Floris A. Zwanenburg

Silicon quantum electronics

This review describes recent groundbreaking results in Si, Si/SiGe, and dopant-based quantum dots, and it highlights the remarkable advances in Si-based quantum physics that have occurred in the past few years. This progress has been possible thanks to materials development of Si quantum devices, and the physical understanding of quantum effects in silicon. Recent critical steps include the isolation of single electrons, the observation of spin blockade, and single-shot readout of individual electron spins in both dopants and gated quantum dots in Si. Each of these results has come with physics that was not anticipated from previous work in other material systems. These advances underline the significant progress toward the realization of spin quantum bits in a material with a long spin coherence time, crucial for quantum computation and spintronics.

High-fidelity readout and control of a nuclear spin qubit in silicon

Detection of nuclear spin precession is critical for a wide range of scientific techniques that have applications in diverse fields including analytical chemistry, materials science, medicine and biology. Fundamentally, it is possible because of the extreme isolation of nuclear spins from their environment. This isolation also makes single nuclear spins desirable for quantum-information processing, as shown by pioneering studies on nitrogen-vacancy centres in diamond. The nuclear spin of a 31P donor in silicon is very promising as a quantum bit: bulk measurements indicate that it has excellent coherence times and silicon is the dominant material in the microelectronics industry. Here we demonstrate electrical detection and coherent manipulation of a single 31P nuclear spin qubit with sufficiently high fidelities for fault-tolerant quantum computing. By integrating single-shot readout of the electron spin with on-chip electron spin resonance, we demonstrate quantum non-demolition and electrical single-shot readout of the nuclear spin with a readout fidelity higher than 99.8 per cent—the highest so far reported for any solid-state qubit. The single nuclear spin is then operated as a qubit by applying coherent radio-frequency pulses. For an ionized 31P donor, we find a nuclear spin coherence time of 60 milliseconds and a one-qubit gate control fidelity exceeding 98 per cent. These results demonstrate that the dominant technology of modern electronics can be adapted to host a complete electrical measurement and control platform for nuclear-spin-based quantum-information processing.

Spectroscopy of few-electron single-crystal silicon quantum dots.

A defining feature of modern CMOS devices and almost all quantum semiconductor devices is the use of many different materials. For example, although electrical conduction often occurs in single-crystal semiconductors, gates are frequently made of metals and dielectrics are commonly amorphous. Such devices have demonstrated remarkable improvements in performance over recent decades, but the heterogeneous nature of these devices can lead to defects at the interfaces between the different materials, which is a disadvantage for applications in spintronics and quantum information processing. Here we report the fabrication of a few-electron quantum dot in single-crystal silicon that does not contain any heterogeneous interfaces. The quantum dot is defined by atomically abrupt changes in the density of phosphorus dopant atoms, and the resulting confinement produces novel effects associated with energy splitting between the conduction band valleys. These single-crystal devices offer the opportunity to study how very sharp, atomic-scale confinement--which will become increasingly important for both classical and quantum devices--influences the operation and performance of devices.

Spin States of the First Four Holes in a Silicon Nanowire Quantum Dot

We report measurements on a silicon nanowire quantum dot with a clarity that allows for a complete understanding of the spin states of the first four holes. First, we show control of the hole number down to one. Detailed measurements at perpendicular magnetic fields reveal the Zeeman splitting of a single hole in silicon. We are able to determine the ground-state spin configuration for one to four holes occupying the quantum dot and find a spin filling with alternating spin-down and spin-up holes, which is confirmed by magnetospectroscopy up to 9 T. Additionally, a so far inexplicable feature in single-charge quantum dots in many materials systems is analyzed in detail. We observe excitations of the empty quantum dot, which cannot correspond to electronic or Zeeman states. We show that the most likely explanation is acoustic phonon emission to a cavity between the two contacts to the nanowire.

Observation of the single-electron regime in a highly tunable silicon quantum dot

We report on low-temperature electronic transport measurements of a silicon metal-oxidesemiconductor quantum dot, with independent gate control of electron densities in the leads and the quantum dot island. This architecture allows the dot energy levels to be probed without affecting the electron density in the leads and vice versa. Appropriate gate biasing enables the dot occupancy to be reduced to the single-electron level, as evidenced by magnetospectroscopy measurements of the ground state of the first two charge transitions. Independent gate control of the electron reservoirs also enables discrimination between excited states of the dot and density of states modulations in the leads.

Pauli Spin Blockade in a Highly Tunable Silicon Double Quantum Dot

Double quantum dots are convenient solid-state platforms to encode quantum information. Two-electron spin states can be detected and manipulated using quantum selection rules based on the Pauli exclusion principle, leading to Pauli spin blockade of electron transport for triplet states. Coherent spin states would be optimally preserved in an environment free of nuclear spins, which is achievable in silicon by isotopic purification. Here we report on a deliberately engineered, gate-defined silicon metal-oxide-semiconductor double quantum dot system. The electron occupancy of each dot and the inter-dot tunnel coupling are independently tunable by electrostatic gates. At weak inter-dot coupling we clearly observe Pauli spin blockade and measure a large intra-dot singlet-triplet splitting > 1 meV. The leakage current in spin blockade has a peculiar magnetic field dependence, unrelated to electron-nuclear effects and consistent with the effect of spin-flip cotunneling processes. The results obtained here provide excellent prospects for realising singlet-triplet qubits.

Resonant tunnelling features in quantum dots

We present a systematic review of features due to resonant electron tunnelling, observable in transport spectroscopy experiments on quantum dots and single donors. The review covers features attributable to intrinsic properties of the dot (orbital, spin and valley states) as well as extrinsic effects (phonon/photon emission/absorption, features in the charge reservoirs, coupling to nearby charge centres). We focus on the most common operating conditions, neglecting effects due to strong coupling to the leads. By discussing the experimental signatures of each type of feature, we aim at providing practical methods to distinguish between their different physical origins. The correct classification of the resonant tunnelling features is an essential requirement to understand the details of the confining potential or to predict the performance of the dot for quantum information processing.We present a review of features due to resonant tunnelling in transport spectroscopy experiments on quantum dots and single donors. The review covers features attributable to intrinsic properties of the dot as well as extrinsic effects, with a focus on the most common operating conditions. We describe several phenomena that can lead to apparently identical signatures in a bias spectroscopy measurement, with the aim of providing experimental methods to distinguish between their different physical origins. The correct classification of the resonant tunnelling features is an essential requirement to understand the details of the confining potential or predict the performance of the dot for quantum information processing.

Single-hole tunneling through a two-dimensional hole gas in intrinsic silicon

In this letter we report single-hole tunneling through a quantum dot in a two-dimensional hole gas, situated in a narrow-channel field-effect transistor in intrinsic silicon. Two layers of aluminum gate electrodes are defined on Si/SiO2 using electron-beam lithography. Fabrication and subsequent electrical characterization of different devices yield reproducible results, such as typical MOSFET turn-on and pinch-off characteristics. Additionally, linear transport measurements at 4 K result in regularly spaced Coulomb oscillations, corresponding to single-hole tunneling through individual Coulomb islands. These Coulomb peaks are visible over a broad range in gate voltage, indicating very stable device operation. Energy spectroscopy measurements show closed Coulomb diamonds with single-hole charging energies of 5–10 meV and lines of increased conductance as a result of resonant tunneling through additional available hole states.

Dynamically controlled charge sensing of a few-electron silicon quantum dot

We report charge sensing measurements of a silicon metal-oxide-semiconductor quantum dot using a single-electron transistor as a charge sensor with dynamic feedback control. Using digitally-controlled feedback, the sensor exhibits sensitive and robust detection of the charge state of the quantum dot, even in the presence of charge drifts and random charge upset events. The sensor enables the occupancy of the quantum dot to be probed down to the single electron level.

Electric field control of magnetoresistance in InP nanowires with ferromagnetic contacts

We demonstrate electric field control of sign and magnitude of the magnetoresistance in InP nanowires with ferromagnetic contacts. The sign change in the magnetoresistance is directly correlated with a sign change in the transconductance. Additionally, the magnetoresistance is shown to persist at such a high bias that Coulomb blockade has been lifted. We also observe the magnetoresistance when one of the ferromagnets is replaced by a nonmagnetic metal. We conclude that it must be induced by a single ferromagnetic contact, and that spin transport can be ruled out as the origin. Our results emphasize the importance of a systematic investigation of spin-valve devices in order to discriminate between ambiguous interpretations.