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We summarise the scientific and technological aspects of the Search for Anomalous Gravitation using Atomic Sensors (SAGAS) project, submitted to ESA in June 2007 in response to the Cosmic Vision 2015-2025 call for proposals. The proposed mission aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a Solar System escape trajectory in the 2020 to 2030 time-frame. SAGAS has numerous science objectives in fundamental physics and Solar System science, for example numerous tests of general relativity and the exploration of the Kuiper belt. The combination of highly sensitive atomic sensors and of the laser link well adapted for large distances will allow measurements with unprecedented accuracy and on scales never reached before. We present the proposed mission in some detail, with particular emphasis on the science goals and associated measurements and technologies.

The electric quadrupole shift is presently the most significant source of uncertainty on the systematic shifts for several single-ion optical frequency standards. We present a simple method for cancelling this shift based on measurements of the Zeeman spectrum of the clock transition. This method is easy to implement and yields very high cancellation levels. A fractional uncertainty of 5 x 10(-18) for the canceled quadrupole shift is estimated for a measurement of the absolute frequency of the 5s (2)S(1/2)-4d (2)D(5/2) clock transition of 88Sr+.

A Cs referenced optical frequency comb system has been used to measure the center frequency of the 5s 2 S 1/2 -4d 2 D 5/2 transition at 445 THz in a single, trapped, and laser-cooled 88 Sr + ion. The transition frequency SD = 444 779 044 095 510 Hz± 50 Hz ͑1͒ is obtained, when corrected for systematic shifts. A detailed calculation of the estimated systematic shifts is presented which yields improved values for the various shift parameters including blackbody and electric quadrupole moment shifts. PACS number(s): 32.30.Jc, 32.80.Pj, 06.20.Fn, 06.30.Ft FIG. 1. Partial energy level diagram of 88 Sr + showing the principal transitions used in cooling, fluorescence detection, repumping, and probing of the reference 445-THz ͑674 nm͒ transition. 70 012507-1 FIG. 2. Schematic diagram of the NRC femtosecond laser frequency comb system.