July 9, 2026 -- CANISIUS is the official name of the new spin-echo neutron interferometer developed at Atominstitut, TU Wien. It enables precise control of neutron waves, something that was previously impossible.

Neutrons cannot be imagined as tiny spheres; they have wave properties, similar to light. This was spectacularly demonstrated in 1974 at the nuclear reactor of the Atominstitut – and it was precisely here that researchers succeeded in exploiting this wave nature of neutrons in a novel way: A measuring device was developed that can use the angular momentum of neutrons in a particularly clever way for experiments. Not only the intrinsic angular momentum – the spin – but also the orbital angular momentum, which is related to the waveform of the neutron, can be adjusted.

Spin-echo neutron interferometer

The spin of neutrons can change in a magnetic field, somewhat like the Earth's axis of rotation slowly changes over time. This phenomenon of spin precession is exploited in certain measuring instruments – so-called spin-echo interferometers. In these instruments, a neutron beam is split into two parts. The two partial beams react differently to a magnetic field and are then recombined. Whether the two partial beams can then be perfectly recombined into a single neutron beam, or whether they might even annihilate each other, depends very sensitively on what happened to the neutrons along the way. Even tiny interactions have a strong, clearly measurable effect on the final result. "This gives us completely new possibilities for precision experiments," says Prof. Hartmut Abele, head of the Neutron and Quantum Physics Group at the Atominstitut.

Another crucial advantage of the new method: Previously, it was often necessary for all the neutrons used to have as close to the same speed as possible. This meant removing any neutrons whose speed didn't match the exact specifications, leaving only a few neutrons available for measurement. The new device works with neutron beams that are entirely non-uniform, composed of neutrons of varying speeds – referred to as "white" neutron beam. "This significantly increases the measurement efficiency," says Niels Geerits, lead author of the presented study. “Consequently we don't need a high-performance neutron source for this new device; we can use it at our own research reactor at the Atominstitut.”

Twisted Waves: “Orbital Angular Momentum” (OAM) states

At the TU Vienna, however, they have now gone a step further: “Our new device CANISIUS can not only measure the neutron waves, but also shape them in a targeted manner,” says Stephan Sponar from the Atomic Institute at the TU Vienna, lead researcher in the development and construction of the new spin-echo instrument. The wave nature of neutrons is exploited to manipulate not the neutron spin, but a different type of angular momentum (OAM). If you separate the two partial beams appropriately and put them back together, you can create a neutron wave that has a spatial rotation, similar to the stream of water flowing down the drain. In this case one speaks of an “orbital angular momentum” of the neutrons.

Ambitious plans

The new interferometer will be used both for materials research, the typical application of spin-echo instruments, and for experiments investigating fundamental questions in quantum theory. "There are a number of fundamental questions, such as those concerning the basic interactions in quantum mechanics or quantum information theory, for which this new method is ideally suited," says Stephan Sponar.

The engineering behind CANISIUS

The new neutron instrument CANISIUS (Coherent Averaging Neutron Instrument for Spin-echo Interferometry and Fundamental Science) at the Atominstitut is a spin echo interferometer, more precisely a so-called SESANS (Spin-Echo Small-Angle Neutron Scattering) instrument. There are currently only a handful of actively operating SESANS instruments worldwide, located, for example, at the TU Delft Reactor Institute (Netherlands), ISIS Neutron and Muon Source (Great Britain), Oak Ridge National Laboratory (ORNL) (USA), and at the Mianyang Research Reactor in China. "This gives the Atominstitut and the TRIGA Center another state-of-the-art instrument at our intensively used 250 kW research reactor," says Andreas Musilek, head of the TRIGA Center, which operates the TU Wien reactor.

To achieve high resolution, neutrons traveling in precisely the same direction are needed: the larger the opening angle of the neutron beam, the less precise the measurement becomes. Therefore, for high resolution, tiny apertures must be used that only allow neutrons with a very specific angle to pass through. This drastically reduces the number of neutrons.

The fundamental physical advantage of SESANS lies in the decoupling of length resolution and geometric beam divergence. As a result, beam deflections—and thus structures—that are significantly smaller than the beam divergence can be resolved. Since the position and angular information is encoded in the neutron spin phase, the geometric trajectory of the neutrons in front of the sample is secondary.