BEGIN:VCALENDAR VERSION:2.0 PRODID:Icfo X-PUBLISHED-TTL:P1W BEGIN:VEVENT UID:69d4b4512ec88 DTSTART:20241118T100000Z SEQUENCE:0 TRANSP:OPAQUE LOCATION:Auditorium and Online (Teams) SUMMARY:ICFO | ROGER TORMO QUERALT CLASS:PUBLIC DESCRIPTION:This thesis describes the design\, fabrication\, and measuremen t of gate-defined quantum dots embedded in suspended carbon nanotubes. The mechanical vibrations of the nanotube couple strongly to the quantum dots by modulating the gate-defined electrostatic potential. The quantum dot c harge states and mechanical vibrations are characterized using transport m easurements and a capacitively coupled superconducting microwave resonator .\nThe samples are fabricated using a novel nanofabrication protocol devel oped within our group\, enabling ultra-clean carbon nanotubes\, as short a s 560 nm\, to be suspended over five gating electrodes. This compact geome try is advantageous for producing near-gigahertz frequency nanotube mechan ical resonators in their ground state of motion at cryostat temperatures o f 10 mK. According to our electrostatic finite-element simulations\, this layout could also enhance the screening of substrate charge fluctuators\, which are a major source of dephasing for quantum dot-based charge qubits. \nThe carbon nanotubes are investigated in a dilution refrigerator at 30 m K. Single\, double\, and triple quantum dots are electrostatically localiz ed within the nanotube through the applied voltages at the gate electrodes . Transport measurements reveal exceptionally high-quality charge stabilit y diagrams in the various quantum dot configurations.\nIn the double quant um dot configuration\, we focus on the inter-dot charge transition where a n electron tunneling between the two dots forms a charge qubit with a gate -tunable energy. The charge qubit is detected by probing a superconducting microwave resonator connected galvanically to a gate electrode and capaci tively coupled to the electric dipole moment of the double quantum dot. Th e microwave cavity consists of a 100 nm thick niobium spiral resonator wit h a resonance frequency of 1.475 GHz and a high characteristic impedance o f 640 &Omega\;. The suspended carbon nanotubes exhibit high-frequency\, hi ghly coherent mechanical modes with typical resonance frequencies ranging from tens to hundreds of MHz. Electron tunneling through the quantum dots embedded in the resonator exerts a back-action force on the mechanical mot ion\, resulting in a mechanical resonance frequency softening. Frequency s oftenings of up to 20% were observed in the first flexural mode of two dis tinct devices\, characterized by the frequencies of 240 MHz and 35 MHz. Fr om the frequency softening\, an electromechanical coupling of 920 MHz and 500 MHz was estimated for each device\, demonstrating operation in the dee p ultrastrong coupling regime. However\, the coupling mechanism in this sy stem relies on an incoherent tunneling process\, which precludes its appli cation in any quantum protocol. The second flexural mode of the carbon nan otube\, characterized by 515 MHz\, presents electromechanical couplings up to 410 MHz when coupled to a charge qubit transition\, confirming operati on in the ultrastrong coupling regime. This significant electromechanical coupling induces non-linear effects down to motional amplitudes roughly te n times the nanotube&rsquo\;s zero-point motion\, which are detected using the microwave cavity that is sensitive to the squared displacement of the mechanical resonator. \; The large non-linearities observed in these devices near the mechanical ground state highlight the potential of carbon nanotube electromechanical systems for quantum applications\, including n anomechanical qubits\, expanded quantum motion delocalisation\, and quantu m sensing.\n \;\nMonday November 18\, 11:00h. ICFO Auditorium \nThesis Director: Prof. Dr. Adrian Bachtold and Dr. Christoffer Moller DTSTAMP:20260407T073753Z END:VEVENT END:VCALENDAR