Qbit It
Varinia Swicegood
I noticed recently that I had edited a posting of an otherwise informed answer solely to replace "q-bit" with "qubit", but then I note that I don't bother to correct the unhyphenated "qbit" to "qubit" when I see it in other similar questions and answers.
Nonetheless there appears to be less than 100 postings that use qbit, while the vast majority, >4k, use qubit, so it might be a don't care in the grand scheme of things. Most of the high-rep users on this site use "qubit" I believe, so I'll go out on a limb and say that "qubit" is preferred but no one will make a stink about "qbit". Maybe some monstrosities like "Q bit" or "quibit" would trigger an edit.
I've personally always edited the other spellings any time I've noticed them, and I'd encourage other people to do the same. It is indeed often quite an annoying janitorial task, but we can hardly do anything about it, as most people using alternative spellings are unlikely to read meta posts anyway.Individual repeated offenders can be asked to use the correct spelling of course, but there's always going to be new people coming in writing "qbit" etc.
I think "qubit" should be used instead of "qbit", "q-bit", or "Q bit" always, unless one of the other spellings becomes more popular overall (in the whole world, not just here on Stack Exchange, and I don't think one of those spellings will become more popular than "qubit" anyway).
However, if you're thinking of going through 100 old posts and editing just "qbit" to "qubit", then what you'll be doing is bumping up 100 old posts to the top of the site's list of questions, and this will make the recent questions (often by new users who are just about to get their first experience of whether or not this is a good place to ask a quantum computing question) get buried.
So please feel free to edit the spelling if you have enough reputation to make edits unilaterally, as long as it's on new questions or an isolated old question (not on 100 old questions at the same time, for example).
Some of these are "just" engineering problems, like shrinking the size down from room-sized 6-qbit systems to something more like the density of an integrated circuit. Or figuring out a way to prevent thermal noise from scrambling the system, without requiring the customer to keep large stocks of liquid Nitrogen (or Helium!) on hand.
Primary among these is error-correction. Part of the inherent nature of the entangled systems used for quantum computing is that they can lose "coherence" spontaneously. Great strides have been made in increasing the entangled lifetime, but you're still very limited in the number of operations that you can perform reliably.
Some techniques for error correction in quantum computations have been developed, but the last article I read on quantum EC indicated that the number of error-correcting qbits required goes up more-or-less logarithmically with the number of active qbits. Note that the initial constant factor may be quite large - it can take 5 physical qbits to represent 1 logical qbit.
To some extent (it remains to be seen how much), this growth in size is going to mitigate against the exponential advantage in speed that quantum computation is supposed to have over conventional computation.
Okay, so you can get a 6 qbit system today, which is way too small to tackle "interesting" problems with. Something like factoring a 2048-digit number is going to require systems with millions or billions of qbits. Sure, you'll get the answer "instantly", but there's no clear path to get anywhere near that level of performance using current techniques. Just loading the problem into the system would probably exceed the coherence lifetime.
Oh, to answer your other questions: I think that most folks are working with quantum storage systems with a single pair of states. In principle, most of these systems could store multiple non-overlapping states per storage unit, but I think a lot more effort is going into making the equipment work reliably at all, rather than maximizing efficiency.
The first "working" 3-qubit NMR quantum computer was built in 1998. The field is still in infancy, and almost all progress is still theoretical and confined to academia, but in 2007 a company called D-Wave Systems presented a prototype of a working 16-qubit, and later during the year 28-qubit adiabatic quantum computer. Their effort is notable since they claim that their technology is commercially viable and scalable. As of 2010, they have 7 rigs, current generation of their chips has 128 qubits. They seem to have partnered with Google to find interesting problems to test their hardware on.
On the other hand, they are quite shy, and to decohere if you look at them funny [1]. If a large amount of power is used, it's used to do the extreme isolation that Qbits need from their environment, such as cooling to 20mK (that is, real cold) temperatures. Whether that is needed (and how much) would depend on the Qbit physical implementation; some (such as superconducting qbits) need that sort of them; others (such as ion trap or photonic-based qubits) don't need that nearly as much...
Assuming the rats pull some industrial espionage, collaboration with the Russians and Chinese, R&D development funded by war money (First Quantum ASIC's anyone?), and chea- ugh... I mean... Artistic liberty! Uh, yeah! Artistic liberty! Totally not cheating.
So that would be about 9 to 12 keys cracked over four years. Ngl, I was expecting a LITTLE more: 1 key per 3 months, but with a careful selection, you could wreck absolute HAVOC on something like the home video game console market, or the prepaid android phone market. (Ever tried rooting a tracfone?)
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