Can We Build a Large-Scale Quantum Computer Using Semiconductor Materials?
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Can We Build a
Large-Scale Quantum Computer Using Semiconductor Materials? B.E. Kane
Abstract The following article is based on the Symposium X presentation given by Bruce E. Kane (University of Maryland) at the 2004 Materials Research Society Spring Meeting in San Francisco. Quantum computing has the potential to revolutionize our ability to solve certain classes of difficult problems. A quantum computer is able to manipulate individual two-level quantum states (“qubits”) in the same way that a conventional computer processes binary ones and zeroes. Here, Kane discusses some of the most promising proposals for quantum computing, in which the qubit is associated with single-electron spins in semiconductors. While current research is focused on devices at the one- and two-qubit level, there is hope that cross-fertilization with advancing conventional computer technology will enable the eventual development of a large-scale (thousands of qubits) semiconductor quantum computer.The author focuses on materials issues that will need to be surmounted if large-scale quantum computing is to be realizable. He argues in particular that inherent fluctuations in doped semiconductors will severely limit scaling and that scalable quantum computing in semiconductors may only be possible at the end of the road of Moore’s law scaling, when devices are engineered and fabricated at the atomic level. Keywords: quantum computing, semiconductors, spintronics.
Introduction One of the most exciting questions facing the physics and materials science communities today is whether it will be possible to construct a large-scale quantum computer.1 Such computers are (currently theoretical) machines which manipulate and process single quantum states in the same way that conventional computers process ones and zeroes. The field of quantum computing has flourished since the realization by Peter Shor in 1994 that quantum computers—if they could be built—could solve certain cryptographic problems that are completely in-
MRS BULLETIN • VOLUME 30 • FEBRUARY 2005
tractable for any conventional computer.2,3 Since then, a wide range of systems have been explored in search of the best “qubit,” or two-level quantum state, on which to base a scalable quantum computer technology. This exploration is still in its infancy: experiments today are typically performed on one or two qubits, while the solution of significant cryptographic problems would require on the order of 10 4 qubits. There is currently no consensus as to which of the many qubits under scrutiny
will be most easily scaled. A good candidate qubit must be a two-level quantum state (such as a spin-1/2 particle) in which it is possible to manipulate and measure the state. Ideally, the qubit should have a very long lifetime relative to the time necessary for performing logic and measurement operations. The lifetime relevant here, usually called the decoherence time, is the time it takes for the information encoded onto the qubit to be lost, typically through interactions of the qubit with its su
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