The goal of qBounce is the experimental verification of Newton's law of gravitation in the micrometer range using highly sensitive measurement methods from quantum mechanics [1,2]. The link between quantum mechanics and gravity used in our experiments are ultracold neutrons, which form bound states in the gravitational potential of the earth. Neutrons are particularly suitable for our measurements due to their electrical neutrality and very low polarizability.
There are numerous motivations for gravitational experiments at small distances. According to the current state of knowledge, all processes on earth and in the universe are subject to four fundamental forces. The force that has been known for the longest time, gravitation, is described by the general theory of relativity. It is a classical field theory and is confirmed by all current experiments. The other three fundamental forces can be described by quantum field theories within the framework of the Standard Model of particle physics. It has not yet been possible to unite gravity with the standard model.
A variety of new theoretical descriptions have been introduced that can provide explanations for various problems in astronomy and cosmology. Some of these prominent and currently unconfirmed theories that would lead to measurable effects in our experiment are given here:
- Large, compactified extra dimensions of space-time were proposed by Arkani-Hamed, Dimopoulos and Dvali [4,5] about a decade ago to solve the hierarchy problem of particle physics. They would change the gravitational potential as follows. In this formula, n is the number of additional dimensions and R is the compactification radius. This proposal increased interest in small-distance gravitational experiments, since at that time compactification radii up to 1 mm were not experimentally ruled out.
- Spin Dependent Interactions
The existence of axions has been proposed to solve the strong CP problem . In addition, axions are considered promising candidates for dark matter. The existence of these hypothetical particles would result in a short-range, spin-dependent force with a monopole-dipole term:
Through astronomical observations and laboratory experiments, axions are experimentally excluded over a wide range of parameters, but not in the so-called axion window, which extends over a mass scale from 10 µeV/c² to 1 eV/c². This corresponds to a length scale of 0.2 µm to 2 cm and is therefore within the sensitivity range of our experiments.
- Other Fifth Forces
Deviations from Newton's law of gravitation are often parameterized generically using a Yukawa-like term:
The two parameters α and λ are the coupling strength and the interaction range. Casimir force measurements and neutron scattering experiments set limits in the submicron range. For ranges greater than 50 µm, the restrictive limits come from pendulum experiments and astronomical observations. Our experiment has the potential to close the gap in between.
 H. Abele, T. Jenke, H. Leeb, J. Schmiedmayer: Ramsey’s method of separated oscillating fields and its application to gravitationally induced quantum phase shifts, opens an external URL in a new window, Phys. Rev. D81, 065019 (2010)
 H. Abele: Gravitation unter dem Mikroskop, Spektrum der Wissenschaft, opens an external URL in a new window, S. 19ff, Sep. 2011
 TU Wien, Pressemitteilung 28/2011 vom 18.04.2011 (http://www.tuwien.ac.at/aktuelles/news_detail/article/6963/, opens an external URL in a new window)
 N. Arkani-Hamed, S. Dimopoulos & G. Dvali: The hierarchy problem and new dimensions at a millimeter, opens an external URL in a new window, Phys. Rev. B429, 263-272 (1998)
 N. Arkani-Hamed, S. Dimopoulos, & G. Dvali: Phenomenology, astrophysics, and cosmology of theories with submillimeter dimensions and TeV scale quantum gravity, opens an external URL in a new window, Phys. Rev. D59, 086004 (1999)
 R. D. Peccei & H. R. Quinn: CP Conservation in the Presence of Pseudoparticles, opens an external URL in a new window, Phys. Rev. Lett. 38, 1440–1443 (1977)