Observation of Ground-State Quantum Beats in Atomic Spontaneous Emission

We report ground-state quantum beats in spontaneous emission from a continuously driven atomic ensemble. Beats are visible only in an intensity autocorrelation and evidence spontaneously generated coherence in radiative decay. Our measurement realizes a quantum eraser where a first photon detection prepares a superposition and a second erases the “which path” information in the intermediate state.

Figure 1

(a) $\pi$-excitation of an $F=3$ to $F^{\prime }=4$ transition with scattering of a first, light grey (red), and second, dark grey (blue) photon, into the $H$ mode; (b) schematic of the apparatus, HWP: half-wave plate, PBS: polarizing beam splitter, BS: beam splitter, APD: avalanche photodiode.

Figure 2

Intensity correlation function of the $H$ mode for 2–3 photons in the $V$ mode with no atoms present and a 5 G magnetic field: (a) $\overline{N}=0.2$ and (b) $\overline{N}=2.0$.

Figure 3

Evolution of the quantum beat as $V$-mode light is mixed with the $H$ mode. Upper and lower extremes correspond to approximately 6 times more detector counts from the $V$ mode than spontaneous emission. The parameters are 0.6 photons in the $V$ mode, a magnetic field of 4 G, and $\overline{N}=1$.

Figure 4

Calculated intensity correlation function with the birefringence background set at 1% (a) and 10% (b) of the $H$-mode photon number; (c)–(e) the three interference terms contributing in (b); curve (a) is plotted against the data of Fig. 2b. Parameters are $\overline{N}=4$, 4 photons in the $V$ mode with no atoms present, and an atomic beam tilt of 1.3 degrees.