Cold atomic ensemble controlled by Raman lasers

Our experimental system shown in Fig. 1a is similar to the one described in our previous work²⁵. The ⁸⁷Rb atoms are trapped by a magneto-optical trap. Two Raman lasers (Stokes and pumping lasers), respectively, couple two ground states (|1〉, |2〉) with the excited state (|3〉). The Raman lasers are set to be two-photon resonance (δ=0) and large single-photon detuning (Δ∼2π × 2.5 GHz) from the excited state. The frequency of the Stokes laser is further locked to the pumping laser with a stable beating frequency (bandwidth is <0.1 kHz) through optical phase-locked loop technique. The shapes of Raman pulses are controlled by two AOMs (Fig. 1a), which are driven by a radio source (Rigol, DG4162). The radio source has a frequency stability smaller than 2 p.p.m. and a maximum frequency output of 160 MHz.

With a bias field B_z about 0.1 G, two-photon Raman transition between magnetic sublevels of |F=1〉 and |F=2〉 is split by 140 kHz, which allows us to selectively transfer population between |F=1, m_F=0〉 and |F=2, m_F′=0〉. Population is measured with the fluorescence collected by a photodiode. To eliminate the total population fluctuation, the populations of |F=1, m_F=0〉 and |F=2, m_F′=0〉 are measured simultaneously in the experiments for normalization.