Decoherence of a two-state atom driven by coherent light

Recent studies of the decoherence induced by the quantum nature of the laser field driving a two-state atom [J. Gea-Banacloche, Phys. Rev. A 65, 022308 (2002); S. J. van Enk and H. J. Kimble, Quantum Inf. Comput. 2, 1 (2002)] have been questioned by Itano [W. M. Itano, Phys. Rev. A 68, 046301 (2003)] and the proposal made that all decoherence is due to spontaneous emission. We analyze the problem within the formalism of cascaded open quantum systems. Our conclusions agree with the Itano proposal. We show that the decoherence, nevertheless, may be divided into two parts—that due to forwards scattering and to scattering out of the laser mode. Previous authors attribute the former to the quantum nature of the laser field.

Figure 1

Schematic diagram of the cascaded system of a two-state atom (Hamiltonian $H_A$) driven by a coherent laser source (Hamiltonian $H_L$). The various inputs and outputs are defined in the text.

Figure 2

Sample Monte Carlo simulations of the Rabi oscillation of the driven two-state atom in the presence of spontaneous emission, total emission rate $\gamma =2{\kappa }_A+2{\kappa }_A^{\prime }$: for a Rabi frequency $\Omega =\sqrt{{\kappa }_L{\kappa }_A}\mid \alpha \mid =2\gamma$ (for coherent state ${\mid \alpha ⟩}_L$) and forward-scattering rate (a) $2{\kappa }_A∕\gamma =0.04$, (b) $2{\kappa }_A∕\gamma =0.4$. The probability $p_e\left(t\right)$ to find the atom in the excited state is plotted as a function of time. Open (closed) triangles mark the times of forwards-scattering (side-scattering) quantum jumps.