Solid State Quantum Technologies

Semiconductor based qubits

Impurities in semiconductors provide the carriers necessary for the operation of transistors. As semiconductor devices get smaller, the exact distribution of these impurities starts to matter affecting the transistors threshold voltage. Getting to the limit of miniaturization in semiconductors we can imagine an array of equally spaced dopants whose charge state can be individually controlled by local gates. This is precisely Kane’s proposal for a Silicon based quantum computer: in this proposal the spin of a dopant nucleus (or the spin of a bound electron) would be used as a quantum bit. Spin rotations driven by AC magnetic fields would be used for single qubit operations while two qubit operations can be produced by spin-spin exchange. Silicon’s very long spin coherence times underlie the interest in this particular semiconductor.

Donors in Si can be considered as hydrogenic atoms within the effective mass approximation but being embedded in a solid many issues have to be taken into account, for instance: (i) mass anisotropy, (ii) valley degeneracy (Si has six equivalent conduction band minima), (iii) central cell corrections (corrections to screening at the dopant position, (iv) confinement in the nanostructure (finite size effects, dielectric mismatch due to the nearby presence of metallic gates or insulating barriers).

The quasi-atomic precision required for the quantum control of donor electrons to work is now starting to be a reality in laboratories around the world. We work on providing a theoretical framework to understand the experiments in this rapidly evolving field.

Recent selected publications
Interface effects on acceptor qubits in silicon and germanium.
J. C. Abadillo-Uriel, M. J. Calderón. Nanotechnology 27, 024003 (2016)arXiv:1509.00699
Donor Wavefunctions in Si Gauged by STM Images.
A. L. Saraiva, J. Salfi, J. Bocquel, B. Voisin, S. Rogge, Rodrigo B. Capaz, M.J. Calderón, Belita Koiller. arXiv:1508.02772Phys. Rev. B 93, 045303 (2016).
Theory of one and two donors in Silicon
A.L. Saraiva, A. Baena, M.J. Calderón, Belita Koiller. J. Phys.: Condens. Matter 27, 154208 (2015) (Special Issue on Single donors). arXiv:1407.8224Labtalk article.
An exchange-coupled donor molecule in silicon
M.F. González-Zalba, A.L.Saraiva, D. Heiss, M.J. Calderón, Belita Koiller, A.J. Ferguson.  Nano Letters 4, 5672 (2014). arXiv:1312.4589