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Fault-tolerant spin-based quantum computers will require fast and accurate qubit read out. This can be achieved using radiofrequency reflectometry given sufficient sensitivity to the change in quantum capacitance associated with the qubit states. Here, we demonstrate a 23-fold improvement in capacitance sensitivity by supplementing a cryogenic semiconductor amplifier with a SQUID preamplifier. The SQUID amplifier operates at a frequency near 200 MHz and achieves a noise temperature below 600 mK when integrated into a reflectometry circuit, which is within a factor 120 of the quantum limit. It enables a record sensitivity to capacitance of 0.07 aF / Hz . The setup is used to acquire charge stability diagrams of a gate-defined double quantum dot in a short time with a signal-to-noise ration of about 38 in 1 μ s of integration time. [ABSTRACT FROM AUTHOR]

The authors demonstrate a silicon-based single-electron transistor (SET) in the few-electron regime. Our structure is similar to a metal-oxide-semiconductor field-effect transistor. The substrate, however, is undoped and could be isotope enriched so that any nonuniformity and spin decoherence due to impurity and nuclear spins can be minimized. A bilayer-gated configuration provides flexibility in manipulating single electrons. The stability chart measured at 4.2 K shows diamondlike domains with a charging energy of 18 meV, indicating a quantum dot of 20 nm in diameter. The benefits of using this enhancement-mode SET in silicon and its potential application for scalable quantum computing are discussed. [ABSTRACT FROM AUTHOR]

We demonstrate experimentally single-electron quantum dots using a single-top-gate transistor configuration on a composite quantum well. The data indicate a 15 meV Coulomb charging energy and a 20 meV orbital energy spacing, which imply a quantum dot of 20 nm in diameter. Combining with the inherent advantage of a large electron g* factor in InAs, our demonstration is significant for a solid state implementation of a scalable quantum computing. [ABSTRACT FROM AUTHOR]

We present experimental studies of the current pumped through a dynamic quantum dot over a wide range of magnetic fields. At low fields we observe a repeatable structure indicating increased confinement of the electrons in the dynamic dot. At higher fields (B>5T), we observe a structure which changes markedly from device to device suggesting that in this regime the transport is sensitive to local disorder. The results are significant for the development of dynamic quantum dot pumps as quantum standards of electrical current. [ABSTRACT FROM AUTHOR]

Using a non-invasive charge detection method, we detect a flow of electrons trapped in dynamic quantum dots. The dynamic quantum dots are defined by surface acoustic waves (SAWs) and move through a long depleted one-dimensional channel. A one-dimensional constriction is placed next to the SAW channel but in a separate circuit; the current induced by the SAWs through this detector constriction is sensitive to the number of electrons trapped in the SAW minima. We observe steps in the detector acoustoelectric current as the number of electrons carried by SAWs are varied as 1; 2; 3 . . . . [ABSTRACT FROM AUTHOR]

We demonstrate the energy- and time-resolved detection of single-electron wave packets from a clock-controlled source transmitted through a high-energy quantum Hall edge channel. A quantum dot source is loaded with single electrons which are then emitted ~150??meV above the Fermi energy. The energy spectroscopy of emitted electrons indicates that at high magnetic field these electrons can be transported over several microns without inelastic electron-electron or electron-phonon scattering. Using a time-resolved spectroscopic technique, we deduce the wave packet size at picosecond resolution. We also show how this technique can be used to switch individual electrons into different electron waveguides (edge channels). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Single-electron circuits of the future, consisting of a network of quantum dots, will require a mechanism to transport electrons from one functional part of the circuit to another. For example, in a quantum computer decoherence and circuit complexity can be reduced by separating quantum bit (qubit) manipulation from measurement and by providing a means of transporting electrons between the corresponding parts of the circuit. Highly controlled tunnelling between neighbouring dots has been demonstrated, and our ability to manipulate electrons in single- and double-dot systems is improving rapidly. For distances greater than a few hundred nanometres, neither free propagation nor tunnelling is viable while maintaining confinement of single electrons. Here we show how a single electron may be captured in a surface acoustic wave minimum and transferred from one quantum dot to a second, unoccupied, dot along a long, empty channel. The transfer direction may be reversed and the same electron moved back and forth more than sixty times-a cumulative distance of 0.25?mm-without error. Such on-chip transfer extends communication between quantum dots to a range that may allow the integration of discrete quantum information processing components and devices. [ABSTRACT FROM AUTHOR]

We use the charged tip of a low-temperature scanning probe microscope to perturb the transport through a graphene nanoconstriction. Maps of the conductance as a function of tip position display concentric halos and, by following the expansion of the halos with back-gate voltage, we are able to identify an elongated domain over the nanoconstriction where they originate. Amplitude modulations of the transmission resonances are correlated with the gradient of the tip-induced potential and we analyze this in terms of modified tunnel coupling between localized states. [ABSTRACT FROM AUTHOR]

Using standard microfabrication techniques, it is now possible to construct devices that appear to reliably manipulate electrons one at a time. These devices have potential use as building blocks in quantum computing devices, or as a standard of electrical current derived only from a frequency and the fundamental charge. To date, the error rate in semiconductor 'tuneable-barrier' pump devices, those which show most promise for high-frequency operation, have not been tested in detail. We present high-accuracy measurements of the current from an etched GaAs quantum dot pump, operated at zero source-drain bias voltage with a single ac-modulated gate at 340MHz driving the pump cycle. By comparison with a reference current derived from primary standards, we show that the electron transfer accuracy is better than 15 parts per million. Highresolution studies of the dependence of the pump current on the quantum dot tuning parameters also reveal possible deviations from a model used to describe the pumping cycle. [ABSTRACT FROM AUTHOR]

Abstract: We present the first results on electron transport through a tuneable-barrier double quantum dot electron pump. This device could yield twice the current to a single-dot pump previously reported [M.D. Blumenthal, B. Kaestner, L. Li, S. Giblin, , T.J.B.M. Janssen, M. Pepper, D. Anderson, G. Jones, D.A. Ritchie, Gigahertz quantized charge pumping, Nat. Phys. 3 (2007) 343], at the same operating frequency. The two quantum wires forming the device were not sufficiently homogeneous for pumping to be observed through both dots simultaneously, but sufficient homogeneity should be attainable in future batches of devices. [Copyright &y& Elsevier]

We demonstrate single-electron pumping in a gate-defined carbon nanotube double quantum dot. By periodic modulation of the potentials of the two quantum dots, we move the system around charge triple points and transport exactly one electron or hole per cycle. We investigate the pumping as a function of the modulation frequency and amplitude and observe good current quantization up to frequencies of 18 MHz where rectification effects cause the mechanism to break down. [ABSTRACT FROM AUTHOR]

Undoped GaAs/AlGaAs heterostructures in which carriers are attracted from the Ohmic contacts by a voltage bias on an insulated top gate allows higher mobilities to be obtained at lower electron densities than is possible with modulation doped heterostructures. However a two level gating scheme and an Ohmic contacting process that maximizes lateral diffusion are necessary to fully exploit the advantages of the undoped system for fabricating lower dimensional mesoscopic structures. Ionized background impurities (at low densities) and interface roughness (at high densities) are found to be the dominant sources of scattering. An approximate length scale set by the number of impurities the interfacial wave function intersects is observed in the magnetoconductance of two-dimensional mesoscopic regions. [ABSTRACT FROM AUTHOR]

This letter presents an experimental investigation into the effect of a surface-acoustic-wave (SAW) on the emission of a single InAs quantum dot. The SAW causes the energy of the transitions within the dot to oscillate at the frequency of the SAW, producing a characteristic broadening of the emission lines in their time-averaged spectra. This periodic tuning of the transition energy is used as a method to regulate the output of a device containing a single quantum dot and we study the system as a high-frequency periodic source of single photons. [ABSTRACT FROM AUTHOR]

The authors report on the experimental demonstration of a GaAs/AlGaAs-based quantum cellular automata cell fabricated using electron beam lithographically defined gates. These surface metallic gates form a pair of double quantum dots, as well as a pair of quantum point contacts (QPCs) that act as noninvasive voltage probes. Measurements at cryogenic temperatures show that an electron transfer in the input dots induces the relocation of a single electron in the output dots. Using the QPCs they were also able to determine the operating limits of the cell. [ABSTRACT FROM AUTHOR]

The authors demonstrate the feasibility of ballistic electron spectroscopy as a tool for mesoscopic physics. A quantum dot is utilized as an energy-selective detector of nonequilibrium ballistic electrons injected into a two-dimensional electron system. In this letter they use a second quantum dot as the electron injector to evaluate the scheme. They propose an application in the study of interacting one- and zero-dimensional systems. [ABSTRACT FROM AUTHOR]

We investigate the effect of applied microwave signals on the electron temperature in a surface-acoustic-wave (SAW) device, using a quantum dot as a thermometer. We find that with the continuous application of the radio-frequency signal at ∼3 GHz, the electron temperature rises from a base temperature of 0.5 K to over 1.3 K, even with a power two orders of magnitude smaller than normally used for acoustic charge transport. As a solution to this problem, we show that this heating can be considerably reduced by pulse modulating the SAW. The methods we have developed to allow this type of electron thermometry also allow them to measure the SAW amplitude from the splitting of conductance peaks. [ABSTRACT FROM AUTHOR]

We report on the control of the spontaneous emission rates in InAs self-assembled quantum dots weakly coupled to the mode of a modified H1 defect cavity in a two-dimensional photonic crystal slab. Changes in sample temperature are used to spectrally tune the exciton emission from a single quantum dot to the monopole mode of the microcavity. A Purcell enhancement of the spontaneous emission rate of up to a factor of 11.4 is seen on-resonance, while suppression by up to a factor of 4.4 is seen off-resonance. Also, a two orders of magnitude increase in the intensity of light detected from the exciton is measured when compared to a quantum dot in bulk GaAs. [ABSTRACT FROM AUTHOR]

We have studied the transport properties of a large graphene double quantum dot under the influence of a background disorder potential and a magnetic field. At low temperatures, the evolution of the charge-stability diagram as a function of the B field is investigated up to 10 T. Our results indicate that the charging energy of the quantum dot is reduced, and hence the effective size of the dot increases at a high magnetic field. We provide an explanation of our results using a tight-binding model, which describes the charge redistribution in a disordered graphene quantum dot via the formation of Landau levels and edge states. Our model suggests that the tunnel barriers separating different electron/hole puddles in a dot become transparent at high B fields, resulting in the charge delocalization and reduced charging energy observed experimentally. [ABSTRACT FROM AUTHOR]

We present a thermometry scheme to extract the temperature of a two-dimensional electron gas by monitoring the charge occupation of a weakly tunnel-coupled 'thermometer' quantum dot using a quantum point contact detector. Electronic temperatures between 120 and 951 mK are measured with an accuracy of better than 1 mK in the best case, and agree with the lattice temperature measured by a resistance thermometer. The thermometer is non-invasive and does not pass a current through the electron system being measured. The tuning is independent of temperature, and the device is robust in a magnetic field up to 5.6 T. [ABSTRACT FROM AUTHOR]

We report quantum dots fabricated on very shallow 2-dimensional electron gases, only 30 nm below the surface, in undoped GaAs/AlGaAs heterostructures grown by molecular beam epitaxy. Due to the absence of dopants, an improvement of more than one order of magnitude in mobility (at 2 × 1011 cm-2) with respect to doped heterostructures with similar depths is observed. These undoped wafers can easily be gated with surface metallic gates patterned by e-beam lithography, as demonstrated here from single-level transport through a quantum dot showing large charging energies (up to 1.75 meV) and excited state energies (up to 0.5 meV). [ABSTRACT FROM AUTHOR]

We investigate the effect of a perpendicular magnetic field on the single-particle charging spectrum of a graphene quantum dot embedded inline with a nanoribbon. We observe uniform shifts in the single-particle spectrum which coincide with peaks in the magnetoconductance, implicating Landau level condensation and edge state formation as the mechanism underlying magnetic field-enhanced transmission through graphene nanostructures. The experimentally determined ratio of bulk to edge states is supported by single-particle band-structure simulations, while a fourfold beating of the Coulomb blockade transmission amplitude points to many-body interaction effects during Landau level condensation of the ν=0 state. [ABSTRACT FROM AUTHOR]