One of the most important discoveries of quantum physics is that particles can propagate in waves along several orbits at the same time. The neutron interferometry is a particularly striking example. When neutrons are fired into a crystal, the neutron wave is split into two parts and then superimposed once again. It is possible to see a distinct interference pattern, indicating that the material has wave properties.
For many years, such neutron interferometers have been crucial reference sources for fundamental physics and precision measurements research. However, their size has so far been limited, as they only consist of a single piece of crystal and function. Although unsuccessful, interferometers constructed from two different crystals have also been tried since the 1990s. Now let's share a successful initiative with you. The attempt was carried out by a team from TU Wien, INRIM Turin and ILL Grenoble using a high precision tip-tilt platform for crystal alignment. This opens up a wide variety of brand new opportunities for quantum measurements, such as investigations into the nature of quantum effects in gravitational fields.
The first neutron interferometry experiments were made from a silicon crystal in 1974 by Helmut Rauch, a longtime professor at the Atomic Institute of the Technical University of Vienna. He was able to see the first neutron interference at the Vienna TRIGA reactor. A few years later, TU Wien installed the permanent interferometry station S18 at the Institut Laue-Langevin (ILL), Grenoble's strongest source of neutrons. The current configuration is still in use.
“The mechanism of the interferometer is comparable to the famous double-slit experiment, where a particle is ejected through the double slit in a wave-like fashion, passes through both slits simultaneously as a wave, and then rides on itself,” says Hartmut Lemmel.
While the neutron interferometer splits the particles into two separate paths with a few centimeters between them, in the double-slit experiment there is only a small distance between the two slits. The particle wave grows to macroscopic size, but by superimposing the two trajectories, a wave pattern is produced, which amply demonstrates that the particle uses both paths simultaneously rather than choosing one.
However, this monolithic architecture is limited by the impossibility of making crystals of any size. “As a result, attempts to build neutron interferometers consisting of two crystals that could be placed farther apart from each other in the 1990s were unsuccessful, according to Lemmel. The two crystals were not properly aligned with each other.”
The interference pattern changes over a full period when an atom moves one of the interferometer's crystals. If one of the crystals is rotated at an angle of about 100 millionths of a degree, the interference pattern is broken. The required angular accuracy is equivalent to detonating a particle 900 kilometers from Vienna to Grenoble and aiming at a pinhead or a evacuation hatch on the moon.
The core technologies were provided by the Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin, a longtime leader in the field of combined optics and X-ray interferometry. Similar in sensitivity, scanning X-ray interferometers are also made of different silicon crystals. In Turin, the sensitivity of a crystal to spatial displacement was exploited to determine the lattice constant of silicon with unprecedented precision. This finding makes it possible to calculate the Avogadro and Planck constants, count the atoms in the macroscopic silicon sphere, and redefine the kilogram.
"What works with discrete crystal X-ray interferometers should work just as well with discrete crystal neutron interferometers, although the required accuracy is even more stringent for neutrons," said INRIM's Enrico Massa. The partnership was ultimately successful in detecting neutron interference in a system. It consists of two different crystals, thanks to the extra built-in laser interferometer, vibration damping, temperature stability, and INRIM's control over the crystals' structure and alignment.
According to Michael Jentschel of the ILL, neutron interferometry has made significant strides in recent years. “Because if you can control two crystals as well as interferometry can be achieved, you can easily extend the distance and increase the size of the entire system.”
This overall size affects the ability of many experiments to accurately measure anything. With unprecedented precision, it will be possible to study fundamental interactions, such as how sensitive neutrons are to hypothetical new forces and gravity in the quantum realm.