Optical and spin properties of Er3+ sites in Si

Abstract

Quantum networks are the quantum analogue of the modern internet, offering the ability to employ more secure protocols than modern communication. At the base of these quantum networks are so-called qubits: physical entities that can be in a simultaneous superposition of two states. For quantum networks, qubits should contain states that can be optically accessed and can store quantum properties for a long time. Nowadays, a number of qubit platforms exist, with spin qubits in Si offering long coherence times due to the low number of nuclear spins in the host crystal. Si is a widely used semiconductor in modern electronic devices and the maturated micro- and nanofabrication techniques can be exploited to miniaturise quantum devices. Additionally, Si and SiO2 form an attractive photonic material system because they provide a large contrast in the refractive index, which is critical for achieving efficient optical confinement. To minimise the photon losses and leverage on the well-established telecommunication networks, the photons excited from the spin states should emit within the telecommunication C-band. In this sense, Er3+:Si is an attractive system because Er3+ ions exhibit a spin transition that can be accessed by photons with frequencies within the telecommunication C-band. Moreover, the optical transition of Er3+ takes place within an inner shell, meaning that the outer shells electrically shield this transition, resulting in narrow and stable optical transitions within the telecommunication C-band.

The upper bounds on the coherence times of the optical and spin transitions of Er3+:Si are, thus far, still unknown. In this thesis, these optical and spin properties of Er3+ ions in Si are investigated with the aim to create an interface between a spin qubit and a flying qubit. Here, the optical measurements include the extraction of inhomogeneous and homogeneous broadening of Er3+:Si over various samples, observing linewidths down to less than 100 MHz and 500 kHz, respectively. The low Er3+ density in natural Si samples showed characteristics of long-lived electron spin states for two sites. The electron spin coherence time of an Er3+ site in a nuclear-spin-free Si crystal was measured to be 0.5 ms by employing a Hahn echo sequence, and this was further extended up to 9 ms using a Carr–Purcell Meiboom–Gill sequence. These optical and spin properties establish that Er3+:Si exhibits fundamentally promising properties for quantum networks.