Yesterday I completed a new paper with Tim Ralph and Bill Munro, entitled "Error tolerance and tradeoffs in loss- and failure-tolerant quantum computing schemes".
One of the biggest problems facing the development of useful quantum computers is the introduction of noise. Broadly speaking, there are two types of noise which are particularly common: loss, and so-called depolarizing noise. Loss noise simply corresponds to individual qubits being lost. Depolarizing noise is where the actual state (i.e. the logical value) of qubits becomes corrupted. One of the big research areas in quantum computing is quantum error correction. This is the study of techniques for tolerating noise in quantum circuits. Recently there have been several extremely promising techniques proposed for dealing with the former type of noise - loss. In this paper we have shown that these techniques generally achieve loss tolerance at the expense of tolerance against depolarizing noise. Since a useful quantum computer will require tolerance against both these types of noise, this limits the use of these loss-tolerant approaches enormously.
The consequence of all this is that unfortunately the requirements for building a quantum computer (in terms of qubit loss) are somewhat more stringent than many of us have been believing.