Researchers from the University of New South Wales and Silicon Quantum Computing Pty Ltd have published a study on the superexchange coupling of donor qubits in silicon. The team demonstrated that by placing four phosphorus donors in a linear chain, one can achieve coherent spin coupling between the end dopants of the chain. This atomic engineering in a solid-state material can functionalize the host with novel phenomena, reduce gate densities, and decrease correlated noise between qubits. The study provides a comprehensive understanding of long-range indirect coupling for donor qubits in silicon, a promising silicon quantum computer building block.
Introduction to Superexchange Coupling of Donor Qubits in Silicon
A team of researchers from the School of Physics at the University of New South Wales and Silicon Quantum Computing Pty Ltd have published a study on the superexchange coupling of donor qubits in silicon. The team, consisting of Mushita M Munia, Serajum Monir, Edyta N Osika, Michelle Y Simmons, and Rajib Rahman, have demonstrated that by placing four phosphorus donors spaced 10-15 nm apart in a linear chain, one can realize coherent spin coupling between the end dopants of the chain. This is analogous to the superexchange interaction in magnetic materials.
The Potential of Atomic Engineering in Solid-State Material
The researchers have shown that atomic engineering in a solid-state material has the potential to functionalize the host with novel phenomena. STM-based lithographic techniques have enabled the placement of individual phosphorus atoms at selective lattice sites of silicon with atomic precision. This enables spin coupling between their bound electrons beyond nearest neighbors, allowing the qubits to be separated by 30-45 nm. The added flexibility in architecture brought about by this long-range coupling not only reduces gate densities but can also reduce correlated noise between qubits from local noise sources that are detrimental to error-correction codes.
The Role of Phosphorus Atoms in Silicon Quantum Computing
Phosphorus atoms are a promising building block of a silicon quantum computer. The platform of phosphorus donor-based quantum computing has been bolstered by key milestone achievements over the last decade, including single-shot spin readout, the realization of single-electron spin and single-nuclear-spin qubits, and more recently two-qubit SWAP gates and a three-qubit donor quantum processor with universal logic operation. The exchange interaction depends on the overlap between the electronic wave functions and ultimately limits the separation of donor qubits to about 10-15 nm in silicon devices.
The Benefits of Spacing Out Qubits
Spacing out the qubits is beneficial from an architectural point of view in fault-tolerant quantum computing, as correlations between the qubits due to local noise sources can be minimized. An increase in separation also relaxes stringent gate-density requirements and offers more independent electrostatic control of the qubits by reducing their capacitive crosstalk. For STM-patterned donors with phosphorus-doped in-plane gates, the density is already low, so this technique is particularly appealing.
The Study of Long-Range Exchange Coupling
In this work, the researchers studied long-range exchange coupling between the end spins of four single-donor 1P quantum dots in a linear chain. With each donor containing a single electron, a superexchange coupling is found to emerge between the donors at the end of the chain. This third-nearest-neighbor interaction enables the qubits to be separated by 30-45 nm. Using atomistic full configuration-interaction (FCI) calculations, the team studied the eigenvalues and eigenvectors of four electron spins across four 1P atoms in silicon.
The Role of Superexchange in Quantum Computing
The researchers also investigated the role of the conduction-band valleys in superexchange for donor separation along different crystallographic directions. They simulated the system using realistic electrostatic potentials produced by the surrounding in-plane STM-patterned gates, where they demonstrated tunability of superexchange with gate voltages, a crucial requirement for the realization of electrically controlled singlet-triplet oscillations. They also commented on the sensitivity of superexchange to charge noise and donor-placement errors, as well as the role of nuclear spins in singlet-triplet oscillations induced by superexchange.
The Future of Quantum Computing with Donor Qubits in Silicon
The study provides a comprehensive understanding of long-range indirect coupling for donor qubits in silicon. Compared with electrostatically defined quantum dots, donor quantum dots in silicon typically have atomic-scale properties with wavefunction length scales an order of magnitude less and a large quantity of orbital-valley energy splittings. The atomistic character of these donors and donor quantum-dot systems needs to be accounted for when considering indirect couplings such as superexchange, as the phenomenon emerges from individual nearest-neighbor exchange couplings.
Superexchange coupling of donor qubits in silicon by Mushita M. Munia, Serajum Monir, Edyta N. Osika, M. Y. Simmons, Rajib Rahman was published on January 22, 2024. The article explores the superexchange coupling of donor qubits in silicon, providing new insights into the field of quantum computing.
