Achieving strong mechanical nonlinearities, minimally invasive detection, and control at the few-phonon level is a
central challenge in the development of mechanical oscillators for quantum technologies, including quantum
information processing (1,2), precision sensing (3), and tests of quantum mechanics (4,5). In our group, we aim to
realize these capabilities using suspended carbon-nanotube mechanical oscillators coupled to electronic quantum
dots. In this talk, I will present results obtained with a device operating in the exotic dispersive ultrastrong-coupling
regime (6,7), where the interaction strength between a nanotube mechanical oscillator and a double-quantum-dot
electronic two-level system (DQD-ETLS) exceeds the bare energy of the oscillator. In this regime, we demonstrate a
mechanical Kerr oscillator with an anharmonicity exceeding the state of the art for mechanical systems by four orders
of magnitude (8). We read out the mechanical states using a superconducting cavity coupled to the square
displacement (x²) of the oscillator (8), paving the way towards future quantum non-demolition (QND) cavity-based
readout of mechanical Fock states (9). I will also show that the decay and decoherence rates of our ETLS charge qubit
outperforms the current state of the art for 2DEG-based systems, reaching the highest coherence values ever
measured in a charge-based DQD-ETLS (10).



[1] A. D. O’Connell, et al. Nature 464 (2010)

[2] Y. Yang, et al. A mechanical qubit, 386 (6723) (2024)

[3] F. Pistolesi, et al. Phys. Rev. X, 11, 031027 (2021)

[4] M.F. Gely et al., AVS Quantum Sci. 3, 035601 (2021)

[5] Oriol Romero-Isart. et, al. Physical Review A, 84 (2011)

[6] C. Samanta et al, Nat. Phys. 19 (2023)

[7] P. Forn-Díaz et al., Rev. Mod. Phys (2019)

[8] C.B. Moller*, R.Tormo-Queralt* (under review)
[9] P. Arrangoiz-Ariola, Nature volume 571 (2019)

[10] P. Scarlino Phys. Rev. Lett. 122, 206802 (2019)