What is Quantum computers: Definition and 59 Discussions

Quantum computing is the exploitation of collective properties of quantum states, such as superposition and entanglement, to perform computation. The devices that perform quantum computations are known as quantum computers. They are believed to be able to solve certain computational problems, such as integer factorization (which underlies RSA encryption), substantially faster than classical computers. The study of quantum computing is a subfield of quantum information science. Expansion is expected in the next few years as the field shifts toward real-world use in pharmaceutical, data security and other applications.Quantum computing began in 1980 when physicist Paul Benioff proposed a quantum mechanical model of the Turing machine. Richard Feynman and Yuri Manin later suggested that a quantum computer had the potential to simulate things a classical computer could not feasibly do. In 1994, Peter Shor developed a quantum algorithm for factoring integers with the potential to decrypt RSA-encrypted communications. Despite ongoing experimental progress since the late 1990s, most researchers believe that "fault-tolerant quantum computing [is] still a rather distant dream." In recent years, investment in quantum computing research has increased in the public and private sectors. On 23 October 2019, Google AI, in partnership with the U.S. National Aeronautics and Space Administration (NASA), claimed to have performed a quantum computation that was infeasible on any classical computer.There are several types of quantum computers (also known as quantum computing systems), including the quantum circuit model, quantum Turing machine, adiabatic quantum computer, one-way quantum computer, and various quantum cellular automata. The most widely used model is the quantum circuit, based on the quantum bit, or "qubit", which is somewhat analogous to the bit in classical computation. A qubit can be in a 1 or 0 quantum state, or in a superposition of the 1 and 0 states. When it is measured, however, it is always 0 or 1; the probability of either outcome depends on the qubit's quantum state immediately prior to measurement.
Efforts towards building a physical quantum computer focus on technologies such as transmons, ion traps and topological quantum computers, which aim to create high-quality qubits. These qubits may be designed differently, depending on the full quantum computer's computing model, whether quantum logic gates, quantum annealing, or adiabatic quantum computation. There are currently a number of significant obstacles to constructing useful quantum computers. It is particularly difficult to maintain qubits' quantum states, as they suffer from quantum decoherence and state fidelity. Quantum computers therefore require error correction.Any computational problem that can be solved by a classical computer can also be solved by a quantum computer. Conversely, any problem that can be solved by a quantum computer can also be solved by a classical computer, at least in principle given enough time. In other words, quantum computers obey the Church–Turing thesis. This means that while quantum computers provide no additional advantages over classical computers in terms of computability, quantum algorithms for certain problems have significantly lower time complexities than corresponding known classical algorithms. Notably, quantum computers are believed to be able to quickly solve certain problems that no classical computer could solve in any feasible amount of time—a feat known as "quantum supremacy." The study of the computational complexity of problems with respect to quantum computers is known as quantum complexity theory.

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  1. .Scott

    Internet Secrets estimated to survive for 15 years

    According to an article in Science News, Canadian Phycisist Michele Mosca has estimated that encrypted information sent over the internet today will be crackable in about 15 years from now. So, if you send valuable information today using the normal encryption methods used today, and someone...
  2. L

    B Quantum Computers and their special properties?

    I believe at the current time quantum computers can't get as much done as fast as normal computers, but do quantum computers have access to information by its own nature that allows it to run special calculations that normal computers can't? In particular physics or biology simulations using...
  3. D

    Quantum computer storage capacity

    I have a question regarding the storage capacity of quantum computers. I read that 32 qubits (4 "quantum bytes" if you will - not sure if that's an actual term or not yet) can store the equivalent of 500mb of data. Is this directly proportional? I.e., would 8 qubits store the equivalent of...
  4. .Scott

    Quantum Computers getting closer

    This New Atlas article on Quantum Brilliance describes the development status of a new quantum data processor that can operate at room temperature. They are predicting a useful commercial device in about 5 years. The device is based on nuclear spin in a diamond substrate. The device is very...
  5. Stal

    Computational Literature suggestions please (Topological Quantum Computers)

    Hi, I will be starting my research in topological QC (based on non abelian anyons following the work of A. Yu. Kitaev). To begin understanding this theory, I need to develop a background in the braiding group used to describe anyons, fault tolerance in quantum computers and probably condensed...
  6. A

    A Are all quantum computers feedforward networks?

    Hi! I am trying to understand how advanced quantum computers have in fact become as well as how advanced they can in principle become. To probe these issues, I am asking the group whether or not quantum computers are restricted to feedforward processing, both currently and in principle? Here is...
  7. iVenky

    I Are quantum computers required to be cold to reduce Brownian motion?

    I understand that based on what I have read online quantum computers are required to be close to absolute zero because it introduces less error. Is it because brownian motion due to thermal agitation of molecules reduces with temperature?
  8. L

    A Quantum computers and Annular Josephson junctions

    Are annular Josephson junctions qubits in some quantum computer right now? https://www.nature.com/articles/425133aI found this article from 2003. What is the progress right now? Also are vortices and fluxons same thing?
  9. J

    Quantum Journal on hardware used to build quantum computers?

    Hi folks, This time is not a book, I would like to know if exists a journal explaining how a qubit is actually built, I mean the hardware not the theory. Indeed I am interested in any hardware related with these technologies. I know this field is still in an early stage and they are still...
  10. ngrunenberg

    B How do quantum computers find a solution?

    Apologies in advance if this is a stupid question, I'm not the brightest. I recently listened to Scott Aaronson's conversation with Lex Fridman, and an interview he did for Scientific American, regarding quantum computing (QC from now on) and have a question regarding how a QC finds a solution...
  11. jedishrfu

    New Book from OReilly: Programming Quantum Computers

    OReilly published a new book, Programming Quantum Computers. From my brief scan of it, the book looks pretty comprehensive. They use circle notation for the Qubit states and have a simulator (QCengine) to go along with the examples on github. The programming language for their algorithm...
  12. J

    What is the difference between quantum computers and quantum accelerators?

    Hello. Could you please explain to me what the difference is between quantum computers and quantum accelerators?
  13. H

    B Taking Quantum Computers into Space

    Can the low temperatures in Space be employed to cool a Quantum computer installed on an operational Space vessel doing away with the need of a super fridge or whatever they use here on Earth? Thank you.
  14. D

    I They physics of phase inversion in Grover's algorithm

    How would this operator be implemented physically if we had a quantum computer? In Grover's algorithm this magical operator is often called "phase inversion". Here is the operator from wiki: https://wikimedia.org/api/rest_v1/media/math/render/svg/07fb23bffa787430b084971c6a108a8f6ff6c2b3 It’s...
  15. Sirsh

    Undergraduate Thesis - FEA and Quantum Computers

    Hey everyone, I've started to do research for my undergraduate thesis topic and I would like to use FEA with an application to quantum computers. I'm thinking of something in regards to mechanical vibrations, however I'm open to consider anything given it's possible at the undergraduate level...
  16. Kara386

    I Quantum Computers: Room Temperature Superposition Explained

    I was reading about quantum computers and the article was saying about how cold everything has to be for qubits to exist in a superposition state. Why do things like the double slit experiment with single photons work at room temperature? Because when you get the interference pattern with single...
  17. W

    A Using quantum-secured communication for data transfering

    Hello! I am wondering if it is theoretically possible to allow a means of data transfer (or internet, etc.) by the idea of quantum entanglement. Correct me if I make any errors in understanding. But, by what I understand, in essence you could for instance run a computation on a quantum computer...
  18. Spinnor

    B IBM’s new five-qubit universal quantum computer

    How significant is this development of a 5 qubit computer by IBM? See, "IBM invites users to test its quantum computer" at http://phys.org/news/2016-05-ibm-users-quantum.html "How IBM’s new five-qubit universal quantum computer works" at...
  19. Quotidian

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    This week, in Sydney, there was the announcement of not one but TWO Quantum Computing centres opening at two rival universities (UNSW and USYD). There was a feature on one of the current affairs shows here with the very charismatic guitar-playing professor of one of the centres, which has had a...
  20. M

    Thermal insulation in quantum computers

    Hello! I was watching a video () about how extreme low temperatures are achieved in a D-Wave computer. As they explain, they have this series of cooling stages, each of which providing a temperature drop over the previous. I was wondering however, how could they avoid thermal propagation through...
  21. Domenico94

    CNOT Quantum Gate Schematics: Physical Implementation

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  22. M

    Chance of building practical quantum computers

    Wiki on quantum tomography says: "The number of experimental configurations (state preparations and measurements) required for quantum process tomography grows exponentially with the number of constituent particles of a system. Consequently, in general, QPT is an impossible task for large-scale...
  23. V

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  24. Elroy

    Exploring Quantum Computers: A Beginner's Journey

    Just as an introduction, I'm attempting to bootstrap myself up in the language of quantum computers. I've got a ways to go, but I am making some headway. If y'all don't mind, I'd like to use these forums to ask what I hope are some relatively simple questions. I'm pretty good to go on the...
  25. moriheru

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  26. R

    Will the future quantum computers use binary, ternary or quaternary?

    Hello. Our current computers use bits, so they use the binary numeral system. But I heard that the future quantum computers will use qubits instead of simple bits. Since in the word "qubit" there is the word "bi" I first thought that this meant that quantum computers would use binary (base...
  27. Jalo

    What role does quantum entanglement play in quantum computers?

    I've been reading about quantum entanglement, and I read in many websites that it plays a very important role on quantum computers. I tried to find why, but the only information I got was that quantum entanglement allowed us to see the value of a qubit without interacting directly with it (and...
  28. K

    Quantum Principles of Quantum Computers

    I understood many maths of QM and even QFT.. but I can't totally understand how a quantum computer can factorize millions of times faster. In normal turing machine, there is 0 and 1. But in quantum computers, there is 0 and 1 and superpositions of it.. meaning it can be 0.0001 or 0.5253 or 0.874...
  29. E

    Is There Evidence Supporting Brains as Quantum Computers?

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  30. F

    Bitcoin Encryption and Quantum Computers (D-Wave)

    Hi, Not sure if this is the right place to post this but... I understand that D-Wave has a developed an adiabetic quantum computer. Google and Lockheed Martin have both purchased these computers from D-Wave recently. Could such a quantum computer (or perhaps future versions of it with...
  31. N

    How long do we need to wait for Quantum Computers?

    I was thinking whether I should put this threat under general physics or engineering... but anyway, here's the problem: how long do we need to wait until quantum computing is put to use? Also, can programmes that run on classical computers run on quantum computers, or do they need an entirely...
  32. E

    Physics How to get into Quantum Computers?

    Hello, I am an undergraduate junior majoring in ECE. Right now I am still taking my core major classes but soon I need to specialize. I am very interested in computer hardware and software and would not mind at all graduating with Computer Engineering. But in the future, maybe after grad school...
  33. R

    Quantum Computers: what are they better at?

    Can someone explain to me what sort of things quantum computers can/could do, based on their differences in their design, that are harder for conventional computers? So far I have a list that consists of 1) integer factorisation. I was hoping someone could extend that for me! :-) After all...
  34. M

    Quantum Computers, how do they work?

    Lately I've been hearing about latest advancements in quantum computers, such as the chinese group breaks distance record for teleporting qubits (As seen on phys.org, Unfortunately i cannot post that link because i am new). I was just wondering if anybody could clear up what a quantum computer...
  35. I

    Why can't quantum computers be used to solve SAT?

    Any boolean formula can be represented efficiently as a reversible circuit (i.e. with at most a polynomial increase in the number of gates), with n input bits and n output bits. If it is an n-input, 1-output formula, the corresponding reversible circuit will have 1 'main' output bit and a series...
  36. L

    Why must Quantum Computers be reversible?

    Although I've read many times that quantum computers are reversible, I am unable to find a understandable explanation as to why (this may be because I study Computer Science). From what I've read, I assume that the answer is linked to the Second Law of Thermodynamics, but I don't know how...
  37. A

    Question about Quantum Computers

    Hello Everyone I was hoping someone can answer how quantum computers work. I understand how digital logic works such as in AND gates and OR gates, but I am not sure how quantum physics wave functions can make this any faster. I was thinking maybe because whenever a piece of matter has the...
  38. G

    Quantum computers and modelling a quantum computer on a classical computer

    First of all, I haven't the slightest idea how a quantum computer actually works but I understand that it is theoretically possible to make them and they could, in theory, be used to compute things that a classical computer would take too long to compute. (i.e. large combinatorial problems like...
  39. D

    When do you think Quantum Computers will be out?

    When do you think Quantum Computers will be out? Most scientists think 20-30 years but we have them in the lab already, they're just 1.5mill and only about 6 qubits.
  40. K

    Programming Quantum Computers - Classical Techniques Obsolete?

    This question has been bugging me. I have a math degree, and my computer knowledge is limited to VERY BASIC programming and being able to build my own PC, so I thought this would be a good place to ask. Note: This question has no "clean cut" forum to fit into. I read ALL the forum...
  41. R

    Paper presentation on Quantum Computers

    Hello there forum! I would like to present a paper on quantum computers. Idk where to start from. I have high school level knowledge on quantum mechanics, semester on quantum physics and also the knowledge the feynman lecture vol 3 would give its readers.
  42. P

    Will Quantum Computers Replace Semiconductors in the Next 5 Years?

    Hi All, we hear about this term quantum computers and electronics which is currently under research. Will there be any major shift in our computer industry from semiconductor to quantum in next 5 years? Is there any significant advancement in this field? Just curious to know:)
  43. B

    Turing machine and quantum computers

    Can anyone help me...from Nielsen-Chuang "quantum computation and quantum information":how might we recognize that a process in nature computes a function not computable by a turing machine?
  44. E

    Quantum Computers: How Hard to Get into Top Programs?

    How hard is it to get into the quantum computer/information theory programs at the top universities? Are they easier to get into than theoretical particle physics or pure math? I would guess it would be just as difficult, but I hardly ever meet or read about people that are interested in that field.
  45. N

    Concerning Quantum computers and information

    I only recently 'discovered' this quantum computer business and quantum information. As I see it, this seems like a drastic departure from regular 'classical' computers. So if we ever get to the point of using these computers, won´t they have to rediscover the entire field of...
  46. T

    Physics Information Theory and Quantum Computers

    I want to be a physics major and I am very interested Astrophysics as a final career path. However recently, after reading a few books about it I have become interested in Information Theory and the prospect of Quantum Computers. I have a very strong Math, Physics, and Computer Science...
  47. K

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  48. N

    Quantum computers, quick question

    Hi, This is probably a very basic question but it's just something I don't seem to understand... I'm just reading up about quantum computers and how they are supposed to work. I don't know all that much about quantum physics so my ideas might be off, but I might know enough to understand...
  49. J

    Equivalent number of flops for quantum computers?

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