Measurement of Enhanced Spin-Orbit Coupling Strength for Donor-Bound Electron Spins in Silicon.

While traditionally considered a deleterious effect in quantum dot spin qubits, the spin-orbit interaction is recently being revisited as it allows for rapid coherent control by on-chip AC electric fields. For electrons in bulk silicon, spin-orbit coupling (SOC) is intrinsically weak, however, it can be enhanced at surfaces and interfaces, or through atomic placement. Here it is showed that the strength of the spin-orbit coupling can be locally enhanced by more than two orders of magnitude in the manybody wave functions of multi-donor quantum dots compared to a single donor, reaching strengths so far only reported for holes or two-donor system with certain symmetry.

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.

Engineering Spin-Orbit Interactions in Silicon Qubits at the Atomic-Scale.

Spin-orbit interactions arise whenever the bulk inversion symmetry and/or structural inversion symmetry of a crystal is broken providing a bridge between a qubit's spin and orbital degree of freedom. While strong interactions can facilitate fast qubit operations by all-electrical control, they also provide a mechanism to couple charge noise thereby limiting qubit lifetimes. Previously believed to be negligible in bulk silicon, recent silicon nano-electronic devices have shown larger than bulk spin-orbit coupling strengths from Dresselhaus and Rashba couplings.

Atomic Engineering of Molecular Qubits for High-Speed, High-Fidelity Single Qubit Gates.

Universal quantum computing requires fast single- and two-qubit gates with individual qubit addressability to minimize decoherence errors during processor operation. Electron spin qubits using individual phosphorus donor atoms in silicon have demonstrated long coherence times with high fidelities, providing an attractive platform for scalable quantum computing. While individual qubit addressability has been demonstrated by controlling the hyperfine interaction between the electron and nuclear wave function in a global magnetic field, the small hyperfine Stark coefficient of 0.

The Use of Exchange Coupled Atom Qubits as Atomic-Scale Magnetic Field Sensors.

Phosphorus atoms in silicon offer a rich quantum computing platform where both nuclear and electron spins can be used to store and process quantum information. While individual control of electron and nuclear spins has been demonstrated, the interplay between them during qubit operations has been largely unexplored. This study investigates the use of exchange-based operation between donor bound electron spins to probe the local magnetic fields experienced by the qubits with exquisite precision at the atomic scale.

Ramped measurement technique for robust high-fidelity spin qubit readout.

State preparation and measurement of single-electron spin qubits typically rely on spin-to-charge conversion where a spin-dependent charge transition of the electron is detected by a coupled charge sensor. For high-fidelity, fast readout, this process requires that the qubit energy is much larger than the temperature of the system limiting the temperature range for measurements. Here, we demonstrate an initialization and measurement technique that involves voltage ramps rather than static voltages allowing us to achieve state-to-charge readout fidelities above 99% for qubit energies almost half that required by traditional methods.

Monolithic Three-Dimensional Tuning of an Atomically Defined Silicon Tunnel Junction.

A requirement for quantum information processors is the in situ tunability of the tunnel rates and the exchange interaction energy within the device. The large energy level separation for atom qubits in silicon is well suited for qubit operation but limits device tunability using in-plane gate architectures, requiring vertically separated top-gates to control tunnelling within the device. In this paper, we address control of the simplest tunnelling device in Si:P, the tunnel junction.

Coherent control of a donor-molecule electron spin qubit in silicon.

Donor spins in silicon provide a promising material platform for large scale quantum computing. Excellent electron spin coherence times of [Formula: see text] μs with fidelities of 99.9% have been demonstrated for isolated phosphorus donors in isotopically pure Si, where donors are local-area-implanted in a nanoscale MOS device.

Exploiting a Single-Crystal Environment to Minimize the Charge Noise on Qubits in Silicon.

Electron spins in silicon offer a competitive, scalable quantum-computing platform with excellent single-qubit properties. However, the two-qubit gate fidelities achieved so far have fallen short of the 99% threshold required for large-scale error-corrected quantum computing architectures. In the past few years, there has been a growing realization that the critical obstacle in meeting this threshold in semiconductor qubits is charge noise arising from the qubit environment.

Engineering long spin coherence times of spin-orbit qubits in silicon.

Electron-spin qubits have long coherence times suitable for quantum technologies. Spin-orbit coupling promises to greatly improve spin qubit scalability and functionality, allowing qubit coupling via photons, phonons or mutual capacitances, and enabling the realization of engineered hybrid and topological quantum systems. However, despite much recent interest, results to date have yielded short coherence times (from 0.

Spin read-out in atomic qubits in an all-epitaxial three-dimensional transistor.

The realization of the surface code for topological error correction is an essential step towards a universal quantum computer. For single-atom qubits in silicon, the need to control and read out qubits synchronously and in parallel requires the formation of a two-dimensional array of qubits with control electrodes patterned above and below this qubit layer. This vertical three-dimensional device architecture requires the ability to pattern dopants in multiple, vertically separated planes of the silicon crystal with nanometre precision interlayer alignment.

Readout and control of the spin-orbit states of two coupled acceptor atoms in a silicon transistor.

Coupling spin qubits to electric fields is attractive to simplify qubit manipulation and couple qubits over long distances. Electron spins in silicon offer long lifetimes, but their weak spin-orbit interaction makes electrical coupling challenging. Hole spins bound to acceptor dopants, spin-orbit-coupled = 3/2 systems similar to Si vacancies in SiC and single Co dopants, are an electrically active spin system in silicon.

Addressable electron spin resonance using donors and donor molecules in silicon.

Phosphorus donor impurities in silicon are a promising candidate for solid-state quantum computing due to their exceptionally long coherence times and high fidelities. However, individual addressability of exchange coupled donors with separations ~15 nm is challenging. We show that by using atomic precision lithography, we can place a single P donor next to a 2P molecule 16 ± 1 nm apart and use their distinctive hyperfine coupling strengths to address qubits at vastly different resonance frequencies.

Characterization of a Scalable Donor-Based Singlet-Triplet Qubit Architecture in Silicon.

We present a donor-based quadruple-quantum-dot device, designed to host two singlet-triplet qubits fabricated by scanning tunnelling microscope lithography, with just two leads per qubit. The design is geometrically compact, with each pair of dots independently controlled via one gate and one reservoir. The reservoirs both supply electrons for the dots and measure the singlet-triplet state of each qubit via dispersive sensing.

Atomically engineered electron spin lifetimes of 30 s in silicon.

Scaling up to large arrays of donor-based spin qubits for quantum computation will require the ability to perform high-fidelity readout of multiple individual spin qubits. Recent experiments have shown that the limiting factor for high-fidelity readout of many qubits is the lifetime of the electron spin. We demonstrate the longest reported lifetimes (up to 30 s) of any electron spin qubit in a nanoelectronic device.

Probing the Quantum States of a Single Atom Transistor at Microwave Frequencies.

The ability to apply gigahertz frequencies to control the quantum state of a single P atom is an essential requirement for the fast gate pulsing needed for qubit control in donor-based silicon quantum computation. Here, we demonstrate this with nanosecond accuracy in an all epitaxial single atom transistor by applying excitation signals at frequencies up to ≈13 GHz to heavily phosphorus-doped silicon leads. These measurements allow the differentiation between the excited states of the single atom and the density of states in the one-dimensional leads.

In Situ Patterning of Ultrasharp Dopant Profiles in Silicon.

We develop a method for patterning a buried two-dimensional electron gas (2DEG) in silicon using low kinetic energy electron stimulated desorption (LEESD) of a monohydride resist mask. A buried 2DEG forms as a result of placing a dense and narrow profile of phosphorus dopants beneath the silicon surface; a so-called δ-layer. Such 2D dopant profiles have previously been studied theoretically, and by angle-resolved photoemission spectroscopy, and have been shown to host a 2DEG with properties desirable for atomic-scale devices and quantum computation applications.

Characterizing Si:P quantum dot qubits with spin resonance techniques.

Quantum dots patterned by atomically precise placement of phosphorus donors in single crystal silicon have long spin lifetimes, advantages in addressability, large exchange tunability, and are readily available few-electron systems. To be utilized as quantum bits, it is important to non-invasively characterise these donor quantum dots post fabrication and extract the number of bound electron and nuclear spins as well as their locations. Here, we propose a metrology technique based on electron spin resonance (ESR) measurements with the on-chip circuitry already needed for qubit manipulation to obtain atomic scale information about donor quantum dots and their spin configurations.

Ultralow-Noise Atomic-Scale Structures for Quantum Circuitry in Silicon.

The atomically precise doping of silicon with phosphorus (Si:P) using scanning tunneling microscopy (STM) promises ultimate miniaturization of field effect transistors. The one-dimensional (1D) Si:P nanowires are of particular interest, retaining exceptional conductivity down to the atomic scale, and are predicted as interconnects for a scalable silicon-based quantum computer. Here, we show that ultrathin Si:P nanowires form one of the most-stable electrical conductors, with the phenomenological Hooge parameter of low-frequency noise being as low as ≈10(-8) at 4.

A surface code quantum computer in silicon.

The exceptionally long quantum coherence times of phosphorus donor nuclear spin qubits in silicon, coupled with the proven scalability of silicon-based nano-electronics, make them attractive candidates for large-scale quantum computing. However, the high threshold of topological quantum error correction can only be captured in a two-dimensional array of qubits operating synchronously and in parallel-posing formidable fabrication and control challenges. We present an architecture that addresses these problems through a novel shared-control paradigm that is particularly suited to the natural uniformity of the phosphorus donor nuclear spin qubit states and electronic confinement.

Suppressing Segregation in Highly Phosphorus Doped Silicon Monolayers.

Sharply defined dopant profiles and low resistivity are highly desired qualities in the microelectronic industry, and more recently, in the development of an all epitaxial Si:P based quantum computer. In this work, we use thin (monolayers thick) room temperature grown silicon layers, so-called locking layers, to limit dopant segregation in highly phosphorus doped silicon monolayers. We present secondary ion mass spectroscopy and atom probe tomography measurements that demonstrate the effectiveness of locking layers in suppressing P segregation.

Bottom-up assembly of metallic germanium.

Extending chip performance beyond current limits of miniaturisation requires new materials and functionalities that integrate well with the silicon platform. Germanium fits these requirements and has been proposed as a high-mobility channel material, a light emitting medium in silicon-integrated lasers, and a plasmonic conductor for bio-sensing. Common to these diverse applications is the need for homogeneous, high electron densities in three-dimensions (3D).

The Impact of Dopant Segregation on the Maximum Carrier Density in Si:P Multilayers.

Abrupt dopant profiles and low resistivity are highly sought after qualities in the silicon microelectronics industry and, more recently, in the development of an all epitaxial Si:P based quantum computer. If we increase the active carrier density in silicon to the point where the material becomes superconducting, while maintaining a low thermal budget, it will be possible to fabricate nanoscale superconducting devices using the highly successful technique of depassivation lithography. In this work, we investigate the dopant profile and activation in multiple high density Si:P δ-layers fabricated by stacking individual layers with intervening silicon growth.