| The at omic co herence effects are the important ba ses of the quantum memory and qua ntum communication. T he electromagnetically i nduced transparency (EIT) effect, which can reduce the absorption of the probe field and enhance the dispersion due to the quantum interference, is usually used to control the group velocity, realize quantum memory, enhance the nonlinearity in vicinity of resonance, realize lasing without inversion, reduce quantum noise, store the correlation photon pairs, and so on. However, the excess n oises introduced by t he i nteraction be tween1ight a nd a toms may change t he noi se prope rty of the out going fi eld. T he excess noi ses ari sing from the atomic noise and phase-to-amplitude conversion noise degrade the fidelity of the output quantum state, which can not satisfy the requests for the high fidelity quantum memory in future. So, it is pretty necessary to explore the noise properties in the atomic coherence effects and give the reasonable method to avoid the damage of these noises.The dissertation reviews the development and research of s ome typical coherence effects, such as coherent population trapping, electromagnetically induced t ransparency, qua ntum memory, e lectromagnetically i nduced absorption, electromagnetically induced absorption in a standing wave, lasing without inversion, and so on. And then it presents the main works, which include t he i nfluence of prob e a nd coupling fi elds'1ine w idths on a tomic coherence, the output noise property of probe under the conditions of EIT and AT s plitting a nd t he out put noi se pro perty for t he four-level atoms w ith ground states perturbed by another coherent light.1. The effect of1aser linewidth on a tomic coherence effect i s studied. The white noise is introduced into the driving current hence changes the laser linewidth, then the laser amplitude noise and phase-to-amplitude conversion noise are measured using homodyne detection method. Taking the laser with injected white noise as probe or coupling field, the experiment on measuring the t ransparency am plitude a nd w idth i s c ompared for t he two c ases. T he result is that the effect of the linewidth variety of the coupling field on EIT is more pronounced than that of probe.2. The noise properties of the outgoing probe through atomic coherent medium are studied. It is demonstrated that under the condition of nonzero detection frequency, tuning the probe detuning makes us to obtain the optimal quantum noi se a tt he posi tion w here i t c orresponds t o t he m inimal phase-to-amplitude noise conversion. The probe detuning should be enhanced to obt ain t he opt imal out put noi se pro perty w hen t he de tection fre quency becomes less. The results on getting the best output quantum state provide us the theoretical certification to enhance the fidelity of quantum memory.3. T he noi se pro perty oft he non-classical s queezed p robe t hrough Autler-Townes splitting four-level medium is stu died. The theory which i s based on the experimental model of A-type t hree-level sy stem w ith perturbation of the coherent switching fi eld not onl y i ncreases the fre edom for manipulating the atomic coherence, but also demonstrates the possibility of double-channel quantum memory. The noise spectrum splits linearly with the d ynamic s tark s plitting a nd th e minimum n oise exits a tt he tw o transparency points s atisfying t wo-photon re sonance conditions. It c an be found that the quantum property is more easily degraded for weak fields due to the influence of the dephasing rate between ground states. The detuning of switching field affects the output amplitude noise and causes an asymmetry M-type c urves of t he noi se spe ctrum, s howing t hat one sque ezing w indow exhibits wider than the other. When the detection frequency equals the Rabi frequency of t he s witching field, w e can a lways obt ain the opt imal output quantum noise at the probe resonance.4. T he di fferences and re lations be tween E IT and A T s plitting a re discussed th eoretically, and t hen the1imit a nd cri terion fo r di stinguishing them i s o btained. Using t he c riterion, t he di fferent i nfluences of th e tw o effects on t he out put noi se prope rties in A-type t hree-level s ystem is compared. We also consider the influence of the medium's optical depth on the output noise. It can be demonstrated that the non-classical state may have higher fi delity i n o ptically t hin m edium w hen AT spl itting oc curs. W hile under the condition of EIT effect, the output noise will approach or even get larger than the shot noise level, which results from the Langevin atomic noise due to the random decay process of atoms. Next we will explore the role of interference factor in quantum memory and quantum state preservation using dressed state method in future.The characterized works of this thesis are as follows:â… . Based on the method injecting the white noise into the laser driving current, we observed the changes of the EIT peak and width with the increase of t he noi se am plitude. It s hows t hat t he I arge I aser1inewidth de grade t he transparency of probe, and it have been demonstrated that the influence of the coupling linewidth on EIT is more pronounced than that of probe.â…¡. We have t heoretically dem onstrated t he opt imal ou tput n oise property a lways o ccurs a t the posi tion c orresponding t o t he minimal phase-to-amplitude noise conversion. The less the detection frequency is, the larger the probe detuning should become, so as to keep the input quantum state nearly unchanged.â…¢. Using the th eoretical model of A-type three-level system with perturbation of the switching field, the double-channel quantum memory for non-classical light is demonstrated. The switching field which modifies the atomic hyperfine transition can modify the EIT resonance and ope ns more than one transparency windows to protect the quantum property of the input field from re sonance absorption. B y set ting t he prope r va lue of t he R abi frequency of the switching field, the maximum squeezing of the input state can still be obtained at nonzero detection frequency.â…£. According to the criterion for di stinguishing EIT and AT splitting, the output quantum properties of input squeezed vacuum are compared for the two cases. We have concluded that the quantum property of non-classical state can be well preserved in either optically thin or optically thick media when AT splitting e ffect occ urs, w hich corresponds t o t he strong-coupling regime. Wile for the case of EIT corresponding to the weak-coupling regime, the output noi se can be a ffected severely due to the influence of the1arge atomic noise, so it will approach or even exceed the shot noise level, which demonstrates that it is be tter t o manipulate t he quantum memory with E IT effect in optically thin medium to avoid large noise.
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