Talk:Qubit

I am a layperson trying to understand an article about using qubits to hide messages or information, ala steganography. After reading this definition, I have a fuzzy idea of some bit floating around in double space like the film, Maxtrix Reloaded. Can you make the concept a little easier to understand for nonphysics geeks like myself? Thanks.

Uh.... are you thinking of Q*bert?

Q*Bert

Omegatron 20:43, Feb 6, 2004 (UTC)

Eliminating the Confusion

As per the requests on the talk page, I dove in and tried to make the Qubit page a little more readable. Any feedback would be greatly appreciated, as this is my first time doing any more than a minor edit on a page.--Nate 20:49, 9 June 2006 (UTC)

ummmm....huh?

So I was trying to learn a little bit more about quantum cryptography and I found this article to be kinda confusing. would someone run a copyedit? TitaniumDreads 10:11, 7 August 2005 (UTC)

Agreed. This article is confusing and does not adequately explain the concept of a Qubit to the average layman (ie, someone without a degree in Quantum Physics and/or Computer Science). I came here to learn how a Qubit differs from a Bit. This article was not helpful. If someone could re-organize and re-write most of what's here, this article could become vastly more accessible to readers. 66.17.118.207 16:44, 22 May 2006 (UTC)

I have seen a couple of explanations all over the Internet, but none of them made me understand even just the third "state" of a Qubit, let alone the exponentially increasing parallel tasks possible. My guess is, that because of the lack of real world examples it is not possible to explain this concept to a layman, comparable to making a Neanderthal man understand how a combustion engine works (no offense to laymen :-) ).--DetlevSchm 03:33, 29 January 2007 (UTC)

Schumacher compression

Schumacher undoubtedly discovered Shumacher compression and is generally accepted as having introduced the term qubit in the 1995 Phys Rev paper Quantum Coding, but historically it is questionable whether one can say he discovered how to interpret states as information. I suspect that something like this may have been known to Feynman. Anyway I'm not a historian of science, but I would appreciate if somebody could look into this. CSTAR 18:18, 20 Jun 2004 (UTC)

I'm stumped

What's a phit? Googling doesn't suggest anything relevant.CSTAR 03:58, 12 Sep 2004 (UTC)

Delete Phit

Please put in a wikilink or some other link to Phit, otherwise we should delete it. No one I have talked to has ever heard of this concept in relation to quantum computing.CSTAR 18:07, 14 Sep 2004 (UTC)

Qudit

Isn't a digit of quantum information called qudigit?

No. The terminology has now settled on qudit.--CSTAR 22:39, 17 Jun 2005 (UTC)

Probability of Qudit States

I have heard of Schrodigner's cat in a box. So that cat is a qudit ... right? But I think physicists are confusing everybody. Can't imagine how a cat to be the superposition of two cats. The more I mull over this the more confused I am. Help!!!! Feb 2006 (HYK, Singapore)

Wha...?

If the limits in the article are the only ones there are, then you could no the probability amplitude of p without knowing the probability amplitude of not-p. That doesn't make sense.

WHY COMPLEX NUMBER?!!!

why in a state of qbit e.g. a|0>+b|0> , a,b are complex number? why these are not just real number between -1,1? please send an e-mail: rastegari.mohammad@yahoo.com

It's just a representation. It's like for colors: you cannot represent a color by a number between zero and one, in fact you need three numbers: a value for red, blue and green. It's the same for states in quantum mechanics: they have a magnitude and a phase, and a convenient way of writing that down is to use complex numbers. UnHoly 06:41, 12 June 2007 (UTC)

Mistake?

I think this is a mistake: The use of entanglement in quantum computing has been referred to as "quantum parallelism", and offers a possible explanation for the power of quantum computing: because the state of the computer can be in a quantum superposition of many different classical computational paths, these paths can all proceed concurrently.

Paralelism: (|0>+|1>)(|0>|1>)=|0>|0>+|0>|1>+|1>|0>+|1>|1> (all combinations of 0 and 1)

Entanglement: |0>|0>+|1>|1> (Not all combinations!!!)

193.144.84.171 11:03, 26 June 2007 (UTC)

You're right. The thing is that no one really 'knows' why quantum computers works. Parallelism does help, but it seems that entanglement is an additional and different ressource. UnHoly 16:53, 26 June 2007 (UTC)

I'm also stumped

This article is almost worthless and unintelligible to me. Exactly what is the definition of a "two-level system"? Is it the same as a "two-state system"? Some examples of two-level systems should be given.

Yes, it is the same. For example, the horizontal and vertical polarizations of a photon form a two-level system. There is already a list of different physical representation at the bottom. UnHoly 06:32, 13 July 2007 (UTC)

And I still don't understand how a quantum computer could work. If you don't know exactly which state the qubit is in, how can you continue into creating bytes, then words, then computer programs? Or if the article means the state can be disambiguated with probabilities, why doesn't the article state this fact, along with the equations/algorithm for doing so?

You always know what your qubits 'are', it's just that they might not fit into a classical worldview. For example, if you have the qubit $|0\rangle +|1\rangle$, then you have the qubit $|0\rangle +|1\rangle$. You don't know what the classical value would be if you measured it, but you know what is your qubit. If you disambiguate (project in quantum-talk) your qubits right ast the beginning, then you get back to classical computing.

The trick for quantum algorithms is then to manipulate the qubits such that the measured value at the end will 'not' be random, but rather reflect the result of some calculation. This is not trivial, and certainly does not look like a normal classical program, and does not include concepts such as bytes or words. See Shor algorithm for an example. UnHoly 06:32, 13 July 2007 (UTC)

And I don't immediately understand the sphere of complex numbers. A complex number has a 2-dimensional representation, so normally two complex numbers would be represented with four dimensions, so why is a 3-dimensional sphere shown?

The requirement of normalization means there are only three degrees of liberty left. See Bloch sphere. This is a pretty standard representation, especially used in optics. UnHoly 06:32, 13 July 2007 (UTC)

This article really needs to be rewritten to make quick, intuitive sense. I already know about the oddities of quantum mechanics, but this article does not explain how any of that relates to computer operation, as far as I can tell.

It does not. Quantum computation is not the same thing. Do not expect to run Quake on a quantum computer. It would not be faster in any sense, even if such a computer could be built with infinite ressource. Feel free to make that clearer if you want, I think the link to quantum information on the first line should be enough. UnHoly 06:32, 13 July 2007 (UTC)

Simnia 21:59, 12 July 2007 (UTC)simnia