Although silicon has been the defining material of classical information processing, it is currently not the main material advanced for quantum information processing. It would however be very compelling to leverage the existing multibillion euro silicon infrastructure for quantum information processing. Silicon spin qubits have already been shown to have excellent single-qubit properties, combining long coherence times with high-fidelity readout and control. The reason silicon qubits are not yet seen as a mainstream platform for quantum computing is mainly due to the lack of convenient coupling and readout mechanisms for the spins that could be used to scale-up to a practical level. This project addresses both deficiencies and aims to enable a long-term future for donor spin qubits in silicon in Europe, for both quantum processing and quantum sensing applications.
There is currently a proliferation of different quantum platforms, but none of them has yet a provable solution to scaling up to millions of qubits that will be eventually needed. Reaching this goal will require fabrication methods that are suitable for large scale production and that are preferably even CMOS compatible. Scaling up also requires high reproducibility of the quantum systems so that their properties, especially decoherence times and resonance frequencies, do not vary between different structures and different fabrication runs. Donor spins in silicon have the potential to solve all these issues. They are both atomic in size, can be easily integrated with additional electronic and photonic circuits on chip, and would allow leveraging the existing huge fabrication infrastructure of conventional silicon electronics. They have shown excellent single qubit properties, combining long coherence times with high-fidelity control and readout. Donor doping of silicon is a decades old, highly refined, technology. Although the placement of donors cannot be controlled with atomic accuracy, technologies exist – and will be advanced in this project – to deterministically place single donors with an accuracy of a few tens of nanometers. One of the goals of the project is to ensure that this implantation accuracy will be enough to realize a multi-qubit platform on a single silicon chip with high yield. As single atoms, the donor spin qubits also have the same properties from qubit to qubit and do not suffer from fabrication imperfections The reproducibility of the donor spin qubits of course requires exquisite control on the uniformity of the substrate lattice to avoid any variation in strain or homogeneity of the lattice. This kind of control is available (more or less exclusively) in silicon due to decades of commercial efforts to grow it with high-purity, and in this project, we advance the atomic control of the surrounding lattice by locally tuning the isotopical composition and strain around the donors. Moreover, we develop interfaces that allow donor spin qubits to exchange quantum information across a chip using acoustic pathways.
Silicon is already in wide commercial use in both photonics and in nano- (NEMS) and microelectromechanical (NEMS/MEMS) devices. These technologies would be the obvious pathway to provide the needed coupling mechanisms between spin qubits, but the full potential of these remains unexplored. In this project, we aim to fill this gap and design, demonstrate and scale-up a silicon quantum computing platform where the computation is done with spin qubits coupled to each other via mechanical (acoustic) pathways. Interestingly, the developed platform can also be used the other way around; with the spins as quantum limited mechanical displacement sensors.
The goal in this project is to develop a complete quantum information platform including qubits, interconnects and scalable control and readout electronics. The platform will be based on embedded atomic spins as qubits, phonons as interconnects, and gate defined quantum dot charge sensors with on-chip multiplexing as readout devices. The project brings together the relevant parties in Europe into a collaboration that forms a new hub for donor spin -based silicon quantum computing. The project network spans single-ion implanters, facilities for spatially resolved isotopic purification of silicon, semi-commercial silicon foundries, start-up companies working on silicon quantum dots and research groups researching both silicon spin quantum computing and quantum acoustics.
