Amplified spontaneous emission is a common noise source in active optical systems, it is generally seen as being an incoherent process. Here we excite an ensemble of rare earth ion dopants in a solid with a {pi}-pulse, resulting in amplified spontane
ous emission. The application of a second {pi}-pulse leads to a coherent echo of the amplified spontaneous emission that is correlated in both amplitude and phase. For small optical thicknesses, we see evidence that the amplified spontaneous emission and its echo are entangled.
We have characterized a novel photon-echo pulse sequence for a double-$Lambda$ type energy level system where the input and rephasing transitions are different to the applied $pi$-pulses. We show that despite having imperfect $pi$-pulses (associated
with large coherent emission due to free induction decay), the noise added is only 0.019$pm$0.001 relative to the shot noise in the spectral mode of the echo. Using this echo pulse sequence in the `rephased amplified spontaneous emission (RASE) scheme cite{Ledingham2010} will allow for generation of entangled photon pairs that are in different frequency, temporal, and potentially spatial modes to any bright driving fields. The coherence and efficiency properties of this sequence were characterized in a Pr:YSO crystal.
We present a fully quantum mechanical treatment of optically rephased photon echoes. These echoes exhibit noise due to amplified spontaneous emission, however this noise can be seen as a consequence of the entanglement between the atoms and the outpu
t light. With a rephasing pulse one can get an echo of the amplified spontaneous emission, leading to light with nonclassical correlations at points separated in time, which is of interest in the context of building wide bandwidth quantum repeaters. We also suggest a wideband version of DLCZ protocol based on the same ideas.
