The Smallest Magnetic Bits and Coherent Magnetic Quantum States
Harald Brune, EPF-Lausanne, Switzerland & FZU-Prague, Czech Republic
Magnetic information storage requires a remanent magnetic state in each individual bit, i.e., its magnetization either pointing up or down in the absence of external magnetic fields. While magnetic bits of hard disks and of magnetic tapes typically contain 10^4 atoms and are governed by classical laws, the smallest possible units to store information magnetically are single atoms, and evidently their magnetic properties are given by quantum mechanics. A magnetic adatom with a large out-of-plane magnetic anisotropy will generally be in a superposition of spin up and spin down. Applying a field can change the occupation numbers of the magnetic quantum levels, but removing it will lead to an almost immediate relaxation to the paramagnetic ground state.
However, we will show that there are a few systems where the magnetic quantum states of individual surface adsorbed atoms are sufficiently well protected from external perturbations that they are long-lived, leading to magnetic hysteresis in single adatoms [1–4]. We call these systems single atom magnets (SAMs) and describe their fascinating Physics and promises for magnetic information storage and for building magnetic quantum labs, where individual SAMs are placed at surfaces to create external fields that are sensed by other atoms .
All known SAMs are rare-earth (RE) elements. Optical spectroscopy measurements reveal very long coherence times of RE ions dilutely dissolved into solids. The present record is 6 ± 1 hours for 151Eu^3+ ions in yttrium orthosilicate . From these studies we conclude that SAMs can also be single atom quantum bits, with coherence times potentially outperforming present solid state realizations of qubits. We discuss this perspective and the application of electron-spin resonance to individual RE adatoms.
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