Questions & Answers on High-Mass Molecule Interference


Here we collect questions that we are being asked by journalists, colleagues and the general public. Feel free to add..

Recent media reports have highlighted our paper on "Quantum superposition of molecules beyond 25 kDa" in Nature Physics and we enjoy the great attention for this experimental effort that took our team three years to build and do. 

Many people around the world contacted us with many questions. We collect a few questions and answers here below.  

  • Q: What do you bring in superposition ?
    A: Very massive organic molecules, each of them composed of up to 2000 atoms. 
  • Every molecule is in a superposition of possible positions. This is what we abbreviate by 'being in two places at once". Hot as they are, no two molecules would ever interfere. It is the superposition and interference of paths.
  • Q: Why do you claim up to 40'000 particles are delocalized at once?
    A: The molecules are composed of atom and the atoms are composed of protons, neutrons and electrons.  A single large molecule with more than 28000 amu mass can thus contain more than 40.000 of these rather elementary constituents.
  • Q: Why these molecules and how do you make them?
    We are talking about perfluoroalkylfunctionalized porphyrin oligomer derivatives. This sounds complex and it is - also in terms of synthesis, which we can not describe here, as you will quickly realize, when you read the supplementary information downloadable with the Nature Physics paper.
    These molecules were designed and synthesized by Marcel Mayor and Patrick Zwick at the University of Basel, with the purpose and result to enhance the mass of the molecule while keeping its electronic polarizability and stickiness (to each other) low.
  • Q: Are all molecules quantum because they are indistinguishable?
    Not at all. On the contrary... there are billions of possible conformers, thousands of vibrational states and on top of that fine structure, hyperfine structure, Zeeman shifts etc. These are really very complex molecules and thermally it is extremely unlikely that they are equal in all states.
  • Q: Don't you say that indistinguishability is the prerequisite for superposition and interference?
    Yes it is. But for every molecules (in a given internal state) we look at the "indistinguishability of paths" through the interferometer. This is what we mean by "wave-like" propagation or quantum delocalization. There is a delocalized wave function that describes the propagation of very molecules and this wave function can occupy much larger spaces than you would think if you assumed a molecule were just a 5 nm large billiard ball.
  • Q: Was this the first ever molecule interference experiment?
    The first molecule diffraction is nearly 100 years old, performed by Estermann and Stern with H2 and we are proud that quantum experiments with hot complex molecules were started with hot fullerenes C60 in our group in 1999.  Since then we have increased the mass of these particles 40 times, devised numerous new interferometers and have developed half a dozen of different sources and metrology applications.
  • Q: What is special about this new experiment?
    It is a very long near-field interferometer, which is capable of analyzing quantum properties of particles with de Broglie wavelengths down to 50 fm. This is almost an order of magnitude smaller than previous experiments.
    A2: The interferometer is universal in the sense that we can use the same machine to study the quantum wave nature of atoms and of molecules composed of thousands of atoms.
  • Q: What do you test?
    A: The validity of the quantum superposition principle for particles of high mass, high temperature and high internal complexity.
  • Q: Why is this interesting or important?
    A: Quantum physics has been around for 100 years and has been confirmed to be the valid theory of the inanimate nature (excluding gravity) throughout all tests. However, if quantum objects can be in superposition of two different energies, two positions, two velocities and directions, why don't we see this in our daily lives? There are simple answers to that (Planck's quantum is small) and more complex answers, such as 'maybe the theory has to be modified for particles of high mass... and maybe because at high mass they are a relevant source of gravity and space time curvature.'
  • Q: What of these aspects are you testing here?
    A: Our experiments set new bounds on parameters that were suggested in attempts to modify the Schrödinger equation by non-linear stochastic terms. Positively said, if you can exclude these terms in a quantum superposition test better than other experiments your test has a higher quantum macroscopicity in one definition. Our experiment improves the value of quantum macroscopicity by an order of magnitude over all previous experiments.