We present data from an induced gallium arsenide (GaAs) quantum wire that exhibits an additional conductance plateau at 0.5(2e2/h), where e is the charge of an electron and h is Planck's constant, in zero magnetic field. The plateau was most pronounced when the potential landscape was tuned to be symmetric by using low-temperature scanning-probe techniques. Source-drain energy spectroscopy and temperature response support the hypothesis that the origin of the plateau is the spontaneous spin-polarization of the transport electrons: a ferromagnetic phase. Such devices may have applications in the field of spintronics to either generate or detect a spin-polarized current without the complications associated with external magnetic fields or magnetic materials.

Measurements are presented of a device designed to cool a 6 microm;{2} region of 2D electron gas using quantum dots. Electrostatic effects are found to be significant in the device, and a model that accounts for them is developed. At ambient electron temperatures above 120 mK the results are consistent with the model and the base temperature of the cooled region is estimated. At an ambient electron temperature of 280 mK, the 6 microm;{2} region is found to be cooled below 190 mK. Below 120 mK the results deviate from predictions, which is attributed to reduced electron-electron scattering rates.

We investigate radio-frequency (rf) reflectometry in a tunable carbon nanotube double quantum dot coupled to a resonant circuit. By measuring the in-phase and quadrature components of the reflected rf signal, we are able to determine the complex admittance of the double quantum dot as a function of the energies of the single-electron states. The measurements are found to be in good agreement with a theoretical model of the device in the incoherent limit. Besides being of fundamental interest, our results present an important step forward towards non-invasive charge and spin state readout in carbon nanotube quantum dots. PACS numbers: 73.63.Fg, 73.63.Kv, 73.23.Hk, 03.67.Lx An important requirement in any quantum information processing scheme is fast manipulation and readout of the quantum system in which the quantum information is encoded. This requires an understanding of the response of the quantum system at finite frequencies which, in the case of an electronic device, involves an understanding of its complex admittance [1,2]. Of particular interest in the context of quantum information processing are double quantum dots which are widely used to define charge and spin qubits [3]. However, while double quantum dots have been investigated in detail over the last decade, experiments to measure and analyze their complex admittance have not yet been performed and this topic has only recently been addressed theoretically [4]. The admittance of quantum dots at finite frequencies is non-trivial as exemplified by recent experiments on single quantum dots [5,6]. The physics is even richer for double quantum dots as internal charge dynamics, i.e. charge transfer between the quantum dots, has to be taken into account. However, the dependence of the admittance on the internal charge dynamics also provides a route towards charge and spin state readout [7].In this work we present a detailed experimental study of the complex admittance of a carbon nanotube double quantum dot which is measured using rf reflectometry techniques. The measurements are compared with a theoretical model of the device where we use a density matrix approach to calculate the double quantum dot admittance. The good quantitative agreement between the experimental and theoretical results allows us to determine the effective conductance and susceptance of the double dot as a function of the energies of the single-electron states. Our measurements thus present a first quantitative analysis of the complex admittance of a double quantum dot. The demonstrated technique also provides the basis for a simple and fast detection scheme for charge and spin state readout in carbon nanotubes -a material with considerable potential for spin-based quantum information processing [8-13] -without the need for a separate charge detector [14].The device we consider is a carbon nanotube grown by chemical vapour deposition on degenerately doped Si ter- minated by 300 nm SiO 2 , see Fig. 1(a). The nanotube is contacted by Au source and drain electrodes which form the outer ...

We make use of spin selection rules to investigate the electron spin system of a carbon nanotube double quantum dot. Measurements of the electron transport as a function of the magnetic field and energy detuning between the quantum dots reveal an intricate pattern of the spin state evolution. We demonstrate that the complete set of measurements can be understood by taking into account the interplay between spin-orbit interaction and a single impurity spin coupled to the double dot. The detection and tunability of this coupling are important for quantum manipulation in carbon nanotubes. DOI: 10.1103/PhysRevLett.106.206801 PACS numbers: 73.63.Kv, 71.70.Ej, 73.23.Hk, 73.63.Fg Spin qubits defined in carbon nanotube quantum dots are of considerable interest for encoding and manipulating quantum information. Because of the absence of hyperfine interaction in the dominant 12 C isotope, spin coherence times are expected to be exceptionally long, while the presence of spin-orbit interaction [1-4] may allow for electrical or even optical [5] control of the spin states. However, before carbon nanotubes can find applications in quantum information processing schemes, we need to understand and control the coupling between individual electron spins and the interaction of the electron spins with their environment.A powerful method to probe the spin system of quantum dots is to measure the electron transport in a double quantum dot device in the spin blockade regime [6]. In this transport regime, the tunneling of an electron between the quantum dots is forbidden by spin selection rules; hence, the current is suppressed. However, the spin blockade can be lifted by the interaction of the electron spins with their environment, and a measurement of the (leakage) current thus directly probes these interactions. The main spin relaxation and decoherence mechanisms in carbon nanotubes that have been considered so far are spin-orbit coupling and hyperfine interaction in 12 C-enriched nanotubes [7,8]. However, a further important consideration in any realistic nanotube device is the presence of bends, impurities, or defects and their coupling to the electron spins. In this work we make use of the excellent spin sensitivity in the spin blockade regime to investigate the spin system coupling of a carbon nanotube double quantum dot and spin impurities in its environment. In a series of detailed measurements, we show how the interplay of a single impurity spin and the spin-orbit interaction affects the spin states of the nanotube double quantum dot and demonstrate that the coupling to spin impurities can be tuned by gate electrodes.The device that we consider is a single-walled carbon nanotube grown with natural isotope ratios contacted by Au electrodes. Side and top barrier gates are used to define and control the double quantum dot; see Fig. 1(a). A typical charge stability diagram of the device is shown in Figs. 1(c) and 1(d) in which the ordered pairs ðn; mÞ indicate the effective electron occupancies of the many-electron double quan...

We investigate a tunable two-impurity Kondo system in a strongly correlated carbon nanotube double quantum dot, accessing the full range of charge regimes. In the regime where both dots contain an unpaired electron, the system approaches the two-impurity Kondo model. At zero magnetic field the interdot coupling disrupts the Kondo physics and a local singlet state arises, but we are able to tune the crossover to a Kondo screened phase by application of a magnetic field. All results show good agreement with a numerical renormalization group study of the device.