CRG-B - Thermal neutron interferometer

S18 - Neutron interferometer

The perfect crystal neutron interferometer is a Mach-Zehnder type interferometer for thermal neutrons. The neutron beam is split into two beam paths which are separated macroscopically in the order of 5cm. The beams are recombined and their interference fringes are measured, revealing the quantum mechanical phase shift between the two beams.


  • Demonstration of basic quantum physics laws
  • Quantum contextuality and entanglement
  • Quantum weak values
  • Quantum Cheshire Cat
  • Decoherence, dephasing and depolarisation
  • Topological phases
  • Measurement of neutron-nuclei scattering lengths
  • Phase tomography


Ultra Small Angle Neutron Scattering extends the range of SANS instruments (e.g. instrument D11) towards smaller scattering angles, smaller momentum transfers or larger structure sizes up to 30µm. Unlike the pinhole geometry of a SANS setup, the USANS setup is sensitive to scattering in only one (horizontal) direction.

Applications and examples

  • Pore size and accessibility of geological samples
  • Colloids
  • Cement hydration
  • Magnetic domains

Instrument layout

A vibration isolated optical bench provides support for two perfect silicon crystals. The first crystal, the monochromator, is located in a thermal beam line (H25) and directs the neutrons of the selected wave length onto the second crystal. The second crystal can be the interferometer or the USANS analyzer crystal. The crystals are in non-dispersive configuration, meaning that all neutrons reflected by the first crystal are accepted by the second.

Two magnetic prisms for polarization can be inserted between the two crystals. The magnetic fields of triangular shape refract the spin-up and -down components to opposite directions, allowing the second crystal to select either the up or down component. 

The setup is surrounded by two housings to ensure stable temperature.