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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Quantum decoherence in qubit devices

Howard E. Brandt

Published May 9, 2003• 0 citations
Publisher

Abstract

A plethora of qubit devices is under consideration for possible development ranging from quantum telereporters to quantum computers. The primary obstacle to the success of these efforts is the phenomenon of quantum decoherence, the rapid vanishing of the off-diagonal components of the reduced density matrix representing the computational degrees of freedom of the device. Following a review of the physics of quantum decoherence, several instructive examples are exposited. Decoherence issues associated with the various possible approaches to quantum computing are addressed. Also, possible generic methods are reviewed for surmounting the decoherence obstacle.

DOI
10.1117/12.488482

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A plethora of qubit devices is under consideration for possible development ranging from quantum telereporters to quantum computers. The primary obstacle to the success of these efforts is the phenomenon of quantum decoherence, the rapid vanishing of the off-diagonal components of the reduced density matrix representing the computational degrees of freedom of the device. Following a review of the physics of quantum decoherence, several instructive examples are exposited. Decoherence issues associated with the various possible approaches to quantum computing are addressed. Also, possible generic methods are reviewed for surmounting the decoherence obstacle.
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The development of quantum walks in the context of quantum computation, as generalisations of random walk techniques, has led rapidly to several new quantum algorithms. These all follow a unitary quantum evolution, apart from the final measurement. Since logical qubits in a quantum computer must be protected from decoherence by error correction, there is no need to consider decoherence at the level of algorithms. Nonetheless, enlarging the range of quantum dynamics to include non-unitary evolution provides a wider range of possibilities for tuning the properties of quantum walks. For example, small amounts of decoherence in a quantum walk on the line can produce more uniform spreading (a top-hat distribution), without losing the quantum speed up. This paper reviews the work on decoherence, and more generally on non-unitary evolution, in quantum walks and suggests what future questions might prove interesting to pursue in this area.
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The development of quantum walks in the context of quantum computation, as generalisations of random walk techniques, led rapidly to several new quantum algorithms. These all follow unitary quantum evolution, apart from the final measurement. Since logical qubits in a quantum computer must be protected from decoherence by error correction, there is no need to consider decoherence at the level of algorithms. Nonetheless, enlarging the range of quantum dynamics to include non-unitary evolution provides a wider range of possibilities for tuning the properties of quantum walks. For example, small amounts of decoherence in a quantum walk on the line can produce more uniform spreading (a top-hat distribution), without losing the quantum speed up. This paper reviews the work on decoherence, and more generally on non-unitary evolution, in quantum walks and suggests what future questions might prove interesting to pursue in this area.
Today's computers work on bits that exist as either 0 or 1. Quantum computers aren't limited to two states; they encode information as quantum bits, or qubits, which can exist in superposition. Qubits represent atoms, ions, photons or electrons and their respective control devices that are working together to act as computer memory and a processor. Because a quantum computer can contain these multiple states simultaneously, it has the potential to be millions of times more powerful than today's most powerful supercomputers. A processor that can use registers of qubits will be able to perform calculations using all the possible values of the input registers simultaneously. This superposition causes a phenomenon called quantum parallelism, and is the motivating force behind the research being carried out in quantum computing. Due to technical obstacles, till date, a quantum computer has not yet been realized. But the concepts and ideas of quantum computing has been demonstrated using various methods like NMR, Ion Trap, Quantum Dot, Optical Methods, etc. A quantum computer manipulates qubits by executing a series of quantum gates, each a unitary transformation acting on a single qubit or pair of qubits. In applying these gates in succession, a quantum computer can perform a complicated unitary transformation to a set of qubits in some initial state. The qubits can then be measured, with this measurement serving as the final computational result. Research must devise a way to maintain decoherence and other potential sources of error at an acceptable level. Probably the most important idea in this field is the application of error correction in phase coherence as a means to extract information and reduce error in a quantum system without actually measuring that system. Thereby, quantum computers will emerge as the superior computational devices and perhaps one day make today's modern computer obsolete.