# SORCE – What it’s all about

The development of optical atomic clocks and optical frequency standards is foundational to the advancement of space-based missions in geodesy, deep-space satellite navigation, and fundamental physics. Optical clocks fluctuate very rapidly ($\sim 10^{14}$ times per second), so the interval between “ticks” of an optical atomic clock is a few femtoseconds. Since the scale of time is divided into such small intervals, optical atomic clocks provide exceptional timing resolution and stability. However, most of these existing clocks are not compact enough to be suitable for space-based applications.

The Stratospheric Optical Rubidium Clock Experiment (SORCE) aims to demonstrate a portable optical atomic clock on a stratospheric balloon platform, as a first step towards a functioning space-based system. This will be the first demonstration of a remotely operated, portable optical atomic clock.

SORCE will utilize the $^5S_{1/2}$ to $^5D_{5/2}$ two-photon transition in rubidium to reach a fractional stability of $5\times10^{-13}/\sqrt{\tau(s)}$ in a 35 x 35 x 25 cm package. A rubidium vapour cell is interrogated with counterpropagating 778 nm laser beams, and a fluorescence signal at 420 nm is detected using a photomultiplier tube.  This signal is used to lock the laser to the atomic transition.

SORCE will fly on a high-altitude balloon as part of the 2019 STRATOS campaign (CSA/CNES) in late August from Timmins, Ontario, Canada.