# Quantum computer Definition and 38 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. ### 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...
2. 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...
3. ### 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...
4. ### 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...

31. ### 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...
32. ### 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...
33. ### 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...
34. ### 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...
35. ### 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...
36. ### What is your reason for studying physics?

What is your reason for studying physics and what do you want to do with it. I actually got thrown into my schools physics program by accident. I began at a community college studying nanotechnology and when i transferred schools it was supposed to be a two-plus two program meaning I would get...
37. ### Help with classes to get into Quantum Computing

Hi I am an undergrad student in an Electrical engineering program and going for my masters and undergrad degree at the same time (special program i am in). I an very interested on the hardware side of quantum computing as to have a future career in this field (eventually will go for a PhD). I...
38. ### The Quantum Computer and beamsplitter

Dear fellows, please help me to imagine the physical working of a quantum computer (QC). I have read here ( http://www.cs.rice.edu/~taha/teaching/05F/210/news/2005_09_16.htm ) that the basic idea behind it is the phenomenon that you can find in the Mach–Zehnder interferometer But how come this...