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.