Molecular orbital calculations of two-electron states for P donor solid-state spin qubits

TL;DR

This paper provides detailed molecular orbital calculations of two-electron states for phosphorus donor pairs in silicon, crucial for understanding and optimizing spin qubits in solid-state quantum computing.

Contribution

It introduces a comprehensive molecular orbital approach to analyze two-electron states, including excited states, exchange coupling, and double occupancy, advancing the modeling of donor-based spin qubits.

Findings

Calculated energy spectrum of two-donor system.

Analyzed dependence of states on strain, position, and separation.

Evaluated exchange coupling and double occupancy probabilities.

Abstract

We theoretically study the Hilbert space structure of two neighbouring P donor electrons in silicon-based quantum computer architectures. To use electron spins as qubits, a crucial condition is the isolation of the electron spins from their environment, including the electronic orbital degrees of freedom. We provide detailed electronic structure calculations of both the single donor electron wave function and the two-electron pair wave function. We adopted a molecular orbital method for the two-electron problem, forming a basis with the calculated single donor electron orbitals. Our two-electron basis contains many singlet and triplet orbital excited states, in addition to the two simple ground state singlet and triplet orbitals usually used in the Heitler-London approximation to describe the two-electron donor pair wave function. We determined the excitation spectrum of the two-donor system, and study its dependence on strain, lattice position and inter donor separation. This allows us to determine how isolated the ground state singlet and triplet orbitals are from the rest of the excited state Hilbert space. In addition to calculating the energy spectrum, we are also able to evaluate the exchange coupling between the two donor electrons, and the double occupancy probability that both electrons will reside on the same P donor. These two quantities are very important for logical operations in solid-state quantum computing devices, as a large exchange coupling achieves faster gating times, whilst the magnitude of the double occupancy probability can affect the error rate.