Optical Pulse Storage In Medium And Selected Release For Two Channels By Fractional STIRAP

Taken as an information carrier, photon has some properties such as huge capacity, high efficiency, fast response which are much more superior to electron. This made optical storage become a frontier field in the quantum information science. Recently researchers have made great progress on theories and experiments about optical storage.In this thesis, we studied an experimental implementation in a three-levelΛ-type system of 87Rb atoms where lower level coherence is created between hyperfine levels (as shown in Fig. 1). In our experiment, the write control optical pulse (λ1 =794.9842 nm) is turned on to store the probe optical pulse (λ2 = 794.9698nm) in the collective atomic medium. It has been demonstrated that maximal coherence between levels can enhance the intensity of the revived probe pulse. Here, we make the write control optical pulse and the probe optical pulse with the same back edge to prepare the Rb atoms with maximal coherence between hyperfine levels ( 5 2 S1/2,F= 1,MF=?1,0 and 5 2 S1/2,F= 2,MF=+1,+2) of which frequency shift is 6.8 GHZ. After a time interval, we turn on the retrieve control pulse at 794.9842 nm; the recovered probe pulse at 794.9698 nm is released. Or we turn on the retrieve control pulse at 794.9698 nm, and then the revived probe pulse at 794.9842 nm is released. That means we experimentally demonstrate that optical pulses stored in an atomic system can be controllably released into two different photonic channels. FIG.1. Diagram of three-levelΛ-type system in 87 Rb atom. ?1 and ? 2 are the Rabi frequency of the write control and the probe pulses, respectively. ? 3 and ? 4 are the Rabi frequency of the retrieve control pulse or the restored pulse, respectively.We consider a three-levelΛ-type system, as shown in Fig. 1. The interaction Hamiltonian in the rotating wave and dipole moment approximations for the three-level system is The elements of the density matrix are given by the Liouville equation: The propagation of optical pulse fields in a medium is described by the equation: where are the coupling constants. We use coordinatesξandτ, which are related to the laboratory coordinates by The equation (2) and the equation (3) form a self-consistent system of equations.We performed numerical simulations of the above self-consistent equations for the case of maximal atomic coherence between hyperfine levels in 87 Rb vapor cell. In Fig 2, we performed the numerical simulations that obtain the released pulse with Rabi frequency ? 4 by the retrieve control pulse with Rabi frequency ? 3. Fig. 2(a) shows the input pulse envelopes with the write control pulse preceding the probe pulse and the retrieve control pulse coming behind of them at the entrance of the medium. The variation of the write control pulse, the probe pulse, the retrieve control pulse and the restored pulse at the exit of the cell is shown in Fig. 2(b). Fig. 2(c) shows the population transfer as a function of time. The initial population is equally distributed between state 1 and 3 . The write control field is first turned on and most of population is prepared to be in the state 1 , then the probe pulse enters into the medium to transfer a half of population from state 1 to 3 , as shown in Fig. 2(c). The coherence termρ1 3, as illustrated in Fig. 2(c), reaches its maximum value for the interval time when states 1 and 3 are half/half populated. We also performed numerical simulations of readout pulse with Rabi frequency ? 3 by the retrieve control pulse with Rabi frequency ? 4, as shown in Fig 3. The theoretical results confirm that optical pulses stored in an atomic system can be controllably released into one of the two different wavelengths. FIG. 2: Numerical simulations of obtaining the released pulse with Rabi frequency ? 4 by the retrieve control pulse with Rabi frequency ? 3. (a) The write control pulse, the probe pulse and the retrieve control pulse at the entrance of the sample cell. (b) The output pulses and the restored pulse at the exit of the cell. (c) Population transfer between 1 and 3 , and the coherence between states 1 and 3 . FIG. 3: Numerical simulations of obtaining the released pulse with Rabi frequency ? 3 by the retrieve control pulse with Rabi frequency ? 4.Now, we turn to a description of the experiment. An energy levels diagram of the experiment is shown in Fig. 1. The experimental arrangement is shown in Fig. 4. The time evolution of the pulse is shown in Fig. 5. The direct comparison among the Fig. 2, Fig. 3 and Fig. 5 shows that the experimental results are in good agreement with numerical simulations. The power of the write control pulse ?1 is 6 mw. The power of the probe pulse ? 2 is 5 mw. The two pulses with the same back edge drive all the atoms of the ensemble into a maximally coherent superposition between the 1 and states 3 . The time delay between the end of the write control pulse and the beginning of the retrieve control pulse ? 3 is 80 ns. The duration of the retrieve control pulse is 30 ns. FIG. 5: (a), (b) are the experimental results of obtaining the released pulse with Rabi frequency ? 4 (λ= 794.9698 nm) by the retrieve control pulse with Rabi frequency ? 3 (λ=794.9842 nm). (a)The write control pulse ?1 , the probe pulse ? 2 and the retrieve control pulse ? 3 at the entrance of the Rb cell, the duration of them are 170 ns, 30 ns, 30ns, respectively. (b)The pulses after Rb cell, ? 4 is the released pulse. (c), (d) are the experimental results of obtaining the released pulse with Rabi frequency ? 3 (λ=794.9842 nm) by the retrieve control pulse with Rabi frequency ? 4 (λ= 794.9698 nm). (c) The pulses at entrance of the cell. (d)The pulses after cell, ? 3 is the released pulse.We also study the storage time of restored pulse at 794.9698nm and 794.9842nm. Fig. 6 shows the intensity of restored pulse versus the storage time. We can note that the amplitude of pulse at 794.9842nm is higher than that at 794.9698nm. We believe that this phenomenon is caused by the population distribution of the hyperfine levels in the system. In experiment the storage time is up to 400ns.In summary, we experimentally demonstrated that the optical signal can be stored into and controllably released from the atomic medium at 794.9698nm or 794.9842nm. We also studied the storage time which can be up to 400ns in the experiment. Such controlled release of stored optical pulses may extend the capabilities of the quantum information storage technique, and can have applications in multichannels all-optical switching, quantum information, quantum networking and image storage systems.