Universal logic with encoded spin qubits in silicon.

Quantum computation features known examples of hardware acceleration for certain problems, but is challenging to realize because of its susceptibility to small errors from noise or imperfect control. The principles of fault tolerance may enable computational acceleration with imperfect hardware, but they place strict requirements on the character and correlation of errors1. For many qubit technologies2-21, some challenges to achieving fault tolerance can be traced to correlated errors arising from the need to control qubits by injecting microwave energy matching qubit resonances. Here we demonstrate an alternative approach to quantum computation that uses energy-degenerate encoded qubit states controlled by nearest-neighbour contact interactions that partially swap the spin states of electrons with those of their neighbours. Calibrated sequences of such partial swaps, implemented using only voltage pulses, allow universal quantum control while bypassing microwave-associated correlated error sources1,22-28. We use an array of six 28Si/SiGe quantum dots, built using a platform that is capable of extending in two dimensions following processes used in conventional microelectronics29. We quantify the operational fidelity of universal control of two encoded qubits using interleaved randomized benchmarking30, finding a fidelity of 96.3% ± 0.7% for encoded controlled NOT operations and 99.3% ± 0.5% for encoded SWAP. The quantum coherence offered by enriched silicon5-9,16,18,20,22,27,29,31-37, the all-electrical and low-crosstalk-control of partial swap operations1,22-28 and the configurable insensitivity of our encoding to certain error sources28,33,34,38 all combine to offer a strong pathway towards scalable fault tolerance and computational advantage.

A Flexible Design Platform for Si/SiGe Exchange-Only Qubits with Low Disorder.

Spin-based silicon quantum dots are an attractive qubit technology for quantum information processing with respect to coherence time, control, and engineering. Here we present an exchange-only Si qubit device platform that combines the throughput of CMOS-like wafer processing with the versatility of direct-write lithography. The technology, which we coin "SLEDGE", features dot-shaped gates that are patterned simultaneously on one topographical plane and subsequently connected by vias to interconnect metal lines. The process design enables nontrivial layouts as well as flexibility in gate dimensions, material selection, and additional device features such as for rf qubit control. We show that the SLEDGE process has reduced electrostatic disorder with respect to traditional overlapping gate devices with lift-off metallization, and we present spin coherent exchange oscillations and single qubit blind randomized benchmarking data.

Quantifying error and leakage in an encoded Si/SiGe triple-dot qubit.

Quantum computation requires qubits that satisfy often-conflicting criteria, which include long-lasting coherence and scalable control1. One approach to creating a suitable qubit is to operate in an encoded subspace of several physical qubits. Although such encoded qubits may be particularly susceptible to leakage out of their computational subspace, they can be insensitive to certain noise processes2,3 and can also allow logical control with a single type of entangling interaction4 while maintaining favourable features of the underlying physical system. Here we demonstrate high-fidelity operation of an exchange-only qubit encoded in a subsystem of three coupled electron spins5 confined in gated, isotopically enhanced silicon quantum dots6. This encoding requires neither high-frequency electric nor magnetic fields for control, and instead relies exclusively on the exchange interaction4,5, which is highly local and can be modulated with a large on-off ratio using only fast voltage pulses. It is also compatible with very low and gradient-free magnetic field environments, which simplifies integration with superconducting materials. We developed and employed a modified blind randomized benchmarking protocol that determines both computational and leakage errors7,8, and found that unitary operations have an average total error of 0.35%, with half of that, 0.17%, coming from leakage driven by interactions with substrate nuclear spins. The combination of this proven performance with complete control via gate voltages makes the exchange-only qubit especially attractive for use in many-qubit systems.

Nanostructured silicon via metal assisted catalyzed etch (MACE): chemistry fundamentals and pattern engineering.

There are a range of different methods to generate a nanostructured surface on silicon (Si) but the most cost effective and optically interesting is the metal assisted wet chemical etching (MACE) (Koynov et al 2006 Appl. Phys. Lett. 88 203107). MACE of Si is a controllable, room-temperature wet-chemical technique that uses a thin layer of metal to etch the surface of Si, leaving behind various nano- and micro-scale surface features or 'black silicon'. MACE-fabricated nanowires (NWs) provide improved antireflection and light trapping functionality (Toor et al 2016 Nanoscale 8 15448-66) compared with the traditional 'iso-texturing' (Campbell and Green 1987 J. Appl. Phys. 62 243-9). The resulting lower reflection and improved light trapping can lead to higher short circuit currents in NW solar cells (Toor et al 2011 Appl. Phys. Lett. 99 103501). In addition, NW cells can have higher fill factors and voltages than traditionally processed cells, thus leading to increased solar cell efficiencies (Cabrera et al 2013 IEEE J. Photovolt. 3 102-7). MACE NW processing also has synergy with next generation Si solar cell designs, such as thin epitaxial-Si and passivated emitter rear contact (Toor et al 2016 Nanoscale 8 15448-66). While several companies have begun manufacturing black Si, and many more are researching these techniques, much of the work has not been published in traditional journals and is publicly available only through conference proceedings and patent publications, which makes learning the field challenging. There have been three specialized review articles published recently on certain aspects of MACE or black Si, but do not present a full review that would benefit the industry (Liu et al 2014 Energy Environ. Sci. 7 3223-63; Yusufoglu et al 2015 IEEE J. Photovolt. 5 320-8; Huang et al 2011 Adv. Mater. 23 285-308). In this feature article, we review the chemistry of MACE and explore how changing parameters in the wet etch process effects the resulting texture on the Si surface. Then we review efforts to increase the uniformity and reproducibility of the MACE process, which is critical for commercializing the black Si technology.

Metal assisted catalyzed etched (MACE) black Si: optics and device physics.

Metal-assisted catalyzed etching (MACE) of silicon (Si) is a controllable, room-temperature wet-chemical technique that uses a thin layer of metal to etch the surface of Si, leaving behind various nano- and micro-scale surface features, including nanowires (NWs), that can be tuned to achieve various useful engineering goals, in particular with respect to Si solar cells. In this review, we introduce the science and technology of MACE from the literature, and provide an in-depth analysis of MACE to enhance Si solar cells, including the outlook for commercial applications of this technology.