Reducing thermal noises by a quantum refrigerator

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Reducing the thermal noises in microwave (MW) resonators can bring about significant progress in many research fields. Recently, a bench-top cooling method using "quantum refrigerators" has been adopted to reduce the thermal noises, reaching around liquid nitrogen temperature. In this study, we investigate the possible cooling limit of the MW resonator by using three-level or four-level systems as the quantum refrigerator. In this refrigerator system, proper light pump makes the multilevel systems concentrated into their ground states, which continuously absorb the thermal photons in the MW resonator. By adiabatic elimination, we give a more precise description for this cooling process. It turns out, though the multilevel systems can be efficiently cooled down, the laser driving also significantly perturbs their energy levels. For three-level refrigerators, such perturbation causes the atom-resonator interaction to become off-resonant, impeding the heat transfer from the MW resonator to the refrigerator, which greatly weakens the cooling effect. We also find that, by using four-level systems as the refrigerator, this issue can be well overcome. Based on practical parameters, our estimation shows the cooling limit could reach the liquid helium temperature.