  Spins confined in quantum dots are a leading candidate for solid-state
quantum bits that can be coherently controlled by optical pulses. There are,
however, many challenges to developing a scalable multibit information
processing device based on spins in quantum dots, including the natural
inhomogeneous distribution of quantum dot energy levels, the difficulty of
creating all-optical spin manipulation protocols compatible with nondestructive
readout, and the substantial electron-nuclear hyperfine interaction-induced
decoherence. Here, we present a scalable qubit design and device architecture
based on the spin states of single holes confined in a quantum dot molecule.
The quantum dot molecule qubit enables a new strategy for optical coherent
control with dramatically enhanced wavelength tunability. The use of hole spins
allows the suppression of decoherence via hyperfine interactions and enables
coherent spin rotations using Raman transitions mediated by a hole-spin-mixed
optically excited state. Because the spin mixing is present only in the
optically excited state, dephasing and decoherence are strongly suppressed in
the ground states that define the qubits and nondestructive readout is
possible. We present the qubit and device designs and analyze the wavelength
tunability and fidelity of gate operations that can be implemented using this
strategy. We then present experimental and theoretical progress toward
implementing this design.
