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Suspended carbon nanotubes coupled to superconducting circuits

Ben H. Schneider

Published Jan 1, 2014• 0 citations
Publisher

Abstract

Carbon nanotubes are unique candidates to study quantum mechanical properties of a nanomechanical resonator. However to access this quantum regime, present detectors are not yet sensitive enough. In this thesis we couple a carbon nanotube CNT mechanical resonator to a superconducting circuit which is a root towards such sensitivity.
DOI
10.4233/uuid:dd1874d2-99d8-4e28-a3f6-272055ac01cd

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The quantum behaviour of mechanical resonators is a new and emerging field driven by recent experiments reaching the quantum ground state. The high frequency, small mass, and large quality-factor of carbon nanotube resonators make them attractive for quantum nanomechanical applications. A common element in experiments achieving the resonator ground state is a second quantum system, such as coherent photons or a superconducting device, coupled to the resonators motion. For nanotubes, however, this is a challenge due to their small size. Here, we couple a carbon nanoelectromechanical (NEMS) device to a superconducting circuit. Suspended carbon nanotubes act as both superconducting junctions and moving elements in a Superconducting Quantum Interference Device (SQUID). We observe a strong modulation of the flux through the SQUID from displacements of the nanotube. Incorporating this SQUID into superconducting resonators and qubits should enable the detection and manipulation of nanotube mechanical quantum states at the single-phonon level.
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Hybrid circuit quantum electrodynamics (QED) involves the study of coherent quantum physics in solid state systems via their interactions with superconducting microwave circuits.Here we present a crucial step in the implementation of a hybrid superconducting qubit that employs a carbon nanotube as a Josephson junction.We realise the junction by contacting a carbon nanotube with a superconducting Pd/Al bi-layer, and implement voltage tunability of the quantum circuit's frequency using a local electrostatic gate.We demonstrate a strong dispersive coupling to a coplanar waveguide resonator by investigating the gate-tunable resonator frequency.We extract qubit parameters from spectroscopy using dispersive readout and find qubit relaxation and coherence times in the range of 10 -200 ns.
This thesis presents an effort towards integration of superconductor-contacted carbon nanotubes into microwave circuits, as a new tool for building quantum circuits. Carbon nanotubes show a wealth of quantum phenomena demonstrated in transport experiments by different groups in the last two decades. With the development of circuit QED, one can study these phenomena using on-chip microwave circuits. Moreover, in superconductor-contacted nanotubes, Andreev reflection of the particles at the interfaces leads to the formation of Andreev bound states. A pair of such states can potentially serve as a new type of a qubit. Here, fabrication and measurement techniques of superconductor contacted carbon nanotubes and superconductor/normal metal bilayer microwave circuits is developed, and their behaviour is studied. In superconductor-contacted nanotubes the full range of contact transparencies is investigated, showing different transport phenomena such as Coulomb blockade, Kondo effect and Fabry-Pérot interference. In addition, the interplay of these effects with multiple Andreev reflection phenomena and the superconducting proximityinduced non-dissipative current transport through the nanotube is studied. The proximity effect is studied using microwave resonators made of superconductor/ normal metal bilayers. Characteristic parameters of the bilayers, such as the critical temperature and the magnetic penetration depth are reliably extracted from the measurements. Finally, the two experiments are brought together in the first step towards using superconductor-contacted nanotubes in quantum microwave circuits. The successful realization of such circuit is achieved, where the Coulomb blockade in a nanotube quantum dot is probed with a microwave field.
Due to their small mass and large mechanical stiffness, free-standing carbon nanotubes turn out to be superb high-frequency mechanical resonators that could find applications in mass and force sensing. With resonance frequencies up to the gigahertz regime, one can envision cooling the mechanical motion of such nanotubes to its quantum mechanical ground state at cryogenic temperatures. This opens the possibility to generate highly nonclassical states of the mechanical motion, similar to those created several years ago with electromagnetic radiation in superconducting microwave resonators. Such states are interesting from a fundamental physics point of view, but potentially also for quantum information and sensing applications. While classical activation of a harmonic oscillator will always produce ``quasiclassical'' coherent states, a necessary ingredient for the production of nonclassical states such as Fock states is a nonlinear element, e.g., superconducting qubits in the case of microwaves. This work predicts that for mechanical oscillations, the spin-orbit coupling of the mechanical motion to a single localized electron spin can do the trick by acting as a nonlinear element for phonons.