Speaker
Description
Over the past few decades, advancements in optical atomic clocks have enabled measurements of time and frequency with unprecedented stability and systematic uncertainty [1,2]. Precision frequency comparisons between macroscopically separated clocks have applications in geodesy [3], probing variations in fundamental constants, and in dark matter searches [4]. Frequency comparisons between independent clock systems are limited by the standard quantum limit (SQL). In contrast, a set of N entangled atomic clocks can achieve a $\sqrt{N}$ stability improvement to surpass the SQL and approach the Heisenberg limit - the ultimate precision possible in quantum theory.
We previously demonstrated this enhancement in a network of two \ion{Sr}{88} clocks [5] on the 674 nm $5S_{1/2} \leftrightarrow 4D_{5/2}$ quadrupole transition using Ramsey spectroscopy, whose stability was mainly limited by the short probe duration of 20~ms due to magnetic field fluctuations. We are now setting up the next generation of the experiment wherein we map the remote Sr-Sr entanglement onto two $^{43}$Ca$^+$ ions. The 729 nm $^{43}$Ca$^+$ $\vert 4S_{1/2}, F=4, m_F=4 \rangle \leftrightarrow \vert 3D_{5/2}, F=4, m_F=3 \rangle$ optical clock transition is field-insensitive at 4.96 G, enabling probe durations at the excited state lifetime limit of $\sim$ 1 s (comparable to the start-of-the-art clocks [1]) and thus improve our stability.
We will present progress towards these clock experiments, including the setup of a 729 nm laser system locked to a high finesse cavity, as well as fibre noise cancellation on a 20~m fibre. We will further present some theoretical work on quantum metrology with Dicke states in the presence of spontaneous decay in larger networks of clocks.
[1] M. C. Marshall et al., Phys. Rev. Lett. 135, 033201 (2025).
[2] E. Oelker et al., Nature Photonics 13, 714–719 (2019).
[3] T. E. Mehlstaubler et al., Reports on Progress in Physics 81, 064401 (2018).
[4] M. S. Safronova et al., Rev. Mod. Phys. 90, 025008 (2018).
[5] B. C. Nichol et al., Nature 609, 689–694 (2022)