May 2025

Abstracts of the Quantum Center Lunch Seminar

Date: Thursday, May 8, 2025
Place: ETH Zurich, Hönggerberg, HPF G 6
Time: 12:00 - 13:30

Accelerated State Expansion of a Nanoparticle in a Dark Inverted Potential  

Gregoire Tomassi - Nanophotonic Systems Laboratory (Quidant Group), ETH Zurich

While the wave packet of a massive particle grows linearly under free dynamics, it grows exponentially in an inverted harmonic potential, offering a pathway to rapidly increase quantum fluctuations to macroscopic dimensions. In this work, we experimentally demonstrate this principle by expanding the center-of-mass thermal state of a 125 nm silica nanoparticle to a position uncertainty of 43.4 nm within 260 μs. This expansion, achieved using an inverted dark potential to minimize decoherence from photon recoil, represents a 952-fold increase, reaching a scale comparable to the nanoparticle’s physical size. This work represents a key advancement toward preparing macroscopic quantum superpositions at unprecedented mass and length scales.

Suspended carbon nanotube quantum dot heat engines

Frederik van Veen - Quantum Devices Group (Perrin Group), ETH Zurich

Low-dimensional nanoscale materials have long been proposed as ideal thermoelectric building blocks1. Recent studies have demonstrated excellent thermoelectric performance and high conversion efficiencies using quantum dots in particle-exchange heat engines2. In this work, we demonstrate the operation of a particle-exchange quantum dot heat engine based on ultra-clean, suspended carbon nanotubes (CNTs). Using Raman spectroscopy, we select pristine, individual CNTs and mechanically transfer them onto thermoelectric devices equipped with embedded heater lines and four-point resistance thermometers. Cryogenic transport measurements over a wide electrostatic gate range confirm the exceptional cleanliness of our devices and the high quality of the CNTs, as evidenced by clear observations of four-fold symmetry and distinct electronic coupling regimes (Coulomb blockade, Kondo, and Fabry-Perot). We measure the steady-state power output of the CNT heat engine through external load resistors across a broad range of energy levels. This allows us to experimentally determine the optimal electronic coupling conditions necessary for maximising quantum dot heat engine performance. Furthermore, we observe a significant enhancement in thermoelectric power output due to the four-fold degeneracy of CNTs, far surpassing values reported in previous studies2. Our findings underline the critical role of entropy in enhancing thermoelectric engine efficiency. Finally, we systematically explore heat engine performance across an extensive temperature range and investigate how vibrational excited states influence thermoelectric transport.

References:
1. Hicks, L. D. & Dresselhaus, M. S. Phys. Rev. B 47, 12727–12731 (1993)
2. M. Josefsson et al., Nature Nanotechnology 12, 920-924 (2018)