Solid-state qubits have the potential for the large-scale integration and for the flexibility of layout for quantum computing. However, their short decoherence time due to the coupling to the environment remains an important problem to be overcome. We propose a new superconducting qubit which incorporates a spin-electronic device: the qubit consists of a superconducting ring with a ferromagnetic π junction which has a metallic contact and a normal Josephson junction with an insulating barrier. Thus, a quantum coherent two-level state is formed without an external magnetic field. This feature and the simple structure of the qubit make it possible to reduce its size leading to a long decoherence time.PACS numbers: 03.67. Lx, 74.50.+r, 74.45.+c, 85.25.Cp The quantum computer is an innovative device in the sense that it would make it possible to solve problems which require unrealistically long computation times on a classical computer [1]. In the quantum computer, the information is stored in a basic element called the qubit, which is a quantum coherent two-level system. The superposition of the two-level state is utilized in the process of quantum computing. For the physical realization of the qubit, various systems have been proposed, e.g., ion traps, nuclear spins, and photons. Among the proposals, solid-state devices have the advantage of large-scale integration and flexibility of layout. On the other hand, a challenging problem for the solid state qubits is the reduction of the decoherence effect, since the solid states qubits in general have a short decoherence time due to their coupling to the environment. In recent years, several qubits based on the Josephson effect have been proposed [2,3,4,5,6,7,8,9,10,11,12,13]. One of the proposals involves a Cooper-pair box type of qubit [2,3,4,5]. In this case, quantum oscillations between the quantum two-level states (Rabi oscillations) have been detected [2,3], and the operation of coupled two qubits has been demonstrated [4,5]. Another example is a flux qubit which uses the superconducting phase. For this proposal, a circuit with a single and relatively large

We study the two-impurity Anderson model for a semiconductor host using the quantum Monte Carlo technique. We find that the impurity spins exhibit ferromagnetic correlations with a range which can be much more enhanced than in a half-filled metallic band. In particular, the range is longest when the Fermi level is located above the top of the valence band and decreases as the impurity bound state becomes occupied. Comparisons with the photoemission and optical absorption experiments suggest that this model captures the basic electronic structure of Ga1−xMnxAs, the prototypical dilute magnetic semiconductor (DMS). These numerical results might also be useful for synthesizing DMS or dilute-oxide ferromagnets with higher Curie temperatures.PACS numbers: 75.50. Pp, 75.30.Hx, 75.40.Mg, The discovery of ferromagnetism in alloys of III-V semiconductors with Mn started an intense research activity in the field of dilute magnetic semiconductors (DMS) [1,2,3]. Room-temperature ferromagnetism in DMS would be a significant development for spintronics device applications. In this respect, it is important to understand the nature of the correlations which develop between magnetic impurities in semiconductors and how they differ from that in a metallic host. With this purpose, we present exact numerical results on the twoimpurity Anderson model for a semiconductor host.In order to study the multiple charge states of Au impurities in Ge, the single-impurity Anderson model of a metallic host was extended to the case of a semiconductor host using the Hartree-Fock (HF) approximation [4]. After the discovery of DMS, the magnetic properties of this model were addressed within HF [5,6]. It was shown that long-range ferromagnetic (FM) correlations develop between Anderson impurities in a semiconductor when the Fermi level is located between the top of the valence band and the impurity bound state (IBS), as illustrated in Fig. 1. The FM interaction between the impurities is mediated by the impurity-induced polarization of the valence electron spins, which are antiferromagnetically coupled to the impurity moments. Within HF, the impurity-host hybridization also induces host split-off states at the same energy as the IBS. When the split-off state becomes occupied, the spin polarizations of the valence band and of the split-off state cancel. This causes the long-range FM correlations between the impurities to vanish. Within the context of DMS, the Anderson Hamiltonian for a semiconductor host was also considered by Krstajić et al. [7], and it was shown that an FM interaction is generated between the impurities due to kinematic exchange. In addition, this model was studied within HF for investigating the multiple charge and spin states of transition-metal atoms in hemoprotein [8].In this paper, we present quantum Monte Carlo (QMC) data on the two-impurity Anderson model for a semiconductor host and make comparisons with the HF results. We find that in a semiconductor the nature of the magnetic correlations between the impurities is differe...

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