Periodic Reporting for period 1 - GRAVITES (Gravitational interferometry with entangled states in optical fibers)
Reporting period: 2023-04-01 to 2024-09-30
The four known fundamental interactions in nature can be described either by Einstein's general theory of relativity or by quantum field theory. In recent decades, physicists have attempted to place these two pillars of modern physics on a common foundation. They have been hampered by the lack of experiments at the interface between them, although both have been independently verified with astonishing precision. In experiments testing quantum field theory, gravitational interactions are typically so weak that they have not yet been observed. Conversely, all experiments to test the correctness of general relativity have so far been performed on scales where quantum effects were negligible. The aim of GRAVITES is to close this gap by performing the first experiments at the interface between quantum physics and general relativity. To this end, this ambitious project will combine four complementary disciplines: Quantum Photonics, Precision Interferometry, General Relativity and Quantum Field Theory. Through the synergy between the research groups, this project is constructing a large fiber optic interferometer sensitive and accurate enough to reveal the influence of gravity on entangled photons. Successful completion of this experiment would be the first insight into the interaction of gravity with the quantum world. Since the sensitivity of the GRAVITES apparatus must exceed that of current large fiber-based quantum interferometers by orders of magnitude, the teams combine cutting-edge technologies in their respective fields to push interferometry with quantum resources to an unprecedented level. Along the way we are also exploring the potential of our interferometer to special relativistic effects like the influence of the rotation of the Earth on our entangled states of light. These developments are also of direct relevance for many other applications such as quantum metrology and quantum sensing. In parallel, the theory teams have been developing comprehensive models of fiber interferometry in curved space-times to describe how gravity influences the propagation of quantum states of light in our interferometer. With this united effort, GRAVITES is in the position to explore new physics that investigates the gravitational properties of quantum superposition and quantum entanglement. This will allow us to create a unique experimental platform for probing how gravity interacts with the quantum world.
The fiber interferometer for GRAVITES consists of two 27-kilometer-long optical fibers wound on vibration-insensitive coils that can be vertically separated by up to two meters with a moving stage. Through careful mechanical design optimized by finite element analysis, our coils show a hundred-fold reduction in sensitivity to external vibrations and our translation stage is capable of rapid height modulation with minimal disturbance to the interferometer. The effect of gravity on our two-photon path entanglement states corresponds to an induced length difference between the coils of about 10 picometers, which is extremely difficult to detect due to ambient and intrinsic noise. To combat them, we use a mixture of cutting edge active and passive stabilization techniques, which in combination provide us with a length stability of better than one picometer. To generate the entanglement, we are building single photon sources based on both spontaneous parametric down conversion and solid state quantum dots supplying more than 10 million entangled states per second. Another important effect that may limit our performance is the relative time delay between the propagation of light along the two different fibers, which is caused by the Earth's rotation. We performed an experiment with an effective area of 715 square meters to quantify its strength for entangled states of light. The results confirm that its influence can be smaller than the expected gravitational signal for a certain geometrical arrangement and winding direction of the two fiber coils. As light propagates through a solid material in standard optical fibers, it effectively samples the molecular motion of its constituents, which is another undesirable effect. For this reason, we investigated how hollow-core optical waveguides, a new type of fiber technology, behave in comparison. Our results show a high-fidelity distribution of entangled photons over kilometer-long distances as well as a strongly reduced thermal noise floor. To understand how gravity affects the propagation of light, we developed a comprehensive model of quantum optics in curved space-times, which models the propagation of single photons through optical fibers, and determines how their spectral properties are modified by the gravitational field. We have also shown in our calculations that the current experimental evidence for some waveguides already rules out the possibility that photons are massive by the standard modifications of Maxwell's equations that are typically considered. The elastic properties of optical waveguides in changing gravitational fields were determined to guide the experimental design and limit the maximum allowable pressure and temperature fluctuations.
This project is dedicated to the study of the interaction between gravity and quantum mechanical systems. Our current understanding of the gravitational redshift is mainly limited to ray-optics models of light propagation, but fiber optics experiments such as the GRAVITES interferometer operate in a regime where the ray-optics description no longer applies. By accurately modelling optical fibers in external gravitational fields, using Einstein’s generalization of Maxwell’s equations, we have obtained a full description of how the spectral properties of light change as it is guided through optical fibers in Earth’s gravitational field. Our first major experimental result was the measurement of the interaction between special relativity and quantum field theory down to rotation rates of a few μrad s−1 using maximally path-entangled states of light. Since the Earth's rotation is also a constant companion in gravitational experiments, a fundamental understanding of this interaction is necessary. To improve on the precision of previous work in this direction by almost three orders of magnitude, we developed a novel optical switching method. This can also be used for even more ambitious but currently unfeasible measurements of gravitational corrections to rotation. The insights gained from our experiment are not only a major step forward in their own right, but were also used for the experimental design for GRAVITES’s main interferometer. Another important experimental achievement of the first 18 months was the successful transmission of entangled quantum light via a hollow-core fiber almost eight kilometers long. This new type of fiber has many advantageous properties for us, such as lower light absorption, less non-linear photon scattering and lower thermal noise. With this experiment, we were able to show for the first time that these fibers also do not have an excessively negative impact on quality characteristics such as purity and clarity. These results motivate further research into their properties for other applications such as quantum communication, where the much lower latency can also provide an advantage.