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Fortschritte der Physik

Quantum Gates by Coupled Quantum Dots and Measurement Procedure in Field-effect-transistor Structure

Tetsufumi Tanamoto

Published Sep 1, 2000• 5 citations
Publisher

Abstract

Coupled quantum-dot system in semiconductor is considered to be one of the promising candidates of quantum computer. It will be most desirable that the quantum computing devices are implemented into the same substrate as the widely used LSI circuits. From this viewpoint, the quantum computer of coupled quantum dots of semiconductors is presented concentrating on the controlled-NOT-gate operation. The decoherence time of the quantum computation and the measurement procedure by field-effect-transistor structure are discussed.

DOI
10.1002/1521-3978(200009)48:9/11<1005::aid-prop1005>3.0.co;2-b

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Coupled quantum-dot system in semiconductor is considered to be one of the promising candidates of quantum computer. It will be most desirable that the quantum computing devices are implemented into the same substrate as the widely used LSI circuits. From this viewpoint, the quantum computer of coupled quantum dots of semiconductors is presented concentrating on the controlled-NOT-gate operation. The decoherence time of the quantum computation and the measurement procedure by field-effect-transistor structure are discussed.
A quantum computer based on an asymmetric coupled-dot system has been proposed and shown to operate as a controlled-NOT gate. The basic ideas are the following. (1) The electron is localized in one of the asymmetric coupled dots. (2) The electron transfer takes place from one dot to the other when the energy levels of the coupled dots are set to be close. (3) The Coulomb interaction between the coupled dots mutually affects the energy levels of the other coupled dots. The decoherence time of the quantum computation and the measurement time are estimated. The proposed system can be realized by developing the technology of the single-electron memory using Si nanocrystals, and a direct combination of the quantum circuit and the conventional circuit is possible.
Semiconductor quantum dots are a promising candidate for future quantum computer devices. Presently, there are three major proposals for designing quantum computing gates based on quantum dot technology: (i) electrons trapped in microcavity; (ii) spintronics; (iii) biexcitons. We survey these designs and show mathematically how, in principle, they will generate 1-bit rotation gates as well as 2-bit entanglement and, thus, provide a class of universal quantum gates. Some physical attributes and issues related to their limitations, decoherence and measurement are also discussed.
We propose an implementation of a universal set of one- and two-quantum-bit gates for quantum computation using the spin states of coupled single-electron quantum dots. Desired operations are effected by the gating of the tunneling barrier between neighboring dots. Several measures of the gate quality are computed within a recently derived spin master equation incorporating decoherence caused by a prototypical magnetic environment. Dot-array experiments that would provide an initial demonstration of the desired nonequilibrium spin dynamics are proposed.
A scheme to possibly implement the quantum computer is proposed based on the idea to build up the single qubit from two coupled quantum dots. The spatial separation of the two logic states of the qubit can greatly reduce the decoherence time, which allows one to perform the coherent quantum logic operations at a fairly high ratio. Together with the realization of the universal quantum logic gates, and the proper scheme of quantum measurement, the present proposal can satisfy all the essential criteria for quantum computing.
A crucial requirement for scalable quantum-information processing is the realization of multiple-qubit quantum gates. Universal multiple-qubit gates can be implemented by a set of universal single qubit gates and any one kind of two-qubit gate, such as a controlled-NOT (CNOT) gate. Semiconductor quantum dot qubits are a leading approach for the physical implementation of quantum computation, due to their potential for large-scale integration. Two-qubit gate operations have been so far only demonstrated in individual electron spin-based quantum dot systems. Due to the relatively short de-coherence time, charge qubits in quantum dots are generally considered to be inferior for going beyond single qubit level. Here, we demonstrate the benchmarking CNOT gate in two capacitively coupled charge qubits, each consisting of an electron confined in a GaAs/AlGaAs double quantum dot. Owing to the strong inter-qubit coupling strength, gate operations with a clock speed up to 5GHz has been realized. A processing tomography shows encouragingly that the universal two-qubit gate operations have comparable fidelities to that of spin-based two-qubit gates. Our results suggest that semiconductor charge qubits have a considerable potential for scalable quantum computing and may stimulate the use of long-range Coulomb interaction for coherent quantum control in other devices.
Since the mid-20th century it has been understood that a general-purpose quan- tum computer would be able to efficiently solve problems that will forever be out-of-reach for conventional computers. Since then, many quantum algorithms have been developed with applications in a wide range of domains including cryptography, simulations, machine learning and data analysis. While this has resulted in substantial attention being paid to the development of quantum com- puters, the best architectures to use in their fabrication is not yet clear. Semiconductor quantum dot devices are a particularly promising candidate for use in quantum computing architectures, as it is anticipated that once the funda- mental building blocks are implemented, they might be massively scalable using the existing lithography techniques of the semiconductor industry. So far, how- ever, it is not yet clear how best to implement the high-fidelity gates required for general-purpose quantum computation. In this thesis, we present and characterise novel theoretical proposals for fast, simple and high-fidelity two-qubit gates using magnetic (exchange) coupling for specific semiconductor quantum dot qubits; namely, the singlet-triplet and resonant-exchange qubits. These two-qubit operations are simple enough that it is feasible for them to be implemented in experiments of the near future. Success- ful implementations would significantly extend the experimentally demonstrable frontier of semi-conductor quantum dot devices as relevant to their use in uni- versal quantum computing architectures. We also develop simple parameter estimation schemes by which it is possible to substantially mitigate the dominant sources of error for our proposed gates; namely, low-frequency charge and magnetic noise. We develop the techniques in the context of pseudo-static magnetic field gradient fluctuations in singlet- triplet qubits, and demonstrate that these techniques lead to a several orders of magnitude improvement in single-qubit coherence times. With minimal effort this could be ported to other qubit architectures.