Matter-wave physics, as conceived by Louis de Broglie in 1923 and formalized by Schrödinger in 1926, will soon be celebrating its centenary. And yet, throughout all decades quantum science has been perceived as a conundrum, even by the greatest minds in the field: Schrödinger still believed in 1952 that we can “never experiment with just one electron or atom or (small) molecule” and Feynman claimed that “nobody understands quantum mechanics”.

How can we understand that the basic principles quantum superposition and entanglement seem defy all common sense in the microscopic world but that we still experience apparently ‘normal’ (non-quantum) phenomena in our daily macroscopic experience?

How complex, massive or connected can an object be a still fall under the laws of quantum physics? Should there be a limit and if so when and why? Is it the coupling to the environment, spontaneous collapse or gravity?

I will introduce the experimental concepts and advanced interferometer technologies that have allowed us to probe the quantum superposition of complex matter, from atoms over molecules and clusters to polypeptide antibiotics and organic molecules beyond 25 kDa. Such devices are very sensitive to external forces, below 10-26 N already today and they have the potential to resolve 10-30 N in the near future.