Many-particle interference

Four photons interfere in a lattice of optical waveguides 
Four photons interfere in a lattice of optical waveguides

Our research is focused on the collective behaviour of identical particles, such as photons with equal colour and polarisation. When such particles impinge on a scattering object, e.g. a multi-mode beam splitter with a finite number of input and output channels, they get spatially redistributed in a manner which depends on both, the structure of the scattering object (such as material properties and symmetries in the design) as well as the type of the particles (whether they are bosons or fermions) and their degree of indistinguishability (how “identical” they are). Usually, the particles do not scatter individually, but exhibit a collective dynamics, called many-particle interference, which arises from the laws of quantum mechanics for identical particles. Calculating the probabilities for all possible output configurations becomes highly non-trivial for an increasing number of particles and input/output modes, as all possible paths of each particle contribute to each configuration. We investigate for which symmetries of the scattering material and the input configuration the calculation can be simplified and so-called suppression laws can be derived. These are relatively simple formulas predicting which output configurations of the particles are ruled out by the symmetry, regardless of other details of the scattering object or the particular distribution of particles at the input.

Semiconductor microcavity with resonant excitation of quantum dots and light collection from the top

Semiconductor microcavity with resonant excitation of quantum dots and light collection from the top

We conduct our research by analytical calculations, numerical simulations as well as photonic experiments. On the theoretical side, we have a close collaboration with the group of Andreas Buchleitner from the University of Freiburg in Germany. On the experimental side, we employ quantum light sources such as parametric fluorescence in nonlinear crystals as well as resonant fluorescence from semiconductor quantum dots. In particular, we make use of the latest GaAs quantum dots in microcavities fabricated by our national research partner Armando Rastelli at the University of Linz. Compared to parametric fluorescence, these light sources carry the advantage of a better controllable number of emitted photons per pump cycle, which should permit a higher contrast of the interference signals. As scattering material we use femtosecond-laser written optical waveguides in glass, which are produced by our collaborator Alexander Szameit at the University of Rostock in Germany.  

We expect that our work will substantially contribute to researchers’ understanding of the impact of symmetries on many-particle interference. Besides the inherent fundamental interest, these results may be useful in the long term for verifying the correct operation of quantum protocols (such as the Boson sampling machines investigated by many other groups) or provide tools for improving the quality and efficiency of information or energy transfer in artificial structures. Finally, our quantum dot devices are also sources of entangled photon pairs of high purity and therefore we investigate their capacity to serve as future light sources for quantum communication, quantum metrology and optical quantum computing. For more details, please have a look at our group’s research on entangled light from quantum dots.


Participating researchers

Robert Keil (Principal Investigator), Christoph Dittel, Julian MünzbergMaximilian Prilmüller, Lukas Kirchner, Gregor Weihs


FWF stand-alone project P 30459 “Many-particle interference in symmetric scattering scenarios” (Robert Keil & Armando Rastelli)

DOC-Fellowship of the Austrian Academy of Sciences “Symmetric Many-Body Quantum Interferences” (Christoph Dittel)


03.04.2018 - Julian Münzberg joins our team as a PhD student. 

22.01.2018 - All known suppression laws can be integrated into a single formalism and the suppression can be clearly linked to mode-permutation symmetries in the scattering unitary. We describe our findings in detail in two papers, which we have just submitted: C. Dittel et al., Totally Destructive Many-Particle Interference arXiv: 1801.07014 and C. Dittel el al., Totally Destructive Interference for Permutation-Symmetric Many-Particle States arXiv: 1801.07019.   

22.12.2017 - Together with the group of Andreas Buchleitner, we investigated how interactions and distinguishability influence the interference conditions at a double-well potential: G. Dufour et al., Many-particle interference in a two-component bosonic Josephson junction: an all-optical simulation, New J. Phys. 19, 125015 (2017), DOI: 10.1088/1367-2630/aa8cf7

06.02.2017 - We found a new suppression law for many-particle interference on hypercube graphs. Please see our publication in Quantum Science and Technology for details: C. Dittel, R. Keil, G. Weihs, Many-body quantum interference on hypercubes, Quantum Sci. Technol. 2, 015003 (2017), DOI: 10.1088/2058-9565/aa540c