In the world's most accurate devices, interferometers, the quantity under investigation (gravitational wave amplitude, magnetic field, energy difference in atomic clocks, etc.) is imprinted to the phase difference in both arms of the interferometer. From the results of the measurements of the phase difference, the desired value is estimated. During the colloquium I will tell you how interferometry is carried out in practice, using Bose-Einstein condensates. I will present the results of my collaboration with the experimental group of Philipp Treutlein from Basel, where, comparing a number of experiments with theory, we understood what factors currently limit precision in the system, how to improve it, and found new values of some atomic constants. The colloquium in the remote form at ZOOM. Please ask the organizers for a link to the meeting.