Donor-based Qubits Research Articles

Machine Learning-Assisted Precision Manufacturing of Atom Qubits in Silicon.

Donor-based qubits in silicon, manufactured using scanning tunneling microscope (STM) lithography, provide a promising route to realizing full-scale quantum computing architectures. This is due to the precision of donor placement, long coherence times, and scalability of the silicon material platform. The properties of multiatom quantum dot qubits, however, depend on the exact number and location of the donor atoms within the quantum dots. In this work, we develop machine learning techniques that allow accurate and real-time prediction of the donor number at the qubit site during STM patterning. Machine learning image recognition is used to determine the probability distribution of donor numbers at the qubit site directly from STM images during device manufacturing. Models in excess of 90% accuracy are found to be consistently achieved by mitigating overfitting through reduced model complexity, image preprocessing, data augmentation, and examination of the intermediate layers of the convolutional neural networks. The results presented in this paper constitute an important milestone in automating the manufacture of atom-based qubits for computation and sensing applications.

Zoo of silicon-based quantum bits

Zoo of silicon-based quantum bits

Donor-based qubits for quantum computing in silicon

Spin-qubits based on impurities such as phosphorus in silicon (Si) have attractive attributes for the development of quantum computing devices. Very long coherence times can be achieved for donor-based qubits in Si due to the availability of isotopically pure 28Si layers where the 29Si atoms, which otherwise lead to decoherence, are largely absent in the active region of the device. Well-behaved single donor qubits in Si can routinely be formed using ion implantation, and the key performance criteria needed to demonstrate the basis of a viable platform for quantum computing have been achieved. The crucial next stage of development is to demonstrate suitable pathways for scale-up that allow patterned arrays of donor qubits to be controllably coupled and that are robust against the inherent donor placement tolerances and material processing constraints that exist. Here, we review progress on the fabrication and measurement of donor-based qubits in silicon via the ion implantation pathway and discuss the key developmental milestones that have been achieved. We also provide an overview of the key scale-up strategies that are being actively pursued to take donor-based quantum computing in Si to the next stage.

Electron spin relaxations of phosphorus donors in bulk silicon under large electric field

Modulation of donor electron wavefunction via electric fields is vital to quantum computing architectures based on donor spins in silicon. For practical and scalable applications, the donor-based qubits must retain sufficiently long coherence times in any realistic experimental conditions. Here, we present pulsed electron spin resonance studies on the longitudinal (T1) and transverse (T2) relaxation times of phosphorus donors in bulk silicon with various electric field strengths up to near avalanche breakdown in high magnetic fields of about 1.2 T and low temperatures of about 8 K. We find that the T1 relaxation time is significantly reduced under large electric fields due to electric current, and T2 is affected as the T1 process can dominate decoherence. Furthermore, we show that the magnetoresistance effect in silicon can be exploited as a means to combat the reduction in the coherence times. While qubit coherence times must be much longer than quantum gate times, electrically accelerated T1 can be found useful when qubit state initialization relies on thermal equilibration.

Ion implantation for deterministic single atom devices

We demonstrate a capability of deterministic doping at the single atom level using a combination of direct write focused ion beam and solid-state ion detectors. The focused ion beam system can position a single ion to within 35 nm of a targeted location and the detection system is sensitive to single low energy heavy ions. This platform can be used to deterministically fabricate single atom devices in materials where the nanostructure and ion detectors can be integrated, including donor-based qubits in Si and color centers in diamond.

Device fabrication and transport measurements of FinFETs built with 28Si SOI wafers toward donor qubits in silicon

We report on the fabrication of transistors in a FinFET geometry using isotopically purified silicon-28-on-insulator (28-SOI) substrates. Donor electron spin coherence in natural silicon is limited by spectral diffusion due to the residual 29Si nuclear spin bath, making isotopically enriched nuclear spin-free 28Si substrates a promising candidate for forming spin quantum bit devices. The FinFET architecture is fully compatible with single-ion implant detection for donor-based qubits and the donor spin-state readout through electrical detection of spin resonance. We describe device processing steps and discuss results on electrical transport measurements at 0.3 K.