Quantum dots defined in carbon nanotubes are a platform for both basic scientific studies and research into new device applications. In particular, they have unique properties that make them attractive for studying the coherent properties of single-electron spins. To perform such experiments it is necessary to confine a single electron in a quantum dot with highly tunable barriers, but disorder has prevented tunable nanotube-based quantum-dot devices from reaching the single-electron regime. Here, we use local gate voltages applied to an ultraclean suspended nanotube to confine a single electron in both a single quantum dot and, for the first time, in a tunable double quantum dot. This tunability is limited by a novel type of tunnelling that is analogous to the tunnelling in the Klein paradox of relativistic quantum mechanics.

We have performed nonlinear transport measurements as a function of a perpendicular magnetic field in a semiconductor Aharonov-Bohm ring connected to two leads. While the voltage-symmetric part of the conductance is symmetric in magnetic field, the voltage-antisymmetric part of the conductance is not symmetric. These symmetry relations are compatible with the scattering theory for nonlinear mesoscopic transport. The observed asymmetry can be tuned continuously by changing the gate voltages near the arms of the ring, showing that the phase of the nonlinear conductance in a two-terminal interferometer is not rigid, in contrast to the case for the linear conductance.PACS numbers: 73.50.Fq, A mesoscopic ring can be used as an electron interferometer in order to compare the electronic phase of electrons traveling through both arms of the ring using the Aharonov-Bohm (AB) effect. However, it has been shown that a two-terminal ring does not allow to measure directly this phase difference in the linear transport [1]: the two-terminal conductance shows AB oscillations with a phase constrained to 0 or π [2,3,4]. This phase rigidity is a consequence of microreversibility [5] showing that the linear conductance of a two-terminal system must be symmetric in magnetic field [1,6]. A direct measurement of the phase difference is possible only in an open multi-terminal geometry [7,8].While the Onsager-Casimir relations hold close to equilibrium (linear conductance), there is no fundamental reason why far from equilibrium the nonlinear conductance should still follow this symmetry, i.e., one could expect G(V, B) = G(V, −B). It is then natural to ask whether the phase rigidity would still hold for the nonlinear transport in a two-terminal ring.In a phase coherent diffusive system, nonlinear conductance is expected when the bias voltage is larger than E T /e, where E T is the Thouless energy [9]. Models developed for non-interacting electrons predict an effect symmetric in magnetic field, which has been observed experimentally through bias voltage induced universal conductance fluctuations [10,11]. The possibility to observe magnetic field asymmetric nonlinear transport has been addressed only very recently both theoretically [12,13] and experimentally [14,15,16,17]. The models proposed there rely on effects of electron-electron interactions in noncentrosymmetric systems. Such behavior could be also expected in AB rings, for which symmetry breaking occurs due to asymmetries in the phase accumulated in each arm of the ring.Here we address the question of the magnetic field symmetries of the nonlinear conductance in a ring used as an Aharonov-Bohm interferometer. We have performed nonlinear d.c. transport measurements in a ring connected to two terminals. The current is fitted by a polynomial function of the bias voltage, with each coefficient of the decomposition showing AB oscillations as a function of magnetic field. While the odd coefficients are symmetric in magnetic field and show strong h/2e oscillations, the even coefficient...

We investigate a Quantum Dot (QD) in a Carbon Nanotube (CNT) in the regime where the QD is nearly isolated from the leads. An aluminum single electron transistor (SET) serves as a charge detector for the QD. We precisely measure and tune the tunnel rates into the QD in the range between 1 kHz and 1 Hz, using both pulse spectroscopy and real -time charge detection and measure the excitation spectrum of the isolated QD.A quantum dot (QD) defined in a carbon nanotube (CNT) is a very interesting and unique physical system for studying individual electron spins 1,2,3,4,5,6 . In particular, the spin relaxation and coherence times are expected to be as long as seconds 1, 2 , which makes this system attractive for quantum information processing. However, both precise control over the tunnel rate into a QD and real -time read out of the charge state of the QD have not been demonstrated yet for CNTs.QDs can be defined in CNTs by using top gates (TGs) as shown in Figure 1. Suitable voltages applied to these TGs create local tunnel barriers in semiconducting CNTs. In this way, single and doubleQDs have been realized 7,8,9 . In this letter we use TGs to precisely tune the tunnel rates into a CNT-QD

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We have performed nonlinear transport measurements as a function of a perpendicular magnetic field in a semiconductor Aharonov-Bohm ring connected to two leads. While the voltage-symmetric part of the conductance is symmetric in magnetic field, the voltage-antisymmetric part of the conductance is not symmetric. These symmetry relations are compatible with the scattering theory for nonlinear mesoscopic transport. The observed asymmetry can be tuned continuously by changing the gate voltages near the arms of the ring, showing that the phase of the nonlinear conductance in a two-terminal interferometer is not rigid, in contrast to the case for the linear conductance. 73.50.Fq, A mesoscopic ring can be used as an electron interferometer in order to compare the electronic phase of electrons traveling through both arms of the ring using the Aharonov-Bohm (AB) effect. However, it has been shown that a two-terminal ring does not allow to measure directly this phase difference in the linear transport [1]: the two-terminal conductance shows AB oscillations with a phase constrained to 0 or π [2, 3, 4]. This phase rigidity is a consequence of microreversibility [5] showing that the linear conductance of a two-terminal system must be symmetric in magnetic field [1,6]. A direct measurement of the phase difference is possible only in an open multi-terminal geometry [7,8].While the Onsager-Casimir relations hold close to equilibrium (linear conductance), there is no fundamental reason why far from equilibrium the nonlinear conductance should still follow this symmetry, i.e., one could expect G(V, B) = G(V, −B). It is then natural to ask whether the phase rigidity would still hold for the nonlinear transport in a two-terminal ring.In a phase coherent diffusive system, nonlinear conductance is expected when the bias voltage is larger than E T /e, where E T is the Thouless energy [9]. Models developed for non-interacting electrons predict an effect symmetric in magnetic field, which has been observed experimentally through bias voltage induced universal conductance fluctuations [10,11]. The possibility to observe magnetic field asymmetric nonlinear transport has been addressed only very recently both theoretically [12,13] and experimentally [14,15,16,17]. The models proposed there rely on effects of electron-electron interactions in noncentrosymmetric systems. Such behavior could be also expected in AB rings, for which symmetry breaking occurs due to asymmetries in the phase accumulated in each arm of the ring.Here we address the question of the magnetic field symmetries of the nonlinear conductance in a ring used as an Aharonov-Bohm interferometer. We have performed nonlinear d.c. transport measurements in a ring con-nected to two terminals. The current is fitted by a polynomial function of the bias voltage, with each coefficient of the decomposition showing AB oscillations as a function of magnetic field. While the odd coefficients are symmetric in magnetic field and show strong h/2e oscillations, the even coefficients are asymm...