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. In addition, we investigate the rich dynamics offered by collective phases of multiple particles.

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 experimentally probe these scenarios with up to four photons, prepared in the desired indistinguishability configuration. On the theoretical side, we have a close collaboration with the group of Andreas Buchleitner from the University of Freiburg in Germany. Our photons are produced by quantum light sources such as spontaneouos parametric down-conversion (SPDC) 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 beam spllitters we use bulk optics, fibers as well as laser-written optical waveguides, which are produced by our collaborator Alexander Szameit at the University of Rostock in Germany.  We further collaborate with Michał Karpiński from the University of Warsaw on matching the spectra of photons from SPDC to those from quantum dots as well as Alessandro Fedrizzi from Heriot-Watt University, Edinburgh, UK on domain-engineered crystal sources achieving record indistinguishability values.

We expect that our work will substantially contribute to researchers’ understanding of the inner workings of many-particle interference. Besides the inherent fundamental interest, these results can 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 photonic 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), Tommaso Faleo, Luis Matheis, Michael Schlosser, Gregor Weihs


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

Master Thesis

We are looking for talented and motivated Master students (internal or external) to work with us on the project. Please contact us, if you are interested.


12.10.2023 -  We have experimentally demonstrated a new multi-particle interference effect: Via engineered pairwise entanglement, N particles can exhibit a strong collective interference signal, which remains invisible if only subsets of the particles are detected. The results, which were obtained in close collaboration with the teams at Freiburg and Heriott-Watt, are presented in this arxiv paper.

01.06.2023 -  Luis Matheis joins our team as a PhD student investigating quantum dot - SPDC interfaces.

24.02.2023 -  Julian Münzberg successfully defends his PhD and moves on to Bernard Gruppe. It has been a pleasure working with you during the last years and good luck in your further career!

10.10.2022 -  Michael Schlosser joins our team as a Master student working towards improved spectral purities of our cone-SPDC source.

19.07.2022 -  We have successfully assembled and tested an active temporal-to-spatial mode demultiplexer, which is fast and efficient at the same time, and published the results in APL Photonics. We use the device to route a sequence of single photons emitted from a GaAs quantum dot into four separate spatial modes. This will become very useful in future many-particle interference experiments.

19.05.2022 -  Franz Draxl successfully defends his Master thesis (results published, see news item above) and moves on to Swarovski Optik. Thanks you and good luck in your further career!

07.09.2021 - Tommaso Faleo joins our team as a PhD student investigating collective multiparticle phases.

17.08.2020 - Franz Draxl joins our team as a Master student working towards temporal demultiplexing of subsequently emitted quantum dot photons.

25.05.2021 - As we show in our latest experiment, destructive many-particle interference can persist even in the presence of inter-particle distinguishability. It is the symmetry of the input state with respect to the unitary scattering network, which determines the interference conditions. Therefore, fully destructive interference can arise, as long as all symmetry-grouped photons remain indistinguishable. The results can be found in this work in PRX Quantum.

26.02.2021 - Together with Alexander Szameit's team we have experimentally investigated three-dimensional photonic quantum walks, where two spatial dimension plus the polarisation of the photons are utilized as degrees of freedom. Among many other cool things, these enable to probe the suppression law on hypercube graphs, which we had studied earlier. The experimental results have been published in Science Advances.

17.08.2020 - Franz Draxl joins our team as a Master student working towards temporal demultiplexing of subsequently emitted quantum dot photons.

09.10.2019 - Christoph Dittel has successfully finished his PhD in our group and has moved on to work with Andreas Buchleitner as a Postdoc. Congratulations and many thanks for everything, Christoph!

02.07.2019 - Robert has written a News & Views article in Nature Physics, reviewing an interesting work on Boson sampling in an integrated waveguide architecture by a team from the University of Bristol.

01.07.2019 - We say goodbye to Maximilian Prilmüller and Lukas Kirchner, who have moved on to the industry (Zumtobel and MED-EL, respectively). Thanks for your nice work and good luck in the future!

16.06.2018 - 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, which we had obtained in collaboration with the Freiburg group and Mattia Walschaers from Paris, in detail in two papers:

  • C. Dittel, G. Dufour, M. Walschaers, G. Weihs, A. Buchleitner, R. Keil, Totally Destructive Many-Particle Interference, Phys. Rev. Lett. 120, 240404 (2018), DOI: 10.1103/PhysRevLett.120.240404
  • C. Dittel, G. Dufour, M. Walschaers, G. Weihs, A. Buchleitner, R. Keil, Totally Destructive Interference for Permutation-Symmetric Many-Particle States, Phys. Rev A 97, 062116 (2018), DOI: 10.1103/PhysRevA.97.062116

The editors of PRA also picked the second paper as their suggestion.

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

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

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