  Scaling up qubits is a necessary step to realize useful systems of quantum
computation. Here we demonstrate coherent manipulations of four individual
electron spins using a micro-magnet method in a quadruple quantum dot - the
largest number of dots used for the single spin control in multiple quantum
dots. We observe Rabi oscillations and electron spin resonance (ESR) for each
dot and evaluate the spin-electric coupling of the four dots, and finally
discuss practical approaches to independently address single spin control in
multiple quantum dot systems containing even more quantum dots.
