Figure 1

Figure 1: A dual-color light beam passes through an atomic ensemble (orange cloud). One of the frequencies is red-detuned and the other is blue-detuned, with respect to the atomic absorption line (orange curve shows the imaginary part of the atomic susceptibility). The atoms induce opposite phase shifts on the light (purple dispersion curve shows the real part of the susceptibility), so that after passing through the ensemble the differential phase shift is proportional to the atom column density in the light path. Since both colors are significantly far from resonance, spontaneous photon scattering and troublesome heating of the atomic ensemble is strongly suppressed.