An efficient quantum memory in 167Er3+:Y2SiO5
Creators: name: Stuart, James
Publisher: name: The Australian National University
Year: 2023

description: This thesis investigates whether a quantum memory suitable for quantum communication applications can be developed using an erbium doped crystal. To assess the potential of the storage material, 167Er3+:Y2SiO5, the performance of two quantum memory protocols are characterised, the Atomic Frequency Comb (AFC) and Rephased Amplified Spontaneous Emission (RASE). As such, this work is a spiritual successor to two previous PhD projects, Kate Ferguson's non-classical demonstration of the RASE protocol using praseodymium, and Milos Rancic's high resolution spectroscopy and demonstration of long hyperfine coherence times in erbium. A telecom compatible quantum memory is vital for the DLCZ quantum repeater protocol, a critical device for quantum communications networks. A quantum memory designed for communications networks will need to meet several requirements: operate in the fibre optic telecommunications band, high recall efficiency, long storage time, and high bandwidth. Erbium is of interest as it has an optical transition within the telecommunications C-band (1530-1565 nm) and Rancic's thesis demonstrated the hyperfine coherence time needed for long storage times, 1.3 s. However, efficient quantum memories using erbium have not been demonstrated to date. This thesis will present an efficient quantum memory using erbium and discuss a pathway to demonstrate all the above criteria simultaneously. Techniques that were developed in Rancic's thesis are expanded in this thesis to create a new memory preparation process. The preparation process uses the long hyperfine lifetimes and large hyperfine splittings found in 167Er3+:Y2SiO5. Using this preparation, two quantum memory protocols were demonstrated, the Atomic Frequency Comb (AFC) and Rephased Amplified Spontaneous Emission (RASE), from Ferguson's thesis. In the AFC experiments, non-classical storage was demonstrated with a delay time of 0.66 us, an efficiency of 22%, and a bandwidth of 6 MHz. In the RASE experiments, an efficiency of 47% was demonstrated with a spin-state storage time of 27 us, and the potential to store 40 temporal modes. The initial results have shown orders of magnitude increases in storage times and efficiency over previous erbium memories. However, the efficiencies shown are not high enough for a quantum repeater demonstration. Cavity-enhancement offers a way to increase the efficiencies of both the AFC and RASE demonstrations. In the AFC chapter, cavity enhancement was discussed as a way to increase the efficiency, theoretically, to 96.6% with a 100 MHz bandwidth. These predicted efficiencies and bandwidths, using erbium, would meet three of the requirements needed for applications in a communications network, while Rancic has already demonstrated the remaining requirement in the same material. The next step for this work will be to realise the predicted efficiency and bandwidth, and then implement hyperfine rephasing for long storage times. In summary, this thesis expands on the works of Ferguson and Rancic to demonstrate quantum memories based in erbium. The demonstrations are promising so far, and proposed improvements to the experiment suggest that a quantum memory fit for quantum networks applications is possible. Furthermore, a pathway to an improved quantum memory is presented. Such a memory could be used in an initial quantum repeater demonstration.