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Neutron Interferometry with Polarized Neutrons as an Additional Degree of Freedom


Our goal is to apply Quantum Information Processing techniques to improve neutron interferometry contrast by making our final measurement insensitive to crystal imperfections.


Schematic view of the experimental setup

Figure 1. Schematic view of the experimental setup.

One approach to achieve this is to employ the Power of One Qubit protocol initially outlined by Knill and Laflamme. This protocol enables us to detect the neutron phase via the neutron spin degree of freedom, which is insensitive to surface defects and blade imperfections. Thus, by entangling the path and spin degrees of freedom, we propose to measure the path-dependent phase difference using the neutron spin. The key step is to introduce an entangling gate that is independent of the blades 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 polarization direction. As a proof of principle experiment, we can 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% enables us to select the incident spin state. Spin rotation inside the interferometer is accomplished using 10 micron thin film permalloy deposited onto a silicon substrate. These films offer 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, 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 serve as spin analyzers that are used downstream of the interferometer to again select one spin orientation. Detection is 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 are shown in Figure 2. As the film is titled, 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 grows 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 is not accomplished. Thus, we observe 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 to the space and heating requirements of the interferometer. We also expect that as we characterize these film rotators further, they may prove useful to other types of neutron experiments as well.

Created February 14, 2011, Updated January 23, 2018