Andrea Morello

Donor atoms in silicon are a versatile platform for experiments in quantum information processing, as
well as quantum foundations. The electron [1] and nuclear [2] spin of a 31 P donor were the first qubits
demonstrated in silicon, and went on to become among of the most coherent qubits in the solid state,
with coherence times exceeding 30 seconds [3], and quantum gate fidelities approaching 99.99% [4].
This talk will provide an overview of the strategies we are devising to scale up beyond singe, isolated
donors. Some of these methods have been already experimentally demonstrated, while others are in
progress.

Using magnetic resonance, we have demonstrated an exchange-based 2-qubit CROT gate for electron
spins [5], in a device where we implanted a high dose of ${}^{31}\text{P}$ donors. Future experiments will focus on
using deterministic, counted single-ion implantation, for which we have recently demonstrated the
capability to detect an individual ion with 99.85% confidence [6]. With nuclear spins, we have achieved
the landmark result of universal 1- and 2-qubit logic operations with >99% fidelity, and prepared a 3-
qubit GHZ entangled state with 92.5% fidelity [7]. We have also demonstrated the coherent electrical
control of an electron-nuclear flip-flop qubit [8], which will greatly facilitate the integration of single-atom
qubits in nanoelectronic devices.

Heavier donors possess a high nuclear spin quantum number and a nonzero electric quadrupole
moment. In the process of operating a single spin-$7/2$ ${}^{123}\text{Sb}$ nucleus, we (re)discovered the phenomenon
of nuclear electric resonance, and applied it for the first time to a single nuclear spin [9]. Encoding
quantum information in the larger Hilbert space of high-spin nuclei will enable interesting options for
logical qubit encoding and quantum error protection [10]. This “scaling up inwards” strategy, combined
with “scaling up outwards” via exchange interaction and electric dipole coupling, provides great flexibility
in the design of a blueprint large-scale donor-based quantum computer in silicon.

[1] J. Pla et al., Nature 489, 541 (2012)
[2] J. Pla et al., Nature 496, 334 (2013)
[3] J. Muhonen et al., Nature Nanotechnology 9, 986 (2014)
[4] J. Muhonen et al., J. Phys: Condens. Matter 27, 154205 (2015)
[5] M. Madzik et al., Nature Communications 12, 181 (2021)
[6] A. Jakob et al., Advanced Materials 34, 2270022 (2022)
[7] M. Madzik et al., Nature 601, 348 (2022)
[8] R. Savytskyy et al., arXiv:2022.04438 (2022)
[9] S. Asaad et al., Nature 579, 205 (2020)
[10] J. Gross, Phys. Rev. Lett. 127, 010504 (2021)