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.

PRS Global Open: A Dream Turning to Reality.

Supplemental Digital Content is available in the text.

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.

Is Double-Blinded Peer Review Necessary? The Effect of Blinding on Review Quality.

BACKGROUND: The review process can be completely open, double-blinded, or somewhere in between. Double-blinded peer review, where neither the authors' nor peer reviewers' identities are shared with each other, is thought to be the fairest system, but there is evidence that it does not affect reviewer behavior or influence decisions. Furthermore, even without presenting author names, authorship is often apparent to reviewers, especially in small specialties. In conjunction with Plastic and Reconstructive Surgery (PRS), the authors examined the effect of double-blinded review on review quality, reviewer publishing recommendation, and reviewer manuscript rating. The authors hypothesized that double-blinded review will not improve review quality and will not affect recommendation or rating. METHODS: Traditionally, PRS peer review has been conducted in a single-blinded fashion. During a 3-month period of standard operation of the Journal, the authors examined reviews, recommendations, and manuscript ratings. Beginning October 1, 2014, PRS started conducting reviews in a double-blinded manner. The authors examined the additional reviews submitted during a 3-month period after the change. Review quality was assessed using the validated Review Quality Instrument. RESULTS: Double-blinding had no effect on reviewer publishing recommendation or manuscript ranking. Review quality did not improve after the implementation of double-blinded review. Blinding was successful 66 percent of the time. The most common reasons for blinding failure were reviewer familiarity with authors' work and author self-citation. CONCLUSIONS: Double-blinding adds considerable work for authors and editorial staff and has no positive effect on review quality. Furthermore, the authors' results revealed no publication bias based on author identity at PRS.