Breakthrough in neutron physics

Particles can move in waves along different paths at the same time – this is one of the most important discoveries in quantum physics. A particularly impressive example is the neutron interferometer: neutrons are projected on a crystal, the neutron wave is divided into two parts, which then overlap each other again. Characteristic interference patterns can be observed which prove the wave properties of matter.

These neutron interferometers have played an important role in precision measurements and basic physics research for decades. However, their size has been limited until now, as they only worked if they were cut from a single piece of crystal. Since the 1990s, attempts have also been made to make interferometers from two separate crystals, but without success. Today, a team from TU Wien, INRIM Turin and ILL Grenoble have achieved just this feat by using a high-precision tip-tilt platform for crystal adjustment. This opens up completely new possibilities for quantum measurements, including research into quantum effects in a gravitational field.

The first step in 1974

The history of neutron interferometry began in 1974 in Vienna. Helmut Rauch, a professor at the Atomic Institute at TU Vienna for many years, created the first neutron interferometer from a silicon crystal and was able to observe the first neutron interference at the TRIGA reactor in Vienna. A few years later, TU Wien installed a permanent interferometer station, S18, at the most powerful neutron source in the world, the Laue-Langevin Institute (ILL) in Grenoble. This configuration is operational to this day.

“The principle of the interferometer is similar to the famous double-slit experiment, in which a particle is projected on a double-slit as a wave, passes through both slits at the same time as a wave and then superimposes on itself, so that a characteristic wave pattern is created by the detector,” explains Hartmut Lemmel (TU Vienna).

However, where the two slits in the double-slit experiment are only a minimal distance apart, the particles in the neutron interferometer are divided into two different orbits with several centimeters between them. The particle wave reaches a macroscopic size – but by superimposing the two paths, you create a wave pattern that clearly proves: The particle did not choose one of the two paths, it used both paths at the same time.

Any inaccuracy can ruin the result

Quantum superpositions in a neutron interferometer are extremely fragile. “Small inaccuracies, vibrations, displacements or rotations of the crystal destroy the effect,” explains Hartmut Lemmel. This is why you typically mill the entire interferometer from a single crystal. In a crystal, all atoms are connected to each other and have a fixed spatial relationship to each other, allowing you to minimize the impact of external disturbances on the neutron wave.

But this monolithic design limits the possibilities as the crystals cannot be made in any size. “In the 1990s, therefore, attempts were made to create neutron interferometers from two crystals, which could then be placed at a greater distance from each other,” explains Lemmel, “but it did not succeed. The alignment of the two crystals against each other did not succeed. the required accuracy. “

Extreme precision requirements

The requirements for accuracy are extreme. When a crystal in the interferometer is displaced by a single atom, the interference pattern changes by an entire period. If one of the crystals is rotated through an angle of the order of one hundred millionth of a degree, the interference number is destroyed. The required angular precision is roughly equivalent to firing a particle from Vienna to Grenoble and aiming at a pinhead, 900 kilometers away – or aiming at a drain cover on the Moon.

The Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin provided the necessary technologies that it had developed over decades in the field of combined optical and X-ray interferometry. Scanning X-ray interferometers are also made up of separate silicon crystals and are just as sensitive. The spatial displacement sensitivity of a crystal was used in Turin to determine the lattice constant of silicon with unprecedented accuracy. This result made it possible to count the atoms in a macroscopic silicon sphere, to determine the constants of the Avogadros and Planck, and to redefine the kilogram.

“Although the required accuracy is even more severe for neutrons, what worked with separate crystal X-ray interferometers should also work with separate crystal neutron interferometers,” says INRIM’s Enrico Massa. With an additional integrated laser interferometer, vibration damping, temperature stabilization and INRIM crystal collection and adjustment monitoring, the collaboration finally succeeded in detecting neutron interference in a system of two separate crystals.

Important for basic research

“This is an important step forward for neutron interferometry,” said Michael Jentschel of ILL. “Because if you can control two crystals well enough for interferometry to be possible, you can also increase the distance and expand the size of the overall system quite easily.”

For many experiments, this total size determines the accuracy that can be obtained in the measurement. It will be possible to study fundamental interactions with unprecedented precision – for example, the sensitivity of neutrons to gravity in the quantum regime and to hypothetical new forces.

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