Shot noise of a quantum dot measured with GHz stub impedance matching

The demand for a fast high-frequency read-out of high impedance devices, such as quantum dots, necessitates impedance matching. Here we use a resonant impedance matching circuit (a stub tuner) realized by on-chip superconducting transmission lines to measure the electronic shot noise of a carbon nanotube quantum dot at a frequency close to 3 GHz in an efficient way. As compared to wide-band detection without impedance matching, the signal to noise ratio can be enhanced by as much as a factor of 800 for a device with an impedance of 100 k$\Omega$. The advantage of the stub resonator concept is the ease with which the response of the circuit can be predicted, designed and fabricated. We further demonstrate that all relevant matching circuit parameters can reliably be deduced from power reflectance measurements and then used to predict the power transmission function from the device through the circuit. The shot noise of the carbon nanotube quantum dot in the Coulomb blockade regime shows an oscillating suppression below the Schottky value of $2eI$, as well an enhancement in specific regions.Comment: 6 pages, 4 figures, supplementar

Optimization of sample-chip design for stub-matched radio-frequency reflectometry measurements

A radio-frequency (rf) matching circuit with an in situ tunable varactor diode used for rf reflectometry measurements in semiconductor nanostructures is investigated and used to optimize the sample-specific chip design. The samples are integrated in a 2-4 GHz stub-matching circuit consisting of a waveguide stub shunted to the terminated coplanar waveguide. Several quantum point contacts fabricated on a GaAs/AlGaAs heterostructure with different chip designs are compared. We show that the change of the reflection coefficient for a fixed change in the quantum point contact conductance can be enhanced by a factor of 3 compared to conventional designs by a suitable electrode geometry

Electrolyte gate dependent high-frequency measurement of graphene field-effect transistor for sensing applications

We performed radiofrequency (RF) reflectometry measurements at 2.4 GHz on electrolyte-gated graphene field-effect transistors (GFETs) utilizing a tunable stub-matching circuit for impedance matching. We demonstrate that the gate voltage dependent RF resistivity of graphene can be deduced even in the presence of the electrolyte which is in direct contact with the graphene layer. The RF resistivity is found to be consistent with its DC counterpart in the full gate voltage range. Furthermore, in order to access the potential of high-frequency sensing for applications, we demonstrate time-dependent gating in solution with nanosecond time resolution.Comment: 14 pages, 4 figure

Clean carbon nanotubes coupled to superconducting impedance-matching circuits.

Coupling carbon nanotube devices to microwave circuits offers a significant increase in bandwidth (BW) and signal-to-noise ratio. These facilitate fast non-invasive readouts important for quantum information processing, shot noise and correlation measurements. However, creation of a device that unites a low-disorder nanotube with a low-loss microwave resonator has so far remained a challenge, due to fabrication incompatibility of one with the other. Employing a mechanical transfer method, we successfully couple a nanotube to a gigahertz superconducting matching circuit and thereby retain pristine transport characteristics such as the control over formation of, and coupling strengths between, the quantum dots. Resonance response to changes in conductance and susceptance further enables quantitative parameter extraction. The achieved near matching is a step forward promising high-BW noise correlation measurements on high impedance devices such as quantum dot circuits.We acknowledge financial support by the ERC project QUEST, the EC project SE2ND, the NCCR QSIT and the Swiss National Science Foundation.This is the author accepted manuscript. The final version is available from NPG via http://dx.doi.org/10.1038/ncomms816

Fast scanning nitrogen-vacancy magnetometry by spectrum demodulation

We demonstrate a spectrum demodulation technique for greatly speeding up the data acquisition rate in scanning nitrogen-vacancy center magnetometry. Our method relies on a periodic excitation of the electron spin resonance by fast, wide-band frequency sweeps combined with a phase-locked detection of the photo-luminescence signal. The method can be extended by a frequency feedback to realize real-time tracking of the spin resonance. Fast scanning magnetometry is especially useful for samples where the signal dynamic range is large, of order millitesla, like for ferro- or ferrimagnets. We demonstrate our method by mapping stray fields above the model antiferromagnet $\alpha$-Fe$_2$O$_3$ (hematite) at pixel rates of up to 100\,Hz and an image resolution exceeding one megapixel.Comment: 11 pages, 5 figure

Metallic nanoparticle contacts for high-yield, ambient-stable molecular-monolayer devices

Accessing the intrinsic functionality of molecules for electronic applications; 1-3; , light emission; 4; or sensing; 5; requires reliable electrical contacts to those molecules. A self-assembled monolayer (SAM) sandwich architecture; 6; is advantageous for technological applications, but requires a non-destructive, top-contact fabrication method. Various approaches ranging from direct metal evaporation; 6; over poly(3,4-ethylenedioxythiophene) polystyrene sulfonate; 7; (PEDOT:PSS) or graphene; 8; interlayers to metal transfer printing; 9; have been proposed. Nevertheless, it has not yet been possible to fabricate SAM-based devices without compromising film integrity, intrinsic functionality or mass-fabrication compatibility. Here we develop a top-contact approach to SAM-based devices that simultaneously addresses all these issues, by exploiting the fact that a metallic nanoparticle can provide a reliable electrical contact to individual molecules; 10; . Our fabrication route involves first the conformal and non-destructive deposition of a layer of metallic nanoparticles directly onto the SAM (itself laterally constrained within circular pores in a dielectric matrix, with diameters ranging from 60 nanometres to 70 micrometres), and then the reinforcement of this top contact by direct metal evaporation. This approach enables the fabrication of thousands of identical, ambient-stable metal-molecule-metal devices. Systematic variation of the composition of the SAM demonstrates that the intrinsic molecular properties are not affected by the nanoparticle layer and subsequent top metallization. Our concept is generic to densely packed layers of molecules equipped with two anchor groups, and provides a route to the large-scale integration of molecular compounds into solid-state devices that can be scaled down to the single-molecule level