The wave-particle duality of matter

In 1807 Thomas Young performed the famous double slit experiment [1]  showing that light propagates like a delocalized wave.  

100 years later it was suggested by Einstein on thermodynamical grounds and in order to explain the photo effect, that light also behaves as a particle [2].

In 1923 Louis de Broglie drew the analogy between light and matter with the hypothesis that every massive object should be associated with a quantum wave, as well [3].

Double-slit experiments with massive objects such as electrons [4], neutrons [5] atoms [6, 7] and large molecules [8] verified this idea in a clear and elegant way.

The convincing power of grating diffraction in illustrating quantum delocalization and interference led to the election of far-field electron diffraction [9, 10] as the “most beautiful experiment in physics”, in a poll by Physics World [11] in 2002.        

 

Our studies, accomplished with chemistry partners around Prof. Mayor  in Basel and our nanotechnology partners around Prof. Cheshnovsky in Tel Aviv, extend this idea and optically visualize for the first time the emergence of a deterministic 2D quantum interference pattern from stochastically arriving single phthalocyanine molecules  [12].

In contrast to  photons which are eliminated in the act of measurement, molecular interferograms can stay for many hours and be potentially interfaced with subsequent manipulation steps.

 

Experimental setup

We launch a molecular beam by microfocusing the light of a diode laser onto a glass window covered a thin layer of phthalocyanines. The width of the laser beam and an optional subsequent first collimation slit define the transverse coherence of the emerging molecular beam. It is chosen such that the quantum wave function associated with the molecules is sufficiently spread out to cover a few slits in the nanomechanical grating.

 

The SiN grating has a thickness of 10 nm, a period of 100 nm and a slit opening of 50 nm. It was written at Tel Aviv University in the group of Prof. Cheshnovsky, using a focussed Ga ion beam.

 

The diffracted molecules travel on to land on a vacuum-air interface, i.e. a 170 µm thin quartz window on which they stay fixed. A diode laser at 660 nm excites the molecules to emit their light into a fluorescence microscope. This enables us to see each individual molecule and with a position accuracy of down to 10 nm.

 

 

Experimental results

Single molecules can bee seen as they arrive stochastically on the detection screen.

As the integration time grows the full deterministic interference pattern becomes visible.

 

The quantum interferogram fans out towars the bottom of the screen, since this is the place where slow molecules arrive. In contrast to their fast counterparts at the top end of the image, the momentum kick associated with the diffraction process leads to a wider position spread.

 

The molecular movie

The movie shows a series of fluorescence images of phthalocyanines after diffraction at the 100 nm nanograting. Each red dot is the image of an individual molecule, seen in fluorescence microscopy. The stochastically arrving molecules from a deterministic interference pattern.

 

 


Earlier molecular far-field experiments

  • First diffraction of hot complex molecules, in particular fullerenes [12]
  • Diffraction of C60 and C70 at standing light wave phase gratings [13]
  • Tracing Heisenberg's uncertainty relation with molecules [14]

References

  1. Young, T. Lectures on Natural Philosophy, 1, (1807).
  2. Einstein, A. Concerning an Heuristic Point of View Toward the Emission and Transformation of Light. Ann. Physik 17: 132-148 (1905).
  3. de Broglie, L. Waves and Quanta, Nature 112: 540-540 (1923).
  4. Davisson, C. and L. H. Germer The scattering of electrons by a single crystal of nickel., Nature 119: 558-560 (1927).
  5. v. Halban Jnr., H. and P. Preiswerk, Preuve Expérimentale de la Diffraction Des Neutrons., C.R. Acad. Sci. Paris 203: 73-75 (1936).
  6. Estermann, I. and Stern, Diffraction of molecular beams, Zeitschrift fiir Physik 61: 95-125 (1930).
  7. Cronin, A. D. et al., Optics and interferometry with atoms and molecules, Rev. Mod. Phys. 81: 1051-1129 (2009).
  8. Arndt, M., et al. Wave-particle duality of C60 molecules., Nature 401(6754): 680-682 (1999).
  9. Tonomura, A,., et al., Demonstration of Single-Electron Buildup of an Interference Pattern. Am. J. Phys., 1989. 57: p. 117-120.
  10. Merli, P., G. Missiroli, and G. Pozzi, On the statistical aspect of electron interference phenomena. American Journal of Physics, 1976. 44: p. 306.
  11. Crease, R.P., The most beautiful experiment in physics. Physics World, 2002.
  12. Juffmann, T., et al., Real-time single-molecule imaging of quantum interference. Nat Nano, 2012. advance online publication.
  13. Nairz, O., et al., Diffraction of complex molecules by structures made of light. Physical Review Letters, 2001. 87(16): p. art. no.-160401.
  14. Nairz, O., M. Arndt, and A. Zeilinger, Experimental Verification of the Heisenberg Uncertainty Principle for Fullerene Molecules. Phys. Rev. A, 2002. 65: p. 65 0321091-4.