We report on low temperature measurements in a fully tunable carbon nanotube double quantum dot. A new fabrication technique has been used for the top-gates in order to avoid covering the whole nanotube with an oxide layer as in previous experiments. The top-gates allow us to form single dots, control the coupling between them and we observe four-fold shell filling. We perform inelastic transport spectroscopy via the excited states in the double quantum dot, a necessary step towards the implementation of new microwave-based experiments.Electron spins in double quantum dots (DQDs) are one of the leading systems for fundamental studies of elementary solid-state qubits 1 . Recent progress has been based on DQDs in 2-dimensional electron gases in GaAs semiconductor heterostructures 2,3 . However, the presence of non-zero nuclear spins

We present a discussion of recent concepts for the construction of a spin quantum computer using endohedral fullerenes. The fullerene molecule is a static, room-temperature trap for atoms with slowly relaxing electron and nuclear spins. The fullerene "containers" can be used to arrange the spins in complex structures such as a linear chain, to form a spin quantum register. We discuss the probable properties of such registers and different strategies to use them in a quantum computer design, including gating and read-out methods.1. Introduction Quantum computation using nuclear spins has been demonstrated in a variety of experiments [1, 2] but is believed to be limited to a small number of qubits [3]. Proposals for realistic spin quantum computer architectures have to address the question of scalability [4]. There are two widely cited concepts fulfilling this criterion, by Kane [5] and by Loss and DiVincenzo [6]. Both concepts are based on electric-field controlled exchange interaction between electron spins, which might however be very difficult [7].Recently, Harneit [8], Suter and Lim [9], and Twamley [10] presented concepts for quantum computation using endohedral fullerenes as spin-qubits, and a microwavepulse controlled magnetic dipolar interaction between qubits. A symbolic drawing synthesizing these concepts is shown in Fig. 1. In this paper, we present the current knowledge about the qubits in question, and we discuss the proposed operational schemes and computer architectures that might be scalable.

We report on photon-assisted tunneling (PAT) experiments in a carbon nanotube quantum dot using microwave frequencies between 20 and 60 GHz. In addition to the basic PAT effect, revealed by the appearance of two extra resonances in the current through the dot, we use PAT for spectroscopy of excited states. The experimental data are in good agreement with simulations.

Using multi-step high performance liquid chromatography (HPLC) it was possible to prepare a chromatographically pure sample of N@C 60 . Invoking EPR spectroscopy of solid samples prepared after each HPLC step, the increase of relative N@C 60 concentration could unambiguously be determined. The UV/Vis spectrum of N@C 60 is indistinguishable within experimental error from that of C 60 , confirming negligible coupling between nitrogen in its atomic ground state and C 60 cage molecular wave functions. The observed large dipolar width of the EPR spectrum of the pure paramagnetic compound indicates weak spin exchange between neighboring paramagnetic centers which is further proof for a very small spin transfer from the encapsulated atom to the fullerene cage.