Talk:Quantum computer

Can't Solve the Halting Problem!

The article was recently changed to say "Ignoring computational and space constraints, it is not known if a quantum computer is capable of solving problems which classical computers cannot." Of course, this is wrong as a quantum computer can be simulated on a classical machine given enough time and space. Recommend we revert back to what it previously said: " If one disregards the question of efficiency (i.e., given enough time and resources) all problems solvable with a quantum computer can also be solved using a traditional computer." —Preceding unsigned comment added by 75.15.129.234 (talk) 20:46, 1 April 2011 (UTC)

You are correct. In accidentally negated one of the statements in an earlier copyedit. I looked up the reference just to be sure, and indeed, what you say is true. Skippydo (talk) 19:31, 2 April 2011 (UTC)

Bad Citation

Citation number 4 is unclear. I know nothing about this and can't follow the citations. Someone who knows something help?Darrell Wheeler (talk) 14:23, 9 April 2011 (UTC)

The book is very well-known in the field. I've given a more helpful citation. Perhaps you'll find this helpful. Skippydo (talk) 19:37, 11 April 2011 (UTC)

"Computer" versus "Computing"

It would seem that an article on quantum /computers/ would give some details on the sort of hardware that is either available, or which theoretically needs to be available, in order to construct a functional "quantum computer." Yet I don't seem to be finding any information like this in the article. Can quantum computers be built on electronic circuitboards using doped silicon chips? Do they require linear particle accelerators or massive installations like the Large Hadron Collider? Can they be made with Tinker Toys(tm) and Legos(tm). Or what?

In fact, this article is about theories of quantum /computing/, and not about quantum /computers/, per se. I suggest changing the title accordingly, or else furnishing the requisite hardware information. —Preceding unsigned comment added by 74.92.174.105 (talk) 00:20, 4 May 2011 (UTC)

• I agree. This article is full of gibberish to someone with a CSEE background, let alone a lay reader. It is in dire need of a better introduction and more real-world examples. This is an encyclopedia, not a PhD fellowship research reference. 67.188.136.186 (talk) 23:16, 19 January 2012 (UTC)

Horrendous & Impossible to read

The basic idea behind this (?) is that regular bits are either OFF (A) or ON (B). And according to my layperson understanding, this qubit has three states (A), (B), and (AB) sort of like blood types. But this article is impossible for a layperson to get basic information like that readily. And I think somewhere is a connection to basic particles of protons (+1), neutrons (0), and electrons (-1).... because those also have 3 states. — Preceding unsigned comment added by 67.244.121.144 (talk) 15:22, 17 June 2011 (UTC)

Retrocausality in quantum computing

I know somewhere I read about retrocausal quantum computer theories; where computations would be completed before they were initiated. Why isn't there any linkage to literature on that? 74.209.54.156 (talk) 10:04, 21 July 2011 (UTC)

Article on counterfactual computation?

"One of the more bizarre properties of QCs was identified by a team at the University of Illinois at Urbana-Champaign. They presented the first demonstration of "counterfactual computation", inferring information about an answer, even though the computer itself did not run" [1] 66.243.213.232 (talk) 01:02, 17 September 2011 (UTC)

this is copied from a book

comparing the text beginning at the first chapter of Quantum Computing by Ximena Byrnes, a 2011 online book with ISBN 978-93-81157-91-6, the text on wikipedia is an almost exact copy. but there is no mention of the book in the sources.

link to book, probably wont work:

http://reader.eblib.com.ezproxy.library.wisc.edu/%28S%28ir3chenla3d1fdhoirwzpvxv%29%29/Reader.aspx?p=735741&o=691&u=UW555H383&t=1317245875&h=703F320C432AF3360021BC30CA61E25A417B03D7&s=10849733&ut=2121&pg=1&r=img&c=-1&pat=n# — Preceding unsigned comment added by 128.104.0.87 (talk) 21:47, 28 September 2011 (UTC)

The introduction to this article predates the publishing of the book by a wide margin. The appropriate question is does the book cite wikipedia. Feel free to paste the offending portion of the book here and we can have a laugh about it. Skippydo (talk) 22:12, 28 September 2011 (UTC)

Yeah, the book should cite wikipedia, not the other way around. But I don't see any citations.

Here is the first five pages of the ebook. Except from the images taken from wikipedia, the book is a plain text document with bolded and enlarged text headings. Looks like something made quickly in microsoft word. And I don't see any citations anywhere.

<<page 1 of 101>>

[the cover of the book, blue with a double helix on top of a keyboard.]

<<page 2 of 101>>

First Edition, 2011

ISBN 978-93-81157-91-6

© All rights reserved. Published by: The English Press 4735/22 Prakashdeep Bldg, Ansari Road, Darya Ganj, Delhi - 110002 Email: info@wtbooks.com

<<this part is sideways on the left margin of every page, so i'll delete it after this>> © Byrnes, Ximena, May 01, 2011, Quantum Computing The English Press, New Delhi, ISBN: 9789381157916

<<page 3 of 101>>

Table of Contents Chapter 1- Introduction to Quantum Computing Chapter 2 - Quantum Superposition Chapter 3 - Quantum Entanglement Chapter 4 - Superconductivity Chapter 5 - Quantum Dot Chapter 6 - Nitrogen-Vacancy Center Chapter 7 - Bose–Einstein Condensate Chapter 8 - Important Concepts in Development of Quantum Computing Chapter 9 - Qubit Chapter 10 - Timeline of Quantum Computing

<<page 4 of 101>>

Chapter- 1

Introduction to Quantum Computing

<<image of bloch spere from wikipedia>> The Bloch sphere is a representation of a qubit, the fundamental building block of quantum computers.

A Quantum computing is also called quantum computer is a device for computation that makes direct use of quantum mechanical phenomena, such as superposition and entanglement, to perform operations on data. Quantum computers are different from traditional computers based on transistors. The basic principle behind quantum computation is that quantum properties can be used to represent data and perform operations on these data. A theoretical model is the quantum Turing machine, also known as the universal quantum computer.

Although quantum computing is still in its infancy, experiments have been carried out in which quantum computational operations were executed on a very small number of qubits (quantum bit). Both practical and theoretical research continues, and many national government and military funding agencies support quantum computing research to develop quantum computers for both civilian and national security purposes, such as cryptanalysis.

If large-scale quantum computers can be built, they will be able to solve certain problems much faster than any current classical computers (for example Shor's algorithm). Quantum computers do not allow the computation of functions that are not theoretically computable by classical computers, i.e. they do not alter the Church– Turing thesis. The gain is only in efficiency.

Basis (bold heading at bottom of page)

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<<page 5 of 101>>

A classical computer has a memory made up of bits, where each bit represents either a one or a zero. A quantum computer maintains a sequence of qubits. A single qubit can represent a one, a zero, or, crucially, any quantum superposition of these; moreover, a pair of qubits can be in any quantum superposition of 4 states, and three qubits in any superposition of 8. In general a quantum computer with n qubits can be in an arbitrary superposition of up to 2n different states simultaneously (this compares to a normal computer that can only be in one of these 2n states at any one time). A quantum computer operates by manipulating those qubits with a fixed sequence of quantum logic gates. The sequence of gates to be applied is called a quantum algorithm. An example of an implementation of qubits for a quantum computer could start with the use of particles with two spin states: "down" and "up" (typically written and , or and ). <<<<<the same symbols as wikipedia uses, but it didn't transfer here>>>> But in fact any system possessing an observable quantity A which is conserved under time evolution and such that A has at least two discrete and sufficiently spaced consecutive eigenvalues, is a suitable candidate for implementing a qubit. This is true because any such system can be mapped onto an effective spin- 1/2 system.

<<same picture that wikipedia has about qubits>>

Bits vs. qubits Qubits are made up of controlled particles and the means of control (e.g. devices that trap particles and switch them from one state to another). Consider first a classical computer that operates on a three-bit register. The state of the computer at any time is a probability distribution over the 23 = 8 different three-bit strings 000, 001, 010, 011, 100, 101, 110, 111. If it is a deterministic computer, then it is in exactly one of these states with probability 1. However, if it is a probabilistic computer, then there is a possibility of it being in any one of a number of different states. We can describe this probabilistic state by eight nonnegative numbers a,b,c,d,e,f,g,h (where a = probability computer is in state 000, b = probability computer is in state 001, etc.). There is a restriction that these probabilities sum to 1.

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so its a very similar copy, with the images copied exactly. The first chapter is mainly a copy and paste of this article. Comparing the wikipedia articles that correspond to the titles of the other chapter looks to give the same result: copying. not everything was copied exactly, but the majority was that i noticed while quickly looking through it. Perhaps somebody copied wikipedia articles into microsoft word, put them together, and used an online image for the cover (the cover image is the first result when searching for "dna keyboard" on google. And its available in many sizes according to google. Interesting. 146.151.54.140 (talk) 00:32, 5 October 2011 (UTC)same person as before

Here's Wikipedia's policy on the issue: [2]. I suspect copyright laws in India aren't so strong and thus pursuing the issue would be a waste of time. Any comments? Skippydo (talk) 19:20, 5 October 2011 (UTC)

is "analog" quantum computation not useful?

Is there any good information on what an "analog" (non-qubit?) quantum computer, or "quantum real computer" would entail being, exactly? Could there be mention of such in this article if it is meaningful. I assume it may entail quantum waves rather than quantum 'digital' particles? 216.227.117.152 (talk) 08:04, 29 September 2011 (UTC)

A quantum computer is analog (and probabilistic) by definition. Skippydo (talk) 19:59, 29 September 2011 (UTC)

Is not the definition of the use of bits, whether qubits or not, digital by definition? 216.227.117.152 (talk) 00:03, 30 September 2011 (UTC)

Digital means two states. A bit, by definition, has only two states. A qubit has an infinite number of states. Skippydo (talk) 17:19, 30 September 2011 (UTC)

Binary means two objective states, but doesn't digital mean relational duality, literally: "representing data as a series of numerical values" even an infinite series? Qubits ultimately normalize objectively into being only one state in relation to one other state as output? I think of the difference as particle (localized: qubit) or wave (nonlocal: analog) 216.227.117.152 (talk) 23:13, 30 September 2011 (UTC)

Church-Turing thesis violated or not?

In this wikipedia article it is claimed that the quantum turing machine does not violate the Church-Turing thesis. However, in this survey: http://www.cs.berkeley.edu/~vazirani/bv.ps the authors claim to give the first formal evidence that the (modern) Church-Turing thesis is violated by the quantum turing machine. In my opinion, this should be investigated and straightened out. Cbswe (talk) 14:21, 17 December 2011 (UTC)

Quantum computers don't violate the Church-Turing thesis, though they might violate a stronger version of thesis, which is sometimes called the extended or physical CTT. --Robin (talk) 05:45, 21 December 2011 (UTC)

(a,b,c,d,e,f,g,h) - Probabilities and vector coordinates

In the 'Bits vs. qubits' section the letter set (a,b,c,d,e,f,g,h) is first used as a set of probabilities and in the next sentence as a set of vector coefficients.

"We can describe this probabilistic state by eight nonnegative numbers a,b,c,d,e,f,g,h (where a = probability computer is in state 000, b = probability computer is in state 001, etc.). There is a restriction that these probabilities sum to 1. The state of a three-qubit quantum computer is similarly described by an eight-dimensional vector (a,b,c,d,e,f,g,h), called a ket. However, instead of adding to one, the sum of the squares of the coefficient magnitudes, | a | ² + | b | ² + ... + | h | ², must equal one."

As pointed out in the text, in the latter case the probabilities are equal to the squares of the coefficients a,b,c..., which is confusing. A different letter sets should be chosen for the probabilities and the vector coefficients, e.g. capital letters (A,B,C,D,E,F,G,H), where A = | a | ² for the probabilities. — Preceding unsigned comment added by Zinger0 (talk • contribs) 20:07, 27 December 2011 (UTC)