  We discuss a measurement-based implementation of a controlled-NOT (CNOT)
quantum gate. Such a gate has recently been discussed for free electron qubits.
Here we extend this scheme for qubits encoded in product states of two (or
more) spins-1/2 or in equivalent systems. The key to such an extension is to
find a feasible qubit-parity meter. We present a general scheme for reducing
this qubit-parity meter to a local spin-parity measurement performed on two
spins, one from each qubit. Two possible realizations of a multiparticle CNOT
gate are further discussed: electron spins in double quantum dots in the
singlet-triplet encoding, and nu=5/2 Ising non-Abelian anyons using topological
quantum computation braiding operations and nontopological charge measurements.
