Electrical Manipulation of Donor Spin Qubits in Silicon and Germanium

Abstract

Many proposals for quantum information devices rely on electronic or nuclear spins in

semiconductors because of their long coherence times and compatibility with industrial

fabrication processes. One of the most notable qubits is the electron spin bound

to phosphorus donors in silicon, which offers coherence times exceeding seconds at

low temperatures. These donors are naturally isolated from their environments to the

extent that silicon has been coined a "semiconductor vacuum". While this makes for

ultra-coherent qubits, it is difficult to couple two remote donors so quantum information

proposals rely on high density arrays of qubits. Here, single qubit addressability

becomes an issue. Ideally one would address individual qubits using electric fields

which can be easily confined. Typically these schemes rely on tuning a donor spin

qubit onto and off of resonance with a magnetic driving field. In this thesis, we measure

the electrical tunability of phosphorus donors in silicon and use the extracted

parameters to estimate the effects of electric-field noise on qubit coherence times. Our

measurements show that donor ionization may set in before electron spins can be sufficiently tuned. We therefore explore two alternative options for qubit addressability.

First, we demonstrate that nuclear spin qubits can be directly driven using electric

fields instead of magnetic fields and show that this approach offers several advantages

over magnetically driven spin resonance. In particular, spin transitions can occur at

half the spin resonance frequency and double quantum transitions (magnetic-dipole

forbidden) can occur.

In a second approach to realizing tunable qubits in semiconductors, we explore

the option of replacing silicon with germanium. We first measure the coherence and

relaxation times for shallow donor spin qubits in natural and isotopically enriched

germanium. We find that in isotopically enriched material, coherence times can exceed

1 ms and are limited by a single-phonon T1 process. At lower frequencies or lower

temperatures the qubit coherence times should substantially increase.

Finally, we measure the electric field tunability of donors in germanium and find

a four order-of-magnitude enhancement in the spin-orbit Stark shift and confirm that

the donors should be tunable by at least 4 times the electron spin ensemble linewidth

(in isotopically enriched material). Germanium should therefore also be more sensitive

to electrically driven nuclear magnetic resonance. Based on these results germanium

is a promising alternative to silicon for spin qubits.