Faster control or longer lifetimes

Abstract

Magnetism at the atomic scale is governed by the laws of quantum mechanics, with individual atoms possessing magnetic moments, called “spin,” that are quantized in discrete levels. Even small quantum spin systems, consisting of only a few coupled atoms, can display complex time dynamics. Uncovering this behaviour and learning how to control spins in these model systems may provide insights into larger, more complex systems that are still poorly understood.

It is experimentally challenging to both measure and control atomic spins in solid-state environments. In this thesis, this is achieved using a scanning tunneling microscope (STM), which can be thought of as an atomically sharp needle ending in a single atom. This tip scans over a surface to produce a topographic image that identifies individual atoms. Over the past decade, the capabilities of atomic-resolution scanning probes such as STM have advanced significantly, particularly through the introduction of pump–probe schemes and electron spin resonance (ESR). However, the potential of these techniques has been limited by short coherence times, meaning that spins lose their quantum properties before they can be fully controlled or utilized.

This thesis explores two approaches to improve coherent control of individual spins in on-surface atoms using an STM equipped with ESR capabilities. First, an arbitrary waveform generator (AWG) is integrated into the STM setup to enable faster and more complex voltage signal generation for spin control and readout. Second, the time dynamics of individual nuclear spins are investigated, as these may exhibit longer coherence times than electron spins. A time-resolved readout of the nuclear spin state is achieved via its hyperfine coupling to the electron spin.

The results show that a high-speed AWG can replicate and combine previously separate techniques for coherent single-atom spin control, allowing for more complex experimental designs. In addition, coherent nuclear spin dynamics are observed for the first time with STM, including initial measurements of nuclear spin lifetimes. By compensating for signal distortions, sub-nanosecond pulse widths are achieved, enabling higher time resolution in spin readout. Further experiments reveal nuclear spin lifetimes on the order of seconds—significantly longer than those of other controllable on-surface spins—making single-shot readout possible. Combined experimental and theoretical analysis indicates that the interaction between electron and nuclear spins ultimately limits the nuclear spin lifetime.