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QP-PQ, quantum probability and white noise analysis

COMPLEXITY CONSIDERATIONS QUANTUM COMPUTATION

Luigi Accardi

Published Mar 1, 2013• 0 citations
Publisher

Abstract

It is usually calimed that quantum computer can outperform classical computer.Is this statement true?We discuss this issue, not in general, but in the context of the most famous algorithm of quantum computation: Shor's algorithm.
DOI
10.1142/9789814460026_0001

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In the last few years, theoretical study of quantum systems serving as computational devices has achieved tremendous progress. We now have strong theoretical evidence that quantum computers, if built, might be used as a dramatically powerful computational tool. This review is about to tell the story of theoretical quantum computation. I left out the developing topic of experimental realizations of the model, and neglected other closely related topics which are quantum information and quantum communication. As a result of narrowing the scope of this paper, I hope it has gained the benefit of being an almost self contained introduction to the exciting field of quantum computation. The review begins with background on theoretical computer science, Turing machines and Boolean circuits. In light of these models, I define quantum computers, and discuss the issue of universal quantum gates. Quantum algorithms, including Shor's factorization algorithm and Grover's algorithm for searching databases, are explained. I will devote much attention to understanding what the origins of the quantum computational power are, and what the limits of this power are. Finally, I describe the recent theoretical results which show that quantum computers maintain their complexity power even in the presence of noise, inaccuracies and finite precision. I tried to put all results in their context, asking what the implications to other issues in computer science and physics are. In the end of this review I make these connections explicit, discussing the possible implications of quantum computation on fundamental physical questions, such as the transition from quantum to classical physics.
In 1982 Feynman noted that the simulation of a quantum process on a classical computer appears to generally involve an exponential slowdown in running time compared to the evolution of the process itself. More recently in quantum computation this effect has been turned around and exploited for computational advantage by formulating quantum processes (quantum algorithms) whose evolution corresponds to the performance of useful computational tasks. Thus the quantum algorithm will engender an exponential speedup in computing time. The most famous such quantum algorithm is Shor's algorithm for factorising whole numbers. It factorises a number having d digits in a time of order less than d whereas no known classical algorithm can perform factorisation in a time bounded by any polynomial in d. In theoretical computer science, computational complexity is generally characterised by classifying computational tasks into complexity classes such as P, BPP and NP. It therefore appears that quantum computers will be able to transgress some of the boundaries set by these classes. (2 pages)
In this paper we will analyse two quantum algorithms that sparked interest in the potential of quantum computers. The first is Lov Grover's algorithm which may be used to conduct a type of database search. The second is Peter Shor's algorithm which may be used to factor large numbers and provides an exponential speed up over the best current classical algorithms. In the context of these two algorithms we will discuss the benefits and weaknesses of quantum computation. We will show that in exchange for a quantum computer's greater speed we must accept an inherent level of uncertainty in our results.
Introduction In an ordinary computer, information is stored in a collection of tiny circuits each of which is designed to have two stable and easily distinguishable configurations: each represents a bit . In our study of quantum cryptography, we have seen how it can be useful to express information not in ordinary bits but in qubits . Whereas a bit can have only two values, say 0 and 1, a qubit can be in any quantum superposition of ∣0〉 and ∣1〉. Moreover, a qubit can be entangled with other qubits. Thus one might wonder whether a quantum computer, in which the basic elements for storing and processing information are qubits, can outperform an ordinary (classical) computer in certain ways. This question was addressed by researchers starting in the 1980s. In terms of practical consequences, perhaps the most dramatic answer has been given by Peter Shor in his 1994 factoring algorithm for a quantum computer, an algorithm that is exponentially faster than any known classical algorithm. As we have seen in Chapter 1, the difficulty of factoring a product of two large primes is the basis of the security of the RSA cryptosystem. So if one could build a large enough quantum computer – and there is no reason in principle why this could not be done – the RSA system would be rendered ineffective. In this chapter we present the basics of quantum computation and then focus on Shor's factoring algorithm.