Speckle patterns

Speckle patterns are granular intensity patterns that result from the random interference of light. Such patterns appear whenever coherent light is randomised, for example when a laser beam reflects off a rough surface.

This is quite a universal phenomenon, although not easily observable under ordinary conditions due to the lack of coherence of natural light. Since the invention of lasers, however, the production of speckle patterns has become extremely simple, and a whole field of optics has emerged around it, particularly in the domain of metrology.

That was the subject of my PhD, where I did lots of interresting things, like measuring displacements 10 times smaller than an atom, or measuring variations in air density of 1 part in a billion! Find out more in my publications.

Arsenical books

Recently I have been involved in a more applied and quite fun project: detecting Arsenic in old books. In the 1800s, bookbinders used a pigment, Emerald Green, for its deep vibrant green colour. However, Emerald Green contains Arsenic, which is highly toxic. It's not that they didn't know: they also used Emerald Green as an insecticide, at the same period! This pigment was also used in clothes and in wall paints. Lots of theater actors died from it, from which people inferred a curse. It might even be what killed Napoleon, who liked the colour so much that he painted parts of his house with it.

The presence of arsenic in book covers has been known for a long time, but only recently has it been recognised as a serious health and safety issue. Big national libraries in Germany and France have started to quanrantine thousands of potentially contaminated books, mainly based on visual identification and bibliographic data. However, there is currently no efficent way to instrumentally detect the presence of arsenic. The exsisting techniques (X ray fluorescence or Raman spectroscopy) are slow, require expensive equipment, specialised personel, and data analysis.

My team and I have develloped an optical technique to solve these issues. We implemented this in a prototype that does the analysis automatically and gives a result in about 10 ms. This technique is less accurate than XRF and raman, but identifies toxic books with 100% true positive rate, 0% false negative, and 10% false positive, which is suitable for large scale testing in libraries. Publication in progress!

Quantum Quack

This is a personal project I have been working on with my friend Rafael Kollyfas. Quantum Quack is a website and iOS app that allows one to make quantum choices. It picks one output in a list of options you provide, using quantum random numbers, rather than ordinary random numbers. This has fascinating implications that vary depending on the different interpretations of quantum mechanics. Here is a summary for the two main ones:

- The Copenhagen interpretation.
This is the "textbook" interpretation, often (somewhat wrongly) considered the most consesual one. In this interpretation, quantum phenomena are fundamentally random. Ordinary random phenomena (such as a random number generator, throwing a dice, etc...) are not fundamentally random, their randomness is only emergent from their high sensitivity to initial conditions. All "ordinary" phenomena, in principle, can be predicted in advance, and randomness is only an illusion. Only quantum phenomena are fundamentally random. However, qantum phenomena never influence the course of things at human scale, because they are confined to the microscopic world. The course of our lifes is entirely determined by the initial conditions of the universe, just like the dice, even you reading this text right now. Therefore, quantum quack allows you to promote a single quantum event from the microscopic to the macroscopic scale, which allows you to escape determinism.

- The Everett interpretation.
The Everett (or many-worlds) interpretation is an alternative interpretation that is actually the most popular one among reseacher working in the foundations of physics. It is often caricatured in popular science, but is actually founded on solid theoretical gounds. In this interpretation, each quantum event splits the wavefunction of the universe into new terms, each of them behaving just as an independent universe, in each of which one possible output of the event occured. The consequences of using quantum quack in this interpretation is, to my opinion, even more fun. If you use it to chose a restaurant for tonight in a list of options, you will experience one of them, but you can find pleasure in the thought that you will also be going to all the other ones in the other branches of the wavefunction. If you use it to play to the lottery, the wave function of the universe will split in as many branches as there are of possible number combinations, and there is one in which you will be a milionaire, with a 100% certainty.

Quantum Quack a website, as well as an iOS app (soon available on other platforms). I host the Privacy Policy here.

The app contains a novelty in the way to generate quantum random numbers, which is by using the smartphone's camera. Indeed, the interaction of light with each pixel of a camera is quantum mechanical by nature, and enough so to produce outputs that are dominated by this quantum interaction. When prompted, the app takes a picture that is fed into an algorithm that selects one of the provided options, in a way that is dominated by the quantum randomness contained in the pixel values.

M. Facchin, S. Khan, K. Dholakia, and G. D. Bruce, “Determining intrinsic sensitivity and the role of multiple scattering in speckle metrology”, Nature Reviews Physics (2024)

M. Facchin, G. D. Bruce, and K. Dholakia, “Measuring picometre-level displacements using speckle patterns produced by an integrating sphere”, Scientific Reports (2023)

M. Facchin, G. D. Bruce, and K. Dholakia, “Measurement of variations in gas refractive index with 10−9 resolution using laser speckle”, ACS Photonics (2022), highlighted in Phys.org

M. Facchin, K. Dholakia, and G. D. Bruce, “Wavelength sensitivity of the speckle patterns produced by an integrating sphere”, J. Phys. Photonics (2021), In J. Phys. Photonics, highlighted in Highlights of 2021 collection

MM. Facchin, G. D. Bruce, and K. Dholakia, “Speckle-based determination of the polarisation state of single and multiple laser beams”, OSA Continuum (2020), editor’s Pick

G. D. Bruce, L. O’Donnell, M. Chen, M. Facchin, and K. Dholakia, “Femtometer- resolved simultaneous measurement of multiple laser wavelengths in a speckle wavemeter”, Opt. Lett. (2020)

A. L. Ariste and M. Facchin, “Superoscillations in solar MHD waves and their possible role in heating coronal loops”, Astronomy & Astrophysics (2018), with a CNRS press release

Contact me at morgan.facchin@icloud.com

Privacy Policy

Welcome to Quantum Quack. This Privacy Policy explains how we collect and use information when you use our mobile application.

1. Information We Collect

We do not collect any information. The app only uses your device’s camera to capture images in real-time to generate quantum random numbers. The images are processed within the app and are not stored or transmitted to any external servers. The options you enter are not recorded.

2. How We Use Information

The images captured by the device’s camera are used solely for the purpose of generating quantum random numbers. These images are processed locally on your device and are not shared or stored beyond the immediate use.

3. Data Sharing and Disclosure

Quantum Quack does not share, sell, or distribute any user data with third parties. All processing occurs locally on your device, and no data is transmitted to external servers.