Single-electron pumps are set to revolutionize electrical metrology by enabling the ampere to be redefined in terms of the elementary charge of an electron. Pumps based on lithographically fixed tunnel barriers in mesoscopic metallic systems and normal/superconducting hybrid turnstiles can reach very small error rates, but only at megahertz pumping speeds that correspond to small currents of the order of picoamperes. Tunable barrier pumps in semiconductor structures are operated at gigahertz frequencies, but the theoretical treatment of the error rate is more complex and only approximate predictions are available. Here, we present a monolithic, fixed-barrier single-electron pump made entirely from graphene that performs at frequencies up to several gigahertz. Combined with the record-high accuracy of the quantum Hall effect and proximity-induced Josephson junctions, quantized-current generation brings an all-graphene closure of the quantum metrological triangle within reach. Envisaged applications for graphene charge pumps outside quantum metrology include single-photon generation via electron-hole recombination in electrostatically doped bilayer graphene reservoirs, single Dirac fermion emission in relativistic electron quantum optics and read-out of spin-based graphene qubits in quantum information processing.

We study an epitaxial graphene monolayer with bilayer inclusions via magnetotransport measurements and scanning gate microscopy at low temperatures. We find that bilayer inclusions can be metallic or insulating depending on the initial and gated carrier density. The metallic bilayers act as equipotential shorts for edge currents, while closely spaced insulating bilayers guide the flow of electrons in the monolayer constriction, which was locally gated using a scanning gate probe.

We report low-temperature transport spectroscopy of a graphene quantum dot fabricated by atomic force microscope nanolithography. The excellent spatial resolution of the atomic force microscope allows us to reliably fabricate quantum dots with short constrictions of less than 15 nm in length. Transport measurements demonstrate that the device is dominated by a single quantum dot over a wide gate range. The electron spin system of the quantum dot is investigated by applying an in-plane magnetic field. The results are consistent with a Landé g-factor ∼ 2 but no regular spin filling sequence is observed, most likely due to disorder.The electronic properties of graphene are of intense current interest because of unique features such as the two-dimensional nature of the graphene lattice and its gapless Dirac spectrum. These properties give rise to novel phenomena such as an anomalous half-integer quantum Hall effect [1, 2], Klein tunneling [3], and an optical transmittance defined solely by the fine-structure constant [4]. Another attraction is the absence of nuclear spin in the dominant carbon-12 isotope, offering the possibility to define spin qubits in graphene quantum dots which do not suffer from the decoherence associated with the hyperfine interaction [5]. The spin system of graphene quantum dots however, is not yet properly understood [6,7].The absence of a band gap in the electronic spectrum and the existence of Klein tunneling means that devices such as quantum dots cannot be defined electrostatically in monolayer graphene. It is possible to introduce a gap in the spectrum of bilayer graphene by applying an electric field and thus define quantum dots using gate electrodes. Typical energies of quantum dots defined in bilayer graphene, however, are too small to readily observe the quantization of the electronic spectrum [8,9]. A confinement gap can be introduced in monolayer graphene by physically defining narrow constrictions in the material. These constrictions can be used as tunable barriers to graphene quantum dots [10]. It has been observed however, that in long constrictions, a confinement gap in combination with a background disorder potential produces a hard transport gap where conduction occurs via of a number of quantum dots in series in the barriers [11,12]. This complicates the transport characteristics and prevents strong coupling between the defined quantum dots and the graphene source and drain electrodes. It is therefore of considerable interest to be able to fabricate graphene quantum dots with short constrictions [13].In this work we explore a fabrication technique -atomic force microscope (AFM) nanolithography -which, due Figure 1: (color online) (a) Atomic force microscope (AFM) friction image and (b) AFM height image of a single quantum dot fabricated in monolayer graphene using AFM nanolithography. The spatial resolution of the technique is ∼ 15 nm for the oxidized lines. The bumps are about 1.5 nm in height.to its excellent spatial resolution allows us to reliably fabricate very short co...

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