Interference Between Atomic Rb (5d_5/2 – 5p_3/2) and (5p_3/2– 5s_1/2) Coherences Observed by Quantum Beating at ~2.1 THz

We observe the interference of atomic coherences in Rb, established among the 7s_1/2, 5d_5/2, 5p_3/2, and 5s_1/2 states in pump-probe experiments and observed directly in the temporal and spectral domains through quantum beating in the ~2-36 THz range. In particular, interference between the (5d_5/2 – 5p_3/2) and (5p_3/2 – 5s_1/2) coherences at 386.3 and 384.2 THz, respectively, is mediated by the 5p_3/2 state and detected through quantum beating in the vicinity of the (5d_5/2 – 5p_3/2) – (5p_3/2 – 5s_1/2) difference frequency of 2.11 THz which is monitored by a parametric four-wave mixing process. The ~2 THz spectral lineshape is that of a generalized Fano profile which can be controlled by varying experimental parameters. A transition of (0.8 ± 0.2) ~π/4 radians in the phase of the coupled oscillator occurs when the laser intensity or the background Rb number density are varied, for example, a phase shift associated with the transformation of a broadband, dissipative oscillator (characterized by a Fano interference window) into a strongly-coupled system resonant at 2.1 THz.