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Advanced Quantum Technologies

Million‐Atom Wave Function Simulations of a Single Donor Qubit Made in Silicon Utilizing Ion Implantation Technology

Haolin Huang, Liam G. Thomas, Muhammad Usman, Rajib Rahman, David N. Jamieson

Published Dec 3, 2025• 0 citations

Abstract

ABSTRACT A large‐scale quantum computer promises quantum algorithms with an exponential speed increase over classical counterparts. Ion implanted donor atom qubits in isotopically enriched have demonstrated leading coherence times and gate fidelities. Large‐scale donor architectures must be tolerant to positional uncertainties owing to ion straggling consistent with the implant energy required to place near surface donor atoms allowing manipulation by surface control electrodes. These issues are addressed with a 40 million silicon atom quantum model based on the atomistic tight‐binding technique in the context of the flip‐flop qubit architecture. The donor electron wavefunction operates in a state of equal superposition between the buried host donor atom and below the surface gate oxide. Key issues include: donor positional uncertainty from ion straggling coupled with non‐uniform control gate potentials, the coupling of donor wave functions to gate‐defined quantum wells, and the effect of bystander donors including the mitigation of non‐functional adventitious channeled ions by re‐implantation employing deterministic ion implantation. An upper constraint of 9 keV ions minimizes straggling allowing quantum superposition and is compatible with existing fabrication techniques. With heavy donors, such as , the constraints are relaxed because, for a given implant depth, the heavy donors straggle less than light donors.

DOI: 10.1002/qute.202500675

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We report a nanofabrication, control and measurement scheme for charge-based silicon quantum computing which utilises a new technique of controlled single ion implantation. Each qubit consists of two phosphorus dopant atoms ~50 nm apart, one of which is singly ionized. The lowest two energy states of the remaining electron form the logical states. Surface electrodes control the qubit using voltage pulses and dual single electron transistors operating near the quantum limit provide fast readout with spurious signal rejection. A low energy (keV) ion beam is used to implant the phosphorus atoms in high-purity Si. Single atom control during the implantation is achieved by monitoring on-chip detector electrodes, integrated within the device structure, while positional accuracy is provided by a nanomachined resist mask. We describe a construction process for implanted single atom and atom cluster devices with all components registered to better than 20 nm, together with electrical characterisation of the readout circuitry. We also discuss universal one- and two-qubit gate operations for this architecture, providing a possible path towards quantum computing in silicon.

We discuss progress towards the fabrication and demonstration of a prototype silicon-based quantum computer. The devices are based on a precise array of 31P dopants embedded in 28Si. Fabrication is being pursued via two complementary pathways – a ‘top-down’ approach for near-term production of few-qubit demonstration devices and a ‘bottom-up’ approach for large-scale qubit arrays. The ‘top-down’ approach employs ion implantation through a multi-layer resist structure which serves to accurately register the donors to metal control gates and single-electron transistor (SET) read-out devices. In contrast the ‘bottom-up’ approach uses STM lithography and epitaxial silicon overgrowth to construct devices at an atomic scale. Techniques for qubit read-out, which utilise coincidence measurements on novel twin-SET devices, are also presented.

Abstract The long coherence time of donor atom nuclear spin states and of its bounded electron in can be exploited to define a qubit. This work is focused on a type of donor‐ and quantum dot‐based qubit, the flip‐flop (FF) qubit, that leverages antiparallel electron‐nuclear spin states of a donor atom controlled by an electric field. It can provide long‐range inter‐qubit interactions in the order of some hundreds of nanometers, thus relaxing the common constraints and tolerances on inter‐qubit distances in donor‐based qubits. Simulation results of linear array (LA) and square array (SA) of four FF qubits are presented to study the effect of noise, idle qubits, and simultaneous gating (parallel gating) on gate fidelity. The impact of noise and qubit mutual coupling for both considered types of array are presented and the obtained fidelity results are compared.