  We describe a method for implementing deterministic quantum gates between two
spin qubits separated by centimeters. Qubits defined by the singlet and triplet
states of two exchange coupled quantum dots have recently been shown to possess
long coherence times. When the effective nuclear fields in the two asymmetric
quantum dots are different, total spin will no longer be a good quantum number
and there will be a large electric dipole coupling between the two qubit
states. We show that when such a double-quantum-dot qubit is embedded in a
superconducting microstrip cavity, the strong coupling regime of cavity quantum
electrodynamics lies within reach. Virtual photons in a common cavity mode
could mediate coherent interactions between two distant qubits embedded in the
same structure; the range of this two-qubit interaction is determined by the
wavelength of the microwave transition.
