It is almost exactly 100 years since De-Broglie made public his outrageous hypothesis regarding Wave-Particle Duality (WPD), where the latter plays a key role in interferometry. In parallel, the Stern-Gerlach (SG) effect, found a century ago, has become a paradigm of quantum mechanics. I will describe the realization of a half- [1-3] and full- [4-5] loop SG interferometer for single atoms [6], and show how WPD, or complementarity, manifests itself. I will then use the acquired understanding to show how this setup may be used to realize an interferometer for macroscopic objects doped with a single spin [5], namely, to show that even rocks may reveal themselves as waves. I emphasize decoherence channels which are unique to macroscopic objects such as those relating to phonons [7,8] and rotation [9]. These must be addressed in such a challenging experiment. The realization of such an experiment could open the door to a new era of fundamental probes, including the realization of previously inaccessible tests of the foundations of quantum theory and the interface of quantum mechanics and gravity, including the probing of exotic theories such as the Diosi-Penrose gravitationally induced collapse. Time permitting, and as an anecdote noting also De-Broglie's less popular assertion, namely, that the standard description of QM is lacking, I will also present our recent work on Bohmian mechanics, which is an extension of De-Broglie's ideas concerning the pilot wave [10].

[1] Y. Margalit et al., A self-interfering clock as a "which path" witness, Science 349, 1205 (2015).

[2] Zhifan Zhou et al., Quantum complementarity of clocks in the context of general relativity, Classical and quantum gravity 35, 185003 (2018).

[3] Zhifan Zhou et al., An experimental test of the geodesic rule proposition for the non-cyclic geometric phase, Science advances 6, eaay8345 (2020).

[4] O. Amit et al., T3 Stern-Gerlach matter-wave interferometer, Phys. Rev. Lett. 123, 083601 (2019).

[5] Y. Margalit et al., Realization of a complete Stern-Gerlach interferometer: Towards a test of quantum gravity, Science advances 7, eabg2879 (2021).

[6] M. Keil et al., Stern-Gerlach interferometry with the atom chip, Book in honor of Otto Stern, Springer (2021).

[7] C. Henkel and R. Folman, Internal decoherence in nano-object interferometry due to phonons, AVS Quantum Sci. 4, 025602 (2022) – invited paper for a special issue in honor of Roger Penrose.

[8] C. Henkel and R. Folman, Universal limit on quantum spatial superpositions with massive objects due to phonons, https://arxiv.org/abs/2305.15230 (2023).

[9] Y. Japha and R. Folman, Role of rotations in Stern-Gerlach interferometry with massive objects, Phys. Rev. Lett. 130, 113602 (2023).

[10] G. Amit et al., Countering a fundamental law of attraction with quantum wave-packet engineering, Phys. Rev. Res. 5, 013150 (2023).