Photon-echo rephasing of spontaneous emission from an ensemble of rare-earth ions

Abstract

This thesis explores the concept of photon-echo-rephasing of the spontaneous emission from an ensemble of atoms. This generates entangled photon pairs, with intermediate storage and on-demand recall of one photon of the pair. In the first instance, this has applications as a single photon source. A more advanced application is as a building-block of a quantum repeater protocol, which is essential for expanding the range and versatility of current quantum communication links. The system used for this experimental demonstration of rephased amplified spontaneous emission (RASE) is a rare-earth ion doped crystal; Pr : Y2SiO5. Rare earth ion crystals are promising candidates for practical quantum information processing devices, largely because they possess long coherence times on both optical and hyperfine transitions. To achieve the rephasing, a novel pulse sequence utilizing four atomic levels is developed and characterized. This pulse sequence allows for the single-photon signals to be resolved spatially, spectrally, and temporally from any background coherent emission associated with the bright driving fields. It is seen that spontaneous emission can indeed be rephased using this sequence, however the degree of correlation measured was not sufficient to prove a non-classical correlation. This is attributed to an insufficient signal to noise ratio. The sources of background noise are identified, and strategies proposed to enable improved correlation measurements in future RASE experiments. The most significant noise contribution is spontaneous emission that differs only slightly in frequency from the signal. Single-photon counters are used for the RASE detection; appropriate for discrete-variable quantum repeater applications, but lacking any frequency resolution. Therefore auxiliary frequency filtering is required. Narrow-band (∼kHz) filtering is performed in the RASE experiment using spectral hole-burning properties of the Pr : Y2SiO5 sample itself. Furthermore, a concept for a dynamic filter is developed and tested. This uses a combination of hole-burning and Stark-shifting. The demonstration involves switching between two MHz-wide transmission windows, 10.2 MHz apart. Each spectral region is switched from an absorption of ∼0.5 dB to 60 dB in a matter of μs. This technique will be particularly useful in future RASE experiments for tunable frequency discrimination. An additional noise source that is found to fundamentally limit the fidelity of RASE in this system is due to uncorrelated spontaneous emission at the same frequency as the RASE signal. This emission is a direct consequence of the low branching ratio of the