quantum computing reproducibility crisis?

While standing in a train to my mother’s house in Brittany, I was catching up on earlier Nature issues and came upon this April issue where, following the retraction of a Nature paper on the topic, Sergey Frolov casts doubt on the possible detection of a new type of quantum particle, the Majorana fermion, whose existence still remains inconclusive. The criticism concentrates on the data analysis of signals where the appearance of a narrow peak should support the hypothesised existence. The article is interesting (to me) as a reflection of someone having published positive, then negative articles on the topic, upon the tendency for authors in the field to cherry-pick experiments where some peaks occur. Among dozens or hundred of experiments where they did not. And calling for open data and more stringent review(er)s on the matter (and others). The arguments in the opinion tribune sound most reasonable but I wonder whether or not other particle physicists share the same concern.

trip to München

While my train ride to the fabulous De Gaulle airport was so much delayed that I had less than ten minutes from jumping from the carriage to sitting in my plane seat, I handled the run through security and the endless corridors of the airport in the allotted time, and reached Munich in time for my afternoon seminar and several discussions that prolonged into a pleasant dinner of Wiener Schnitzel and Eisbier.  This was very exciting as I met physicists and astrophysicists involved in population Monte Carlo and parallel MCMC and manageable harmonic mean estimates and intractable ABC settings (because simulating the data takes eons!). I wish the afternoon could have been longer. And while this is the third time I come to Munich, I still have not managed to see the centre of town! Or even the nearby mountains. Maybe an unsuspected consequence of the Heisenberg principle…

Le Monde puzzle [#849]

A straightforward Le Monde mathematical puzzle:

Find a pair (a,b) of integers such that a has an odd number d of digits larger than 2 and ab is written as 10^d+1+10a+1. Find the smallest possible values of a and of b.

I ran the following R code

d=3
for (a in 10^(d-1):(10^d-1)){
  c=10^(d+1)+10*a+1
  if (a*trunc(c/a)==c)
       print(c(a,c))}

which produced a=137 (and b=83) as the unique case. For d=4, I obtained a=9091 and b=21, for d=6, a=909091, and b=21, for d=7, a=5882353 and b=27, while for d=5, my code did not return any solution. While d=8 took too long to run, a prime factor decomposition of 10⁹+1 leads to (with the schoolmath R library)

> for (d in 3:10) print(c(d,prime.factor(10^(d+1)+1)))
[1]   3  73 137
[1]    4   11 9091
[1]    5  101 9901
[1]      6     11 909091
[1]       7      17 5882353
[1]     8     7    11    13    19 52579
[1]     9   101  3541 27961
[1]   10   11   11   23 4093 8779

which gives a=52631579 and b=29 for d=8 and also explains why there is no solution for d=5. The corresponding a has too many digits!

This issue of Le Monde Science&Médecine leaflet had more interesting entries, from one on “LaTeX as the lingua franca of mathematicians”—which presumably made little sense to any reader unfamiliar with LaTeX—to the use of “big data” tools (like news rover) to analyse data produce by the medias, to  yet another tribune of Marco Zito about the “five sigma” rule used in particle physics (and for the Higgs boson analysis)—with the reasonable comment that a large number of repetitions of an experiment is likely to exhibit unlikely events, and an also reasonable recommendation to support “reproduction experiments” that aim at repeating exceptional phenomena—, to a solution to puzzle #848—where the resolution is the same as mine’s, but mentions the principle of Dirichlet’s drawers to exclude the fact that all prices are different, a principle I had never heard off…

Alésia sunset

Mark Girolami came on Monday for a short visit at CREST this week, to discuss further the Russian roulette with Nicolas and I (and evacuate some of my “worries”), exploit the potential links with vanilla Rao-Blackwellisation, and look at other directions of common interest. In the conversation, we spent a while pondering about the “sign problem”, namely the difficulty with signed unbiased estimates of positive normalising constants. Quickly bumping into the impossibility of simulating from a negative density. Not that we had high expectations of solving in a single afternoon an NP hard problem, and one of the major unsolved problems in the physics of many-particle systems… Although Mark had made the “mistake” of picking a Monday for his visit, reducing considerably the potential for wine bars and great restaurants in the area, we undertook to play Russian roulette with sea-shells, at a brasserie in the shadow of Alésia church, without any of us being hit by a bacterial bullet. (Mark then played the Parisian roulette by biking back to the north of Paris and his hotel, again managing to foil the automotive bullet!)

Monte Carlo workshop (Tage 1 & 2)

Gathering with simulators from other fields (mostly [quantum] physicists) offers both the appeal of seeing different perspectives on simulation and the diffiulty of having to filter alien vocabulary and presentation styles (generally assuming too much background from the audience). For instance; while the first talk on Tuesday by Gergely Barnaföldi about using GPUs for simulation was quite accessible, showing poor performances of the (CPU based) Mersenne twister., when using Dieharder as the evaluator. (This was in comparison with GPU-based solutions.) This provided an interesting contrapoint to the (later) seminar by Frederik James on random generators. (Of course, I did have some preliminary background on the topic.)

On the opposite, the second talk by Stefan Schäfer involved hybrid Monte Carlo methods but it took a lot of efforts (for me) to translate back to my understanding of the notion, gathered from this earlier Read Paper of Girolami and Calderhead, with the heat-bath and leapfrog algorithms. One extreme talk in this regard was William Lester’s talk on Wednesday morning on quantum Monte Carlo and its applications in computational chemistry where I could not get past the formulas! Too bad because it sounded quite innovative with notions like variational Monte Carlo and diffusion Monte Carlo… Nice movies, though. On the other hand, the final talk of the morning by Gabor Molnar-Saska on option pricing was highly pedagogical, defining everything and using simple examples as illustrations. (It certainly did not cure my misgivings about modelling the evolution of stock prices via pre-defined diffusions like Black-and-Scholes’, but the introduction was welcome, given the heterogeneity of the audience.) Both talks on transportation problems were also more accessible (maybe because they involved no pysics!)

The speakers in the afternoon sessions of Wednesday also made a huge effort to bring the whole audience up-to-date about their topic, like protein folding and high-energy particle physics (although everyone knows about the Higgs boson nowadays!). And ensemble Kalman filters (x2). In particular, Andrew Stuart did a great job with his simulation movies. Even the final talk about path-sampling for quantum simulation was mostly understandable, at least the problematic of it.  Sadly, at this stage, I still cannot put a meaning on “quantum Monte Carlo”… (Incidentally, I do not think my own talk reached much of the audience, missing convincing examples I did not have time to present:)