Towards Single-Ion Detection and Single-Photon Storage in Rare-Earth-Ion-Doped Crystals

Solid materials doped with rare-earth ions are considered an attractive platform for quantum information applications. One of the main reasons for this is the exceptionally long optical and hyperfine coherence times of the 4fn states, due to the shielding provided by the outer lying 5s and 5p electrons. This enables a large number of quantum operations before the system loses its coherence. Another reason is the wide inhomogeneous line broadening (∼ GHz), compared to the narrow homogeneous linewidth of individual ions (∼ kHz), which gives rise to a large number of spectrally separated ions that can, in principle, be individually addressed. In order to make use of this feature, a reliable detection (readout) of single ions is required. The excited states of 4f − 4f transitions have relatively long lifetimes ranging from hundreds of microseconds to milliseconds. This leads to a low spontaneous emission rate, which renders a direct detection of an individual ion challenging. Part of the research presented in this thesis addresses this issue by utilizing the Purcell effect to enhance the spontaneous emission rate of the ions. This can be achieved by placing nanocrystals doped with rare earth ions inside a microcavity. The density of states inside the cavity is modified compared to the density of states in free space, and as a consequence, the lifetime of the excited state of the ion becomes shorter. Y2O3 nanocrystals doped with Nd3+ ions were investigated, the Nd3+ ions being candidate for a readout ion that could potentially be used to probe the state of co-doped qubit ions in a non-destructive measurement. A fiber based microcavity was constructed for use at cryogenic temperatures. A side of fringe locking scheme was implemented to increase the stability of the cavity. Scanning cavity microscopy was demonstrated and used at room temperature to locate the nanocrystals, and cavity coupling to the 4f − 4f transitions of the Nd3+ ions was achieved. Cryogenic measurements were performed and cavity enhanced detection of an ensemble of a few ions was demonstrated.Another part of this research was focused on quantum memories. A quantum memory capable of storing a single photon and being able to retrieve it on demand with high fidelity are necessary for long distance quantum communication. The atomic frequency comb (AFC) scheme is an interesting quantum memory schemes that has been routinely demonstrated in rare-earth-ion-doped systems. To achieve on-demand storage, the standard AFC scheme is usually combined with bright optical pulses to transfer a stored photon into and out of the spin levels; referred to as the spin-wave storage scheme. However, these bright pulses create a high optical background, which makes it challenging to store and recall single photons. In this thesis, an extension of the standard AFC protocol that utilizes the linear Stark effect to perform noise-free, on-demand storage without the need for spin transfer pulses, is presented. The modified protocol was experimentally implemented in Pr3+:Y2SiO5 using weak coherent states as a memory input. A signal-to-noise ratio (SNR) of 570 was achieved using an average of 0.1 photons per pulse for storage, limited by the detector dark current. An order of magnitude higher SNR was estimated for storage of single-photon Fock states. This SNR, to the best of the author’s knowledge, is unsurpassed for the presented technique. A standard AFC analytical model was modified to incorporate the presented technique technique, and to investigate the storage performance of other materials for practical use in quantum networks. The Stark shift technique was also combined with the spin-wave storage scheme, providing another tool that could improve the performance of spin-wave storage at the single-photon level