In the framework of the ERC advanced research project QUEST, we have developed a versatile high-frequency setup that works in the frequency window 1-10 GHz and allows for high-resolution reflectance and noise measurements. In order to apply this to high impedance devices, such as quantum dots, we have developed impedance matching circuits based on on-chip coplanar transmission line resonators, so called stub-tuners, as well as LC circuits.1 Recently, a second setup has been added. The new high-frequency setups are very powerful as they allows us to measure the radiation emitted from a device, i.e. to do noise measurements and noise correlations. But additionally, they also enables us to measure the ac admittance of a device at GHz frequencies and to perform dispersive readout of double quantum-dot charge qubits.2 First, due to the high frequency a large bandwidth is available, allowing for fast measurements. On top of this, the rf measurement is also sensitive to the capacitive / inductive part of a device, which can conveniently be measured by monitoring frequency shifts.2 We have embarked on this and have studied simple QDs, double QDs and recently also graphene pn junctions.3 In addition, we are also in the position to drive devices at high frequency. We have, for example, demonstrated charge pumping in a SNW QD device at GHz frequency.4

These pictures show a view into a interior of a dry dilution refrigerator from BlueFors Cryogenics, which we use for high-frequency measurements. There are three measurement stations, one for 1-1.5 GHz, the other for 2.5-4.0 GHz, and the latter for 4.0-6.0 GHz, that can be used for reflectometry and noise measurements.
  1. T. Hasler, M. Jung, V. Ranjan, G. Puebla-Hellmann, A. Wallraff and CS, Phys. Rev. Appl. 4 (5), 054002 (2015).
  2. V. Ranjan, G. Puebla-Hellmann, M. Jung, T. Hasler, A. Nunnenkamp, M. Muoth, C. Hierold, A. Wallraff and CS, Nat. Commun. 6, 7165 (2015).
  3. V. Ranjan, S. Zihlmann, P. Makk, K. Watanabe, T. Taniguchi, C. Schönenberger, Phys. Rev. Appl. 7, 54015 (2017).
  4. S. d’Hollosy, M. Jung, A. Baumgartner, V. A. Guzenko, M. H. Madsen, J. Nygard and CS, Nano Lett. 15 (7), 4585-4590 (2015).

Funding: ERC-QUEST     keyword: QUEST

2018
  • Blocking-state influence on shot noise and conductance in quantum dots
    M. -C. Harabula, V. Ranjan, R. Haller, G. Fülöp, and C. Schönenberger.
    Phys. Rev. B  97, 115403 (2018)
    [arXiv:1801.00286 ] [Abstract ]

    Quantum dots (QDs) investigated through electron transport measurements often exhibit varying, state-dependent tunnel couplings to the leads. Under speci c conditions, weakly coupled states can result in a strong suppression of the electrical current and they are correspondingly called blocking states. Using the combination of conductance and shot noise measurements, we investigate blocking states in carbon nanotube (CNT) QDs. We report negative di erential conductance and super- Poissonian noise. The enhanced noise is the signature of electron bunching, which originates from random switches between the strongly and weakly conducting states of the QD. Negative differential conductance appears here when the blocking state is an excited state. In this case, at the threshold voltage where the blocking state becomes populated, the current is reduced. Using a master equation approach, we provide numerical simulations reproducing both the conductance and the shot noise pattern observed in our measurements.

2017
  • Contactless Microwave Characterization of Encapsulated Graphene p-n Junctions
    V. Ranjan, S. Zihlmann, P. Makk, K. Watanabe, T. Taniguchi, and C. Schönenberger.
    Phys. Rev. Appl.  7(5), 54015 (2017)
    [arXiv:1702.02071 ] [Abstract ]

    Accessing intrinsic properties of a graphene device can be hindered by the influence of contact electrodes. Here, we capacitively couple graphene devices to superconducting resonant circuits and observe clear changes in the resonance-frequency and -widths originating from the internal charge dynamics of graphene. This allows us to extract the density of states and charge relaxation resistance in graphene p-n junctions without the need of electrical contacts. The presented characterizations pave a fast, sensitive and non-invasive measurement of graphene nanocircuits.

2015
  • Clean carbon nanotubes coupled to superconducting impedance-matching circuits
    V. Ranjan, G. Puebla-Hellmann, M. Jung, T. Hasler, A. Nunnenkamp, M. Muoth, C. Hierold, A. Wallraff, and C. Schönenberger.
    Nature Communications  6, 7165 (2015)
    [arXiv:1505.04681 ] [Abstract ]

    Coupling carbon nanotube devices to microwave circuits offers a significant increase in bandwidth 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-bandwidth noise correlation measurements on high impedance devices such as quantum dot circuits.

2014
  • Electrolyte gate dependent high-frequency measurement of graphene field-effect transistor for sensing applications
    W. Fu, El M. Abbassi, T. Hasler, M. Jung, M. Steinacher, M. Calame, C. Schönenberger, G. Puebla-Hellmann, S. Hellmüller, T. Ihn, and A. Wallraff.
    Appl. Phys. Lett.  104, 13102 (2014)
    [arXiv:1401.0381 ] [Abstract ]

    We performed radiofrequency (RF) reflectometry measurements at 2.4 GHz on electrolyte-gated graphene field-effect transistors, 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.