Qubit, qubit, qbit, or q-bit?

Dave Bacon (The Pontifftook on David Mermin in the latest issue of Physics Today over the spelling of the word ‘qubit.’  In his recently published book Quantum Computer Science Mermin takes issue with the conventional spelling, suggesting that ‘qbit’ is more appropriate.  Dave and David, in their dueling letters, also debate the merits of ‘q-bit’ which sounds too much like ‘Q-tip’ for some people.  At one point Mermin suggests ‘Qbit’ is perhaps the most appropriate.  However, there is one serious problem with that argument.  van Enk has actually proposed a definitional difference between ‘qubit’ and ‘Qubit’ and even ‘qubit.’  The first is an actual physical qubit – a photon, a spin-1/2 particle, etc.  The last is defined as being a physical qubit on a noiseless channel.  That is, the italics denote an information resource.  The middle form employing capitalization (which van Enk applies to cbits, ebits, refbits, etc.) is a way to represent the encoding of a single logical qubit with two physical qubits.  So suppose we have the following:

|0>(L) = |0>|1>
|1>(L) = |1>|0>

Alice can encode the logical qubit by performing

|0>(a|0>+b[exp{iP}]|1>) ->  exp{iP}(a|0>(L)+b|1>(L))

If you can’t read that, read the paper


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