What is Quantum computer: Definition and 94 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. E

    Physics Opinions about a career path in quantum computing

    Hi, I'm 20 and I'm courrently persuing a barchelor's degree in Computer Science in Italy and now I'm in the middle of the second year. Thinking about a career path I've always been sure to specialize in Cyber Security but I recently came across Quantum Computing that I find fascinating. After...
  2. Greg Bernhardt

    Quick tour of Google's Quantum Computer

    I thought this was a fun and quick video of an engineer giving a tour of their quantum computer. Only about 30 seconds, but it looks well, quite beautiful! The engineer mentioned they achieved milestone 2 which has something to do with size. I also haven't heard of the field "Quantum Metrology"...
  3. S

    More Nobel prizes in quantum information?

    Alain Aspect, John Clauser & Anton Zeilinger have rightfully received the Nobel prize for their contributions to quantum information, as they were three of the main pioneers of quantum information. However, is it now impossible or very unlikely that other physicists working on this field (e.g...
  4. 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...
  5. Bob Walance

    B A homebrew quantum computer simulator

    A while ago I started writing a quantum computer simulator in order to learn more about quantum computing. It certainly has helped me. The simulator is written in Python and the development was done on a Raspberry Pi 4. It has also been tested on a pc. In order to see it do something useful I...
  6. M

    I Schrödinger's astronaut as a quantum computer?

    Consider the following scenario: a space station is put into orbit, which is absurdly well shielded from all sorts of radiation, a.k.a. "a box". You cannot make any effective observation of what the astronaut inside is up to. (This postulate may be implausible, but in the age of "weakly...
  7. VictorMedvil

    How Do You Build A Quantum Computer?

    So I have heard that IBM and Google have constructed Quantum computers, how does one actually construct a Quantum computer? I would like as much detail as possible on the construction of one of these devices and how it works exactly, I wanted to buy a Spin Q Quantum computer and I understand how...
  8. porton

    I Are electrons universal problem solvers?

    Existence of an universal problem solver, a polynomial-time NP-complete algorithm is a $1000000 prize question. But suppose that we were able to know something "simple", e.g. an electron state or electron wave function exactly. Would we be able to solve complex mathematical problems (like...
  9. 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...
  10. J

    I Many-worlds implies computers are not conscious?

    Consider the following thought-experiment in the many-worlds interpretation. Suppose that I have a reversible conscious observer AI and a particle with +1/2 spin in the z direction. Next the observer measures the spin in the x-direction and therefore spits into a version that measures +1/2 and...
  11. 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?
  12. P

    A Translating/encoding a continuous variable model into a qubit model

    I've read these two pages that discuss going from qubit to continuous variable - https://arxiv.org/abs/quant-ph/0008040 and https://arxiv.org/abs/1907.09832 . I'm curious if anyone knows some papers that discuss going the other way around? I.e. qubitizing a continuous variable model? Any insight...
  13. U

    I Will thermal disturbance change an atom's spin?

    If thermal motion (collision of atoms) changes the direction of an atom, will that change the direction of spin? If so, how much time does it take from the change in the atom orientation to the change in the spin?
  14. J

    I How does a quantum computer work?

    What is the principle of operation of a quantum computer on a silicon chip? What are a qubits and how they connected and interacted between themselves? How does CMOS technology work in a quantum computer?
  15. Auto-Didact

    Google: Demonstrating Quantum Supremacy

    Here is the paper, published today in Nature: Arute et al. 2019, Quantum supremacy using a programmable superconducting processor
  16. Bob Walance

    B Quantum computer vs supercomputer performance

    I've been learning about quantum computing for the last year and I can hardly believe what I read and hear. However, assuming that no big technical hurdles get in the way, I do think that the promises will become reality someday. It is becoming a bit clearer to me, but it's a very difficult...
  17. Haorong Wu

    I What does U\dagger H U represent?

    Hi, there. I'm reading a paper about ion trap. There are several Hamiltonians mentioned in the paper. First, assume ##H_0=H_{\text{internal}} + H_{\text{osc}}##, where ##H_{\text{internal}}## is the Hamiltonian for internal states for a ion, and ##H_{\text{osc}}## is the Hamiltonian describing...
  18. Haorong Wu

    I Questions about an optical photon quantum computer

    Hi. I'm learning the optical photon quantum computer from Nielsen's QCQI. Since I'm not familiar with quantum optics, I got some questions about it.Q1. In page 288, the book reads: A laser outputs a state known as a coherent state ##\left | \alpha \right > = e^{- \left | \alpha \right | ^2 /2 }...
  19. platosuniverse

    B Is spacetime a quantum error correcting code?

    This is a fascinating discussion. I know some people don't want to debate this or they can't debate it but the truth doesn't care about your feelings. This isn't speculative, it's backed by Scientific research. First paper. Is Spacetime an Error Correcting Code. Published in the Journal of High...
  20. A

    I Arrow of time and its reversal on a IBM quantum computer

    I would like to know what the implications of this paper are https://arxiv.org/abs/1712.10057. They say " Here we show that, while in nature the complex conjugation needed for time reversal is exponentially improbable, one can design a quantum algorithm that includes complex conjugation and...
  21. C

    Quantum Computer Security

    I was wondering, I recently discovered, but don't have the article, that there is a security technology that involves the superposition of photons across a fiber optic link. I was wondering could a glass register more adequately create a quantum computer for peoples usage than an actual atomic...
  22. jedishrfu

    One Step Closer to a Universal Quantum Computer

    Researchers have demonstrated holonomic quantum gates under zero-magnetic field at room temperature, which could enable the realization of fast and fault-tolerant universal quantum computers. A quantum computer is a theoretical machine with the potential to solve complex problems much faster...
  23. M

    B Understanding the Intricacies of Quantum Computers and Their Limitations

    I have no idea how a quantum computer works. The quantum computer evidently exploits the laws of MQ, and therefore the post could be interesting. There are 2 difficulties, for my comprehension. The first is that MQ works on a statistical basis, and it is not clear how a quantum system can...
  24. R

    Programs Prospects of Pursuing Graduate Studies in Quantum Computing

    Hello, I am close to finishing my undergraduate degree in Computer Engineering, and I am very interested in pursuing graduate studies. For a long time, I have been passionate about computer science and I've been looking into the research done in various labs in the schools that I'm considering...
  25. J

    A Shor's algorithm - need to uncompute auxiliary qubits?

    Due to required reversibility, classical function (f(a)=y^a \mod N) in Shor's algorithm needs a lot of auxiliary qubits. I was afraid that their later treatment might influence the computation - and just got confirmation from Peter Shor himself: that we need to "uncompute" these auxiliary...
  26. J

    Programs CE + Phys or CE + Math: Interested in Quantum Computing & AI

    Hey all, I'm a student in university who wants to Double Major in Computer Engineering and Physics or Computer Engineering and Mathematics. Ideally I want to get some time in working on quantum computers and some time in working on advanced AI, so one of the big things I want to do is pick the...
  27. ISamson

    Quantum Computer advantages

    Hello, I have found out that in the future we might have quantum computers, which will be more powerful than today's computers. Instead of trying every option available to the computer, quantum computers will be able to try many different possible solutions at the same time. What would be some...
  28. T

    Courses for Quantum Computing

    Hello, I’m currently an Undergraduate Computer Engineer at UIC and minoring in physics. I have become more and more interested in Quantum Computing and its related fields. I plan on continuing my masters in Electrical engineering (Nano - technology). I was wondering if you could help me out in...
  29. 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...
  30. O

    How powerful would a dream quantum computer be?

    Just for fun let's imagine a dream PQC (Personal Quantum Computer) from the future: Specifications: - 4MQ Processer (4MQ = 4 Mega Qubits) - 1TB Time Crystal Memory - 2PB Atomic Storage Hard Drive (Peta Byte=1000TB) - 3D Quantum Dot Screen with built in Holographic Display - All in one Quantum...
  31. A

    B Need Listing of Photon and Particle Quantum States

    Hi, I am learning quantum entanglement. I am interested to create an up to date list of all known : - Photon Quantum States - Particle Quantum States - Classically entagled photon states I guess that there is an organization out there that already have this info. If someone can point me into...
  32. C

    I Superfluidic Quantum Computer?

    Would it be possible to make a quantum computer with fluidic logic (https://en.m.wikipedia.org/wiki/Fluidics), using a superfluid, like liquid Helium-4, as the the "current"?
  33. J

    I How are quantum bits implememted?

    Its very easy to pull a logic gate or transistor circuit of the internet for example and begin to understand how it represents physical bits, by either having 5V across the circuit or below 3.3V. But I was wondering what are some of the methods for representing qubits from a hardware...
  34. munirah

    How to differentiate GHZ state and W state?

    Good day, I try to differentiate GHZ-state and W-state using three tangle. Suppose The value three tangle for GHZ-state equal to 1 while W-state equal to 0. I used three tangle formula, $$\tau_{ABC}=\tau_{A(BC)}-\tau_{AB}-\tau_{AC}=2(\lambda^{AB}.\lambda^{AB}+\lambda^{AC}.\lambda^{AC})$$...
  35. raphampm

    I Role of entanglement in general purpose quantum computing

    I've been doing a course on Quantum Computing and I haven't managed to figure out so far how entanglement would be a useful resource on a general purpose quantum computer. By general purpose quantum computer I mean some theoretical device that could possibly replace current classical computers...
  36. munirah

    Concurrence and three tangle

    Good day, May anyone help me to confirm the formula use for three tangle using concurrence. From my reading, three tangle, $$\tau=\tau_{A(BC)}-\tau_{AB}-\tau_{AC}$$ and it can be related to concurrence $$\tau=C^2_{A(BC)}-C^2_{AB}-C^2_{AC}$$ and I used formula for C is...
  37. munirah

    How can I make singular matrix become nonsingular matrix?

    << Mentor Note -- thread moved from Homework Help forums to General Math >> Good day, I run coding in Mathematica. But, I get singular matrix A at certain loop. In theory, how can I make matrix A become orthogonal A=\begin{pmatrix} 0& 0 & 0 & 0 & 0 & 0 & 0 & 0\\ 0& 0 & 0 & 0 & 0 & 0 & 0 &...
  38. munirah

    Term of Tripartite: Explaining Three Qubit System & Entanglement

    Good day, I get confuse about the term used. Can anyone help me to explain: Tripartite system. Three qubit system. Tripartite entanglement. I really need help for me clarify these term. Thank you very much for helping me.
  39. munirah

    How to calculate negativity measurement of quantum state?

    Good day, From my reading according to negativity for tripartite state, it is given as below; $$N_{ABC}(\rho)=(N_{A-BC}N_{B-AC}N_{C-AB})^{1/3}$$ with $$N_{I-JK}=-2\Sigma_i\sigma_i(\rho^{TI})$$ where $$\sigma_i(\rho^{TI})$$ being the negative eigenvalues of $$\rho^{TI}$$, the partial...
  40. munirah

    Parameter of SU(4) and SU(2)

    Homework Statement Good day, From my reading, SU(4) have 15 parameter and SU(2) has 3 paramater that range differently with certain parameter(rotation angle). And all the parameter is linearly independent to each other. My question are: 1. What the characteristic of each of the parameter? 2...
  41. munirah

    How can I do when I make a Log2 towards zero?

    Good day, Homework Statement I want to make a measurement on qubit by using formula von Neumann entropy using Mathematica given as below; Homework Equations (ρ)=−Tr(ρlog2ρ) The Attempt at a Solution The ρ={{0.5,0},{0,0.5}}My problem is, when I make the log2{{0.5,0},{0,0.5}} I get the...
  42. munirah

    How to get the matrices in partial trace

    Homework Statement Good day, I want to ask the matrix that obtained from below formula and example. $$tr_A(L_{AB})=\sum_i [(\langle i|\otimes id)L_{AB}(|i\rangle\otimes id)]$$ this formula above can be represented as in matrix form below, $$tr_A(L_{AB})=...
  43. anorlunda

    I Making a quantum computer do Shor's algorithm

    I think I understand quantum superposition and entanglement, and a qubit. I just finished reading Scott Aaronson's brilliant blog post "Shor I'll Do It" that allowed me to understand Shor's Algorithm and how it relates to QM. But now I'm missing the next step. How does one "wire up" a number...
  44. gleem

    IBM's invitation to use its quantum computer

    You can now register with IBM to use their quantum computer on line. see “IBM Quantum Experience” website. Of course you have to convince them that you are worthy of that experience.
  45. 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...
  46. B

    Quantum Good self study books for physics of quantum computing

    Hello everyone, I'm an undergraduate double majoring in physics and math. I have one year left after this one, then I plan to go to grad school for theoretical physics (not sure what field yet). My professor keeps suggesting the physics side of quantum computing as a good career route and I'm...
  47. 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...
  48. S

    Quantum Computing and Math prequisities

    Hello, I am a Physics undergraduate, currently beginning my third (out of four) year of studies. I have already taken courses about Real and Complex Analysis, Linear Algebra, ODEs and PDEs (for Maths), Classical Mechanics (Lagrangian and Hamiltonian formalism), Electromagnetism, Special...
  49. J

    What is a quantum computer?

    I'm new to physics and absolutely love it so far. I've always loved theoretical physics and the like, so, I've also heard about a thing called quantum computers. What are these? How do they work? Why are they so special and different from past computing processes? (I've just began learning...
  50. F

    Good Books on Quantum Computing and Quantum Mechanics

    Hello guys, I will be starting my M.S Program in Solid State Electronics and was also curious about looking into quantum computing/superconductors. So can you guys suggest introductory levels books on quantum computing and and superconductors? I have an okay background in Physics (just...
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