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


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

    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.


  • Microwave Photodetection in an Ultraclean Suspended Bilayer Graphene p–n Junction
    M. Jung, P. Rickhaus, S. Zihlmann, P. Makk, and C. Schönenberger.
    Nano Letters, 16(11):6988-6993, nov 2016. [DOI] arXiv:1702.01529

    We explore the potential of bilayer graphene as a cryogenic microwave photodetector by studying the microwave absorption in fully suspended clean bilayer graphene p–n junctions in the frequency range of 1–5 GHz at a temperature of 8 K. We observe a distinct photocurrent signal if the device is gated into the p–n regime, while there is almost no signal for unipolar doping in either the n–n or p–p regimes. Most surprisingly, the photocurrent strongly peaks when one side of the junction is gated to the Dirac point (charge-neutrality point CNP), while the other remains in a highly doped state. This is different to previous results where optical radiation was used. We propose a new mechanism based on the phototermal effect explaining the large signal. It requires contact doping and a distinctly different transport mechanism on both sides: one side of graphene is ballistic and the other diffusive. By engineering partially diffusive and partially ballistic devices, the photocurrent can drastically be enhanced.

  • Subgap resonant quasiparticle transport in normal-superconductor quantum dot devices
    J. Gramich, A. Baumgartner, and C. Schönenberger.
    Applied Physics Letters, 108(17):172604, apr 2016. [DOI] arXiv:1601.00672

    We report thermally activated transport resonances for biases below the superconducting energy gap in a carbon nanotube quantum dot (QD) device with a superconducting Pb and a normal metal contact. These resonances are due to the superconductor’s finite quasi-particle population at elevated temperatures and can only be observed when the QD life-time broadening is considerably smaller than the gap. This condition is fulfilled in our QD devices with optimized Pd/Pb/In multi-layer contacts, which result in reproducibly large and “clean” superconducting transport gaps with a strong conductance suppression for subgap biases. We show that these gaps close monotonically with increasing magnetic field and temperature. The accurate description of the subgap resonances by a simple resonant tunneling model illustrates the ideal characteristics of the reported Pb contacts and gives an alternative access to the tunnel coupling strengths in a QD.

  • Cooper-Paare tunneln durch einen Quantenpunkt
    Andreas Baumgartner, Jörg Gramich, and Christian Schönenberger.
    Physik in unserer Zeit, 47(2):62-63, mar 2016. [DOI]

    Elektronische Bauteile aus Supraleitern und Quantenpunkten zeigen eine Vielzahl von neuen und fundamentalen physikalischen Eigenschaften und stellen neue quantentechnologische Anwendungen in Aussicht. Kuerzlich ist es gelungen, den wohl grundlegendsten Transportprozess in einer solchen Struktur in Experimenten zu identifizieren, naemlich den direkten Transport von Elektronen aus einem Supraleiter durch einen Quantenpunkt, das sogenannte Andreev-Tunneln. Das Verstaendnis dieses Prozesses liefert die Grundlage fuer zukuenftige Anwendungen, die quantenmechanische Phaenomene in elektronischen Bauteilen ausnutzen werden.


  • Shot Noise of a Quantum Dot Measured with Gigahertz Impedance Matching
    T. Hasler, M. Jung, V. Ranjan, G. Puebla-Hellmann, A. Wallraff, and C. Schönenberger.
    Physical Review Applied, 4(5):54002, nov 2015. [DOI] arXiv:1507.04884.pdf

    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 carbonnanotube quantum dot at a frequency close to 3 GHz in an efficient way. As compared to wideband 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 kOmega. 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 as an enhancement in specific regions

  • Gigahertz Quantized Charge Pumping in Bottom-Gate-Defined InAs Nanowire Quantum Dots
    S. d’Hollosy, M. Jung, A. Baumgartner, V. A. Guzenko, M. H. Madsen, J. Nygård, and C. Schönenberger.
    Nano Letters, 15(7):4585-4590, jul 2015. [DOI] arXiv:1509.01574

    Semiconducting nanowires (NWs) are a versatile, highly tunable material platform at the heart of many new developments in nanoscale and quantum physics. Here, we demonstrate charge pumping, that is, the controlled transport of individual electrons through an InAs NW quantum dot (QD) device at frequencies up to 1.3 GHz. The QD is induced electrostatically in the NW by a series of local bottom gates in a state of the art device geometry. A periodic modulation of a single gate is enough to obtain a dc current proportional to the frequency of the modulation. The dc bias, the modulation amplitude and the gate voltages on the local gates can be used to control the number of charges conveyed per cycle. Charge pumping in InAs NWs is relevant not only in metrology as a current standard, but also opens up the opportunity to investigate a variety of exotic states of matter, for example, Majorana modes, by single electron spectroscopy and correlation experiments.

  • 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, may 2015. [DOI] arXiv:1505.04681

    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.

  • Entanglement Detection with Non-Ideal Ferromagnetic Detectors
    P. Rozek, P. Busz, W. Klobus, D. Tomaszewski, A. Grudka, A. Baumgartner, C. Schönenberger, and J. Martinek.
    Acta Physica Polonica A, 127(2):493-495, feb 2015. [DOI]

    Entangled states are essential in basics quantum communication protocols and quantum cryptography. Ferromagnetic contacts can work as a spin detector, giving possibility of converting information about electron spin to the electric charge, and therefore, detection of entangled states with the electric current measurements is possible. Method of conrming entanglement with non-ideal detectors is presented, the impact of decoherence and noise on states and quality of entanglement is discussed. Entanglement witness (EW) operator method is compared with the CHSH inequalities approach. Required spin polarization for the EW is lower than for the CHSH inequalities. System with asymmetric spin polarizations of detectors was analyzed, including the CHSH inequalities and the EW method.


  • Local electrical tuning of the nonlocal signals in a Cooper pair splitter
    G. Fülöp, S. d’Hollosy, A. Baumgartner, P. Makk, V. A. Guzenko, M. H. Madsen, J. Nygård, C. Schönenberger, and S. Csonka.
    Physical Review B, 90:235412, dec 2014. [DOI] arXiv:1409.0818

    A Cooper pair splitter consists of a central superconducting contact, S, from which electrons are injected into two parallel, spatially separated quantum dots (QDs). This geometry and electron interactions can lead to correlated electrical currents due to the spatial separation of spin-singlet Cooper pairs from S. We present experiments on such a device with a series of bottom gates, which allows for spatially resolved tuning of the tunnel couplings between the QDs and the electrical contacts and between the QDs. Our main findings are gate-induced transitions between positive conductance correlation in the QDs due to Cooper pair splitting and negative correlations due to QD dynamics. Using a semi-classical rate equation model we show that the experimental findings are consistent with in-situ electrical tuning of the local and nonlocal quantum transport processes. In particular, we illustrate how the competition between Cooper pair splitting and local processes can be optimized in such hybrid nanostructures.

  • Entanglement witnessing and quantum cryptography with nonideal ferromagnetic detectors
    W. Kobus, A. Grudka, A. Baumgartner, D. Tomaszewski, C. Schönenberger, and Jan Martinek.
    Phys. Rev. B, 89:125404, mar 2014. [DOI] arXiv:1310.5640

    We investigate theoretically the use of non-ideal ferromagnetic contacts as a mean to detect quantum entanglement of electron spins in transport experiments. We use a designated entanglement witness and find a minimal spin polarization of η>1/3–√≈58 required to demonstrate spin entanglement. This is significantly less stringent than the ubiquitous tests of Bell’s inequality with η>1/2–√4≈84. In addition, we discuss the impact of decoherence and noise on entanglement detection and apply the presented framework to a simple quantum cryptography protocol. Our results are directly applicable to a large variety of experiments.

  • 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, jan 2014. [DOI] arXiv:1401.0381

    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.