![]() Fourth, the CAC must be robust in the orbital environment against, for example, the variation in Earth’s magnetic field and impacts from high-energy particles. Third, all operations of the CAC must be automated and all units must be maintained without any manual-adjustments. Second, the CAC must pass mechanical, thermal, and electromagnetic compatibility tests specified for space missions. First, because of the limited resources on board a spacecraft, weight, volume, and power consumption must be greatly reduced compared with ground-based fountain clocks. Operating a CAC in orbit has great challenges. Under the support of the China Manned Space Program (CMSP), we started a mission called Cold Atom Clock Experiment in Space (CACES) in 2011 with the goal of operating a rubidium CAC in space. The PHARAO clock is expected to operate in space with a frequency stability of 1.0 × 10 −13 τ −1/2 ( τ is the average time in second) and an accuracy below 3 × 10 −16 (ref. For example, the ACES mission, which consists of a caesium CAC called PHARAO, a hydrogen maser, as well as a package for frequency comparisons and distribution, aims to search for drifts in fundamental constants and measure the gravitational red shift with improved precision 20, 21, 22, 23, 24, 25, 26. Several projects on space CACs, such as ACES, PARCS, and RACE, have been proposed in the last few decades 19. Nevertheless, testing while in orbital operation is required to gauge the long-term operation of a space CAC. These methods provide a microgravity environment ranging from several seconds (drop tower, parabolic flight) to several minutes (sounding rocket). Moreover, other space applications in cold atom physics such as cold atom interferometry, optical clocks, and cold atom sensors also benefit from the techniques used in space CACs 14.Įxperiments related to cold atoms in microgravity have been successfully demonstrated in a drop tower, parabolic flights, and a sounding rocket 15, 16, 17, 18. Applying CACs in space is of great interest, not only in constructing the next-generation TKS and GNSS, but also in permitting deep space surveys and conducting more accurate tests of fundamental physics 9, 10, 11, 12, 13. Primary caesium fountain standards currently reach an uncertainty around 2 × 10 −16, and the improved accuracy and stability of optical clocks motivates a future redefinition of the SI second 7.Ĭurrently, the best performing space atomic clocks used in the GNSS are those at a frequency stability of a few parts in 10 15 per day 8. A variety of CACs have been demonstrated on the ground, notably atomic fountain clocks 2, 3, 4 and optical frequency standards based on neutral atoms in a lattice or trapped ions 5, 6. The width of the central Ramsey fringe for a cold atom clock (CAC) is almost two orders of magnitude narrower than that for their hot atom counterparts. ![]() The atoms are first cooled by lasers, and then interrogated by a microwave field typically with the Ramsey method. Laser cooling of atoms provides an approach to improve the performance of atomic clocks further 1, particularly in applications that require precision time-keeping over long time scales. Traditional atomic clocks which use hot atoms, however, have almost reached their limits especially in regard to long-term stability. ![]() Modern time keeping systems (TKS) on Earth and the global navigation satellite system (GNSS) rely heavily on atomic clocks.
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