn’t include anything they said about whether the article had in fact been overturned, which is kind of the crucial point. Presumably because they were unanimous that it hadn’t, and this is not the story the journalist (Daniel Garisto) wanted to tell. You see, a story about people proving eachother wrong is much more exciting than reality, which is science being built by results adding to eachother. Actual overturnings are quite rare, as they should, because papers that can be overturned shouldn’t be published in the first place.

The story’s fundamental problem is that it never clarifies what is “real quantum mechanics”. It mentions that Stückelberg developed a real-valued quantum mechanics in 1960; it can reproduce complex quantum mechanics exactly, and hence it is not falsifiable. It never mentions this, of course, because then it would be obvious that this is not the theory that was falsified in 2021. What was falsified was Wotters’ real quantum mechanics, that was introduced in 1990. Bizarrely the story even interviews Wotters, but never mentions this! It introduces him as “a quantum information theorist at Williams College”. Because again if it had mentioned that, then the article could hardly be overturned by a new real quantum mechanics that was introduced in 2025.

All I’m saying is well-known by everybody involved, who doubtless told it to the journalist, who doubtless couldn’t care less.

This is only the superficial, names-and-dates part of the story. To get to the substance, head over to Miguel’s blog, which explains it in detail. He promises to do this not only for this story, but every single time Quanta writes about quantum information or quantum foundations. Let’s hope his stamina lasts!

A couple of years ago I was complaining about the lack of direct support for SDPs with complex numbers, and documenting my efforts to get it working on SeDuMi via YALMIP. That worked, but in the meantime I’ve stopped using MATLAB (and thus YALMIP), and started solving SDPs in Julia via JuMP.

No problem, I did once, I can do it again. Funnily enough, the work was pretty much the complement of what I did for YALMIP: YALMIP already had a complex interface for SeDuMi, it was just disabled. JuMP didn’t have one, I had to write it from scratch. On the other hand, I had to write the dualization support for YALMIP, but not JuMP, because there we get it for free once the interface is working. The codebases couldn’t be more different: not only Julia is a much better programming language than MATLAB, but also YALMIP is a one-man project which has been growing by accretion for decades1When I asked Löfberg, YALMIP’s author, about a particularly tangled piece of spaghetti, he told me that he had no idea, his code was as much of a mystery to him as it was to me.. In contrast, JuMP is a much newer project, and has been written by many hands. I has a sane, modular design by necessity.

That was done, but it was pointless, as SeDuMi is just not a competitive solver. To get a real benefit I’d need to add complex support to the best solvers available. My favourite one is Hypatia, which was in a similar situation: the only thing lacking was the JuMP interface. I contacted the devs, and they quickly wrote the interface themselves. Amazing! But that was also the end of the line: to the best of my knowledge no other solver could handle complex numbers natively.

I thought maybe I can hack the solver myself, as the algorithm is really simple. I looked a bit around, and found the perfect target: COSMO, an ADMM solver written in Julia. It was harder than I thought because COSMO fiddled directly with LAPACK, and that requires dealing with a FORTRAN interface from the 70s. Nevertheless I did it, and my PR was accepted. Luckily the interface came for free, as COSMO is written to work with JuMP directly. But it was also a bit pointless, because it turns out COSMO is not really good either, and it’s mostly abandoned.

I decided to go for the best ADMM solver available: SCS. The problem is, it’s written in C2For some reason I always end up writing C, and I always question my life choices that led me to it.. But I had a strong motivation to do it: I was working on a paper that needed to solve some huge complex SDPs that only SCS could handle. Getting a speedup there was really worth it. And it turned out to be much easier than I thought: the codebase is rather clean and well-organized. The only difficulty was dealing with LAPACK, but I had already learned how to do it from my work with COSMO. It just took ages for my PR to be accepted, so long that my paper was already done by then, and I had no intention of running the calculations again.

With the solver done, I went to work on the JuMP interface, which was just released today. As part of my evil plan to get people to switch to Julia, I didn’t write the YALMIP interface. I mean, writing MATLAB code for something that I’ll never use personally? No way. In any case, the new version of SCS showed a 4x speedup on an artificial problem designed to display the advantage, and a 75% speedup on a real complex SDP I had lying around.

Is that the end of my crusade? I really wanted to add support to a second-order solver, as these are faster and more precise (at the cost of not being able to handle the largest problems). The best one available is MOSEK. But it’s a closed source solver owned by a private, for-profit corporation, so nope. I actually tried to contribute a bug report to them once, and as a result I got personally insulted by MOSEK’s CEO.

An interesting possibility is Clarabel. It is written in Julia, so it should be really easy to hack. And in principle it should be much faster than Hypatia, as it uses the specialized algorithm for symmetric cones, whereas Hypatia uses the generic one that can also handle non-symmetric cones. But my benchmarks showed that it is not competitive with Hypatia, so until that changes there is no point, and I did enough pointless work.

An old friend of mine, Jacques Pienaar, wrote to me last Friday asking whether this paper by Wang et al. is bullshit, or is he going crazy. Don’t worry, Jacques, you’re fine. The paper is bullshit.

It claims to experimentally demonstrate the violation of a Bell inequality using unentangled photons. Which is of course impossible. It’s a simple and well-known theorem that separable states cannot violate a Bell inequality. Let me prove it here again for reference: let $\rho_{AB} = \sum_\lambda p_\lambda \rho^A_\lambda \otimes \rho^B_\lambda$ be a separable state shared between Alice and Bob, who measure it using POVMs $\{A^a_x\}_{a,x}$ and $\{B^b_y\}_{b,y}$. Then the conditional probabilities they observe are
\begin{align*}
p(ab|xy) &= \tr[\rho_{AB} (A^a_x \otimes B^b_y)] \\
&= \sum_\lambda p_\lambda \tr(A^a_x \rho^A_\lambda) \tr(B^b_y \rho^B_\lambda) \\
&= \sum_\lambda p_\lambda p(a|x \lambda) p(b|y \lambda),
\end{align*} so we directly obtain a local hidden variable model for them, with hidden variables $\lambda$ distributed according to $p_\lambda$, with response functions $p(a|x\lambda)$ and $p(b|y \lambda)$. Therefore they cannot violate any Bell inequality.

The authors certainly know this. Well, I know Mario Krenn and Anton Zeilinger personally, and I know that they know. The others I can safely presume. This means that the paper is not merely mistaken. It is bullshit.

But what have they done?, you ask. How have they obtained the result they claim? Honestly, I don’t know, and it’s not my problem. When you contradict a well-known theorem you’re the one who has to explain why the theorem is wrong, or why it doesn’t apply to your situation. They don’t explain it. I read the paper, and all they have to say on this subject is the first sentence of the abstract “Violation of local realism via Bell inequality […] is viewed to be intimately linked with quantum entanglement”. They also talk about “the ninety-year endeavor in the violations of local realism with entangled particles.” Apparently it’s not a theorem, just an opinion? Or tradition?

So I’m perfectly justified in washing my hands. I can’t contain my curiosity, though. Is the quantum state they used actually entangled? Or is the violation of a Bell inequality just fake? The state does seem to be separable, so there’s only option left: there is no violation. Their setup is inherently probabilistic, they use four photon sources, which can in total generate either 0, 2, 4, 6, or 8 photons. They postselect on the detection of four photons. Well, well. It’s well-known that you can fake a violation of a Bell inequality via post-selection, if the detection efficiency is $\le 2/3$. What is their efficiency? They don’t reveal. But they do seem to be aware that their experiment is not halal, as they write “We expect that tailored loopholes and local hidden variable to the work reported here can be identified.”

UPDATE: Wharton and Price just uploaded a comment to the arXiv, confirming that the Bell violation is faked through postselection.

UPDATE 2: Now Cieśliński et al. uploaded another comment to the arXiv. They agree with Wharton and Price’s analysis, and additionally show that one can make their experimental setup produce a real Bell violation by adding on/off switches to optionally block the pumping field going to the second downconverters. But then they become part of the measurement device, not state preparation, and the state at this point is entangled.

I am bothered by the language of this comment. Which does not even call itself a comment. Despite refuting everything about the paper, they call it “brilliant” and “outstanding”. They also can’t even bring themselves to directly say the obvious, that a Bell violation with separable states is impossible. They write that “Moreover, we provide an analysis which shows that the statement in their title, about unentangled photons, cannot be upheld.” They also repeatedly talk about the “conjecture” by Wang et al. that they violated a Bell inequality. That’s ridiculous, they didn’t “conjecture” anything, they directly and repeatedly claim a Bell violation. I hope they grow some self-respect and that in the next version of the comment they use proper scientific language. UPDATE 4: I’m glad to report that the updated version of the comment was in fact soberly written.

UPDATE 3: Sabine Hossenfelder made a YouTube video about the paper. I find it mysterious why she should care about it, since she’s a superdeterminist. In any case, the video is embarrassing. Doubly so because it will have orders of magnitude larger audience than the comments. She gives the paper a “0 out of 10 in the bullshit meter”, dismisses the correct explanation of postselection by demonstrating she has no idea what postselection is, and concludes with “of course there’s always the possibility that I just don’t understand it”. Indeed, Dr. Hossenfelder, on this point you are correct.

If you have used the NPA hierarchy to bound a Tsirelson bound, and want to recover a state and projectors that reproduce the computed expectation values, life is easy. The authors provide a practical method to do so, just compute a projector onto the span of the appropriate vectors. Now if you’re using its generalization, the PNA hierarchy, and want to recover a state and operators, you’re out of luck. The authors only cared about the case where full convergence had been achieved3In particular when the rank loop or flatness condition is respected., i.e., when the operator constraints they wanted to impose like $A \ge 0$ or $[A,B] = 0$ were respected. They didn’t use a nice little result by Helton, which implies that as long as you’re using full levels of the hierarchy2So no level 1+AB or stuff like this you can always recover a solution respecting all the moment constraints, which are stuff like $\mean{A} \ge 0$ or $\mean{A^2 B} = 2$. This in turn implies that if you only have moment constraints, no operator constraints, then the hierarchy always converges at a finite level! This is the only sense in which non-commutative polynomial optimization is simpler than the commutative case, so it is a pity to lose it.

In any case, going from Helton’s proof to a practical method to recover a solution requires shaving yaks. Therefore, I decided to write it up in a blog post, to help those in a similar predicament, which most likely include future me after I forget how to do it.

For concreteness, suppose we have a non-commutative polynomial optimization problem with a single3The multivariate case is essentially the same, it’s just a pain in the ass to write down the moment matrix, as the simplest nontrivial case will be 7 x 7 Hermitian operator $A$. Suppose we did a complete level 2, constructing the moment matrix associated to the sequences $(\id, A, A^2)$ with whatever constraints you want (they don’t matter), and solved the SDP, obtaining a 3×3 Hermitian matrix4I’m not assuming it is real. Usually it is, but I want to discuss the most general case here, and sometimes it is indeed complex.
\[ M = \begin{pmatrix} \mean{\id} & \mean{A} & \mean{A^2} \\
& \mean{A^2} & \mean{A^3} \\
& & \mean{A^4} \end{pmatrix} \]Now we want to recover the solution, i.e., we want to reconstruct a state $\ket{\psi}$ and operator $A$ such that e.g. $\bra{\psi}A^3\ket{\psi} = \mean{A^3}$.

The first step is to construct a matrix $K$ such that $M = K^\dagger K$. This can always be done since $M$ is positive semidefinite, one can for example take the square root of $M$. That’s a terrible idea, though, because $M$ is usually rank-deficient, and using the square root will generate solutions with unnecessarily large dimension. To get the smallest possible solution we compute $M$’s eigendecomposition $M = \sum_i \lambda_i \ketbra{m_i}{m_i}$, and define $K = \sum_{i;\lambda_i > 0} \sqrt{\lambda_i}\ketbra{i}{m_i}$. Of course, numerically speaking $\lambda_i > 0$ is nonsense, you’ll have to choose a threshold for zero that is appropriate for your numerical precision.

If we label each column of $K$ with an operator sequence, i.e., $K = (\ket{\id}\ \ket{A}\ \ket{A^2})$, then their inner products match the elements of the moment matrix, i.e., $\langle \id | A^2 \rangle = \langle A | A \rangle = \mean{A^2}$. This means that we can take $\ket{\psi} = \ket{\id}$, and our task reduces to constructing an operator $A$ such that
\begin{gather*}
A\ket{\id} = \ket{A} \\
A\ket{A} = \ket{A^2} \\
A = A^\dagger
\end{gather*}This is clearly a linear system, but not in a convenient form. The most irritating part is the last line, which doesn’t even look linear. We can get rid of it by substituting it in the first two lines and taking the adjoint, which gives us
\begin{gather*}
A\ket{\id} = \ket{A} \\
A\ket{A} = \ket{A^2} \\
\bra{\id}A = \bra{A} \\
\bra{A}A = \bra{A^2}
\end{gather*}To simplify things further, we define the matrices $S_A = (\ket{\id}\ \ket{A})$ and $L_A = (\ket{A}\ \ket{A^2})$ to get
\begin{gather*}
A S_A = L_A \\
S_A^\dagger A = L_A^\dagger
\end{gather*}which is nicer but not quite solvable. To turn this into a single equation I used my favourite isomorphism, the Choi-Jamiołkowski. Usually it’s used to represent superoperators as matrices, but it can also be used one level down to represent matrices as vectors. If $X$ is a $m \times n$ matrix, its Choi representation is
\[ |X\rangle\rangle = \id_n \otimes X |\id_n\rangle\rangle,\] where
\[ |\id_n\rangle\rangle = \sum_{i=0}^{n-1}|ii\rangle. \] This is the same thing as the vec function in col-major programming languages. We also need the identity
\[ |XY\rangle\rangle = \id \otimes X |Y\rangle\rangle = Y^T \otimes \id |X\rangle\rangle \] with which we can turn our equations into
\[ \begin{pmatrix} S_A^T \otimes \id_m \\
\id_m \otimes S_A^\dagger \end{pmatrix} |A\rangle\rangle = \begin{pmatrix} |L_A\rangle\rangle \\
|L_A^\dagger\rangle\rangle \end{pmatrix} \]where $m$ is the number of rows of $S_A$. Now the linear system has the form $Ax = b$ that any programming language can handle. In Julia for instance you just do A \ b .

It’s important to emphasize that a solution is only guaranteed to exist if the vectors come from a moment matrix coming from a full level of the hierarchy. And indeed we can find a counterexample when this is not the case. If we were dealing instead with
\[ M = \begin{pmatrix} \mean{\id} & \mean{A} & \mean{B} & \mean{BA} \\
& \mean{A^2} & \mean{AB} & \mean{ABA} \\
& & \mean{B^2} & \mean{B^2A} \\
& & & \mean{AB^2A} \end{pmatrix} \] then a possible numerical solution for it is
\[ M = \begin{pmatrix} 1 & 1 & 0 & 0 \\
& 1 & 0 & 0 \\
& & 1 & 0 \\
& & & 1 \end{pmatrix} \] This solution implies that $\ket{\id} = \ket{A}$, but if we apply $B$ to both sides of the equation we get $\ket{B} = \ket{BA}$, which is a contradiction, as the solution also implies that $\ket{B}$ and $\ket{BA}$ are orthogonal.

I also want to show how to do it for the case of variables that are not necessarily Hermitian. In this case a full level of the hierarchy needs all the operators and their conjugates, so even level 2 is annoying to write down. I’ll do level 1 instead:
\[ M = \begin{pmatrix} \mean{\id} & \mean{A} & \mean{A^\dagger} \\
& \mean{A^\dagger A} & \mean{{A^\dagger}^2} \\
& & \mean{AA^\dagger} \end{pmatrix} \] The fact that this is a moment matrix implies that $\mean{A} = \overline{\mean{A^\dagger}}$, which is crucial for a solution to exist. As before we construct $K$ such that $M = K^\dagger K$, and label its columns with the operator sequences $K = (\ket{\id}\ \ket{A}\ \ket{A^\dagger})$. The equations we need to respect are
\begin{gather*}
A\ket{\id} = \ket{A} \\
A^\dagger \ket{\id} = \ket{A^\dagger}
\end{gather*} or more conveniently
\begin{gather*}
A\ket{\id} = \ket{A} \\
\bra{\id} A = \bra{A^\dagger}
\end{gather*} We use again the Choi-Jamiołkowski isomorphism to turn this into a single equation
\[ \begin{pmatrix} \ket{\id}^T \otimes \id_m \\
\id_m \otimes \bra{\id} \end{pmatrix} |A\rangle\rangle = \begin{pmatrix} \ket{A} \\
\overline{\ket{A^\dagger}} \end{pmatrix} \]and we’re done.

I’m happy to announce the 0.3 release of Ket.jl. This is a library to do quantum information and optimization in Julia; it is the second project I alluded to in my Julia post. The goal is to free the new generations from the tyranny of MATLAB by providing the means of production in a completely open-source ecosystem.

You might have noticed that this is version 0.3, and correctly assumed that this is not the first release. It has been publicly accessible, yet unannounced for 6 months already, due to a fundamental problem: it still needed MATLAB to do the crucial task of upperbounding Tsirelson bounds. This has now been solved by replacing the dependence Moment with Erik Woodhead’s package QuantumNPA. Another problem that has been solved is that the entire package is designed to be capable of handling arbitrary precision when desired. This was not possible with Moment due to MATLAB being an antediluvian abomination, but was rather easy with QuantumNPA.

With these problems solved, Ket is still far from ready, but it is in a good state to be built upon. Therefore I think this is a good moment to announce it publicly, to attract users and collaborators. I want to emphasize that this is a collaborative effort; I haven’t programmed even half of the library. It wouldn’t have happened without the invaluable help of Sébastien Designolle, Carlos de Gois, Lucas Porto, and Peter Brown.

We have written Ket to use it for our own research, but we’d like it to be useful for the wider community. Just not so wide that we don’t understand the code anymore. Also, we’d like to keep our focus on quantum information and optimization; we’re intentionally not including anything to do quantum simulation, since this job is already well taken care of by QuantumOptics.jl and QuantumToolbox.jl. We’re also not going to do quantum computing, that’s Yao.jl‘s job.

Now it’s time to hear from you: found a bug? Want a feature? Please open an issue in Github. Want to contribute some code? Also please open an issue to discuss it before doing a pull request.

My review about semidefinite programming was accepted by Reviews of Modern Physics. Great! After acceptance, nothing happens for three months. Then, the tragedy: we get the proofs. Now I understand what took them so long. It takes time to thoroughly destroy the text, the equations, and the references.

First, the text. They changed everything to US spelling, which is understandable. What is less understandable is that they eliminated any adjectives such as “unfortunate”, “convenient”, “brief”, “important”, “independent”, “natural”, “interesting”, etc. Apparently the text was not dry enough, they must scrub it clean of any hint that this review was written by human beings that might have opinions about the subject. For some mysterious reason, “above” is a forbidden word, only “aforementioned” is good enough. They also made a point of changing absolutely everything to passive voice, sentence structure or understandability be damned. This is just the tip of the iceberg, they have a huge list of arcane style rules that include not writing “we” in abstracts, not writing “e.g.” anywhere, replacing slashes with and/or, and some bizarre rule about hyphens that I don’t understand but ruined half of the hyphenated expressions5Quoting: “Please note that it is RMP style to remove the hyphen from an otherwise hyphenated adjectival compound when the compound follows the noun that it modifies.” Please don’t explain me what this means.. The result is that the text now reads as if it was written by a robot having an epileptic attack.

The worst part, though, is that they wantonly rewrote sentences in the middle of proofs of theorems, presumably because they felt their formulation was more elegant. The only disadvantage is that it made the proofs wrong. I would have thought it is obvious that you shouldn’t rewrite text that you don’t understand, but ignoring this is at least consistent with their pattern of breathtaking incompetence.

Second, the equations. LaTeX is not good enough for them. No, they use some proprietary abomination to typeset the paper for printing, and have some conversion script to map LaTeX into their format. Which will randomize the alignment of the equations, and rewrite every inline fraction $\frac{a}{b}$ as $a/b$. Which wouldn’t be so bad, if it didn’t change $\frac{a}{b}c$ to $a/b\,c$. But hey, what’s a little ambiguity next to conforming to the style rules?

Then, the bibliography. The pricks have some really strange rules about linking to the published versions only by DOIs, that somehow involve randomly removing some of the DOIs we had included, and removing links that are not DOIs. Such as the links to the solvers and libraries the readers can use to implement the algorithms we describe. Who would care about that, right? Certainly not the people who would read a review about SDPs?

As a bonus point, these morons still haven’t figured out Unicode in bloody 2024. Apparently é is their favourite glyph, so I work at the department of “Fésica”, Antonio Acín is sometimes named Acén, Máté Farkas is Mété Farkas, García-Sáez became Garcéa-Séez, Károly Pál is Kéroly Pél, and so on, and so on, and so on.

So no, I give up. I have neither the time nor the will to go through this huge review again and correct everything they fucked up. My intention was to just let it stay wrong, but thankfully I have a young and energetic co-author, Alex, who was determined to go through the review word-by-word and fix all the errors they introduced. The text can’t be fixed, though, as the mutilation there was intentional. So I’m officially writing off the APS version. The “published” version on the APS website will be the pile of shit that they wrote. The carefully written and typeset version that we wrote is the one on the arXiv.

In the future, I hope to never publish with APS again. My dream typesetting is the one done by Quantum, which is none at all. I don’t need to pay some ignorant to butcher my paper, nor do I need to waste my time putting Humpty Dumpty together again.

I’ve just finished writing yet another referee report. It’s not fun. It’s duty. Which got me wondering: am I doing my part, or am I a parasite? I get much more referee requests than I have the time to do, and I always feel a bit guilty to decline one. So the question has a practical implication, can I decline with a clear conscience, or should I grit my teeth and try to get more refereeing done?

To answer that, first I have to find out how many papers I have refereed. That’s impossible, I’m not German. My records are spotty and chaotic. After a couple of hours of searching, I managed to find 77 papers. These are certainly not all, but I can’t be missing much, so let’s stick with 77.

Now, I need to compute the refereeing burden I have generated. I have submitted 33 papers for publication, and each paper usually gets 2 or 3 referees. Let’s call it 2.5. Then the burden is 82.5, right? Well, not so fast, because my coauthors share the responsibility for generating this refereeing burden. Should I divide by the average number of coauthors then? Again, not so fast, because I can’t put this responsibility on the shoulders of coauthors that are still not experienced enough to referee. On the same light, I should exclude from my own burden the papers I published when I shouldn’t be refereeing. Therefore I exclude 3 papers. From the remaining 30, I count 130 experienced coauthors, making my burden $30*2.5/(130/30) \approx 17.3$.

Wow. That’s quite the discrepancy. I feel like a fool. I’m doing more than 4 times my fair share. Now I’m curious: am I the only one with such a unbalance, or does the physics community consists 20% of suckers and 80% parasites?

More importantly, is there anything that can be done about it? This was one of the questions that were discussed in a session about publishing in the last Benasque conference, but we couldn’t find a practicable solution. Even from the point of view of a journal it’s very hard to know who the parasites are, because people usually publish with several different journals, and the numbers of papers in any given journal is too small for proper statistics.

For example, let’s say you published 3 papers in Quantum, with 4 (experienced) coauthors on average, and each paper got 2 referee reports. This makes your refereeing burden 1.5. Now let’s imagine that during this time the editors of Quantum asked you to referee 2 papers. You declined them both, claiming once that you were too busy, and another time that it was out of your area of expertise. Does this make you a parasite? Only you know.

Let’s imagine then an egregious case, of someone that published 10 papers with Quantum, got 20 requests for refereeing from them, and declined every single one. That’s a $5\sigma$ parasite. What do you do about it? Desk reject their next submission, on the grounds of parasitism? But what about their coauthors? Maybe they are doing their duty, why should they be punished as well? Perhaps one should compute a global parasitism score from the entire set of authors, and desk reject the paper if it is above a certain threshold? It sounds like a lot of work for something that would rarely happen.

I’ve just been to the amazing Quantum Redemption conference in Sweden, organized by my friend Armin Tavakoli. I had a great time, attended plenty of interesting talks, and had plenty of productive discussions outside the talks as well. I’m not going to write about any of that, though. Having a relentlessly negative personality, I’m going to write about the talk that I didn’t like. Or rather, about its background. The talk was presenting some developing ideas and preliminary results, it was explicitly not ready for publication, so I’m not going to publish it here2I have presented several times preliminary results that included embarrassing mistakes, and I’d be mortified if someone in the audience decided to preserve them for posterity.. But the talk didn’t make sense because its background doesn’t make sense, and that is well-published, so it’s fair game.

I’m talking about the paper Quantum mechanics and the covariance of physical laws in quantum reference frames by my friends Flaminia, Esteban, and Časlav. The basic idea is that if you can describe a particle in a superposition from the laboratory’s reference frame, you can just as well jump to the particle’s reference frame, from which the particle is well-localized and the laboratory is in a superposition. The motivations for doing this are impeccable: the universality of quantum mechanics, and the idea that reference frames must be embodied in physical systems. The problem is that you can’t really attribute a single point of view to a superposition.

By linearity, the members of a superposition will evolve independently, so why would they have a joint identity? In general you affect some members of a superposition without affecting the others, there is no mechanism transmitting information across the superposition so that a common point of view could be achieved. The only sort of “interaction” possible is interference, and that necessitates erasing all information that differentiates the members of the superposition, so it’s rather unsatisfactory.

In any case, any reference frame worth of the name will be a complex quantum system, composed of a huge amount of atoms. It will decohere very very quickly, so any talk of interfering a superposition of reference frames is science fiction. Such gedankenexperimente can nevertheless be rather illuminating, so I’d be curious about how they describe a Wigner’s friend scenario, as there the friend is commonly described as splitting in two, and I don’t see a sensible of attributing a single point of view to the two versions. Alas, as far as I understand their quantum reference frames formalism was not meant to describe such scenarios, and as far as I can tell they have never done so.

This is all about interpretations, of course. Flaminia, Esteban, and Časlav are all devout single-worlders, and pursue with religious zeal the idea of folding back the superpositions into a single narrative. I, on the other hand, pray at the Church of the Larger Hilbert Space, so I find it heresy to see these highly-decohered independently-evolving members of a superposition as anything other than many worlds.

People often complain that all this interpretations talk has no consequences whatsoever. Well, here is a case where it unquestionably does: the choice of interpretation was crucial to their approach to quantum reference frames, which is crucial to their ultimate goal of tackling quantum gravity. Good ideas tend to be fruitful, and bad ideas sterile, so whether this research direction ultimately succeeds is an indirect test of the underlying interpretation.

You might complain that this is still on the metatheoretical level, and is anyway just a weak test. It is a weak test indeed: the Big Bang theory was famously created by a Catholic priest, presumably looking for a fiat lux moment. Notwithstanding its success, I’m still an atheist. Nevertheless, weak evidence is still evidence, and hey, if you don’t like metaphysics interpretations are really not for you. If you do like metaphysics, however, you might also be interest in metatheory ;)

A couple of days ago I finally released the first Julia project I had alluded to, a technique to compute key rates in QKD using proper conic methods. The paper is out, and the github repository is now public. It’s the first paper from my new research group in Valladolid, and I’m very happy about it. First because of the paper, and secondly because now I have students to do the hard work for me.

The inspiration for this paper came from the Prado museum in Madrid. I was forced to go there as a part of a group retreat (at the time I was part of Miguel Navascués’ group in Vienna), and I was bored out of my mind looking at painting after painting2Don’t get me wrong, the museum has some interesting stuff like Goya’s The Executions, but if you go there you realize that the vast majority of painting consisted of rich people paying to get their own portraits. I don’t have a problem with that, it’s just not interesting to look at. Oh, a 17th century nobleman. Oh, another 17th century nobleman. Now a 18th century noblewoman. Argh.. I then went to the museum cafe and started reading some papers on conic optimization to pass the time. To my great surprise, I found out that there was an algorithm capable of handling the relative entropy cone, and moreover it had already been implemented in the solver Hypatia, which to top it off was written in Julia! Sounded like Christmas had come early. ¿Or maybe I had a jamón overdose?

Life wasn’t so easy, though: the relative entropy cone was implemented only for real matrices, and the complex case is the only one that matters2One day I need to find out why mathematicians, computer scientists, and engineers seem to have such an allergy of complex numbers. Only physicists seem to care about them. And it’s not even our fault, the guy who invented Hermitian matrices, Charles Hermite, was a mathematician, not a physicist.. I thought no problem, I can just do the generalization myself. Then I opened the source code, and I changed my mind. This cone is a really nasty beast. The PSD cone is a child’s birthday in comparison. I was too busy with other projects at the time to seriously dedicate to it, so I wrote to the developers of Hypatia, Chris Coey and Lea Kapelevich, asking whether they were interested in doing the complex case. And they were! I just helped a little bit with testing and benchmarking.

Now I can’t really publish a paper based only on doing this, but luckily the problem turned out to be much more difficult: I realized that the relative entropy cone couldn’t actually be used to compute key rates. The reason is somewhat technical: in order to solve the problem reliably one cannot have singular matrices, it needs to be formulated in terms of their support only (the technical details are in the paper). But if one reformulates the problem in terms of the support of the matrices, it’s no longer possible to write it in terms of the relative entropy cone3This is not the case for the positive semidefinite cone; if you reformulate the problem in terms of the support of the matrices it is still an SDP, and life is easy..

I had to come up with a new cone, and implement it from scratch. Now that’s enough material for a paper. To make things better, by this time I was already in Valladolid, so my students could do the hard work. Now it’s done. ¡Thanks Andrés, thanks Pablo, thanks Miguel!

I’m quite happy, this is pretty much the best grant available in Spain, it gives me a lot of money for 5 years, including a PhD student and a postdoc. But the reason I’m posting about it here is to share some information about the grant system that I believe is not widely known.

My grant proposal was evaluated with 98.73 points out of 100. Sounds very high, until you learn that the cutoff was 97.27. I sincerely believe that my grant proposal was excellent and deserved to be funded, as self-serving as this belief may be, but I can’t believe there was a meaningful difference between my proposal and one that got 97 points. There was clearly too many good proposals, and the reviewers had to somehow divide a bounded budget between them. I think it’s unavoidable that the result is somewhat random.

I have been on the other side before: I’ve had before grants that had been highly evaluated and nevertheless rejected. I think now I can say that it was just bad luck. I have also been on the reviewing side: twice I received some excellent grants to evaluate, and gave very positive evaluations to them, sure that they would be funded. They weren’t.

Everyone that has applied to a grant knows how much work it is, and how frustrating is it to be rejected after all. Still, one should keep in mind that rejection doesn’t mean you are a bad researcher. It is the norm, there’s just way too little money available to fund everyone that deserves it.