1.1 Trapped neutral atom as quantum memories

Photons are ideal carrier of quantum information for communication, but storing them for a long time is difficult. In optical quantum communication, a quantum network can consist of spatially separated quantum memories to store and manipulate information which is encoded in internal quantum state of physical system such as photons, ions, atoms, etc. [1]. In order to operate a successful long-distance quantum communication and quantum information processing, the quantum memory should have a high-storage efficiency, which is defined as the ratio of input photon energy to long coherence time. Different physical systems such as cold atoms trapped in optical lattices [2–8] or cold ions in electrostatic traps [9] are used as quantum memories. Ions are electrically charged and therefore can be tightly confined in deep traps for a long time. However, the strong Coulomb repulsion

limits the number of ions that can be precisely controlled in a single trap. In contrast, more promising experiments could be made with the cold trapped neutral atoms. The neutral atoms usually interact only at very short range and can be collected in large ensembles without perturbing each other, and therefore the decoherence and losing the quantum information due to the interaction with the environment are low. In other words, single qubits [1] or even multi-qubits can be encoded in atomic states which afford long coherence times. Cooling and trapping of neutral atoms is one of the challenging techniques to achieve a higher signalto-noise ratio and to stabilize the system over long periods for more balanced memory efficiency and fully control of all physical degrees of freedom with long coherence times. To keep the atom in trap, it is necessary to raise the atomic trap depth to be comparable to or even larger than the thermal energy of background atoms. Although, in principle, there is no fundamental limit to the lifetime with sufficiently deep traps, there are practical limitations. Optical traps for neutral atoms cannot be arbitrarily deep since both the trap depth and the photon scattering rate, which causes decoherence, scale proportional to the optical power. Even in the absence of collisional losses as the main reason to lose the trapped atom, the heating due to fluctuations of the trapping potential can eventually cause trapped atoms to escape. As a result, the trapping system used to confine the atoms is switched off during the storage. Nevertheless, the residual electric and magnetic field still limits the lifetime of the quantum memory [10, 11]. For a quantum memory in a magnetooptical trap (MOT), the atomic diffusion imposes a strict limitation on the storage time, which in turn limits the maximum distance for quantum communication in practical applications. On the contrary, the optical dipole trap can offer an array of spatial forms which can be rapidly switched as well [12]. Optical dipole traps rely on the induced atomic electric dipole interaction with electric component of the trapping light. The power needed for trapping depends on the desired trap depth and the detuning from the nearest optical transitions. Small detuning gives deeper traps, with a depth scaling as 1/Δ, where Δ is the detuning from the nearest strong electronic transition. This must be balanced against the photon scattering rate, which causes heating and decoherence and scales as 1/Δ<sup>2</sup> [13]. When the light frequency is far-detuned [12, 14, 15], a nearly conservative potential well with less influence from spontaneous photon scattering is created for the atom. The AC-Stark shift in the atomic state from the induced dipole gives sensitivity to intensity noise of lasers and atomic position. The nonuniform spatial intensity profile results in the intensity-dependent AC-Stark shift and defines the shape of associated atom trap. Therefore, the AC-Stark shift affects the qubit level, and fluctuation in laser field leads to broad the qubit line.

In red-detuned far-off resonance optical dipole trap (FORT) [16, 17], the laser beam is focused to attract atoms to the region, where the intensity is high. The large detuning efficiently suppresses the effect of scattered photons. On the other hand, the blue-detuned FORT [18, 19] can confine the atom in the dark region of the bluedetuned laser beams. The blue-detuned FORT has several advantages over the bright trap of the red-detuned laser. As the atoms are trapped in the "dark" place, the photon scattering rate due to the trapping laser can be greatly reduced, while in the bright trap, this rate can be reduced only by increasing the detuning of the trapping laser. A quantum memory of this sort thus has a potential storage time of seconds [13]. In conclusion all the abovementioned issues, such as linear and nonlinear scattering, recoil heating, intensity fluctuation, and pointing instabilities of the trapping beams, result in dipole-force fluctuations, and collisions with the background gas lead to heating up the qubit atom and therefore escaping from the trap [20]. This motivates the researchers to look for a way to create more stable quantum memories with longer storage time.

Localized Excitation of Single Atom to a Rydberg State with Structured Laser Beam for Quantum… DOI: http://dx.doi.org/10.5772/intechopen.82319
