Russian Physicists Discover Method to Increase Number of Atoms in Quantum Sensors
Physicists from the Institute of Spectroscopy of the Russian Academy of Sciences and HSE University have successfully trapped rubidium-87 atoms for over four seconds. Their method can help improve the accuracy of quantum sensors, where both the number of trapped atoms and the trapping time are crucial. Such quantum systems are used to study dark matter, refine navigation systems, and aid in mineral exploration. The study findings have been published in the Journal of Experimental and Theoretical Physics Letters.
Quantum sensors are devices that leverage the effects of quantum mechanics to study matter. They enable the detection of minute changes in gravitational and magnetic fields and allow for highly precise measurements of Earth's acceleration and rotation. Advancements in this field of modern applied physics have the potential to redefine the standards of accuracy in measuring physical quantities.
However, atoms cannot simply be placed in a sensor, as thermal motion prevents them from remaining in place for even a minute. To confine atoms within a specific area, scientists slow them down by cooling them through multiple stages using various techniques. The first stage involves cooling and trapping atoms in magneto-optical traps (MOTs), which are created using laser light and magnetic fields. Creating magnetic field distributions in compact devices requires the use of an atomic chip.
'Each cooling stage reduces the number of atoms in the sensor's working volume, which in turn decreases the accuracy of the device. Therefore, it is crucial to collect as many atoms as possible during the preparation of the initial ensemble to ensure that the accuracy of the quantum sensor remains high after all cooling stages.' This is how Daria Bykova, a doctoral student and teacher at the HSE Faculty of Physics, explains the key aspect of the problem.
Primary cooling to a temperature of around a hundred microkelvins significantly slows the thermal motion of atoms, helping retain them in a designated area of space. A decrease in temperature is achieved through laser radiation: exposure to a laser beam causes atoms to lose kinetic energy and move more slowly. Together, laser radiation and a magnetic field hold atoms in place long enough to conduct experiments, effectively forming a trap from which atoms cannot easily escape. In the next stage, which does not involve a laser field, atoms are cooled to a temperature of about a hundred nanokelvins, which is another thousand times lower.
'One could say that we use laser radiation to "push" the atoms toward the centre of the trap. They are trapped by a magnetic field and the pressure of light,' comments Bykova.
An atomic chip is an effective technology that enables researchers to reduce the size of quantum sensors and improve their energy efficiency. It generates a magnetic field near its surface, which is essential for creating traps, and allows for the cooling and localisation of atom ensembles near it.
At the Department of Laser Spectroscopy of the Institute of Spectroscopy (RAS), students and doctoral students from HSE University created traps using atomic chip technology. This configuration allowed them to retain atoms in the designated area for 4 seconds, a duration considered long in quantum technologies.
The researchers experimentally demonstrated that when using an atomic beam to load atoms into a MOT on a chip, the number of trapped atoms increases significantly compared to loading from atomic vapour in a vacuum chamber. The researchers also confirmed their ability to effectively control the loading of the atomic trap. They were able to adjust the position of the atomic beam using laser fields. This combination of technologies has significantly increased the loading speed while maintaining an ultra-high vacuum in the atomic chip area, compared to previous experiments.
'We discovered the optimal loading conditions in the MOT and trapped 4.9×10⁷ atoms, a number sufficient for stable operation. The ensemble's lifetime is 4.1 seconds, which is long enough to carry out the subsequent stages of deeper cooling and create a prototype of a quantum sensor,' explained Anton Afanasyev, Associate Professor at the Joint Department of Quantum Optics and Nanophotonics with the Institute for Spectroscopy (RAS) of the HSE Faculty of Physics, Senior Research Fellow at the Institute of Spectroscopy (RAS).
The study was supported by the HSE Academic Fund and carried out at the Department of Laser Spectroscopy of the Institute of Spectroscopy (RAS).
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