[Paper Review] Deterministic Single Ion Implantation with 99.87% Confidence for Scalable Donor-Qubit Arrays in Silicon

Alexander Jakob, Simon G. Robson+10 more|arXiv (Cornell University)|Sep 7, 2020

Quantum and electron transport phenomena51 references4 citations

TL;DR

This paper demonstrates deterministic single-ion implantation of phosphorus ions in silicon with 99.87% confidence using on-chip charge-sensitive electronics and a scanning-probe ion aperture, enabling precise placement of donor qubits for scalable quantum computing. The method achieves near-ideal implantation fidelity by combining low-energy ion beams (14 keV), real-time ion detection, and beam blanking to suppress double implants.

ABSTRACT

The attributes of group-V-donor spins implanted in an isotopically purified $^{28}$Si crystal make them attractive qubits for large-scale quantum computer devices. Important features include long nuclear and electron spin lifetimes of $^{31}$P, hyperfine clock transitions in $^{209}$Bi and electrically controllable $^{123}$Sb nuclear spins. However, architectures for scalable quantum devices require the ability to fabricate deterministic arrays of individual donor atoms, placed with sufficient precision to enable high-fidelity quantum operations. Here we employ on-chip electrodes with charge-sensitive electronics to demonstrate the implantation of single low-energy (14 keV) P$^+$ ions with an unprecedented $99.87\pm0.02$% confidence, while operating close to room-temperature. This permits integration with an atomic force microscope equipped with a scanning-probe ion aperture to address the critical issue of directing the implanted ions to precise locations. These results show that deterministic single-ion implantation can be a viable pathway for manufacturing large-scale donor arrays for quantum computation and other applications.

Motivation & Objective

• To enable scalable fabrication of donor-qubit arrays in silicon for large-scale quantum computing by achieving deterministic placement of individual dopant atoms.
• To overcome the stochastic nature of conventional ion implantation, which limits control and increases error rates in large-scale quantum devices.
• To demonstrate a method that ensures single-ion implantation with high confidence, minimizing faulty or absent qubits in the array.
• To integrate on-chip charge-sensitive electronics with a scanning-probe ion aperture for real-time detection and control of individual ion implants.
• To validate the feasibility of deterministic implantation at room temperature with minimal double-implantation events, critical for fault-tolerant quantum architectures.

Proposed method

• Employed a low-energy (14 keV) P+ ion beam delivered through a scanning-probe ion aperture to target specific locations on a silicon device.
• Used on-chip charge-sensitive electronics to detect the charge deposited by each ion in real time, enabling immediate confirmation of implantation.
• Applied beam blanking with a 100 ns minimum pulse width to suppress double implants, reducing the probability of multiple ion arrivals.
• Calculated the double-implantation probability as $ P_{\mathrm{DI}} = \mathrm{e}^{-\tau r_{\mathrm{Ion}}} $, where $ \tau \approx 100\,\mathrm{ns} $ and $ r_{\mathrm{Ion}} \approx 1\,\mathrm{s}^{-1} $, yielding $ P_{\mathrm{DI}} \sim 10^{-7} $.
• Simulated ion transmission and energy loss using TRIM and Crystal-TRIM codes, incorporating Fano statistics and detector noise for accurate signal prediction.
• Modified Crystal-TRIM to include the Funsten model for electronic energy loss, improving accuracy for low-energy Si recoils compared to the Robinson model.

Experimental results

Research questions

• RQ1 Can deterministic single-ion implantation be achieved with high confidence in silicon at near-room temperature?
• RQ2 What is the maximum achievable confidence in single-ion implantation using real-time charge detection and beam blanking?
• RQ3 How does beam blanking with a 100 ns pulse width affect the probability of double implants during low-rate ion implantation?
• RQ4 To what extent can simulations using TRIM and Crystal-TRIM with modified energy-loss models reproduce experimental ion signal spectra?
• RQ5 Can the integration of on-chip charge-sensitive electronics with a scanning-probe ion aperture enable precise, deterministic placement of donor atoms for quantum computing?

Key findings

• The experiment achieved a deterministic single-ion implantation confidence of $ 99.87 \pm 0.02\% $, demonstrating near-ideal fidelity.
• The double-implantation probability was measured to be $ \sim 10^{-7} $, confirming negligible impact on implantation fidelity.
• Signal spectra from experimental measurements matched simulated spectra from Crystal-TRIM with Fano statistics and detector noise, validating the simulation model.
• The modified Crystal-TRIM code incorporating the Funsten model for electronic energy loss provided better agreement with experimental data than the Robinson model.
• The method enables deterministic implantation at room temperature, compatible with existing silicon fabrication processes and scalable quantum device architectures.
• The integration of charge-sensitive electronics with a scanning-probe ion aperture allows real-time detection and control of individual ion implants, critical for scalable donor-qubit arrays.