Available online at http://www.idealibrary.com on doi:10.1006/spmi.2002.1036 Superlattices and Microstructures, Vol. 31, Nos 2–4, 2002 Qubit system in capacitively coupled semiconductor quantum dots TETSUFUMI TANAMOTO † Corporate R & D Center, Toshiba Corporation, 1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan (Received 11 March 2002) We theoretically discuss a quantum-dot quantum computer based on charged states. The tunneling Hamiltonian of a weakly coupled quantum-dot array in the Coulomb blockade region is shown to be reduced that of nuclear magnetic resonance spectroscopy. We numer- ically discuss the region in which this scheme is realized. We also discuss a measurement procedure using a field-effect transistor structure. c  2002 Elsevier Science Ltd. All rights reserved. Key words: quantum computer, coherence, quantum dot, MOSFET, NMR, measurement. 1. Introduction Because of various kinds of quite new aspects of quantum computers, many intensive studies concerning quantum computation have been being carried out [1]. In this article, we theoretically discuss the possibility of a quantum computer made by using a semiconductor quantum-dot array. The electrical and optical proper- ties of quantum dots have been investigated for many years and the quantum-dot system is widely considered to be one of the most promising candidates for the quantum computer [2–8]. Among those proposals, from the viewpoints of solid state electronics and decoherence problems, a spin-based quantum-dot quantum com- puter is considered to be most preferable [3]. However, in the spin-based quantum-dot quantum computers, there are some restrictions on the materials in order to control spins of electrons in the quantum dots freely. At this point, in quantum-dot quantum computers based on charged states of quantum dots, there is the merit that the latter will be able to be constructed from many kinds of materials as long as charging effects can be observed. At present, we do not know whether this merit overcomes the demerit of the short decoherence time of charged states. We think that we should investigate every kind of possibility of quantum computation from the many candidates. In the recent paper [8], we showed the general scheme of a quantum-dot quantum computer based on the charged state by using a field-effect transistor (FET) structure. Here, we discuss this quantum-dot quantum computer more extensively. We set e = 1 and k B = 1. 2. The Hamiltonian for the coupled quantum dot system A qubit based on charged states is composed of two quantum dots coupled via a thin tunneling barrier and a gate electrode that is attached to a thick insulating material (Fig. 1). The quantum dots are assumed to be sufficiently small for charging effects to be observed [9–11]. N αi and N βi are the numbers of excess electrons from the neutral states in two quantum dots. One excess charge is assumed to be inserted from a † E-mail: 0749–6036/02/020151 + 07 $35.00/0 c  2002 Elsevier Science Ltd. All rights reserved.