Rare-earth quantum magnets

Quantum computers utilize the superposition principle of quantum objects: the property of being in a combination of different states simultaneously. They promise groundbreaking capabilities due to their inherent exponential scaling and are attracting attention across many fields of physics and beyond. Several routes are pursued to realize a universal quantum computer, amongst others employing trapped ions, superconducting circuits and dopants in semiconductors.

The potential of rare-earth doped crystals for quantum technologies has been recognized early on. A focus of our QPS group is on rare-earth fluorides as a platform for quantum computer. For example, LiY_1-xHo_xF₄ and LiY_1-xTb_xF₄ exhibit a plethora of quantum phenomena ranging from quantum annealing to long lived coherent oscillations.

Our current scheme for universal quantum computer is based on Kramers rare-earth ions doped in crystals. Specifically, we are exploring erbium in LiYF₄ and other host materials as candidates for solid-state qubits, due to optical transitions in the telecom band. The various energies and symmetries of different crystal field states, alongside with optical access offer a promising platform for a solid-state quantum computer. We use our unique combination of high-resolution optical spectroscopy and a high-brilliance infrared source (the Swiss Light Source synchrotron) to gain an understanding of the nature of the individual states. Ultimately, we aim to control the electronic and nuclear degrees of freedom of these states coherently, meaning we will be able to manipulate the states of individual rare-earth atoms in the solid, to produce faster and less error-​prone quantum logic gates.

Universal quantum computing using electronuclear wavefunctions of rare-earth ions
M. Grimm, A. Beckert, G. Aeppli, M. Müller
PRX Quantum 2, 010312 (2021)

Optical response from terahertz to visible light of electronuclear transitions in LiYF₄:Ho³⁺
G. Matmon, S. A. Lynch, T. F. Rosenbaum, A. J. Fisher, G. Aeppli
Phys. Rev. B 94, 205132 (2016)