It has become one of the major problems of theoretical physics to understand the interplay between our most successful theories, quantum mechanics and general relativity. A resolution of this problem can only be driven by experiments or observations at the interface of the two theories. In addition, the race in the development of space-based quantum technologies, where quantum resources are generated and probed locally or are exchanged over thousands of kilometers through the inhomogeneous gravitational field of Earth, fuels the need to understand the influence of general relativistic effects on quantum resources also from a practical point of view.

A particular example of an interesting fundamental effect at the interface of quantum mechanics and general relativity is the generation of entanglement between the internal energy structure of a quantum system and its external (motional) degrees of freedom due to gravitational time dilation or redshift. These entanglement dynamics due to gravity have been proposed to be witnessed in atom interferometry, with single photons in Mach-Zehnder interference, photon pairs in Hong-Ou-Mandel interference and phonons in Bose-Einstein condensates.

QuantumFrontiers researchers at the Bremen Centre of Applied Space Technology and Microgravity have now investigated the entanglement dynamics of photons and quantum memories due to gravity in Mach-Zehnder and Hong-Ou-Mandel interferometry setups. They provide an experimental proposal and a feasibility study to witness the effect in Hong-Ou-Mandel experiments whose necessary spatial extensions are dramatically smaller than those of proposed experiments that only employ photons. Such an experiment would represent an experimental test of theoretical modeling combining a multi-particle effect predicted by the quantum theory of light and an effect predicted by general relativity.

On the applied side, the article published in the scientific journal Quantum, represents the first analysis of relativistic gravitational effects on space-based quantum memories which are expected to be an important ingredient for global quantum communication networks.

**Original publication **

Entanglement dynamics of photon pairs and quantum memories in the gravitational field of the earth

Roy Barzel, Mustafa Gündoğan, Markus Krutzik, Dennis Rätzel, and Claus Lämmerzahl

Quantum **8**, 1273 (2024)

https://doi.org/10.22331/q-2024-02-29-1273