1 QUANTUM INFORMATION TRANSFER BETWEEN TWO QUBITS: AN INTRODUCTION Nguyen Van Hieu and Nguyen Bich Ha Institute of Materials Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Vietnam Abstract. This article is the beginning of a series of theoretical works of the authors and their research group to study the physical mechanism of the quantum information (QI) transfer between two qubits. As a simple example a two-spin-qubit system with the spin-spin direct coupling and without the interaction with the environment is investigated. The rate equations are exactly solved and the interpretation of the expressions of their solution as a physical mechanism of the QI transfer between two qubits is presented. This exact solution is used to study the entanglement conservation in several classes of quantum states of the system. A general method for studying arbitrary systems of two coupled spin-qubits interacting with the environment is also proposed. Keywords: Quantum information, qubit, exchange interaction, density matrix. PACS numbers: 03.65.Yz, 03.67.Hk, 42.50.Ex I. INTRODUCTION One of the very promising perspectives of nanotechnology is the possibility to manufacture the nanostructured devices and systems for processing the quantum information (QI). The basic element of any devices of this kind is quantum bit or qubit for short. Each qubit is a two-state quantum system. It is called also a two-level system with the tacit consent that both levels are non-degenerate. Qubits are QI carriers: QI encoded into a qubit is its wave function (for a pure state) or its density matrix (for a mixed state). The interaction of qubits causes the mutual dependence of their reduced density matrices, which can be considered as physical mechanism of QI transfer or transmission between them [1- 6].