This brief review discusses electronic properties of mesoscopic graphene-based structures. These allow controlling the confinement and transport of charge and spin; thus, they are of interest not only for fundamental research, but also for applications. The graphene-related topics covered here are: edges, nanoribbons, quantum dots, pn-junctions, pnp-structures, and quantum barriers and waveguides. This review is partly intended as a short introduction to graphene mesoscopics.

Massless Dirac particles cannot be confined by an electrostatic potential. This is a problem for making graphene quantum dots but confinement can be achieved with a magnetic field and here general conditions for confined and deconfined states are derived. There is a class of potentials for which the character of the state can be controlled at will. Then a confinement-deconfinement transition occurs which allows the Klein paradox to be probed experimentally in graphene dots. A dot design suitable for this experiment is presented.

We study hole spin resonance in a p-channel silicon metal-oxide-semiconductor field-effect transistor. In the sub-threshold region, the measured source-drain current reveals a double dot in the channel. The observed spin resonance spectra agree with a model of strongly coupled two-spin states in the presence of a spin-orbit-induced anti-crossing. Detailed spectroscopy at the anticrossing shows a suppressed spin resonance signal due to spin-orbit-induced quantum state mixing. This suppression is also observed for multi-photon spin resonances. Our experimental observations agree with theoretical calculations.PACS numbers: 73.63. Kv, 73.23.Hk, The silicon-based metal-oxide-semiconductor fieldeffect transistor (MOSFET) is a key element of largescale integrated circuits that are at the core of modern technology. Looking into the future, a universal faulttolerant quantum computer also requires a huge number of physical qubits, on the order of 10 8 or more [1,2]. As such, a qubit integrated with the standard Si MOS-FET architecture would be truly attractive from the perspectives of scaling up and leveraging existing technologies. One example of such a qubit is the spin of an impulity/defect in the channel of a Si MOSFET. Indeed, spin qubits defined in Si nano-devices are not only compatible with current silicon technology, but are also known to be one of the most quantum coherent among known qubit designs [3][4][5][6][7][8][9][10][11][12][13].Although there are many studies of impurities and defects in Si [14], single impurity/defect in the channel of a Si MOSFET has only recently been studied experimentally, by single-electron tunneling [15][16][17][18][19]. Spins of such defects are difficult to characterize because of their weakly-interacting nature. Controlling the spins of impurities in a MOSFET, as well as in a gate-confined quantum dot, can be achieved much more easily in a pchannel MOSFET than an n-channel. The reason is that the larger spin-orbit interaction (SOI) of a hole (-like) spin enables the spin resonance by an oscillatory electric field, instead of a magnetic field, at microwave frequencies under typical sub-Tesla static magnetic fields. Such electrically-driven spin resonance (EDSR) has been demonstrated in III-V devices [20][21][22][23], as well as in Si [24][25][26], while SOI effects in gate-confined Si quantum dots have been investigated in the spin blockade region [27]. However, systematic investigations of EDSR * E-mail address: k-ono@riken.jp † these authors contributed equally to this work under the direct influence of SOI have not been performed in Si, the material that provides an ideal stage for studying SOI due to the minor presence of nuclear spins.In this work we study sub-threshold transport and EDSR in a short p-channel Si MOSFET, and quantitatively reveal the effects of SOI and EDSR on lifting the spin blockade. Specifically, our transport measurements demonstrate that there are two effective dots in the channel, which allow us to identify a spin blockade regime and explore spin reso...

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 examine a graphene quantum dot formed by combining an electric and a uniform magnetic field. The electric field creates a smooth quantum well potential while the magnetic field induces an exponential tail to the dot states. The states peak in the well and the electrostatic barrier region as a result of the Klein tunneling effect. Coupling between dot states which peak in different regions can be achieved with the electric and magnetic fields. The tunability of this dot with moderate external fields could be used for designing quantum devices in monolayer graphene.