The Quantum-Classical Interface

Quantum physics is one of the best founded and experimentally best verified theories of nature, ever conceived. However, it suffers from the apparent contradiction in interpretational issues around the nature of reality and locality.

How can we understand that microscopic objects must be described by superpositions of postions, momenta or energy states while we only perceive definite values in our everyday world? 

  • Is quantum decoherence the answer to why we observe a classical world? But at what point does a measurement outcome become 'truly' unique?
  • Is there an objective cut between quantum and classical physics and if so where should this be? Is it rather fundamental or technical?
  • Do wave functions collapse spontaeneously, every now and then and the faster the more massive a particle is? If so, how big would a particle have to be, for us to observe wave function collapse on finite time scales?
  • Is gravity an agent in this game? Do our quantum equations change for supermassive bodies in mesoscopically delocalized quantum states? 
  • How can we compare the macroscopicity of different quantum systems?


Quantum-Physics at the Interface to Biology and the Life Sciences

In our daily lives we often tend to make distinctions between inorganic and organic matter, the inanimate and the animate world or even non-conscious objects and conscious beings. Is this a distinction of complexity only? If so, can we still quantum delocalize proteins, DNA, viruses or cells?

These questions have triggered a long-term experimental journey in our lab, with even more questions:

  • It has been difficult to prepare neutral beams of size-selected massive particles in the mass range of 1000 − 1000000 amu. Life defines and selects the size and shape of biomolecules by their functionality. Can we exploit this ’nature-made nanotechnology’ for quantum experiments?
  • Cold quanta have been studied by many groups for many years. Can we get new insights in a complexity range that is closer to our daily lives, with molecules composed of many hundreds of covalently bound atoms and internal temperatures exceeding several hundred Kelvins.
  • The rich internal structure of biomolecules opens unique interaction channels with the environment. What new decoherence mechanisms will we find?
  • Can we exploit quantum interferometry to provide more accurate statements about biomolecular properties?

In our conceptional and experimental research work we are working towards gaining a better understanding of these questions.


  • C. Brand, S. Eibenberger, U. Sezer, M. Arndt,
    Matter-wave physics with nanoparticles and biomolecules,
    arXiv:1703.02129v1 [quant-ph] 6 Mar 2017   (2017).
  • Insight review: Testing the limits of quantum mechanical superpositions
    Markus Arndt & Klaus Hornberger
    Nature Physics 10, 271-277 (2014); DOI: 10.1038/nphys2863
  • Colloquium: Quantum interference with clusters and molecules
    K. Hornberger, S. Gerlich, S. Nimmrichter, P. Haslinger and M. Arndt
    Rev. Mod. Phys. 84, 157-173 (2012); DOI: 10.1103/RevModPhys.84.157
  • Macroscopicity of Mechanical Quantum Superposition States,
    S. Nimmrichter, K. Hornberger,
    Phys. Rev. Lett. 110,  160403 (2013).
  • Testing spontaneous localization theories with matter-wave interferometry
    S. Nimmrichter, K. Hornberger, P. Haslinger, and M. Arndt
    Phys. Rev. A 83, 043621 (2011); DOI: 10.1103/PhysRevA.83.043621