Study On Coherent Cooling Of Trapped Quantum Systems

The motion of a single trapped ion, atom or a nanomechanical resonator can be treated as quantum harmonic motion at low temperature. Ground state cooling of such systems is a fundamental ingredient toward quantum state preparation, quantum coherent manipulation and quantum information process. The present cooling method of choice for trapped systems is the sideband cooling. But during last few years, there are a variety of new cooling methods have been proposed by exploiting the quantum coherent effects. This kind of new cooling methods have several advantages in comparisons with traditional sideband cooling. The thesis mainly investigates coherent cooling schemes of trapped ion systems,trapped atoms in an optical resonator, and nanomechanical resonators. The main results and the creative points are listed as follows:1.We have proposed a double dark-state cooling scheme of a trapped ion using standing waves. In our scheme, the off-resonant heating by carrier transition is eliminated due to the property of the standing wave, and the blue sideband heating is eliminated by the EIT effect. Henceforth, all the dominant heating transitions vanish, and the whole system will be eventually cooled to a double dark-state. Compared with former cooling methods,the final mean phonon number can be lower than the single-photon recoil energy, and is robust to the fluctuations of the laser strength.2.We have proposed a cooling scheme of a trapped ion using quantum interference between different pathways. In our scheme, the carrier transition is eliminated by the EIT effect, and the blue sideband heating is eliminated by the quantum destructive interference between different pathways. The system can be successfully cooled to its ground state as well. The cooling scheme is also a subrecoil cooling method and robust to fluctuations of the laser strength. Moreover, in the new scheme, fast cooling rate can be achieved and there dimensional simultaneous cooling can be realized.3.We have proposed a coherent cooling method of a trapped atom in an optical cavity. The cooling scheme is based on the quantum coherent effect between the cavity and the atom, namely, the cavity induced transparency(CIT). In the cooling scheme, the carrier transition is successfully suppressed by the CIT effect. Furthermore, we use the EIT effect to eliminate the blue sideband heating. Further theoretical analysis and numerical simulations show that the final phonon number of the new cooling method is lower than pervious cooling methods in the strong coupling regime.4.We attempt to generalize the idea and the method of the coherent cooling to nanomechanical resonator systems. We have proposed a ground state cooling method of a singlephoton cavity optomechanical system coupled with a single atom. We find that the heating transition due to the radiative pressure can be successfully suppressed due to the coherent effects between the Λ-type three level atom and the cavity photon. The nanomechanical resonator can be cooled to its ground state in strong coupling regime. Compared with former cooling methods, there is less requirements for the linewidth of the atom and cavity,which is easier to be applied in the experiment.