Rare earth quantum magnets

Quantum computers (QC) utilize the superposition principle of quantum objects: the property of being in a combination of different states simultaneously. QC promise groundbreaking research capabilities due to their inherent exponential scaling and are attracting attention across many fields of physics and beyond. The quest for a universal quantum computer spans different fields of research and several different paradigms now exist using single atoms in traps, superconducting circuits and dopants in semiconductors.

One focus of our QT group is on rare-earth (RE) fluorides as a platform for QC. LiY_1-xHo_xF₄ for example is a RE fluoride which exhibits a plethora of quantum phenomena ranging from quantum annealing to long lived coherent oscillations [18-21]. The potential of RE doped crystals for QT has been recognized early on and significant progress was recently shown [22-26]. We explore LiY_1-xHo_xF₄ and other suitable RE:LiYF₄ materials as candidates for solid-state qubits. The various energies and symmetries of different crystal field states, alongside with optical access offer a promising platform for a solid-state QC. We use our unique combination high-resolution optical spectroscopy and a high-brilliance infrared source (the Swiss Light Source Synchrotron) to gain thorough understanding of the nature of the individual states. Next, we aim to control the electronic and nuclear degrees of freedom of these states coherently, meaning we will be able to maniplate both the energy and the phase of individual RE atoms in the solid.

[1] Charles H. Bennett and David P. DiVincenzo, Quantum information and computation, Nature

404, 247 (2000).

[2] I. U. I. Manin, Vychislimoe i nevychislimoe, Sovetskoye Radio (1980).

[3] R. P. Feynman, Simulating Physics with Computers, International Journal of Theoretical

Physics 21, 467{488 (1982).

[4] David Deutsch, Quantum theory, the Church{Turing principle and the universal quantum com-

puter, Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering

Sciences 400, 97{117 (1985).

[5] Iulia Buluta, Sahel Ashhab, and Franco Nori, Natural and artiffcial atoms for quantum compu-

tation, Reports on Progress in Physics 74, 104401 (2011).

[6] R. Versluis et al., Scalable Quantum Circuit and Control for a Superconducting Surface Code,

Phys. Rev. Applied 8, 034021 (2017).

[7] Bjoern Lekitsch et al., Blueprint for a microwave trapped ion quantum computer, Science

Advances 3, (2017).

[8] Charles D. Hill et al., A surface code quantum computer in silicon, Science Advances 1, (2015).

[9] Rahul Nandkishore and David A. Huse, Many-Body Localization and Thermalization in Quan-

tum Statistical Mechanics, Annual Review of Condensed Matter Physics 6, 15{38 (2015).

[10] P. W. Anderson, Absence of Diffusion in Certain Random Lattices, Phys. Rev. 109, 1492{1505

(1958).

[11] N F Mott, The Basis of the Electron Theory of Metals, with Special Reference to the Transition

Metals, Proceedings of the Physical Society. Section A 62, 416 (1949).

[12] D. M. Silevitch, G. Aeppli, and T. F. Rosenbaum, Probing many-body localization in a disordered

quantum magnet, ArXiv e-prints (2017).

[13] Christoph Simon, Towards a global quantum network, Nature Photonics 11, 678{680 (2017).

[14] P. E. Hansen, T. Johansson, and R. Nevald, Magnetic properties of lithium rare-earthfluorides:

Ferromagnetism in LiErF4 and LiHoF4 and crystal-field parameters at the rare-earth and Li

sites, Phys. Rev. B 12, 5315{5324 (1975).

[15] A H Cooke et al., Ferromagnetism in lithium holmiumfluoride-LiHoF4 . I. Magnetic measure-

ments, Journal of Physics C: Solid State Physics 8, 4083 (1975).

[16] J E Battison et al., Ferromagnetism in lithium holmium fluoride-LiHoF4 . II. Optical and

spectroscopic measurements, Journal of Physics C: Solid State Physics 8, 4089 (1975).

[17] H. M. R_nnow et al., Quantum Phase Transition of a Magnet in a Spin Bath, Science 308,

389{392 (2005).

[18] J. Brooke et al., Quantum Annealing of a Disordered Magnet, Science 284, 779{781 (1999).

[19] S. Ghosh et al., Coherent Spin Oscillations in a Disordered Magnet, Science 296, 2195{2198

(2002).

[20] S. Ghosh et al., Entangled quantum state of magnetic dipoles, Nature 425, 48 (2003).

[21] M. A. Schmidt et al., Using thermal boundary conditions to engineer the quantum state of a

bulk magnet, Proceedings of the National Academy of Sciences 111, 3689{3694 (2014).

[22] P. Babkevich et al., Neutron spectroscopic study of crystal-field excitations and the effect of the

crystal field on dipolar magnetism in LiRF4 (R = Gd, Ho, Er, Tm, and Yb), Phys. Rev. B 92,

144422 (2015).

[23] Milos Rancic et al., Coherence time of over a second in a telecom-compatible quantum memory

storage-material, Nature Physics 14, 50 (2017).

[24] Manjin Zhong et al., Optically addressable nuclear spins in a solid with a six-hour coherence

time, Nature 517, 177 (2015).

[25] Antonio Ortu et al., Simultaneous coherence enhancement of optical and microwave transitions

in solid-state electronic spins, Nature Materials 17, 671{675 (2018).

[26] N Kukharchyk et al., Optical coherence of 166Er: 7LiYF4 crystal below 1 K, New Journal of

Physics 20, 023044 (2018).