Cold atomic clocks have proven long term performance in outer space

Chinese scientists launched a cold atomic clock into orbit. Relative error of measuring time such a clock was approximately 10-13 (one second for 160 thousand years), after 15 months of flight features hours remained at the same level. Article published in Nature Communications.

In these days of precise time measurement is mainly used atomic clocks based on transitions between energy levels of atoms. These clocks are installed at base stations of mobile communications and in the services of the exact time; without atomic clocks would be impossible to operate navigation systems (GPS and GLONASS), in which the distance to the point determined by the time of signal reception from satellites. Read more about the work of such navigation systems you can listen to the story of theoretical physicist Emil Akhmedov. In addition, the modern definition of the second also relies on atomic clocks: in the international system of units SI the second is the period of time during which occurs 9192631770 oscillations that meets the resonance frequency of an energy transition between levels of the hyperfine structure of the ground state of the atom of cesium-133 (133Cs).

However, the standard atomic clock, working with “hot” atoms are not the most accurate. For example, the national time standards (cesium clocks) are about one billionth of a second per day, that is, the relative error of frequency measurement of the transition of atom in them is approximately 10-14. On the other hand, the error of “cold atom clocks”, or rather, hours with cold atoms (cold atomic clock, CAC), is less than 10-18. Placed in these clocks the atoms are cooled with lasers to temperatures of the order of several hundreds of nanokelvins, and then read the oscillation frequency of the atoms, applying to them an oscillating magnetic field and measuring the population of the energy levels (this technique is called interferometry by Ramsay, Ramsey interferometry). Sorry, still don’t know how such a watch will behave in space. To conduct such a review, should significantly reduce the energy consumption, volume and weight hours, to meet the compatibility standards for manufactured electromagnetic fields, thermal radiation and mechanical load for space missions, as well as protect the watch from fluctuations in the Earth’s magnetic field and the effects of cosmic rays.

A group of scientists under the leadership of Lian Liu (Liang Liu) was first conducted such a review. To do this, they have produced a cold atomic clock, tying in an optical trap about 50 million atoms of rubidium-87 and measuring using interferometry Ramsay the transition frequency between the levels with the values of the orbital quantum number L = 1 and L = 2. To reduce the influence of external effects, physics has pumped out the installation to a pressure of about 10-12 atmospheres, screened her three layers of mu-metal and changed by means of servomotoru the configuration of layers so that fluctuations of the external magnetic field does not exceed five nanotesla. If such shielding is not performed, the watch would have quickly failed — magnetic field at the Earth’s orbit reaches 80 microtesla, that is, it should lead to splitting of energy levels and change the crossover frequency as a result of the Zeeman effect. Making sure that the hours meet all the requirements for spacecraft, scientists have found them on the satellite Tiangong-2 and launched into orbit September 15, 2016 (the acceleration of free fall of the order of 10-4g).

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