The peculiar nature of electron scattering in graphene is among many exciting theoretical predictions for the physical properties of this material. To investigate electron scattering properties in a graphene plane, we have created a gate-tunable potential barrier within a single-layer graphene sheet. We report measurements of electrical transport across this structure as the tunable barrier potential is swept through a range of heights. When the barrier is sufficiently strong to form a bipolar junction (n-p-n or p-n-p) within the graphene sheet, the resistance across the barrier sharply increases. We compare these results to predictions for both diffusive and ballistic transport, as the barrier rises on a length scale comparable to the mean free path. Finally, we show how a magnetic field modifies transport across the barrier.

Transport through potential barriers in graphene is investigated using a set of metallic gates capacitively coupled to graphene to modulate the potential landscape. When a gate-induced potential step is steep enough, disorder becomes less important and the resistance across the step is in quantitative agreement with predictions of Klein tunneling of Dirac fermions up to a small correction. We also perform magnetoresistance measurements at low magnetic fields and compare them to recent predictions.

We perform electrical transport measurements in graphene with several sample geometries. In particular, we design "invasive" probes crossing the whole graphene sheet as well as "external" probes connected through graphene side arms. The four-probe conductance measured between external probes varies linearly with charge density and is symmetric between electron and hole types of carriers. In contrast measurements with invasive probes give a strong electron-hole asymmetry and a sublinear conductance as a function of density. By comparing various geometries and types of contact metal, we show that these two observations are due to transport properties of the metal/graphene interface. The asymmetry originates from the pinning of the charge density below the metal, which thereby forms a p-n or p-p junction, depending on the polarity of the carriers in the bulk graphene sheet. Our results also explain part of the sublinearity observed in conductance as a function of density in a large number of experiments on graphene, which has generally been attributed to short-range scattering only.

We demonstrate a hybrid architecture consisting of a quantum dot circuit coupled to a single mode of the electromagnetic field. We use single wall carbon nanotube based circuits inserted in superconducting microwave cavities. By probing the nanotube-dot using a dispersive read-out in the Coulomb blockade and the Kondo regime, we determine an electron-photon coupling strength which should enable circuit QED experiments with more complex quantum dot circuits.PACS numbers: 73.63.Fg An atom coupled to a harmonic oscillator is one of the most illuminating paradigms for quantum measurements and amplification [1]. Recently, the joint development of artificial two-level systems and high finesse microwave resonators in superconducting circuits has brought the realization of this model on-chip [2,3]. This "circuit Quantum Electro-Dynamics" architecture allows, at least in principle, to combine circuits with an arbitrary complexity. In this context, quantum dots can also be used as artificial atoms [4,5]. Importantly, these systems often exhibit many-body features if coupled strongly to Fermi seas, as epitomized by the Kondo effect. Combining such quantum dots with microwave cavities would therefore enable the study of a new type of coupled fermionicphotonic systems.Cavity quantum electrodynamics [6] and its electronic counterpart circuit quantum electrodynamics[1] address the interaction of light and matter in their most simple form i.e. down to a single photon and a single atom (real or artificial). In the field of strongly correlated electronic systems, the Anderson model follows the same purified spirit [7]. It describes a single electronic level with onsite Coulomb repulsion coupled to a Fermi sea. In spite of its apparent simplicity, this model allows to capture non-trivial many body features of electronic transport in nanoscale circuits. It contains a wide spectrum of physical phenomena ranging from resonant tunnelling and Coulomb blockade to the Kondo effect. Thanks to progress in nanofabrication techniques, the Anderson model has been emulated in quantum dots made out of two dimensional electron gas[8], C60 molecules [9] or carbone nanotubes [10]. Here, we mix the two above situations. We couple a quantum dot in the Coulomb blockade or in the Kondo regime to a single mode of the electromagnetic field and take a step further towards circuit QED experiments with quantum dots. * To whom correspondence should be addressed: kontos@lpa.ens. fr FIG. 1: a. Schematics of the quantum dot embedded in the microwave cavity. The transmitted microwave field has different amplitude and phase from the input field as a result of its interaction with the quantum dot inside the cavity. The quantum dot is connected to "wires" and capacitively coupled to a gate electrode in the conventional 3-terminal transport geometry. b. Scanning electron microscope (SEM) picture in false colors of the coplanar waveguide resonator. Both the typical coupling capacitance geometry of one port of the resonator and the 3-terminals geometry are visib...