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Semiconductor spins are one of the few qubit realizations that remain a serious candidate for the implementation of large-scale quantum circuits. Excellent scalability is often argued for spin qubits defined by lithography and controlled via electrical signals, based on the success of conventional semiconductor integrated circuits. However, the wiring and interconnect requirements for quantum circuits are completely different from those for classical circuits, as individual direct current, pulsed and in some cases microwave control signals need to be routed from external sources to every qubit. This is further complicated by the requirement that these spin qubits currently operate at temperatures below 100 mK. Here, we review several strategies that are considered to address this crucial challenge in scaling quantum circuits based on electron spin qubits. Key assets of spin qubits include the potential to operate at 1 to 4 K, the high density of quantum dots or donors combined with possibilities to space them apart as needed, the extremely long-spin coherence times, and the rich options for integration with classical electronics based on the same technology.npj Quantum Information (2017) 3:34 ; doi:10.1038/s41534-017-0038-y INTRODUCTIONThe quantum devices in which quantum bits are stored and processed will form the lowest layer of a complex multi-layer system. 1-3 The system also includes classical electronics to measure and control the qubits, and a conventional computer to control and program these electronics. Increasingly, some of the important challenges involved in these intermediate layers and how they interact have become clear, and there is a strong need for forming a picture of how these challenges can be addressed.Focusing on the interface between the two lowest layers of a quantum computer, each of the quantum bits must receive a long sequence of externally generated control signals that translate to the steps in the computation. Furthermore, given the fragile nature of quantum states, large numbers of quantum bits must be read out periodically to check whether errors occurred along the way, and to correct them. 4 Such error correction is possible provided the probability of error per operation is below the accuracy threshold, which is around 1% for the so-called surface code, a scheme which can be operated on two-dimensional (2D) qubit arrays with nearest-neighbor couplings. 5,6 The read-out data must be processed rapidly and fed back to the qubits in the form of control signals. Since each qubit must separately interface with the outside world, the classical control system must scale along with the number of qubits, and so must the interface between qubits and classical control.The estimated number of physical qubits required for solving relevant problems in quantum chemistry or code breaking is in the 10 6 -10 8 range, using currently known quantum algorithms and quantum error correction methods. 7,8 For comparison, state-

A nonreactive spreading protocol for measuring contact angles of buffered water droplets on low-energy surfaces is described. The protocol consists of immersing the sample (prior to contact-angle measurement) in a buffer solution of the same pH as that of the buffered water droplet that will be used to measure the contact angle. Contact-angle titration data acquired using this protocol for acidic self-assembled monolayers of alkanethiolates on gold exhibit a smooth transition in contact angle between plateau regions at low and high pH. This is in contrast with data acquired for identical monolayers using the more common reactive spreading protocol, for which a plateau region could not be obtained at high pH. It is postulated that the buffer pretreatment leaves the surface in a partially deprotonated state even though the surface is not macroscopically wet; this postulate is supported by infrared reflection-absorption spectroscopy data which show a conversion from protonated to deprotonated carboxylate in a dry acid-containing monolayer after exposure to a basic buffer solution. The nonreactive spreading protocol has proved particularly useful for deriving equilibrium parameters, e.g. pKa values for surface acid groups, from contact-angle titration data. Mixed monolayers of 11 -mercaptoundecanoic acid with the alkanethiols nonanethiol, decanethiol, undecanethiol, and dodecanethiol exhibited well-defined contact-angle titration curves from which p_Ka values for acid dissociation in the monolayers could be obtained. A strong dependence of pKa on alkanethiol chain length was observed, with pXa values ranging from approximately 6.5 for a mixed monolayer with nonanethiol to approximately 11.5 for a mixed monolayer with dodecanethiol. The latter value reflects a shift of 6.7 pK units relative to the pKa for butyric acid (a representative alkanoic acid) in bulk water. The pKa shifts are interpreted in terms of two physical processes, one involving solvation of carboxylate anions in the monolayer microenvironment and another involving interfacial potentials at the monolayersolution interface.

Quantum computation requires many qubits that can be coherently controlled and coupled to each other [1]. Qubits that are defined using lithographic techniques are often argued to be promising platforms for scalability, since they can be implemented using semiconductor fabrication technology [2][3][4][5]. However, leading solidstate approaches function only at temperatures below 100 mK, where cooling power is extremely limited, and this severely impacts the perspective for practical quantum computation. Recent works on spins in silicon have shown steps towards a platform that can be operated at higher temperatures by demonstrating long spin lifetimes [6], gate-based spin readout [7], and coherent singlespin control [8], but the crucial two-qubit logic gate has been missing. Here we demonstrate that silicon quantum dots can have sufficient thermal robustness to enable the execution of a universal gate set above one Kelvin. We obtain singlequbit control via electron-spin-resonance (ESR) and readout using Pauli spin blockade. We show individual coherent control of two qubits and measure single-qubit fidelities up to 99.3 %. We demonstrate tunability of the exchange interaction between the two spins from 0.5 up to 18 MHz and use this to execute coherent two-qubit controlled rotations (CROT). The demonstration of 'hot' and universal quantum logic in a semiconductor platform paves the way for quantum integrated circuits hosting the quantum hardware and their control circuitry all on the same chip, providing a scalable approach towards practical quantum information.Spin qubits based on quantum dots are among the most promising candidates for large-scale quantum computation [2,9,10]. Quantum coherence can be maintained in these systems for extremely long times [11] by using isotopically enriched silicon ( 28 Si) as the host material [12]. This has enabled the demonstration of singlequbit control with fidelities exceeding 99.9% [13,14] and the execution of two-qubit logic [15][16][17][18]. The potential to build larger systems with quantum dots manifests in the ability to deterministically engineer and optimize qubit locations and interactions using a technology that greatly resembles today's complementary metal-oxide semiconductor (CMOS) manufacturing. Nonetheless, quantum error correction schemes predict that millions to billions of qubits will be needed for practical quantum informa-tion [19]. Considering that today's devices make use of more than one terminal per qubit [20], wiring up such large systems remains a formidable task. In order to avoid an interconnect bottleneck, quantum integrated circuits hosting the qubits and their electronic control on the same chip have been proposed [2,3,21]. While these architectures provide an elegant way to increase the qubit count to large numbers by leveraging the success of classical integrated circuits, a key question is whether the qubits will be robust against the thermal noise imposed by the power dissipation of the electronics. Demonstrating a universal gate set at elevat...

Quantum dots take a shortcut toward practical quantum information.