March 2022
Abstracts of the QSIT/Quantum Center, ETH Zurich Lunch Seminar, Thursday, March 3, 2022
Quantum optomechanics with a levitated nanoparticle in free space
Massimiliano Rossi – Photonics Laboratory (Novotny group), ETH Zurich
A nanoparticle levitated in electromagnetic fields is a promising testbed for physics at the interface between the classical and the quantum realm. The light-matter interaction between the nanoparticle and the trapping field generates an optomechanical coupling, which provides a way to control the nanosphere motion at the quantum regime. We optically levitate a nanosphere in cryogenic free space and measure interferometrically the scattered light to retrieve information about the center of mass motion. Building from this measurement, we exert a feedback force to cool this motion down to a state with an average occupancy of only 0.65 phonons, close to the quantum ground state. At the same time, the mechanical motion imprints correlations on the quadratures of the scattered light. By performing homodyne state tomography, we reconstruct these correlations, which manifest as ponderomotive squeezing of light quadrature fluctuations below the vacuum noise. The quantum control featured in our platform offers a route to investigating quantum mechanics at macroscopic scales.
Superconducting devices in twisted bilayer graphene
Elías Portolés - Nanophysics Group (Ensslin group), ETH Zurich
Magic-angle twisted bilayer graphene (MATBG) hosts a number of correlated states of matter that can be tuned by electrostatic doping [1]. Superconductivity has drawn considerable attention and the mechanism behind it is a topic of active discussion. MATBG has been experimentally characterized by numerous transport and scanning-probe experiments. The material has also emerged as a versatile platform for superconducting electronics, as proven by the realization of monolithic Josephson junctions [2, 3]. In this talk, I will describe the path towards realizing such junctions as well as the physics they exhibit. Furthermore, I will discuss our recent realization in this material of a SQUID (Superconducting QUantum Interference Device), made of two Josephson junctions [4]. The device allows us to observe magneto-oscillations of the critical current, demonstrating long-range coherence agreeing with an effective charge of 2e for the superconducting charge carriers. We can tune to both asymmetric and symmetric SQUID configurations by electrostatically controlling the critical currents through the junctions. With this tunability, we study the inductances in the device, finding values of up to 2μH. Finally, we directly observe the current-phase relation of one of the Josephson junctions of the device. Our results show that superconducting devices in MATBG can be scaled up and used to reveal properties of the material.
[1] Cao, Y. et al. Unconventional superconductivity in magic-angle graphene superlattices. Nature 556, 43–50 (2018)
[2] de Vries, F.K. et al. Gate-defined Josephson junctions in magic-angle twisted bilayer graphene. Nature Nanotechnology 16, 760–763 (2021)
[3] Rodan-Legrain, D. et al. Highly tunable junctions and non-local Josephson effect in magic-angle graphene tunnelling devices. Nature Nanotechnology 16, 769–775 (2021)
[4] Portolés E. et al. A Tunable Monolithic SQUID in Twisted Bilayer Graphene. arXiv:2201.13276 (2022)