A. J. Rimberg

Controlled deposition of individual single-walled carbon nanotubes on chemically functionalized templates

Carbon nanotubes offer great promise as molecular wires because they exhibit high electrical conductivity and chemical stability. However, constructing nanotube-based electronic devices requires a controlled means of assembling the tubes. We report procedures both for producing individual short SWNT segments and for their reliable deposition on chemically functionalized nanolithographic templates. Using this method, we have positioned individual nanotubes at specific locations and orientations in such a way that the nanotubes contact metal electrodes. This discovery is potentially very important for fabrication of simple electrical circuits with nanotubes, and provides a new tool for study of electron transport in nanotubes. q 1999 Elsevier Science B.V. All rights reserved.

Real-time detection of electron tunnelling in a quantum dot

Nanostructures in which strong (Coulomb) interactions exist between electrons are predicted to exhibit temporal electronic correlations. Although there is ample experimental evidence that such correlations exist, electron dynamics in engineered nanostructures have been observed directly only on long timescales. The faster dynamics associated with electrical currents or charge fluctuations are usually inferred from direct (or quasi-direct) current measurements. Recently, interest in electron dynamics has risen, in part owing to the realization that additional information about electronic interactions can be found in the shot noise or higher statistical moments of a direct current. Furthermore, interest in quantum computation has stimulated investigation of quantum bit (qubit) readout techniques, which for many condensed-matter systems ultimately reduces to single-shot measurements of individual electronic charges. Here we report real-time observation of individual electron tunnelling events in a quantum dot using an integrated radio-frequency single-electron transistor. We use electron counting to measure directly the quantum dots tunnelling rate and the occupational probabilities of its charge state. Our results provide evidence in favour of long (10 µs or more) inelastic scattering times in nearly isolated dots.

Analogue Hawking Radiation in a dc-SQUID Array Transmission Line

We propose the use of a superconducting transmission line formed from an array of direct-current superconducting quantum interference devices for investigating analogue Hawking radiation. Biasing the array with a space-time varying flux modifies the propagation velocity of the transmission line, leading to an effective metric with a horizon. Being a fundamentally quantum mechanical device, this setup allows for investigations of quantum effects such as backreaction and analogue space-time fluctuations on the Hawking process.

Quantum Hall effect in InAs/AlSb quantum wells

We demonstrate via low‐temperature electron transport measurements the realization of a high‐mobility (≳300 000 cm2/V s) two‐dimensional electron gas in unintentionally doped InAs/AlSb single 120 A quantum wells grown on GaAs substrates by molecular beam epitaxy. Magnetoresistance and Hall measurements at T∼0.4 K show a well‐formed quantum Hall effect, with effects due to spin splitting observed at filling factors as high as ν=17. The electron densities of these wells could be reduced by a factor ∼5 by using the negative persistent photoconductivity of these samples.

Introduction of a dc bias into a high-Q superconducting microwave cavity

We report a technique for applying a dc voltage or current bias to the center conductor of a high-quality factor superconducting microwave cavity without significantly disturbing selected cavity modes. This is accomplished by incorporating dc bias lines into the cavity at specific locations. The measured S-matrix parameters of the system are in good agreement with theoretical predictions and simulations. We find that at 4 K the quality factor of the cavity degrades by less than 1% under the application of a dc bias.

A cavity-Cooper pair transistor scheme for investigating quantum optomechanics in the ultra-strong coupling regime

We propose a scheme involving a Cooper pair transistor (CPT) embedded in a superconducting microwave cavity, where the CPT serves as a charge tunable quantum inductor to facilitate ultra-strong coupling between photons in the cavity and a nano- to meso-scale mechanical resonator. The mechanical resonator is capacitively coupled to the CPT, such that mechanical displacements of the resonator cause a shift in the CPT inductance and hence the cavityʼs resonant frequency. The amplification provided by the CPT is sufficient for the zero point motion of the mechanical resonator alone to cause a significant change in the cavity resonance. Conversely, a single photon in the cavity causes a shift in the mechanical resonator position on the order of its zero point motion. As a result, the cavity-Cooper pair transistor coupled to a mechanical resonator will be able to access a regime in which single photons can affect single phonons and vice versa. Realizing this ultra-strong coupling regime will facilitate the creation of non-classical states of the mechanical resonator, as well as the means to accurately characterize such states by measuring the cavity photon field.

Single-electron transistor strongly coupled to an electrostatically defined quantum dot

A system is described in which an Al-based superconducting single-electron transistor (S–SET) is fabricated directly above an electrostatically defined quantum dot formed in a two-dimensional electron gas. This arrangement allows the coupling capacitance between the S–SET central island and the dot to be comparable to the tunneling capacitances of the dot. As a result, the measured sensitivity of the S–SET referred to charge on the dot is 1.2×10−4 e/Hz, about an order of magnitude better than previously reported results. The increased sensitivity makes this system ideally suited for broadband measurements such as study of individual tunneling events.

Realization of a single-Cooper-pair Josephson laser

We have embedded a voltage-biased Cooper-pair transistor (CPT) in a high-Q superconducting microwave cavity. When the energy given to a tunneling Cooper pair by the voltage bias is equal to a multiple of the cavity photon frequency, the cavity is pumped to a strongly nonequilibrium state. The cavity photons act back on the CPT, allowing us to enter a regime of strongly correlated electronic-photonic transport. We directly observe the effects of photonic backaction on Cooper-pair transport, and see clear evidence for single-emitter lasing in the form of emission dominated by stimulated transport processes.

LOW-DENSITY HIGH-MOBILITY ELECTRON GAS IN WIDE PARABOLIC GAAS/ALXGA1-XAS WELLS

Remotely doped wide parabolic GaAs/AlxGa1−xAs wells are used to create thick (≳ 1000 A) layers of high‐mobility (≳ 2×105 cm2/V s) electron gas with three‐dimensional densities below (by a factor ∼3) the metal‐insulator transition for doped GaAs. The temperature dependences of the Hall mobility and sheet density show no qualitative changes in a series of three samples spanning the metal‐insulator transition. Shubnikov–de Haas oscillation measurements are used to determine the width of the electron gas layers.

Universal Quantum Fluctuations of a Cavity Mode Driven by a Josephson Junction

We analyze the quantum dynamics of a superconducting cavity coupled to a voltage-biased Josephson junction. The cavity is strongly excited at resonances where the voltage energy lost by a Cooper pair traversing the circuit is a multiple of the cavity photon energy. We find that the resonances are accompanied by substantial squeezing of the quantum fluctuations of the cavity over a broad range of parameters and are able to identify regimes where the fluctuations in the system take on universal values.