Gravitational redshift in quantum communication processes

• Applicant: David Edward Bruschi, Ph.D.
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas ; Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 548528714

The main objective of the GREQO project is to obtain a concrete model of realistic photons that propagate on a curved spacetime, and to understand how their quantum state is affected by curvature. In particular, the GREQO project is looking into the following concrete objectives: i) Modelling the evolution of the quantum state of a photon propagating in a 3+1-dimensional weakly curved spacetime. Modelling realistic photons that travel between two users placed at different heights will require us to obtain the following: extension of existing 1-D toy-model results to the full 3+1-dimensional case, including photon wave-packets sent with arbitrary initial angle; upgrade of the theory from that of the scalar field to Spin-1 field, thereby introducing polarization/helicity, which also become defining degrees of freedom of the quantum state; computation of gauge invariant figures of merit quantifying the effects on the polarization; application to setups where photons are reflected by multiple nodes; inclusion of the effects of gravity as a new term into the radar equation, which quantifies the probability of photon detection by a receiver when the signal propagates through free space between Earth and a satellite. ii) Characterization of the state-transformation between input location and final location as a quantum channel by computing relevant measures of coherence, channel capacity and entanglement. Modelling the process as a channel will allow us to interpret the action of gravity on the quantum state using the theory of quantum channels. This will allow us to: provide bounds on the transmissible information through empty space when photons are employed as information carriers; find optimal configuration in different scenarios optimizing over the photon’s initial parameters and the parameters of the sender and receiver; inform on the potential additional side channels that can be used to attack quantum key distribution protocols based on photon exchange in space; characterize also the distribution and use of multipartite entangled states of propagating photons in concrete protocols. iii) Feasibility analysis. As a final objective we will apply the results of this project to at least one concrete satellite-based setup. We will focus on a setup where the source is placed on a small, cost-effective Cubesat, which is a member of the fully functional family of small (nano)satellites, deployed at LEO orbits. Our aim will be to estimate the overall magnitude of the effects and to compare it with other existing sources of noise and loss. This objective will lead to further application for funding to continue these studies.

• DFG Programme: Research Grants
• International Connection: Austria
• Cooperation Partner: Professor Andreas Wolfgang Schell, Ph.D.