Talk:Qubit

"childhood of the binary chip" ?????

What is this, some kind of sequel to Canticle for Liebowitz? Seriously, someone rewrite this article that knows what they are talking about. Otherwise I'm going to just assume that anyone selling me "quantum computers" is blowing smoke up my ass. — Preceding unsigned comment added by 198.233.187.6 (talk) 18:56, 4 January 2018 (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)

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)

None taken :-) Philsown (talk) 21:12, 11 May 2012 (UTC)

Whether you wanna study Quantum Computing first learn basic physics and linear algebra, don't try to be a genius without a previous necessary background. — Preceding unsigned comment added by 181.54.169.243 (talk) 03:32, 6 February 2013 (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) —Preceding unsigned comment added by 165.21.154.116 (talk • contribs) 13:39, 28 February 2006

Yes, the idea of Schrodinger's cat is to highlight the fact that a real cat cannot be in a superposition of alive and dead even though things like atoms can be in superpositions states. GavinMorley (talk) 15:44, 6 June 2012 (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)

Layperson

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. —Preceding unsigned comment added by Lorikeets (talk • contribs) 17:38, 24 June 2006

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 —Preceding unsigned comment added by 80.191.109.129 (talk • contribs) 22:33, 11 June 2007

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)

As opposed to a 2-tuple??? (m,p). In any case, this isn't a forum...Tgm1024 (talk) 23:52, 26 June 2019 (UTC)

h,, — Preceding unsigned comment added by 60.225.75.60 (talk) 06:52, 14 November 2013 (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)

Wrong. The Bloch sphere has only two degrees of freedom. — Preceding unsigned comment added by 217.95.171.91 (talk) 16:31, 22 May 2018 (UTC)

Besides, mathematically, sphere's are technically 2 dimensional constructs (circles are 1 dimensional). The "OP" is confusing a sphere with a "ball".Tgm1024 (talk) 23:59, 26 June 2019 (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

WHOA! Interleaved-comments in a talk page? USENET and email works with that, even forums. Here? Recipe for confusion!Tgm1024 (talk) 23:56, 26 June 2019 (UTC)

Physical representation

In article Trapped_ion_quantum_computer#Components_of_a_quantum_computer, it mentioned there are two ways to form a qubit using the electronic states of an ion:

1. hyperfine qubits
2. optical qubits

Are they also included in section 'Physical representation' of this article? I'm confused. - Justin545 (talk) 10:08, 4 May 2008 (UTC)

I meant if the section didn't include them, can someone help? I'm not good at that. I don't even know if they have been included in the section. Including the two types of qubit should improve the completion of the article. - Justin545 (talk) 11:23, 4 May 2008 (UTC)

Is a quiet qubit a variation in the qubit, qutrit, qudit progression?

The statement A quiet qubit refers to a qubit that can be efficiently decoupled from the environment. is problematic since a quiet qubit was a particular superconducting qubit proposed by Ioffe et al but never realized as proposed. It is not a variation in the in the qubit, qutrit, qudit progression since it does not encode more or fewer states than a qubit. Moreover, a good qubit is decoupled from the environment. Suggest that this comment be removed. Shadesofgrey (talk) 17:38, 14 December 2009 (UTC)

Be WP:BOLD and remove the sentence. (I agree with its removal too.) --Robin (talk) 22:58, 14 December 2009 (UTC)

I agree so I removed it. GavinMorley (talk) 15:30, 6 June 2012 (UTC)

Is Bloch Sphere sentence wrong?

In the Bloch sphere paragraph: "Represented on such a sphere, a classical bit could only be on the z-axis at the top or (a single point) on the equator of the sphere." Surely a classical bit would be only on "North Pole" or "South Pole" of the sphere, and a point on the equator woud be a superposition with equal probabilities of being |0> or |1>? Should it be changed or am I being stupid?--Hermajesty21 (talk) 16:11, 22 December 2010 (UTC)

I'm gonna be bold and change it, this among other things agrees with me. --Hermajesty21 (talk) 12:10, 23 December 2010 (UTC)

Kwantum

Kwantum is also a chain of stores in the Netherlands. (disambiguation) —Preceding unsigned comment added by 195.240.86.253 (talk) 22:21, 16 April 2011 (UTC)

Needs a better introduction

A qubit is an item of quantum data, not information.

A bit is defined as a unit of information, so a qubit is a unit of information also, rather than data. See: http://www.thefreedictionary.com/a+bit GavinMorley (talk) 15:38, 6 June 2012 (UTC)

thefreedictionary also defines "information" as Processed, stored, or transmitted data, however a lot of people use information to explicitly mean "interpreted data".

I suggest that instead of saying "A qubit can be thought of as a superposition or two states", which is true but entirely confusing to non-physicists that the following statement is used instead "A qubit can be thought of as being an analogue value varying between two extremes, for example, polarisation angles from 0º to 90º."

I think we also need some simple statements describing how qubits relate to real world information. I suggest the following: Qubits can be used in an analogue manner, where the precise value of a qubit is a genuine analogue value. For example, polarisation angles between 0º and 90º may relate to a real world value if you multiply the angle by 4, which can result in a real angle between 0º and 360º. Qubits can also be used in a discrete manner, where the qubit takes one of a specific number of states. For example, polarisation angles of 0, 30, 60 and 90º. In this case, one qubit relates directly to 2 bits of data by assigning each of the four binary states associated with the 2 bits to one angle each. Yeatesi (talk) 09:37, 5 June 2012 (UTC)

An "analogue value" would be classical information, which a qubit is not. Also if you measure a qubit you can only get one of two results, unlike with an "analogue value". GavinMorley (talk) 15:38, 6 June 2012 (UTC)

This is why there needs to be a better set of statements describing how qubits relate to the real world, and, possibly, the electronic, binary, world. If a qubit can ONLY take the value of true or false, then how does it differ from a "classical" analogue value that can only be measured by checking if it is above or below a threshold? If it is not different to that, then that should be stated, as that would then be a clear definition of what a qubit physically is. 195.59.43.240 (talk) 14:20, 11 June 2012 (UTC)

Quantum systems do not follow our everyday classical intuition so it can indeed be difficult to describe qubits in simple language. A qubit is quite different to a "classical analogue value that can only be measured by checking if it is above or below a threshold". Here you are saying there is some problem with the measurement apparatus that prevents us from properly measuring the classical analogue value. If it were not for this then you would just have a classical analogue value. In describing qubits we are not limited by problems with measurement apparatus. When measured perfectly a qubit can only take one of two values, but before it is measured it can be in any superposition of these two values. Another feature of qubits is that they can be entangled with other qubits. This means their states can be correlated more strongly than is possible in our everyday classical intuition. GavinMorley (talk) 17:06, 16 June 2012 (UTC)

In the table in the "physical representation", one of the items is polarisation of light. Light has a polarisation angle, which is an analogue value. When the value of the qubit is read, it is read as either horizontal, or not horizontal (vertical). This is a classic case of having a genuine classical analogue value being compared to a threshold; it either satisfies the horizontal measurement, or it does not. If you are not limited by measurement apparatus, then why are you binning the measurement into true or false states, rather than into the actual superposition state? 195.59.43.240 (talk) 08:38, 18 June 2012 (UTC)

The polarization angle of light can indeed be used to encode an analogue value. However, when photon polarization is used as a qubit it is used differently. For example, if a photon's polarization was very close to horizontal, then a measurement would give you "horizontal" with a very high probability and "vertical" with a very small probability. The choice is truely random, subject to those probabilities. According to quantum theory, this unpredictability is not due to some noise or imperfect equipment. In the case of the analogue value being compared with a threshold, the result here would always be "horizontal". GavinMorley (talk) 14:25, 18 June 2012 (UTC)

What is the unpredictability due to, if not random noise processes and/or imperfect apparatus? 195.59.43.240 (talk) 17:04, 19 June 2012 (UTC)

That is a good question that quantum researchers have been wondering about for decades. Some interpretations of quantum mechanics say there are 'hidden variables' which determine these apparantly unpredictable outcomes. More commonly it is said that we can know the probabilities when measuring a quantum superposition, but the choice is truly random. Experiments to date are not able to distinguish between these interpretations. There is a commercial product providing quantum random numbers: http://www.idquantique.com/true-random-number-generator/products-overview.html GavinMorley (talk) 16:35, 26 June 2012 (UTC)

Example at Bloch Sphere

I find in the example ${|0\rangle +i|1\rangle } \over {\sqrt {2}}$ the first part coherent where $\alpha =\cos \left({\frac {\pi }{4}}\right)={\frac {1}{\sqrt {2}}}$.

At $\beta =e^{i\phi }\sin \left({\frac {\pi }{4}}\right)$ part, the $\sin \left({\frac {\pi }{4}}\right)={\frac {1}{\sqrt {2}}}$ is correct too and computing $\ln(i)={\frac {\pi }{2}}$ we can observe that $\phi ={\frac {\pi }{2}}$. Here is the explanation why it is pointing to the positive y axis.

unsigned

I have no problem with $\alpha =\cos \left({\frac {\pi }{4}}\right)={\frac {1}{\sqrt {2}}}$ , cosine (or sine also, in this case) for 90 degrees or pi/4 radians, but could somebody just please explain ${|0\rangle +i|1\rangle }$, I have to assume, "i" to be the solution to $x^{2}=-1$, but what's "rangle ?" "|0>" !? I would be very helpful to me - and others, I presume Boeing720 (talk) 03:21, 30 November 2017 (UTC)

arXiv Citation

I cited P. Shor's paper as it is on arXiv, is there a better place I should cite this from as arXiv isn't actually a journal? KM4JWL (talk) 14:39, 15 April 2016 (UTC)

Quantum computing for dummies

I admit that i have yet to grasp the very concept. My physics education ended with the rudiments of quantum physics. So my dumb question is: How is a quantum computer even possible if everything is probabilities? One presumes that computing requires exact results. How does a qubit, which can have probabilistic values, yield useful results? I don't see the answer to that, at least in non-expert terms, in this article. Tmangray (talk) 00:35, 5 May 2016 (UTC)

Must agree with you. But also what's ment by for instance "|0>" ? I must assume for instance |x|^2 is "the absolute value of x raised to 2" , but the first one ??? And I have studied math from trigonometry (incl addition formulas, laps , sine and cosine theorem , radians degrees etc) to differential calculus, as well as from complex numbers to statistics and probabilities + static mechanic & dynamic mechanic outside of the math subject etc. If I don't get "|0>", how many other readers will ? Please use some examples at the very least. Boeing720 (talk) 02:06, 25 November 2017 (UTC)

Quantum computing capabilities today

Should this article include information about the current state of quantum computing? I'm trying to learn the concept, having not gotten past sophomore engineering physics, and what I'm reading confuses me. A recent Wired article says that Google is aiming to create a 50 qubit supercomputer this year [[1]], yet the D-Wave Systems article says they had a 128 qubit chipset as far back as 2011. Who is using the term wrong? Timtempleton (talk) 22:11, 23 June 2017 (UTC)

Transclusion of Draft article "Physical and logical qubits"?

I wrote a draft article, Draft:Physical and logical qubits. Should we transclude or otherwise incorporate some of its text here? --Daviddwd (talk) 23:36, 15 October 2018 (UTC)

Just do it, or else your question will go unanswered for years. wikipedia policy is (kindof) "just do it" and "be brave". · · · Omnissiahs hierophant (talk) 16:57, 22 February 2021 (UTC)

References

(even though this is a talk page)

Is "weirdness" an appropriate technical adjective to include in the opening of a scientific topic?

I would say "no". Could we replace this with a quoted or synthesized scientific definition? Asides from the word itself being inappropriate, it's just obscuring whatever actual information should go there.

I'm not saying overly technical. Just accurate and objective. Perhaps "weirdness" can be replaced with something like "special properties such as X". 71.179.84.225 (talk) 00:45, 5 December 2018 (UTC)

some necessary precisions to make some concepts understandable

when talking about the bloch sphere representation , it is unclear why overall phase of the state e^{i ψ} has no physically observable consequences . Some details would be appreciated (or some links to another page) .

also the section standard representation is unclear on the way of building the four-dimensional vector basis from the two-dimensional vector basis . it's a tensor product right ? I had to figure it out but it's not obvious .

Stilgarnat (talk) 09:43, 22 September 2019 (UTC)

The phase disappears when you take the absolute value (distance from origin) of a complex number. See the Born Rule for details. This is why it is not directly observable. It is however possible to design a circuit that rotates the phase into view, for example the gate/circuit called diffusion operator does exactly this ($U_{s}$ in the Grover's algorithm if you read super-carefully). · · · Omnissiahs hierophant (talk) 16:36, 17 August 2021 (UTC)

Physical implementation

I think that the Photon / Time-bin encoding / Time of arrival / Early-Late type of qubit should be removed from this list. These things are not full qubits because only the relative phase of the basis states is a tunable property. Superposition cannot be achieved with these because there is nearly always a 50/50% chance of a |0> or a |1> being detected. They have their place in quantum information science but not as qubits in a quantum computer. Polymath uk (talk) 16:07, 7 July 2020 (UTC)

Then do it! · · · Omnissiahs hierophant (talk) 16:54, 17 August 2021 (UTC)

I disagree; please provide sources for the claimed imperfections; arXiv:1305.3592 contains a proposal for universal quantum computation with time-bin qubits, doi:10.1364/JOSAB.24.000226 showed how to do cluster-state quantum computation, and doi:10.1103/PhysRevApplied.13.034013 demonstrated a two-qubit gate for that encoding. Recent reviews such as doi:10.1088/1361-6633/aad5b2 discuss it among the standard photonic encodings. --Qcomp (talk) 18:47, 17 August 2021 (UTC)

Thanks for your input :D I think Qcomp is very likely correct. · · · Omnissiahs hierophant (talk) 19:15, 17 August 2021 (UTC)

Seperate page for Qudits

I think that the overall explanation of qudits should be rerouted or expanded into its own page considering the mathematics and theory behind higher order D-Dimensional states is somewhat different than standard qubit stuff, what does everyone think? Twiximus (talk) 18:09, 16 Oct 2022 (UTC) — Preceding unsigned comment added by 98.97.58.13 (talk)

quantum entanglement

The qubit itself is an exhibition of quantum entanglement. Yes, more than one of these can be entangled (as written in the article), but a selling point for using qubits is to reduce computational stress with the phenomenon of local simult processing. 72.174.131.123 (talk) 02:25, 14 January 2024 (UTC)