The size of silicon transistors used in microelectronic devices is shrinking to the level at which quantum effects become important1. Although 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 computers2, 3, 4 and spintronic devices5. An electron spin in silicon can represent a well-isolated quantum bit with long coherence times6 because of the weak spin–orbit coupling7 and the possibility of eliminating nuclear spins from the bulk crystal8. However, the control of single electrons in silicon has proved challenging, and so far the observation and manipulation of a single spin has been impossible. Here we report the demonstration of single-shot, time-resolved readout of an electron spin in silicon. This has been performed in a device consisting of implanted phosphorus donors9 coupled to a metal-oxide-semiconductor single-electron transistor10, 11—compatible with current microelectronic technology. We observed a spin lifetime of ~6seconds at a magnetic field of 1.5tesla, and achieved a spin readout fidelity better than 90 per cent. High-fidelity single-shot spin readout in silicon opens the way to the development of a new generation of quantum computing and spintronic devices, built using the most important material in the semiconductor industry.
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The projective, single-shot readout of a qubit is a crucial step in both circuit-based and measurement-based quantum computers12. For electron spins in the solid state, this has only been achieved in GaAs/AlGaAs quantum dots coupled to charge detectors13, 14, 15. The spin readout was achieved using 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 site11, as shown in Fig. 1a. The strong coupling inherent to this arrangement is responsible for the high charge transfer signals that ultimately allow fast and high-fidelity single-shot spin readout. As a charge detector, we use here the silicon single-electron transistor10 (SET), a nonlinear nanoelectronic device consisting of a small island of electrons tunnel-coupled to source and drain reservoirs, electrostatically induced beneath an insulating SiO2 layer. A current can flow from source to drain only when the electrochemical potential of the island assumes specific values16, 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 externally applied magnetic field B, when the spin-up state |↑ has a higher energy than the spin-down state |↓, by an amount EZ = gμBB, where g≈2 is the spin gyromagnetic ratio and μB is the Bohr magneton. The Zeeman splitting EZ must be larger than the thermal and electromagnetic broadening of electron states in the SET island. Therefore we perform the experiment in high magnetic fields, B>1T, and with very low electron temperatures, Tel≈200mK.