Single-atom spin qubits in silicon

The idea of using the spin of a single donor atom in silicon to encode quantum information goes back to the Kane proposal [1] in 1998. The proposal was motivated by two observations: (i) Silicon is one of the most promising materials to host spin qubits in solid state, owing to the very weak spin-orbit coupling, and to the possibility to eliminate decoherence from nuclear spins by isotopic purification; (ii) A trillion-dollar worth industry already exists, that has developed extraordinary tools to manufacture silicon nanoscale devices in a reliable and cost-effective way.

The proposal appeared ambitious, visionary but very challenging at the time, because it relied upon the non-trivial assumption that the progress in the fabrication of classical silicon devices could be harnessed to pursue quantum information goals. Indeed, over a decade of intense efforts has been necessary before the first breakthroughs in silicon quantum technologies could be demonstrated.

I will present the first experimental demonstration of a qubit based on a single phosphorus atom in silicon. The atom is coupled to a silicon Single-Electron Transistor, and the whole device is fabricated retaining standard CMOS technologies such as ion implantation [2] and metal gates fabricated on top of high-quality silicon oxide [3].

In a single-atom device, we have demonstrated single-shot readout [4] and coherent control [5] of the donor electron spin, as well and the spins of the 31P nucleus and of a strongly-coupled 29Si nucleus. All three qubits exhibit excellent coherence and high-fidelity readability, with the nuclear ones being accessible through a quantum nondemolition measurement.

These results represent major milestones in the search for a scalable and coherent quantum computer platform, and confirm the vision of silicon as the choice material for both quantum and classical technologies.

[1] B. E. Kane, Nature 393, 133 (1998).
[2] D. N. Jamieson et al., Appl. Phys. Lett. 86, 202101 (2005).
[3] A. Morello et al., Phys. Rev. B 80, 081307(R) (2009).
[4] A. Morello et al., Nature 467, 687 (2010).
[5] J. J. Pla et al., Nature 489, 541 (2012).