Quantum gates by coupled quantum dots and measurement procedure in Si MOSFET

We investigated the quantum gates of coupled quantum dots, theoretically, when charging effects can be observed. We have shown that the charged states in the qubits can be observed by the channel current of the MOSFET structure.Comment: 3 pages, 2 figures, use revtex.sty. 'The Eleventh International Conference on Nonequilibrium Carrier Dynamics in Semiconductors (HCIS-11)',July 19-23, 199

Implementation of standard quantum error correction codes for solid-state qubits

In quantum error-correcting code (QECC), many quantum operations and measurements are necessary to correct errors in logical qubits. In the stabilizer formalism, which is widely used in QECC, generators $G_i (i=1,2,..)$ consist of multiples of Pauli matrices and perform encoding, decoding and measurement. In order to maintain encoding states, the stabilizer Hamiltonian $H_{\rm stab}=-\sum_i G_i$ is suitable because its ground state corresponds to the code space. On the other hand, Hamiltonians of most solid-state qubits have two-body interactions and show their own dynamics. In addition solid-state qubits are fixed on substrate and qubit-qubit operation is restricted in their neighborhood. The main purpose of this paper is to show how to directly generate the stabilizer Hamiltonian $H_{\rm stab}$ from conventional two-body Hamiltonians with Ising interaction and XY interaction by applying a pulse control method such as an NMR technique. We show that generation times of $H_{\rm stab}$ for nine-qubit code, five-qubit code and Steane code are estimated to be less than 300 ns when typical experimental data of superconducting qubits are used, and sufficient pulse control is assumed. We also show how to prepare encoded states from an initial state $|0....0>$. In addition, we discuss an appropriate arrangement of two- or three-dimensional arrayed qubits.Comment: 14 pages, 4 figure

The Effect of Local Structure and Non-uniformity on Decoherence-Free States of Charge Qubits

We analyze robustness of decoherence-free (DF) subspace in charge qubits when there are a local structure and non-uniformity that violate collective decoherence measurement condition. We solve master equations of up to four charge qubits and a detector as two serially coupled quantum point contacts (QPC) with an island structure. We show that robustness of DF states is strongly affected by local structure as well as by non-uniformities of qubits

Superconductive Phonon Anomalies in High-TcT_c Cuprates

We consider the effects on phonon dynamics of spin-lattice coupling within the slave-boson mean-field treatment of the extended $t$-$J$ model. With no additional assumptions the theory is found to give a semi-quantitative account of the frequency and linewidth anomalies observed by Raman and neutron scattering for the 340$cm^{-1}$ $B_{1g}$ phonon mode in $YBa_2Cu_3O_7$ at the superconducting transition. We discuss the applicability of the model to phonon modes of different symmetries, and report a connection to spin-gap features observed in underdoped YBCO. The results suggest the possibility of a unified understanding of the anomalies in transport, magnetic and lattice properties.Comment: heavily revised version of previous paper, including systematic treatment of effect of $t$ term, coupling constant derivation and calculation of phonon linewidth broadening. Revised Figs. 3 and 4 still only available by sending fax # to [email protected]

Steady-state solution for dark states using a three-level system in coupled quantum dots

Quantum dots (QDs) are one of the promising candidates of interconnection between electromagnetic field and electrons in solid-state devices. Dark states appear as a result of coherence between the electromagnetic fields and the discrete energy levels of the system. Here, we theoretically solve the steady-state solutions of the density matrix equations for a thee-level double QD system and investigate the condition of the appearance of a dark state. We also numerically show the appearance of the dark state by time-dependent current characteristics.Comment: 5 pages, 5 figure

N\'eel and singlet RVB orders in the t-J model

The N\'eel and the singlet RVB orders of the {\it t-J} model in a 2D square lattice are studied in the slave-boson mean-field approximation. It is shown that the N\'eel order parameter takes the maximum value at the finite temperature and disappear at the lower temperature for a certain range of doping. It is also shown that the N\'eel and the singlet RVB orders coexist at low temperature. This suggests the possibility of the coexistence of the N\'eel and the superconducting orders.Comment: RevTeX, 8 pages, 1 postscript figure. To appear in Physica C, Volume 257, issue 38