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Interferometry: Neutron Interferometry with Polarized Neutrons as an Additional Degree of Freedom


The goal is to apply Quantum Information Processing techniques to improve the performance of perfect-crystal neutron interferometers by making the final measurement insensitive to crystal imperfections.


Schematic view of the experimental setup

Figure 1. Schematic view of the experimental setup. The red line indicates the neutron paths. Three light blue parts represent the three blades of the interferometer.

Neutron interferometers measure the quantum entanglement between two neutron states traveling along separate paths. The quantum entanglement is generated using diffraction in crystal lattices. Crystal imperfections effectively cause quantum state noise and lowers the achievable state separation from the ideal case. We have investigated using a protocol that enables us to detect the neutron phase via the neutron spin degree-of-freedom, which, unlike that of the path degree-of-freedom, is insensitive to surface defects and lattice imperfections. Thus, by entangling the path and spin degrees-of-freedom, we measure the path-dependent phase difference using the neutron spin. The key step is to introduce an entangling gate that is independent of the diffracting crystal and dependent on the magnetic properties of a material placed in one path of the neutron interferometer. This is accomplished by rotating the neutron spin in one beam path away from the polarization direction, allowing its magnetization to process about a material's internal magnetization, and then rotating it back to the original polarization direction. As a proof of principle experiment, we equivalently measure the phase accrual from the precession about the vertical guide field rather than a material. Our experimental setup is outlined in Figure 1. 

A transmission supermirror spin-polarizer selectively transmits one neutron spin orientation to the interferometer and reflects the other away from the apparatus. An aluminum DC coil spin flipper with efficiency >99% enabled us to select the incident spin state. Spin rotation inside the interferometer was accomplished using 10-micron thin film permalloy deposited onto a silicon substrate. These films offered the advantage that they are magnetized without requiring an active current supply, which would introduce heating near the interferometer and cause temperature gradients that are destructive to the measurement. When placed in an ambient magnetic field of greater than 6 gauss (0.6 mT), the films are magnetized to 1 Tesla in-plane. By tilting the films with respect to the neutron beam, a mutual angle is created between the spin polarization direction and the permalloy magnetization, and Larmor precession occurs. By tuning the tilt angle based on film thickness, the neutron can exit the film having achieved a particular desired rotation (e.g., 90 degrees). Saturated Heusler crystals served as spin analyzers that are used downstream of the interferometer to again select one spin orientation. Detection was achieved using 3He detectors of >99% efficiency.

Polarization vs. permalloy film tilt
Figure 2: Polarization vs. permalloy film tilt. At approximately 55 degrees tilt the permalloy acts, as desired, just like a 90 degree spin rotator.

Experimental data for neutron polarization vs. film tilt angle for a single 10-micron film is shown in Figure 2. As the film was tilted, the mutual angle between it and the neutron spin orientation grows and the neutron experiences progressively larger rotations. At 50-degree tilt, a 90-degree spin rotation is induced. As the tilt grew to near 60 degrees, the vertical size of the neutron beam exceeds the vertical projection of the film, and thus rotation of the full beam was not accomplished. Thus, we observed dephasing of the neutron beam. The permalloy films are good replacements for the bulky and hot DC coil flippers, which achieve exceptional efficiencies but cannot be applied due to the space and heating requirements of the interferometer. We expect that these film rotators may prove useful to other types of neutron experiments as well.

Created February 14, 2011, Updated March 10, 2021