Telecom Band Quantum Memory in the Solid-State Towards Quantum Repeater Networks

Abstract

Atomic defects in the solid state are ideal single photon sources and quantum memories, making them promising candidates for quantum nodes in a quantum repeater network. However, most of these defects emit photons with wavelengths outside the telecom band, resulting in prohibitively high photon loss rates for practical long-distance quantum networks. While there are significant efforts towards wavelength conversion for these emitters, this thesis focuses on erbium ions, which have an optical transition in the telecom band, allowing for intrinsically very low photon losses. We develop and characterize this erbium-based quantum memory, focusing on two main properties: its memory characteristics and the indistinguishability of the photons it emits.To achieve a long-lived quantum memory, we chose a solid-state host with a low nuclear spin concentration. This magnetically quiet environment allows for an erbium electron spin coherence in excess of 200 μs. To achieve indistinguishable single photon emission, we introduce the erbium ion in a non-polar symmetry site in the solid state, resulting in reduced sensitivity to electric field noise. We also chose a solid-state host with no intrinsic rare-earth ions, which reduces background photon emission, allowing for higher single photon purity. Furthermore, we leverage mature silicon nanophotonic techniques to couple the ions to small mode-volume, low-loss optical cavities. Through the Purcell effect, we reduce the optical lifetime by more than two orders of magnitude and demonstrate a near lifetime-limited optical coherence. We then leverage these improvements to demonstrate the first indistinguishable single photon emission from a rare-earth ion with a visibility of V = 80% in a Hong-Ou-Mandel interference experiment. The demonstration of indistinguishable photon emission and long-lived spin coherence are key fundamental steps towards the implementation of long-distance quantum repeater networks.