1. Introduction
Quantum coherence and interference (Scully & Zubairy, 1997) are leading edge topics in quantum optics and laser physics, and have led to many important novel effects, such as coherent population trapping (CPT) (Arimondo & Orriols, 1976, Alzetta et al., 1976, Gray et al., 1978), lasing without inversion (LWI) (Harris, 1989, Scully et al., 1989, Padmabandu et al., 1996), electromagnetically induced transparency (EIT) (Boller et al., 1991, Harris, 1997, Ham et al., 1997, Phillips et al., 2003, Fleischhauer et al., 2005, Marangos, 1998), high refractive index without absorption (Scully, 1991, Scully & Zhu 1992, Harris et al., 1990), giant Kerr nonlinear effect (Schmidt & Imamoglu, 1996), and so on. In particular, EIT, which can dramatically modify the absorption and dispersion characteristics of an optical medium, plays an important role in quantum optics. In the last two decades, EIT has attracted great attention and has been successfully applied to ultraslow and stopping light (Kasapi et al., 1995, Hau et al., 1999, Kocharovskaya et al., 2001, Liu et al., 2001, Turukhin et al., 2002), quantum switching (Ham & Hemmer, 2000), quantum memory (Ham et al., 1997; ibid., 1998), quantum entanglement generation (Lukin & Imamoglu, 2000), and quantum computing (Lukin & Imamoglu, 2001).
It is well established that light is the fastest information carrier in nature. However, controlling light for localized application is very difficult. Thus, manipulation of light velocity becomes a crucial task in optical and quantum information processing (Nielsen & Chuang, 2000). Recently light localization using EIT has been demonstrated for stationary light (Bajcsy et al., 2003). Stationary light gives novel effects to nonlinear quantum optics in the context of lengthening light-matter interaction time. Compared with ultraslow light, where the medium’s length is a limiting factor, stationary light is free from spatial constraint. For example, the interaction time using ultraslow light in a semiconductor quantum dot, whose spatial dimension is less than a few tens of nanometers, is much less than nanosecond. By using a stationary light technique, however, we can enormously increase the interaction time of the light with such a nano optical medium. In this chapter, we discuss stationary light based on the EIT-induced ultraslow light phenomenon. We theoretically investigate how to dynamically manipulate multicolor (MC) stationary light in the multi double lambda-type system by simply changing the parameters of control fields, and demonstrate ultralong trapping of light, which is different from the conventional quantum mapping phenomenon. Quantum coherent control of the stationary light has potential applications to various quantum optical processing such as quantum nondemolition measurement and quantum wavelength conversion.
This chapter is organized as follows. In section 2, quantum coherent control of two-color stationary light is described. In section 3, quantum manipulation of MC stationary light is presented. In section 4, we give the results of MC stationary light, with discussions. Finally, section 5 offers conclusions.
2. Quantum coherent control of light
2.1. Model and theory
In this section, we present dynamic control of two-color stationary light in a double-lambda type four-level system using EIT. Figure 1 shows the energy level diagram. We assume that initially one weak probe quantum field
For theoretical analytical purposes, we introduce a quantum field
where
Under the typical adiabatic condition for the slow light propagations (Fleischhauer & Lukin, 2000, Zibrov et al., 2002), we introduce new field operators
where
Using spatial Fourier transformation
where
where
Now we study temporal dynamics of the two-color coupled fields. Initially the control field + (-=0) is turn on and a probe pulse A+ with a Gaussian shape (
From Eq. (7), the stationary light condition (
Thus the group velocity of the coupled light can be easily controlled by manipulating the control field Rabi frequencies. If the stationary light condition is satisfied, the two coupled lights can be completely stopped. Ignoring small terms proportional to
where
2.2. Numerical simulations and discussions
Here we numerically demonstrate quantum manipulation of a traveling light pulse for the two-color stationary light by solving Eqs. (2) and (3). For simplicity we ignore the weak decay rate
Compared with the standing-wave grating case (Bajcsy et al., 2003, Andre et al., 2005), the group velocity
At time
Figure 3 shows numerical simulation of the two-color stationary light for case (a) mentioned above (also see Figure 2(1)). For simplification, we set
frequencies for the control fields. Figure 3(c) and Figure 3(d) show the top view of the fields A+ and A- propagation, respectively. Figure 3(e) and Figure 3(f) show the temporal evolutions of the field amplitudes A+ and A-, respectively. Figure 3(g) and Figure 3(h) show space-time evolution of the fields A+ and A-, respectively. These figures show that when the control pulse
Figure 4 shows numerical simulation of the two-color stationary light for case (b) (see Figure 2(2)). Similar to Figure 3, Figure 4(c) and Figure 4(d) show the top view of the coupled fields. Figure 4(e) and Figure 4(f) show the temporal evolutions of the coupled field amplitudes. Figure 4(g) and Figure 4(h) show space-time evolution of the coupled fields. As seen in Figure 4(c) and 4(d), when
In summary, we have demonstrated two-color stationary light and quantum wavelength conversion using a quantum-mechanically reversible process between photons and atomic coherence in a double-
3. Quantum manipulation of MC stationary light
If we choose a multilevel system, MC stationary light should be possible. In this section, we show quantum coherent control of multiple travelling light pulses in an optically dense medium by generalizing the approach we used in the previous section. Figure 5 shows an energy level diagram of the present MC stationary light. The control fields with Rabi frequencies
We assume that initially all the atoms stay in the ground state
atoms are driven by one classical control field
For a theoretical analysis of MC stationary light based on the multi double lambda-type scheme in Figure 5, we introduce quantum fields
where
Using Eq. (12) and adding the relaxation constants and Langevin forces associated with the atomic relaxation processes, we derive the Heisenberg equations for the atomic operators
where
In Eqs. (13)-(15), we ignore the influences of the weak populations of excited states
where
We assume a typical adiabatic condition for slow light propagation
where the fluctuation forces are:
Eqs. (21) and (22) can be solved by using spatial Fourier transformation. Assuming that initial field
where
4. Results and discussion
In this section, we analyze quantum evolution of the MC field. Under the slow light condition and taking into account weak relaxation processes between the two ground levels
where
4.1. Traveling MC field
Ifwhere the spatial dispersion will be
4.2. Stationary MC field
Eg. (26) shows that all the laser fields propagate and evolve together with one group velocity
This is the MC stationary light condition. Obviously, this relation generalizes the results obtained for two-color stationary light. The amplitudes of these coupled fields can be manipulated by varying the Rabi frequencies of the corresponding control fields:
For a Gaussian shape of the initial input probe pulse, we have the following amplitudes
Eq. (32) shows that the amplitude
The maximum MC field stationary time is determined by relaxation constant
Using the conditions
by satisfying the particular relationship to the initial probe field
From Eqs. (34) and (35), we can know that the condition for minimum spreading is independent of the total number of the control fields M+ and M-. Putting Eq. (31) into Eq. (27), we find the important relation for the minimum spatial spreading of the MC stationary light pulses:
4.3. MC-wavelength conversion
After optical trapping of the initial probe pulse by using MC stationary light, it is possible to generate an arbitrary forward traveling light field
The electric field amplitude of the m-th component in the MC light at time
with the temporal duration
5. Conclusion
We first demonstrated the two-color stationary light and quantum wavelength conversion using quantum coherence resulting from strongly coupled slow light through EIT in a double-lambda system. Then we generalized the approach to the MC light fields in the multi double Λ coherent atomic medium driven by the M++M- control intensive laser fields and showed how to manipulate the MC light field within the adiabatic limit. The results show that the MC light fields can be controlled by simply adjusting the control fields’ parameters for (1) MC stationary light, (2) selection of propagation direction (forward or backward), and (3) MC wavelength conversion. The maximum stationary time and minimum spatial spreading of the MC field have also been discussed. On-demand quantum manipulation of the MC light field can greatly increase the interaction time of the light and medium, and holds promise for applications in optical buffer, controllable switching, and quantum optical information processing.
We acknowledge that this work was supported by the CRI program (Center for Photon Information Processing) of the Korean Ministry of Education, Science and Technology via National Research Foundation.
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