Cold atoms and precision sensors in space

features [PhySICS In SPACE] ColD AToMS AnD PrECISIon SEnSorS In SPACE >>> DoI 10.1051/epn:2008305 w. ertmer 1, e. rasel 1, C. salomon 2, s. schiller 3, g.m. tino 4 and l. Cacciapuoti 5 IQO Hannover, 2 ENS Paris,3 Heinrich-Heine-Universität Düsseldorf,4 University of Firenze,5 European Space Agency 1 C ooling atoms, which means slowing them down to very low velocity, enables scientists to realise accurate physical measurements based on quantum transitions between atomic energy levels. A well-known spin-off is the astonishing precision of atomic clocks. Measurement accuracy improves as the interaction time between atoms and excitation field increases. Laser cooling can reduce the velocity of the atoms and increase the interaction times, but it cannot cope with Earth gravity without perturbing the measurement. In ground-based laboratories, atomic fountains help to circumvent gravity and extend the measurement duration. This naturally led scientists to envisage experimenting with ultra-cold atoms in space where long and unperturbed evolution times are possible in a freely falling laboratory. Such an environment enables one to exploit the potential of cooling atoms close to stand still permitting: • extension of the measurement time by one to three orders of magnitude in a perturbation-free environment; • quantum evolution unbiased by gravity; • reduction of the kinetic energy by up to three orders of magnitude, down to the sub-pico-Kelvin regime. atomic clocks A pioneering step will take place in 2013, when the Columbus module will receive the Atomic Clock Ensemble in Space (ACES) payload [1], carrying ultra-stable atomic clocks and a high-precision time transfer system. Using this accurate time reference in space, the ESA-led ACES mission will perform new tests of general relativity, search for possible minute violations of Einstein’s equivalence principle, and develop several applications in Earth observation and geodesy. Clocks are basic instruments for science, technology and industry. For several decades, the most accurate and stable clocks have been atomic clocks using the hyperfine transition of the neutral Cs atom at 9.1 GHz as quantum reference. This transition also defines the unit of time. Every Europhysics News reader has played with a GPS receiver! In this global navigation system, each GPS satellite carries a set of precise atomic clocks. The orbiting satellites send electromagnetic timing signals to re- ceivers on Earth that can determine their position with submeter accuracy by a simple triangulation method. Compared to GPS clocks, the time scale provided by ACES in space will have a hundred-fold increase in precision. Indeed in the PHARAO clock, developed by the French space agency CNES in the frame of the ACES mission, caesium atoms are cooled to a temperature of 1 μK, corresponding to velocities of about 7mm/s (Fig 1). For free-falling atoms, these low velocities cannot be maintained on the Earth surface because of the gravity acceleration. In space, there is no such limit and atoms in free expansion can be probed for several seconds. In this way, we expect the space clock to reach a relative frequency stability and accuracy of 1 part in 1016, which corresponds to an error of less than one second over 300 million years. While the accuracy of cold Cs atom clocks is already very high, recent advances in laser and quantum technology have opened the way to the realization of even more accurate and stable clocks. In optical clocks, the reference oscillator is a laser whose frequency is continuously controlled by comparison with the atomic transition. It is expected that clocks operating in the optical domain of the electromagnetic spectrum rather than in the microwave domain (with a ~105 times larger frequency) will bring at least two orders of magnitude improvement, reaching the 10-18 stability and accuracy level. Two development lines are currently investigated: optical clocks based on a single atomic ion in an electrodynamic trap and optical lattice clocks, based on ensembles of ~104 neutral atoms trapped in the periodic potential produced by an intense laser. Present accuracy of ion clocks reaches a few parts in 1017 [2], while lattice clocks have been evaluated to one part in 1016 [3]. Both laboratory clock types exhibit frequency instability significantly lower than that of Cs clocks, reaching levels as low as 1•10-16 after few hours of averaging. When operated in space, optical clocks can enable high-precision experiments, e.g. measurements of the gravitational redshift due to various solar-system bodies, of the Shapiro gravitational time delay, and accurate tests of the space-time independence of fundamental constants. One particularly attractive 䉴 䉳 FIg. 1: (left) working principle of the PHarao clock (Projet d’Horloge atomique à refroidissement d’atomes en orbite); Cs atoms are launched in free flight along the ultra-high vacuum tube where they are probed on the clock transition by the resonant field in a microwave cavity. (right) engineering model of the clock interrogation tube; the clock is presently under test at Cnes premises in toulouse. europhysicsnews number 3 • volume 39 • 33 Article available at http://www.europhysicsnews.org or http://dx.doi.org/10.1051/epn:2008305 features 䉴 application of clocks in space is a high-precision mapping of the gravitational potential at the Earth’s surface. This technique, which will be first demonstrated by the ACES mission, is based on the precision measurement of the gravitational red-shift between clocks on the ground, continuously compared via a master clock in space. Implementing optical clocks in space requires a concerted development effort of research groups, industry, and space agencies. Size, mass, power consumption, reliability, and performance of the clock shall reach a level compatible with the individual applications. The Space Optical Clock (SOC) project, supported by ESA and national space agencies, is developing this technology, currently focusing research efforts towards a transportable prototype of an optical clock based on Sr atoms. atom interferometry sensors Atomic clocks are not the only instruments which benefit from weightlessness! Clocks belong to a larger class of inertial sensors based on ultra-cold atoms and on the interference of atomic matter-waves. Inertial sensors using atom interferometry provide a new tool for the precise measurement of acceleration, rotations, and faint forces [3]. According to the principle of these sensors, the measured physical quantity is converted into a frequency, which can be measured with the highest accuracy. Atomic gravimeters, gyroscopes, and gravity gradiometers have now reached on the ground a level of performance competitive with classical instruments and surpassing them in some cases. The most important features of these instruments are represented by the precisely known calibration factor and the good long-term stability, p (...truncated)