Researchers from the ARC Centre for Quantum Computation and Communication Technology (CQC2T) working with Silicon Quantum Computing (SQC) have situated the ‘sweet spot’ for positioning qubits in silicon to scale up atom-based quantum processors. Creating quantum bits, or qubits, by exactly putting phosphorus atoms in silicon is a world-leading approach in the growth of a silicon quantum computer. This method has been pioneered by CQC2T Director Prof. Michelle Simmons. In their research, published in Nature Communications, precision placement has proven to be essential for developing robust interactions—or coupling—between qubits. The team found that there is a special angle, or sweet spot, within a particular plane of the silicon crystal where the interaction between the qubits is most resilient to a valley interference effect. This was located using scanning tunnelling microscope (STM) lithography techniques developed at UNSW, to observe the atomic-scale details of the interactions between the coupled atom qubits, including the valley interference between the atoms and the envelope anisotropy.
Tunneling is a fundamental quantum process with no classical equivalent, which can compete with Coulomb interactions to give rise to complex phenomena. Phosphorus dopants in silicon can be placed with atomic precision to address the different regimes arising from this competition. However, they exploit wavefunctions relying on crystal band symmetries, which tunneling interactions are inherently sensitive to. Here we directly image lattice-aperiodic valley interference between coupled atoms in silicon using scanning tunneling microscopy. Our atomistic analysis unveils the role of envelope anisotropy, valley interference and dopant placement on the Heisenberg spin exchange interaction. We find that the exchange can become immune to valley interference by engineering in-plane dopant placement along specific crystallographic directions. A vacuum-like behaviour is recovered, where the exchange is maximised to the overlap between the donor orbitals, and pair-to-pair variations limited to a factor of less than 10 considering the accuracy in dopant positioning. This robustness remains over a large range of distances, from the strongly Coulomb interacting regime relevant for high-fidelity quantum computation to strongly coupled donor arrays of interest for quantum simulation in silicon.
The electrical measurements were carried out at 4.2 K in an STM (Scienta Omicron LT-STM). Both sample fabrication and measurements are done in UHV with a pressure lower than 10−10 mbar. The tunnel current I was measured as a function of the bias voltage U using ultralow noise electronics including a transimpedance amplifier. The differential conductance dI/dU shown in Supplementary Note 2 was obtained by numerical differentiation. Spatially resolved measurements of donor pairs quantum state were acquired using the multi-line scan technique, where the topography is recorded at U = −1.45 V during the first pass, and played during the second pass in open-loop mode with the current I recorded at the bias mentioned in the caption of the corresponding figures. The sample fabrication described above results in the donor pairs to be measured in the sequential transport regime, with a first tunnel barrier with tunnel rate Γin occurring from the highly doped substrate annealing, and the second tunnel barrier with tunnel rate Γout being a combination of the Si overgrowth after P deposition and the vacuum barrier, mainly dominated by the latter and tip-sample distance. Additional information regarding STM images and spectroscopy analysis can be found in the Supplementary Note 1.
B. Voisin, J. Bocquel, A. Tankasala, M. Usman, J. Salfi, R. Rahman, M. Y. Simmons, L. C. L. Hollenberg & S. Rogge
1. Centre for Quantum Computation and Communication Technology, School of Physics, The University of New South Wales, Sydney, NSW, 2052, Australia
B. Voisin, J. Bocquel, J. Salfi, M. Y. Simmons & S. Rogge
2. Electrical and Computer Engineering Department, Purdue University, West Lafayette, IN, USA
A. Tankasala & R. Rahman
3. Centre for Quantum Computation and Communication Technology, School of Physics, The University of Melbourne, Parkville, Victoria, 3010, Australia
M. Usman & L. C. L. Hollenberg
4. School of Computing and Information Systems, Melbourne School of Engineering, The University of Melbourne, Parkville, Victoria, 3010, Australia
5. School of Physics, The University of New South Wales, Sydney, NSW, 2052, Australia