The size of silicon transistors used in microelectronic devices is shrinking to the level where quantum effects become important 1 . While this presents a significant challenge for the further scaling of microprocessors, it provides the potential for radical innovations in the form of spin-based quantum computers 2-4 and spintronic devices 5 . An electron spin in Si can represent a well-isolated quantum bit with long coherence times 6 because of the weak spin-orbit coupling 7 and the possibility to eliminate nuclear spins from the bulk crystal 8 . However, the control of single electrons in Si has proved challenging, and has so far hindered the observation and manipulation of a single spin. Here we report the first demonstration of single-shot, time-resolved readout of an electron spin in Si. This has been performed in a device consisting of implanted phosphorus donors 9 coupled to a metal-oxide-semiconductor single-electron transistor 10,11 -compatible with current microelectronic technology. We observed a spin lifetime approaching 1 second at magnetic fields below 2 T, and achieved spin readout fidelity better than 90%. High-fidelity single-shot spin readout in Si opens the path to the development of a new generation of quantum computing and spintronic devices, built using the most important material in the semiconductor industry.The projective, single-shot readout of a qubit is a crucial step in both circuit-based and measurement-based quantum computers 12 . For electron spins in solid state, this has only been achieved in GaAs/AlGaAs quantum dots coupled to charge detectors 13-15 . The spin readout was achieved utilizing spin-dependent tunnelling, in which the electron was displaced to a different location depending on its spin state. The charge detector, electrostatically coupled to the electron site, sensed whether the charge had been displaced, thereby determining the spin state. Here we apply a novel approach to charge sensing, where the detector is not only electrostatically coupled, but also tunnel-coupled to the electron site 11 , as shown in Fig. 1a. As a charge detector we employ here the silicon single-electron transistor 10 (SET), a nonlinear nanoelectronic device consisting of a small island of electrons tunnel-coupled to source and drain reservoirs, electrostatically induced beneath an insulating SiO 2 layer. A current can flow from source to drain only when the electrochemical potential of the island assumes specific values 16 , resulting in a characteristic pattern of sharp current peaks as a function of gate voltage (Fig. 1e). The shift in electrochemical potential arising from the tunnelling of a single electron from a nearby charge centre into the SET island is large enough to switch the current from zero to its maximum value. This tunnelling event becomes spin-dependent in the presence of a large magnetic field, when the spin-up state | ↑ has a higher energy than the spin-down state | ↓ , by an amount larger than the thermal and electromagnetic broadening of electron states in the SET isla...

A single atom is the prototypical quantum system, and a natural candidate for a quantum bit, or qubit--the elementary unit of a quantum computer. Atoms have been successfully used to store and process quantum information in electromagnetic traps, as well as in diamond through the use of the nitrogen-vacancy-centre point defect. Solid-state electrical devices possess great potential to scale up such demonstrations from few-qubit control to larger-scale quantum processors. Coherent control of spin qubits has been achieved in lithographically defined double quantum dots in both GaAs (refs 3-5) and Si (ref. 6). However, it is a formidable challenge to combine the electrical measurement capabilities of engineered nanostructures with the benefits inherent in atomic spin qubits. Here we demonstrate the coherent manipulation of an individual electron spin qubit bound to a phosphorus donor atom in natural silicon, measured electrically via single-shot read-out. We use electron spin resonance to drive Rabi oscillations, and a Hahn echo pulse sequence reveals a spin coherence time exceeding 200 µs. This time should be even longer in isotopically enriched (28)Si samples. Combined with a device architecture that is compatible with modern integrated circuit technology, the electron spin of a single phosphorus atom in silicon should be an excellent platform on which to build a scalable quantum computer.

The spin of an electron or a nucleus in a semiconductor [1] naturally implements the unit of quantum information -the qubit -while providing a technological link to the established electronics industry [2]. The solid-state environment, however, may provide deleterious interactions between the qubit and the nuclear spins of surrounding atoms [3], or charge and spin fluctuators in defects, oxides and interfaces [4]. For group IV materials such as silicon, enrichment of the spinzero 28 Si isotope drastically reduces spin-bath decoherence [5]. Experiments on bulk spin ensembles in 28 Si crystals have indeed demonstrated extraordinary coherence times [6][7][8]. However, it remained unclear whether these would persist at the single-spin level, in gated nanostructures near amorphous interfaces. Here we present the coherent operation of individual 31 P electron and nuclear spin qubits in a top-gated nanostructure, fabricated on an isotopically engineered 28 Si substrate. We report new benchmarks for coherence time (> 30 seconds) and control fidelity (> 99.99%) of any single qubit in solid state, and perform a detailed noise spectroscopy [9] to demonstrate that -contrary to widespread belief -the coherence is not limited by the proximity to an interface. Our results represent a fundamental advance in control and understanding of spin qubits in nanostructures.It is well known that the Si/SiO 2 interface hosts a variety of defects that act as charge and spin fluctuators. Spin resonance experiments have documented the deleterious effects of the Si/SiO 2 interface on the coherence of donors in 28 Si, implanted at different depths [10]. Theoretical models suggest that magnetic fluctuation from paramagnetic spins at the interface cause the decohering noise [4], and recent work advocates the use of 'clock transitions' in 209 Bi donors [11] to obtain a spin qubit that is to first-order insensitive to magnetic noise. Fluctuations of interface charges or gate voltages can also cause decoherence, if there is a physical mechanism for electric fields to couple to the spin qubit states. Evidence of such effects was found for instance in carbon nanotube valley-spin qubits [12]. For donors in silicon, fluctuating electric fields can couple to the spin states by modulating the hyperfine coupling [13, 14] or the g-factor [15]. Here we operate single-atom spin qubits in isotopically purified 28 Si, with a residual 29 Si concentration of 800 ppm. Minimizing the effect 29 Si nuclear spin fluctuations allowed us not only to set new benchmarks for qubit performance in solid state, but also to uncover the microscopic origin of residual decoherence mechanisms, specific to a gated nanostructure.A substitutional P atom in Si behaves to a good approximation like hydrogen in vacuum, with energy levels renormalized by the effective mass and the dielectric constant of the host material [16]. Both the bound electron (e − ) and the nucleus ( 31 P) possess a spin 1/2 and constitute natural qubits with simple spin up/down eigenstates, which we denote ...

Quantum computers have the potential to revolutionize aspects of modern society from fundamental science and medical research 1-3 to data analysis 4,5 . The successful demonstration of such a machine depends on the ability to perform high-fidelity control and measurement of individual qubits 6 -the building blocks of a quantum computer. Errors introduced by quantum operations and measurements can be mitigated by employing quantum error correction protocols 7 , provided that the probabilities of the

Nitrogen-vacancy (NV -) color centers in diamond were created by implantation of 7 keV 15 N (I = ½) ions into type IIa diamond. Optically detected magnetic resonance was employed to measure the hyperfine coupling of the NV -centers. The hyperfine spectrum from 15 NV -arising from implanted 15 N can be distinguished from 14 NVcenters created by native 14 N (I = 1) sites. Analysis indicates 1 in 40 implanted 15 N atoms give rise to an optically observable 15 NV -center. This report ultimately demonstrates a mechanism by which the yield of NV -center formation by nitrogen implantation can be measured.

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We demonstrate a method for the controlled implantation of single ions into a silicon substrate with energy of sub-20-keV. The method is based on the collection of electron-hole pairs generated in the substrate by the impact of a single ion. We have used the method to implant single 14-keV 31 P ions through nanoscale masks into silicon as a route to the fabrication of devices based on single donors in silicon.

Control of individual spin qubits through local electric fields, suitable for large-scale silicon quantum computers.