  Electron spins in semiconductor quantum dots are good candidates of quantum
bits for quantum information processing. Basic operations of the qubit have
been realized in recent years: initialization, manipulation of single spins,
two qubit entanglement operations, and readout. Now it becomes crucial to
demonstrate scalability of this architecture by conducting spin operations on a
scaled up system. Here, we demonstrate single-electron spin resonance in a
quadruple quantum dot. A few-electron quadruple quantum dot is formed within a
magnetic field gradient created by a micro-magnet. We oscillate the wave
functions of the electrons in the quantum dots by applying microwave voltages
and this induces electron spin resonance. The resonance energies of the four
quantum dots are slightly different because of the stray field created by the
micro-magnet and therefore frequency-resolved addressable control of the
electron spin resonance is possible.
