Decoherence and error correction

• Decoherence in general arises from interactions with the environment, which typically has the effect of measuring the system and thus collapsing a quantum computation. In addition, we have to be careful about leaving temporary qubits floating around. We can expect them to be entangled with the rest of the system, and thus an observation of the ``dust'' left behind by intermediate computations could effect a measurement of the system, invalidating later stages. Thus, one emphasis in research on quantum computation has been on how to efficiently avoid leaving any garbage floating about.
• As noted above, error detection/correction is difficult in the quantum environment since we cannot reliably clone an arbitrary qubit. Further, any intermediate measurement of the system for error control is likely to invalidate our computation. There are, however, approaches using polarization encoding schemes for error control.