The Quantum Pontiff

Qubit, qbit, q-bit, or Q*Bert?

My grandfather liked to write letters to the editor. I think I inherited this disease from him. Here are the contents of a recent letter I wrote to the editor of Physics Today which I hope some of you may find amusing.

I greatly enjoyed reading N. David Mermin’s last two Reference Frame columns on factoring and quantum computing (“What has quantum mechanics to do with factoring?”, Physics Today, April 2007, page 8 and “Some curious facts about quantum factoring”, Physics Today, October 2007, page 10.) However as the proud one-time owner of the California license plate “QUBIT” (which I had occasion to park beside a “QUARK” New Mexico license plate), I feel it is my duty to cheerfully disagree with Mermin’s choice to denote quantum bits as “qbits.” His slander of the traditional shortened spelling of quantum bit as “the vulgar spelling qubit” is nothing less than laying down the gauntlet to the proud worldwide community of heterographic homophone lovers, many of whom I call my friends.

Mermin’s arguments are deceptively enticing [1]: that “Qubit” violates the English rule that “qu” should be followed by a vowel, that no one would ever call the ear appliance a “Qutip,” and finally that Dirac with good reason called them “q-numbers” not “qunumbers.” (One reason might have been enough, but cube it, and certainly no one will argue back at you.)

Let us rebut these arguments one by one. First of all, if Mermin is serious about his respelling of the shortened form of quantum bits, then most certainly he should choose “q-bit” instead of “qbit,” as the only word with a consonant following a “q” in the oxford English dictionary is the crippling child of circumstance “qwerty” to describe a keyboard. And, having settled on “q-bit” one is faced with quite a dilemma. Writing down “q-bit” immediately reminds one of “Q-tip” and, worse for those of us from the video game age, of “Q-Bert” (or “Q*Bert”), the name of an addictive video game featuring an eponymous alien who hopped on a tricolored pyramidal staircase. Now while Mermin may find no discomfort in hearing echoes of “Q-tip” in writing down “Q-bit,” I would think that the majority of physicists (not to mention computer scientists) would prefer not to be reminded of ear infections or even of hopping aliens when they transcribe their serious scientific work. Finally, I note that both “q-number” and “Q-tip” arose at the same time (“Q-tips” originally being called “Baby Gays,” were renamed to “Q-tips” in 1926: the “q” stood for “quality”) With no disrespect to Dirac (who seemed to have a thing for full letter initials), history, as measured by the success of “q-number” in today’s scientific nomenclature versus the ubiquity “Q-tip” to describe cotton swaps, has clearly decided that the “q-” prefix works better for brand names than for scientific terms. In short: let us keep the corporate branding of science off our beloved quantum bits.

Lastly, and most importantly for the progress of science, the fact that “qubit” is an intentional homophone of “cubit” is not vulgar but rather a blessing in disguise. “Cubit” is an English unit of length, roughly equal to the length of a forearm. And it is this “roughly” which should offend the heart of any physicist. Like the “foot” the “cubit” is an imprecise unit whose lack of precision makes me think it deserves to die (can you use “cubit” on your physics tests? no!), and I can think of no better death than its replacement by “qubit,” a precise unit of quantum information.

(As an addendum to the beautify of this homophone, one should not overlook the fact that “cubit” is most often associated with lengths in biblical texts, and, despite Mermin’s claim to the contrary, there are English words with “qu” followed by a vowel, albeit proper nouns, notably “Qumran” which is the settlement nearest where the dead sea scrolls were discovered.)

Long live “qubits.” And may “qbits” or their dirty cousins “Q-bits” only rear their head in a future quantum computing company or in a medical journal describing bits of cotton swap lodged in the ear.

[1] “From Cbits to Qbits: Teaching computer scientists quantum mechanics,” American Journal of Physics — January 2003 — Volume 71, Issue 1, p. 23.


  1. #1 mick
    November 27, 2007

    Beautifully written Dave.

  2. #2 Madalyn
    November 27, 2007

    I suppose it makes sense for a physicist, but surely you mean a consonant followed by a “q” instead of a constant following a “q”.

  3. #3 Dave S.
    November 27, 2007

    First of all, if Mermin is serious about his respelling of the shortened form of quantum bits, then most certainly he should choose “q-bit” instead of “qbit,” as the only word with a constant following a “q” in the oxford English dictionary is the crippling child of circumstance “qwerty” to describe a keyboard.

    [nitpick mode]

    I think you mean “consonant” there, not ‘constant’.

    [/nitpick mode]

    [trivia mode]

    The letter ‘w’ is arguably not always a consonant. In some words, like cwm, its a vowel. It’s also considered by some a sort of pseudo-vowel in a word like ‘sow’.

    [/trivia mode]

  4. #4 dileffante
    November 27, 2007

    I agree, though I never noticed that much “hope” in Q*bert; I thought the alien was just hopping. I don’t like nitpicking, but since the issue is spelling…

  5. #5 Dave Bacon
    November 27, 2007

    Doh. Consonant.

    Ah true, I hadn’t thought about the “w” being a vowel. Maybe by definition we need to make the “w” in “qwerty” a vowel?

  6. #6 lylebot
    November 27, 2007

    I have no problem with the word “qubit”, but I will say that Mermin’s characterization of it as “orthographically preposterous” (in some other essay I read once) made me laugh out loud. Though I agree with you that qbit is no less orthographically preposterous.

  7. #7 anon
    November 27, 2007

    If you’ve not sent the letter yet:
    –it returns (sorry): “off of our beloved quantum bits”
    –and is it really “hoped” and “hoping” not “hopped” and “hopping”? says the double p is the way. I’m not just being pedantic here, I initially read it as versions of “hope.”

    Otherwise, great!

  8. #8 Dave Bacon
    November 27, 2007

    Doh, doh, and more doh. Fixed. I don’t think I’ll ever cure myself of “off of” since I’m certain thats par for the course where I grew up.

  9. #9 Jonathan Vos Post
    November 27, 2007

    querty. w as a consonant. Hmmmm. They speak of “semivowels.” But that doesn’t go far enough. I always thought of both w and y as more like:

    (|consonant> + |vowel>)/sqrt(2).

    Or, more to the point, as initial letters with the kets turned into bras (I tried naively typing that, but the browser balked).

    I mean, w can be a vowel when detected by a hiker in Wales, in mid-CWM. Or a consonant in Woo-woo!

    |turkey> or |duck>?

    Other than that, a VERY fine Letter to the Editor. Which counts as a Least Publishable Unit, in you CV.

  10. #10 Bilal Shaw
    November 27, 2007

    I think the whole issue should be settled democratically. Where do I cast my vote for “qubit”? My advisor is on Mermin’s campaign trail on this.

  11. #11 Steven
    November 28, 2007

    I’d like to suggest that “ququmber” could be introduced as a novophasm for a cucumber of exactly 1 cubit length.

  12. #12 JohnQPublic
    November 28, 2007

    I’m certainly out of my league here, but is it more complicated to emphasis the quantum aspect of the bit rather than it’s just a multi-state bit? “Quantum” now has a mysteriousness to it for the layman. Isn’t the useful attribute of the q-bit the fact that it contains more than 2 states? So, why not emphasis that?

  13. #13 Jonathan Vos Post
    November 28, 2007

    Sink the quball, you scratch. That means you’ve scored -1 balls in the game of 8-ball. Or is that E8-ball?

  14. #14 Dave Bacon
    November 28, 2007

    Hey JohnQPublic,
    more than 2 states? Well this is the bone of contention when I posed by guide to quantum theory in ten minutes here. “State” is a very confusing word. I would say a qubit has two configurations, but our description of it has multiple values.

  15. #15 Abbie
    November 29, 2007

    since Qumran is a semitic word, the Q is a k-ish sound and the u is just a vowel; kum-ran. By that logic, qubit would be pronounced kuh-bit.

    I don’t think either qbit or qubit can be rationalized. I would vote for q-bit, but it doesn’t really matter. The pronunciations are readily apparent, so who cares?

  16. #16 The Vlad
    November 29, 2007

    “that no one would never call the ear appliance a “Qutip,””

    “no one would ever” ???

  17. #17 Dave Bacon
    November 29, 2007

    Me fail english? That’s unpossible!


  18. #18 pete
    November 30, 2007

    Time to fess up and say I find it hard to pronounce qubit as a homophone for cubit; I always end up pronouncing it qwbit (with qw the IPA /kw/, the qw in qwerty).

    And that /kw/-pronuciation seems to be the naive reading of British people. Which is what tends to make me a Merminite, for fear that the great unwashed will turn the cool sound q-bit into a Canadian provence. (And since I’ve always called Q-Tips cotton wool buds, the similarities don’t worry me.)

    Still, it’s good to see the most heinous scientific issues of our time are being debated in full Keep a stiff upper lip, and all that gubbins…

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