Journée "Fondements de la physique quantique"
Discussion Day "Foundations of Quantum Physics"

Final Programme Final

Quantum physics in the first person, present tense (Michel Bitbol)
Does Quantum Physics offer a (complete or incomplete) representation of the “real world out there”? A powerful critique of the representational conception of knowledge has been formulated in biology by the autopoietic theory of living beings, in the cognitive science by the theory of enaction, and in philosophy by phenomenology. These three disciplines approach knowledge from the standpoint of the knower, namely in the first person. And they show how the illusion of a representation is generated by the process of anticipation and adaptation of the knower that stands for “knowledge”. Quantum physics might well be a case wherein the classical representational conception of knowledge has been defeated, and the first-person foundation of knowledge has been laid bare. This was already suggested by Bohr, and more explicitly claimed by several contemporary physicists, such as David Mermin and Christopher Fuchs in their “QBist” interpretation of quantum physics. Here, I will show that an extreme first-personal conception of quantum knowledge (not only from the standpoint of the knower, but rather from the standpoint of her present state), easily makes sense of standard “paradoxes” of quantum physics. It also accommodates effortlessly the non-standard uses of “generalized quantum theory” in the human sciences.



De Broglie pilot-wave theory and randomness (Aurélien Drezet)
During this talk  we will review a old and serious problem of the standard de Broglie-Bohm approach which is how to justify the probability rules of quantum mechanics (the so called Born's rule) from a purely deterministic dynamics.


Interaction between two walkers and de Broglie double solution (Thomas Durt)
Recently*, we elaborated a theoretical model according to which  elementary particles and/or droplets are represented by solitonic solutions of a non-linear equation, in the mind of de Broglie's double  solution program**. The essential ingredient of our model is the factorisation ansatz according to which the wave describing the walker  (and/or particle) and its environment factorizes into the product of a pilot wave associated to the environment with a peaked solitonic wave  associated to the walker (and/or particle). This model originally aimed at describing elementary particles, but it appeared that it  could also be applied in order to describe the phenomenology of  walkers*,**. In the case of elementary particles, our model allowed us to predict  the appearance of a pseudo-gravitational interaction similar to  Newton's gravity. In the case of droplets, we predicted that the  interaction potential between two droplets is proportional to the  Helmholtz Green function, which implies that attractive and repulsive  interactions are present, which appeared to provide a good agreement  with the experimental observations*. Memory also plays a role in our  approach because we showed** that in order to satisfy the de  Broglie-Bohm guidance  equation, it is necessary that the intrinsic  movement of the soliton possesses a stochastic contribution (in agreement with what de Broglie called the ``hidden thermostat''  hypothesis) a possibility explored in the 50's by Bohm and Vigier. The scope of our talk is to provide a survey of these ideas and to  briefly address some open questions like e.g. ``Is it correct to  describe a bouncing droplet by a soliton?'', ``How to falsify  experimentally our model?'', or ``To which extent is it necessary to  invoke quantum mechanics in this approach?'' and so on.
*T. Durt, Generalized guidance equation for peaked quantum solutons  and effective gravity, EPL 114 (2016) 10004.
**L. de Broglie's double solution and self-gravitation, Annales de la  Fondation Louis de Broglie, Volume 42, special issue (2017) 43,  http://aflb.ensmp.fr/AFLB-421/table421.htm.

Recovering the quantum formalism from physically realist axioms (Philippe Grangier and Alexia Auffèves).
We present a heuristic derivation of Born's rule and unitary transforms in Quantum Mechanics, from a simple set of axioms built upon a physical phenomenology of quantization [1, 2]. This approach naturally leads to the usual quantum formalism, within a new realistic conceptual framework that is discussed in details. Physically, the structure of Quantum Mechanics appears as a result of the interplay between the quantized number of "modalities" accessible to a quantum system, and the continuum of "contexts" that are required to define these modalities. Mathematically, the Hilbert space structure appears as a consequence of a specific "extra-contextuality" of modalities, closely related to the hypothesis of Gleason's theorem, and consistent with its conclusions.
Refs.
[1] A. Auffeves and P. Grangier, Found. Phys. 46, 121 (2016)  https://arXiv.org/abs/1409.2120
[2] A. Auffeves and P. Grangier, Sci. Rep. 7, 43365 (2017)  https://arXiv.org/abs/1610.06164.

Experimental measurements of bounds on higher-order interferences (Thomas Kauten)

Thomas Kauten, Robert Keil, Thomas Kaufmann, Benedikt Pressl, Sebastian Gstir, Christoph Dittel, Rene Heilmann, Toni Eichelkraut, Alexander Szameit, Caslav Brukner & Gregor Weihs
Quantum mechanics is a fundamental theory to describe the physics of microscopic objects, but there are still questions whether this theory is complete or not. If the answer to this question is no, axioms could be violated. One of the fundamental axioms of quantum mechanics is Born’s rule, which claims that the description of nature is probabilistic, i.e. P(r,t) = |Ψ(r,t)|2. Combined with the quantum mechanical superposition of wavefunctions this results in interference terms, which contain all the possible pairings of the states in the superposition, but do not allow higher-order interference terms with more than two constituents.
In this work, we will present results of improved versions of an experiment, which was proposed by Sorkin in 1994 [1], and was experimentally implemented for the first time in 2010 [2], with the goal of putting a bound on the potential magnitude of higher-order interference terms. For this we employed different types of switchable multi-path interferometers (a free-space bulk optical [3], a hybrid bulk-integrated [4] and a fully integrated interferometer) in different measurement regimes (classical, semi-classical and quantum). Improved power and phase stabilization and increased throughput enables us to provide an upper bound on the potential magnitude of these higher order interference terms which is two orders of magnitude smaller compared to previous works [5]. The discussion of our experiment also contains the characterization and compensation of systematical errors in our measurement setup, such as the detector nonlinearity [6]. Additionally, we will use our integrated interferometers in the future to test for the possibility of quantum mechanical wavefunctions based on quaternions or octonions rather than complex numbers [7].
Our experiments confirm quantum mechanics by ruling out higher order interference terms to an extent that is more than four orders of magnitude smaller than the expected pair wise interference.
Refs.
[1] R. D. Sorkin, “Quantum mechanics as quantum measure theory”, Mod. Phys. Lett. A 9, 3119-3128 (1994).
[2] U. Sinha, C. Couteau, T. Jennewein, R. Laflamme, and G. Weihs, “Ruling Out Muti-Order Interference in Quantum Mechanics”, Science 329, 418-421 (2010).
[3] T. Kauten, R. Keil, T. Kaufmann, B. Pressl, C. Brukner, and G. Weihs, “Obtaining tight bounds on higher-order interferences with a 5-path interferometer”. New J. Phys., 19, 033017 (2017).
[4] R. Keil, T. Kaufmann, T. Kauten, S. Gstir, C. Dittel, R. Heilmann, A. Szameit, and G. Weihs, “Hybrid waveguide-bulk multi-path interferometer with switchable amplitude and phase”. APL Photonics, 1, 081302 (2016).
[5] I. Söllner, B. Gschösser, P. Mai, B. Pressl, Z. Vörös, and G. Weihs, “Testing Borns Rule in Quantum Mechanics for Three Mutually Exclusive Events”, Found. Phys. 42, 742-751 (2012).
[6] T. Kauten, B. Pressl, T. Kaufmann, and G. Weihs, “Measurement and modeling of the nonlinearity of photovoltaic and Geiger-mode photodiodes”, Rev. Sci. Instrum. 85, 063102 (2014).
[7] A. Peres, “Proposed Test for Complex versus Quaternion Quantum Theory”, Phys. Rev. Lett. 42, 683-686 (1979).

Can the properties of a quantum system vanish ? (Alex Matzkin)
The Weak Measurements approach is a framework based on implementing non-destructive, non-perturbing measurements on a quantum system as the system evolves from an initially prepared state to a final state obtained by performing a standard measurement. The result of a weak measurement is called the “weak value” of the weakly measured observable. Weak values have peculiar features. In particular, a vanishing weak value means that the corresponding system property cannot be found at the location where the observable was weakly measured. This may have striking consequences, like not being able to detect a particle inside an interferometer although it went in and out, or observing a spatial separation between a particle and one of its properties. I will describe situations in which quantum properties appear to “vanish” and I will put forward an explanation based on the analysis of null weak values.
Refs.
Q. Duprey and A. Matzkin Phys. Rev. A 95, 032110 (2017)
Q. Duprey, S. Kanjilal, U. Sinha, D. Home and A. Matzkin, arXiv:1703.02959 (2017)