CRG-B - Thermal neutron interferometer

Pedagogical animations

Six animations are shown :

  1. without phase shifter (-> no contrast)
  2. normal
  3. with absorber
  4. with spin flipper, vertical analyzer (-> no contrast)
  5. with spin flipper, horizontal analyzer
  6. with spin flipper, varying analyzer

Movie 1 - Neutron interferometry with polarized neutrons

Without phase shifter (-> no contrast)

A neutron interferometer consists of three perfect crystal silicon blades. The first blade splits the neutrons coherently into two beam paths. The second blade acts as a mirror. The third blade superimposes the components from both paths, creating two exit beams labelled O and H.
The animation shows an empty interferometer without phase shifter. O and H beam show constant intensity.


One of the beam paths contains a phase shifter which is constantly growing in thickness, thereby shifting the relative phase between the two components. Due to constructive and destructive interference respectively the intensity oscillates between the two exit beams.

With absorber

One of the beam paths contains a detector. The detector may click or not, in both cases we know which path the neutron has taken. Therefore the interference fringes vanish.
The red color of the neutron represents a classically localized state. Inside the interferometer the neutron is in a superposition of both path states, indicated by gray color.

With spin flipper, vertical analyzer (-> no contrast)

One of the beam paths contains a spin flipper which turns the incident spin up state into a spin down state.
The exiting beams are a superposition of up and down state, each coming from a distinct beam path. If the detectors analyze the spin state in vertical direction (like in this example) the which way information is retrieved and the interference pattern vanishes.

With spin flipper and horizontal analyzer

Same setup as before but with horizontal spin analyzers which cannot distinguish between spin up and spin down state. The interference pattern is recovered again.

With spin flipper and varying analyzer

The interferometer creates an enganglement between two degrees of freedom, each forming a two level system, namely the spin degree of freedom (with states up and down) and the path degree of freedom (path I and II in the interferometer). Various combinations of phase shifts and spin analyzer angles allow the measurement of certain correlations and Bell inequalities, proving that the prepared state is of pure quantum nature and cannot be explained by classical hidden variable theories.

Ref.: Hasegawa et al, nature 425, 45 (2003).

(Flaw in the animation: In this experiment the spin filter should not rotate between vertical and horizontal but within the horizontal plane.)