Characterizing a Quantum Memory Incorporating an Entangled Light Source

Abstract

It has been proposed that secure communication over large distances can be achieved using quantum repeater protocols. Two key components which are required for a repeater are an entangled light source and a quantum memory. The approach investigated here, is to use a protocol which incorporates both a memory and entangled light source using rephased amplified spontaneous emission (RASE). To be a viable repeater, RASE must achieve second long storage times, and efficiencies exceeding 90\%. The focus of this thesis was to investigate the mechanisms limiting the efficiency of RASE. In this work, RASE was performed in the rare-earth ion-doped crystal Pr:YSO, and the coherence was stored on the ground hyperfine transition for 5 $\mu s$. Rare-earth ion-doped crystals are an ideal platform for RASE because they offer long optical and hyperfine coherence times. The hyperfine transitions are of particular interest, because the memory storage times required for repeater can be engineered. RASE works by inverting an ensemble of two-level atoms to create the optical gain necessary for amplified spontaneous emission (ASE). Detection of ASE heralds the ensemble into a collective state which is entangled with the emitted field. Because the ensemble is inhomogeneously broadened, the collective state will then begin to dephase. The state can be rephased, after a tunable delay time, and the entanglement readout as a second optical field (RASE). In this work, the metric chosen to confirm entanglement was the inseparability criterion. In previous work, the two emitted fields of light were proven to be entangled with significant confidence, however, the rephasing efficiency was limited to 3\%. The explanation for this was a mismatch between the first and second pass of the beam through the crystal, which was the consequence of a retroreflective mirror being positioned incorrectly. In this thesis, the beam mismatch, caused by the position of the mirror, was shown to not affect the RASE experiment. Instead, the RASE experiment is more likely to be affected by a spatial mode mismatch which is caused by the distortion of the control pulses. The effect of these distortions was observed in the efficiency measurement of RASE. According to the theory, the efficiency of RASE should consistently improve with optical depth. Experimentally, however, in the regime where the two fields were entangled, the efficiency was maximized at 8.3% when the optical depth was 0.8. The performance then began to deteriorate when the optical depth exceeded 1. This result, therefore, informs how the RASE experiment should be designed in the future. Specifically, care should be taken to correct for the effects of any spatial mode distortions, in order for RASE to operate at the threshold efficiency required for a quantum repeater.