arXiv subjects

Hannes R. Firgau

Publications and source records attributed to Hannes R. Firgau.

8 recordsLinked to original sources

arXiv2025

Tomography of entangling two-qubit logic operations in exchange-coupled donor electron spin qubits

Scalable quantum processors require high-fidelity universal quantum logic operations in a manufacturable physical platform. Donors in silicon provide atomic size, excellent quantum coherence and compatibility with standard semiconductor processing, but no entanglement between donor-bound electron spins has been demonstrated to date. Here we present the experimental demonstration and tomography of universal 1- and 2-qubit gates in a system of two weakly exchange-coupled electrons, bound to single phosphorus donors introduced in silicon by ion implantation. We surprisingly observe that the exchange interaction has no effect on the qubit coherence. We quantify the fidelity of the quantum operations using gate set tomography (GST), and we use the universal gate set to create entangled Bell states of the electrons spins, with fidelity ~ 93%, and concurrence 0.91 +/- 0.08. These results form the necessary basis for scaling up donor-based quantum computers.

quant-ph↗

arXiv2025

Scalable entanglement of nuclear spins mediated by electron exchange

The use of nuclear spins for quantum computation is limited by the difficulty in creating genuine quantum entanglement between distant nuclei. Current demonstrations of nuclear entanglement in semiconductors rely upon coupling the nuclei to a common electron, which is not a scalable strategy. Here we demonstrate a two-qubit Control-Z logic operation between the nuclei of two phosphorus atoms in a silicon device, separated by up to 20 nanometers. Each atoms binds separate electrons, whose exchange interaction mediates the nuclear two-qubit gate. We prepare and measure a nuclear Bell state with a fidelity of 76 +/- 5 $\%$ and a concurrence of 0.67 +/- 0.05. With this method, future progress in scaling up semiconductor spin qubits can be extended to the development of nuclear-spin based quantum computers.

quant-ph↗

arXiv2023

Scalable Atomic Arrays for Spin-Based Quantum Computers in Silicon

Semiconductor spin qubits combine excellent quantum performance with the prospect of manufacturing quantum devices using industry-standard metal-oxide-semiconductor (MOS) processes. This applies also to ion-implanted donor spins, which further afford exceptional coherence times and large Hilbert space dimension in their nuclear spin. Here we demonstrate and integrate multiple strategies to manufacture scale-up donor-based quantum computers. We use $^{31}$PF$_{2}$ molecule implants to triple the placement certainty compared to $^{31}$P ions, while attaining 99.99$\,$% confidence in detecting the implant. Similar confidence is retained by implanting heavier atoms such as $^{123}$Sb and $^{209}$Bi, which represent high-dimensional qudits for quantum information processing, while Sb$_2$ molecules enable deterministic formation of closely-spaced qudits. We demonstrate the deterministic formation of regular arrays of donor atoms with 300$\,$nm spacing, using step-and-repeat implantation through a nano aperture. These methods cover the full gamut of technological requirements for the construction of donor-based quantum computers in silicon.

cond-mat.mtrl-sci↗

arXiv2022

Near-Surface Electrical Characterisation of Silicon Electronic Devices Using Focused keV Ions

The demonstration of universal quantum logic operations near the fault-tolerance threshold establishes ion-implanted near-surface donor atoms as a plausible platform for scalable quantum computing in silicon. The next technological step forward requires a deterministic fabrication method to create large-scale arrays of donors, featuring few hundred nanometre inter-donor spacing. Here, we explore the feasibility of this approach by implanting low-energy ions into silicon devices featuring an enlarged 60x60 $μ$m sensitive area and an ultra-thin 3.2 nm gate oxide - capable of hosting large-scale donor arrays. By combining a focused ion beam system incorporating an electron-beam-ion-source with in-vacuum ultra-low noise ion detection electronics, we first demonstrate a versatile method to spatially map the device response characteristics to shallowly implanted 12 keV $^1$H$_2^+$ ions. Despite the weak internal electric field, near-unity charge collection efficiency is obtained from the entire sensitive area. This can be explained by the critical role that the high-quality thermal gate oxide plays in the ion detection response, allowing an initial rapid diffusion of ion induced charge away from the implant site. Next, we adapt our approach to perform deterministic implantation of a few thousand 24 keV $^{40}$Ar$^{2+}$ ions into a predefined micro-volume, without any additional collimation. Despite the reduced ionisation from the heavier ion species, a fluence-independent detection confidence of $\geq$99.99% was obtained. Our system thus represents not only a new method for mapping the near-surface electrical landscape of electronic devices, but also an attractive framework towards mask-free prototyping of large-scale donor arrays in silicon.

cond-mat.mes-hall↗

arXiv2022

Observing hyperfine interactions of NV centers in diamond in an advanced quantum teaching lab

The negatively charged nitrogen-vacancy (NV$^-$) center in diamond is a model quantum system for university teaching labs due to its room-temperature compatibility and cost-effective operation. Based on the low-cost experimental setup that we have developed and described for the coherent control of the electronic spin (Sewani et al.), we introduce and explain here a number of more advanced experiments that probe the electron-nuclear interaction between the \nv electronic and the \NN~and \CC~nuclear spins. Optically-detected magnetic resonance (ODMR), Rabi oscillations, Ramsey fringe experiments, and Hahn echo sequences are implemented to demonstrate how the nuclear spins interact with the electron spins. Most experiments only require 15 minutes of measurement time and can, therefore, be completed within one teaching lab.

physics.ed-ph↗

arXiv2020

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

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.

cond-mat.mes-hall↗

arXiv2020

Coherent control of NV- centers in diamond in a quantum teaching lab

The room temperature compatibility of the negatively-charged nitrogen-vacancy (NV-) in diamond makes it the ideal quantum system for a university teaching lab. Here, we describe a low-cost experimental setup for coherent control experiments on the electronic spin state of the NV- center. We implement spin-relaxation measurements, optically-detected magnetic resonance, Rabi oscillations, and dynamical decoupling sequences on an ensemble of NV- centers. The relatively short times required to perform each of these experiments (<10 minutes) demonstrate the feasibility of the setup in a teaching lab. Learning outcomes include basic understanding of quantum spin systems, magnetic resonance, the rotating frame, Bloch spheres, and pulse sequence development.

physics.ed-ph↗

arXiv2019

Coherent electrical control of a single high-spin nucleus in silicon

Nuclear spins are highly coherent quantum objects. In large ensembles, their control and detection via magnetic resonance is widely exploited, e.g. in chemistry, medicine, materials science and mining. Nuclear spins also featured in early ideas and demonstrations of quantum information processing. Scaling up these ideas requires controlling individual nuclei, which can be detected when coupled to an electron. However, the need to address the nuclei via oscillating magnetic fields complicates their integration in multi-spin nanoscale devices, because the field cannot be localized or screened. Control via electric fields would resolve this problem, but previous methods relied upon transducing electric signals into magnetic fields via the electron-nuclear hyperfine interaction, which severely affects the nuclear coherence. Here we demonstrate the coherent quantum control of a single antimony (spin-7/2) nucleus, using localized electric fields produced within a silicon nanoelectronic device. The method exploits an idea first proposed in 1961 but never realized experimentally with a single nucleus. Our results are quantitatively supported by a microscopic theoretical model that reveals how the purely electrical modulation of the nuclear electric quadrupole interaction, in the presence of lattice strain, results in coherent nuclear spin transitions. The spin dephasing time, 0.1 seconds, surpasses by orders of magnitude those obtained via methods that require a coupled electron spin for electrical drive. These results show that high-spin quadrupolar nuclei could be deployed as chaotic models, strain sensors and hybrid spin-mechanical quantum systems using all-electrical controls. Integrating electrically controllable nuclei with quantum dots could pave the way to scalable nuclear- and electron-spin-based quantum computers in silicon that operate without the need for oscillating magnetic fields.

cond-mat.mes-hall↗