Highly tunable exchange in donor qubits in silicon

Yu Wang,Lloyd C L Hollenberg,Archana Tankasala,Gerhard Klimeck,Michelle Y Simmons,Rajib Rahman

doi:10.1038/npjqi.2016.8

Abstract

AbstractIn this article we have investigated the electrical control of the exchange coupling (J) between donor-bound electrons in silicon with a detuning gate bias, crucial for the implementation of the two-qubit gate in a silicon quantum computer. We found that the asymmetric 2P–1P system provides a highly tunable exchange curve with mitigated J-oscillation, in which 5 orders of magnitude change in the exchange coupling can be achieved using a modest range of electric field (3 MV/m) for ~15-nm qubit separation. Compared with the barrier gate control of exchange in the Kane qubit, the detuning gate design reduces the gate density by a factor of ~2. By combining large-scale atomistic tight-binding method with a full configuration interaction technique, we captured the full two-electron spectrum of gated donors, providing state-of-the-art calculations of exchange energy in 1P–1P and 2P–1P qubits.

Highlights

Donor qubits in silicon are promising candidates for spin-based quantum computation as they have exceptionally long T1 and T2 times4–6 and offer both electron and nuclear spins for encoding quantum information5–8 utilising commonly used silicon device technology
With recent demonstration of single qubits in silicon with both electronic and nuclear spins of donors,5,7 the biggest challenge is to demonstrate two-qubit gates based on the exchange interaction
That we have explored the range of exchange energies that can be accessed in donor qubits for various donor separations, we investigate the electric field control of exchange in the 1P–1P and 2P–1P systems

Summary

INTRODUCTION

Donor qubits in silicon are promising candidates for spin-based quantum computation as they have exceptionally long T1 (refs 1–3) and T2 times and offer both electron and nuclear spins for encoding quantum information utilising commonly used silicon device technology. We introduce an alternative design for an exchange gate in a two-qubit donor system, which allows flexibility in device fabrication and in tuning the exchange coupling In principle, this new design can (1) eliminate the need for additional J-gates between the donors, (2) function with a range of donor separations, (3) provide an ~ 5 orders of magnitude J-tunability within a modest E-field range of ~ 3 MV/m and lowered ‘Off’ state exchange and (4) mitigate the J-oscillations with donor separations. Placed on either side of the donor qubits, the detuning gates eliminate the need for a sensitive tunnel barrier control by the J-gate Instead, this design realises a tilt in the potential landscape of the two qubits, as shown in Figure 1d,e, and relaxes the more stringent engineering requirements of donor separations and gate widths of the Kane architecture, leading to a reduced overall gate density in the computer

RESULTS

DISCUSSION

MATERIALS AND METHODS