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arXiv:quant-ph/9902055

Models of Quantum Computers and Decoherence Problem

I. V. Volovich

Published Feb 15, 1999• 6 citations
arXivHTMLPDF

Abstract

Mathematical models of quantum computers such as a multidimensional quantum Turing machine and quantum circuits are described and its relations with lattice spin models are discussed. One of the main open problems one has to solve if one wants to build a quantum computer is the decoherence due to the coupling with the environment. We propose a possible solution of this problem by using a control of parameters of the system. This proposal is based on the analysis of the spin-boson Hamiltonian performed in the stochastic limit approximation.

DOI
10.48550/arxiv.quant-ph/9902055

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Mathematical models of quantum computers such as a multidimensional quantum Turing machine and quantum circuits are described and its relations with lattice spin models are discussed. One of the main open problems one has to solve if one wants to build a quantum computer is the decoherence due to the coupling with the environment. We propose a possible solution of this problem by using a control of parameters of the system. This proposal is based on the analysis of the spin-boson Hamiltonian performed in the stochastic limit approximation.
The physics and the mathematics of computation are examined to provide a foundation and perspective for the investigation of the quantum mechanics of computation. Our purpose is to explore the fundamental limits and constraints imposed on computation by Nature through the laws of physics and the mathematics of computational complexity. Inasmuch as information storage and transmission are an integral part of computation, their physical bounds are considered. The computer is viewed both physically and mathematically as a dynamical system, and is depicted in terms of the basic Turing machine paradigm. Three fundamental classes of the Turing machine are defined; the deterministic, stochastic and quantum Turing machines. Hamiltonian models and physical realizations of quantum computing are described. Quantum computers can perform some tasks which have no classical analogue, but they cannot compute functions that are non-computable by classical means. Some classically intractable problems can be solved with quantum computers.
During the last decade the field of quantum information has seen considerable progress both theoretically and experimentally. The building block of this theory is the so-called qubit, the quantum counterpart of the classical bit. It turns out that spin degrees of freedom of quantum particles are among the most promising candidates for quantum information processing and computation. Thus, a deep understanding of the different processes that govern the dynamics of these objects is of fundamental importance. For instance, the decoherence process, caused the coupling of the qubits to their surrounding environment, is the main obstacle to quantum information processing: it leads them to lose their quantum coherence and thus behave classically. In addition, quantum systems exhibit correlations that have no classical counterpart. This phenomenon is called entanglement and is the vital resource for quantum teleportation and quantum computing. Decoherence and entanglement dynamics were extensively studied within the framework of the Markovian approximation using the master equation approach. However, due to the non-commuting character of quantum observables, only few models are known to be exactly solvable. Moreover, many spin systems display a strongly non-Markovian behavior. This thesis is devoted to the development of new techniques for deriving the exact dynamics of spin qubits, coupled through Heisenberg and/or Ising interactions to spin environments that have internal dynamics. The basic idea behind these techniques is to use the underlying symmetries exhibited by the Hamitonian operators of the composite systems. This allows for the study of problems related to decoherence and entanglement.
We study the observation of quantum Turing machines by allowing interactions between a quantum machine and its environment during the computation, whereas a quantum Turing machine --original model introduced by Deutsch-- remains isolated. We show that the introduction of observations leads to a weakening of the well formedness conditions of quantum Turing machines such that any (reversible or not) classical Turing machine is a special instance of a quantum machine. Moreover, observation of quantum Turing machines provides a formal solution to the halting process problem: the impossibility to know whether a given quantum Turing machine has actually reached its halting state. It also provides a more realistic abstract architecture of a quantum computer while most of the physical proposals of quantum computers are based on an hybrid classical-quantum architecture. However, we show that a natural formalisation of an observable quantum Turing machines leads to an over-powerful model solving undecidable problems. As a consequence, we introduce a restricted version of observable quantum Turing machines and we show that, under this restriction, any observable quantum Turing machine can be efficiently simulated by a quantum Turing machine. Finally, we discuss the potential application of the observable quantum Turing machine in the quest of a universal quantum Turing machine while recent papers have pointed out that a classical control is a key feature of a universal quantum machine.
Quantum computing promises revolutionary advancements across various fields, but its practical realization is fundamentally limited by the phenomenon of quantum decoherence. Decoherence refers to the loss of quantum superposition and entanglement due to interactions with the surrounding environment, leading to computational errors. This paper presents a comprehensive modeling and prediction framework for quantum decoherence, aiming to provide a deeper understanding of this critical process. The framework utilizes a mathematically rigorous approach based on the principles of quantum mechanics, specifically focusing on the master equation formulation of decoherence. We derive a set of equations that accurately describe the evolution of the quantum state under the influence of various environmental noise sources. Furthermore, we explore methods for estimating decoherence rates and predicting the decoherence time, which is a crucial parameter for assessing the viability of quantum computations. The model incorporates key concepts such as the Bloch sphere representation, the Lindblad master equation, and the concept of decoherence channels. We demonstrate the applicability of the model to various quantum systems and discuss the potential for developing effective decoherence mitigation strategies. The ultimate goal is to provide a tool that can guide the design and optimization of quantum computers, ultimately paving the way for robust and reliable quantum computation.
This review outlines the principles of operation of quantum computers and their elements. The theory of ideal computers that do not interact with the environment and are immune to quantum decohering processes is presented. Decohering processes in quantum computers are investigated. The review considers methods for correcting quantum computing errors arising from the decoherence of the state of the quantum computer, as well as possible methods for the suppression of the decohering processes. A brief enumeration of proposed quantum computer realizations concludes the review.