Page 67 - ILL Annual Report 2019

SCIENTIFIC HIGHLIGHTS
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a)
b)
1.2 0.8 0.4
phase shifter,
detector O–beam
0
0 5 10 15 20
Rotation Frequency (kHz)
The goal of our present experiment was to reveal the purely quantum mechanical aspect of spin-rotation coupling as an extension of the Sagnac effect in a neutron interferometer experiment [4], as suggested by [3], using the instrument S18 (figure 1). Our set-up is shown in figure 2a. The first interferometer plate coherently splits a beam of polarised neutrons. In path I, the neutron
spin interacts with a magnetic field of variable rotation frequency. The field amplitude is tuned according to the frequency to produce cyclic spin evolution paths. Initial and final spin states differ only in their phase. The last interferometer plate acts as a projector, comparing the additional phase with that of the reference beam in
path II. In our experiment, only the outgoing O-beam (figure 2a) is of importance. Our final results are presented in figure 2b. Since the results are in the form of a phase shift the Bloch-equations are insufficient to describe them, so we must rely on the Schrödinger equation.
Figure 2
a) Schematic of set-up: monochromatised neutrons, with polarisation vec⃑tor parallel to the direction of propagation (+y), enter the interferometer. Inside the interferometer, a rotating magnetic field B (Ω, t) is generated in path I, rotating on a plane perpendicular to the neutron beam. After recombination at the final interferometer plate, the neutrons in the forward direction (O-beam) are detected in a 3He counter tube. Spin directions are indicated with blue arrows, the magnetic field in violet.
The phase shifter plate consists of a slab of sapphire and is rotated to record interferograms by inducing additional relative phase shifts χ between the paths.
b) Final results: linearly fitted phase of the interferograms relative to the static case dependent on the rotation frequency of the magnetic field.
As predicted by theory, the phase shift is found to be linearly dependent on the rotation frequency of the magnetic field. The observed phase shift can also be described by a combination of dynamical and geometric phases in the inertial frame. However, our present set-up focuses on a specific configuration where the quantum mechanical extension of the Sagnac effect is revealed.
This means that we can give an alternative explanation for the results in terms of the Mashhoon effect of spin-rotation coupling in the rotating frame. A rotating observer has to apply different forces on a particle depending on its spin state in order to accelerate it.
This is why ref. [3] interprets this effect as the ‘inertia of intrinsic spin’. Spin-rotation coupling can likewise be used to determine the rotation of a system.
Figure 3
Cartoonistic impression of the neutron interferometer experiment by Armin Danner.
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Phase Shift (rad)