Magnetic Field Enhanced Coherence Length in Cold Atomic Gases

We study the effect of an external magnetic field on coherent backscattering of light from a cold rubidium vapor. We observe that the backscattering enhancement factor can be increased with $B$. This surprising behavior shows that the coherence length of the system can be increased by adding a magnetic field, in sharp contrast with usual situations. This is mainly due to the lifting of the degeneracy between Zeeman sublevels. We find good agreement between our experimental data and a full Monte Carlo simulation, taking into account the magneto-optical effects and the geometry of the atomic cloud.

Figure 1

Principle of the experiment. An external magnetic field $\mathbf{B}$ is applied orthogonally to the incident wave vector ${\mathbf{k}}_i$. A circularly polarized laser beam is shined at the cold atomic cloud, and the backscattered far-field intensity distribution is recorded on a charge-coupled device (detection in the parallel helicity channel).

Figure 2

CBS enhancement factor of light on a cold Rb atomic cloud, measured in the parallel helicity channel $h\parallel h$, as a function of the transverse magnetic field $B$ (circles); it shows a dramatic increase at nonzero magnetic field, whereas time-reversal symmetry is broken. For each $B$ value, the laser is tuned to resonance with the $m_F=3\to m_{F^{\prime }}=4$ transition. The solid line is the result of the Monte Carlo simulation (no adjustable parameter). The inset shows two (angularly averaged) experimental CBS peak profiles for $B=0$ (1) and $B=43\mathrm{G}$ (2).

Figure 3

Results of the Monte Carlo simulations. (a) shows the increase of the ratio of coherent over incoherent intensity with $B$. (b) Contribution of single scattering (triangles), incoherent (crosses), and coherent (circles) multiple scattering to the total intensity in the backward direction. Lines are drawn to guide the eyes.

Figure 4

Plot of $p_0=L_{\varphi }/\ell$ vs $B$, as extracted from our Monte Carlo calculation, as a function of the magnetic field $B$. In sharp contrast with the usual behavior, $p_0$ increases with $B$, roughly linearly (the solid line is drawn to guide the eye). This is due to the lifting of the Zeeman degeneracy. The point dispersion reflects the numerical accuracy.