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 investigate the possibility of achieving the strong coupling regime of cavity quantum electrodynamics using rare earth ions as impurities in monolithic optical resonators. We conclude that due to the weak oscillator strengths of the rare earths, i
t may be possible but difficult, to reach the regime where the single photon Rabi frequency is large compared to both the cavity and atom decay rates. However reaching the regime where the saturation photon and atom numbers are less than one should be much more achievable. We show that in this `bad cavity regime, transfer of quantum states and an optical phase shift conditional on the state of the atom is still possible, and suggest a method for coherent detection of single dopants.
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.
It has recently been discovered that the optical analogue of a gradient echo in an optically thick material could form the basis of a optical memory that is both completely efficient and noise free. Here we present analytical calculation showing this
is the case. There is close analogy between the operation of the memory and an optical system with two beam splitters. We can use this analogy to calculate efficiencies as a function of optical depth for a number of quantum memory schemes based on controlled inhomogeneous broadening. In particular we show that multiple switching leads to a net 100% retrieval efficiency for the optical gradient echo even in the optically thin case.
