Spin-based Quantum Computing in Silicon
Spin qubits in silicon are excellent candidates for scalable quantum information processing  due to their long coherence times and the enormous investment in silicon MOS technology. I will discuss qubits based upon the electron and nuclear spins associated with single phosphorus (P) dopant atoms in silicon [2-5] and also more recent work based upon electron spins confined in Si-MOS quantum dots [6-9]. In each case, single-shot electron spin readout is performed using an adjacent single electron transistor and the process of spin-to-charge conversion, showing spin lifetimes of order seconds [2, 7] for the electrons and many minutes for the nuclear spins . Control of individual electron and nuclear spins is achieved by spin resonance using an on-chip microwave transmission line .
In isotopically enriched Si-28 all of these spin qubits show control fidelities FC above 99%, consistent with some fault-tolerant QC error correction codes. Specifically the P donor electron spin qubit has FCe > 99.6% , the 31P nuclear spin qubit has FCn > 99.99% , and the Si-MOS quantum dot electron spin qubit has FCe > 99.6% . Using dynamical decoupling sequences the coherence times for the P atom qubits can reach T2eCPMG = 0.5 s for the electron and T2nCPMG = 30 s for the nuclear spin.
In the SiMOS quantum dot qubits the electron g*-factor can be tuned using a gate voltage, leading to a Stark shift in the qubit operation frequency of > 10 MHz , allowing individual addressability of many qubits. Most recently we have demonstrated the exchange coupling of two SiMOS qubits to realize CNOT gates  for which over 100 two-qubit gates can be performed within a coherence time of 8 μs.
 D.D. Awschalom et al., “Quantum Spintronics”, Science 339, 1174 (2013).
 A. Morello et al., “Single-shot readout of an electron spin in silicon”, Nature 467, 687 (2010).
 J.J. Pla et al., “A single-atom electron spin qubit in silicon”, Nature 489, 541 (2012).
 J.J. Pla et al., “High-fidelity readout and control of a nuclear spin qubit in Si”, Nature 496, 334 (2013).
 J.T. Muhonen et al., “Storing quantum information for 30 seconds in a nanoelectronic device”, Nature Nanotechnology 9, 986 (2014).
 S.J. Angus et al., “Gate-defined quantum dots in intrinsic silicon”, Nano Lett. 7, 2051 (2007).
 C.H. Yang et al., “Spin-valley lifetimes in a silicon quantum dot with tunable valley splitting”, Nature Communications 4, 2069 (2013).
 M. Veldhorst et al., “An addressable quantum dot qubit with fault-tolerant control fidelity”, Nature Nanotechnology 9, 981 (2014).
 M. Veldhorst et al., “A two-qubit logic gate in silicon”, arXiv:1411.5760.
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