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1. Fundamental physics with a state-of-the-art optical clock in space

   https://doi.org/10.1088/2058-9565/ac7df9  

   Derevianko, Andrei; Gibble, Kurt; Hollberg, Leo; Newbury, Nathan R; Oates, Chris; Safronova, Marianna S; Sinclair, Laura C; Yu, Nan (July 2022, Quantum Science and Technology)

   Abstract Recent advances in optical atomic clocks and optical time transfer have enabled new possibilities in precision metrology for both tests of fundamental physics and timing applications. Here we describe a space mission concept that would place a state-of-the-art optical atomic clock in an eccentric orbit around Earth. A high stability laser link would connect the relative time, range, and velocity of the orbiting spacecraft to earthbound stations. The primary goal for this mission would be to test the gravitational redshift, a classical test of general relativity, with a sensitivity 30 000 times beyond current limits. Additional science objectives include other tests of relativity, enhanced searches for dark matter and drifts in fundamental constants, and establishing a high accuracy international time/geodesic reference. 

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2. Towards satellite tests combining general relativity and quantum mechanics through quantum optical interferometry: progress on the deep space quantum link

   https://doi.org/10.1140/epjqt/s40507-025-00370-1  

   Mohageg, Makan; Anastopoulos, Charis; Brasher, Olivia; Gallicchio, Jason; Hu, Bei Lok; Jennewein, Thomas; Johnson, Spencer; Lin, Shih-Yuin; Ling, Alexander; Lohrmann, Alexander; et al (December 2025, EPJ Quantum Technology)

   The Deep Space Quantum Link (DSQL) is a space-mission concept that aims to explore the interplay between general relativity and quantum mechanics using quantum optical interferometry. This mission concept was formally presented to the United States National Academy of Science Decadal Survey as a research campaign for Fundamental Physics in 2022. Since then, advances have been made in the space-based quantum optical technologies required to conduct a DSQL-type mission. In addition, other research efforts have defined alternative measurement concepts to explore the same scientific questions motivating the DSQL mission. This paper serves as an update to the community on the status of the DSQL mission concept and related research and technology development efforts. 

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3. Goals and feasibility of the deep space quantum link

   https://doi.org/10.1117/12.2593986  

   Mazzarella, Luca; Mohageg, Makan; Strekalov, Dmitry V.; Zhai, Aileen J.; Israelsson, Ulf E.; Matsko, Andrey; Yu, Nan; Anastopoulos, Charis; Carpenter, Bradley; Gallicchio, Jason R.; et al (August 2021, Proceedings of the SPIE)

   Deacon, Keith S.; Meyers, Ronald E. (Ed.)

   In this article, we review the proposed experiments for the Deep Space Quantum Link (DSQL) mission concept aiming to probe gravitational effects on quantum optical systems. Quantum theory and general relativity are the two most successful frameworks we have to describe the universe. These theories have been validated through experimental confirmations in their domains of application— the macroscopic domain for relativity, and the microscopic domain for quantum theory. To date, laboratory experiments conducted in a regime where both theories manifest measurable effects on photons are limited. Satellite platforms enable the transmission of quantum states of light between different inertial frames and over distances impossible to emulate in the laboratory. The DSQL concept proposes simultaneous tests of quantum mechanics and general relativity enabled by quantum optical links to one or more spacecrafts. 

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4. Testing quantum theory on curved spacetime with quantum networks

   https://doi.org/10.1103/PhysRevResearch.7.023192  

   Borregaard, Johannes; Pikovski, Igor (May 2025, Physical Review Research)

   Quantum technologies present new opportunities for fundamental tests of nature. One potential application is to probe the interplay between quantum physics and general relativity—a field of physics with no empirical evidence yet. Here we show that quantum networks open a new window to test this interface. We demonstrate how photon mediated entanglement between atomic or atomlike systems can be used to probe time-dilation-induced entanglement and interference modulation. Key are nonlocal measurements between clocks in a gravitational field, which can be achieved either through direct photon interference or by using auxiliary entanglement. The resulting observable depends on the interference between different proper times, and can only be explained if both quantum theory and general relativity are taken into account. The proposed protocol enables clock interferometry on kilometer-scale separations and beyond. Our work thus shows a realistic experimental route for a first test of quantum theory on curved spacetime, opening up new scientific opportunities for quantum networks. Published by the American Physical Society2025 

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5. Optical coherence between atomic species at the second scale: improved clock comparisons via differential spectroscopy

   https://doi.org/10.48550/arXiv.2109.09540  

   Kim, M.E.; McGrew, W.F.; Nardelli, N.V.; Clements, E.R.; Hassan, Y.S.; Zhang, X.; Valencia, J.L.; Leopardi, H.; Hume, D.B.; Fortier, T.M.; et al (April 2022, ArXivorg)

   Comparisons of high-accuracy optical atomic clocks \cite{Ludlow2015} are essential for precision tests of fundamental physics \cite{Safronova2018}, relativistic geodesy \cite{McGrew2018, Grotti2018, Delva2019}, and the anticipated redefinition of the SI second \cite{Riehle2018}. The scientific reach of these applications is restricted by the statistical precision of interspecies comparison measurements. The instability of individual clocks is limited by the finite coherence time of the optical local oscillator (OLO), which bounds the maximum atomic interrogation time. In this letter, we experimentally demonstrate differential spectroscopy \cite{Hume2016}, a comparison protocol that enables interrogating beyond the OLO coherence time. By phase-coherently linking a zero-dead-time (ZDT) \cite{Schioppo2017} Yb optical lattice clock with an Al+ single-ion clock via an optical frequency comb and performing synchronised Ramsey spectroscopy, we show an improvement in comparison instability relative to our previous result \cite{network2020frequency} of nearly an order of magnitude. To our knowledge, this result represents the most stable interspecies clock comparison to date. 

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