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Séminaire MCBT : mardi 4 octobre 2022 à 11h00

Ilya Golokolenov

Titre : Single mesoscopic phononic mode thermodynamics


Institut Néel, Salle E424
Résumé : In recent decades, the laws of thermodynamics have been pushed down to smaller and smaller scales, within the theoretical field of stochastic thermodynamics and state-of-art experiments performed on microfabricated mesoscopic systems. But these measurements concern mostly thermal properties of electrons and photons. In this talk we report on the measurements of thermal fluctuations of a single mechanical mode in-equilibrium with a heat reservoir. The device under study is a nanomechanical beam with a first flexural mode resonating at 3.8 MHz, cooled down to temperatures in the range from 100 mK to 400 mK. The technique is constructed around a microwave opto-mechanical setup using a cryogenic High Electron Mobility Transistor (HEMT), and is based on two parametric amplifications implemented in series: an in-built opto-mechanical ‘blue-detuned’ pumping plus a Traveling Wave Parametric Amplifier (TWPA) stage. We demonstrate our ability to resolve energy fluctuations of the mechanical mode in real-time up to the fastest relevant speed given by the mechanical relaxation rate. The energy probability distribution is then exponential, matching the expected Boltzmann distribution. The variance of fluctuations is found to be k_B T^2 with no free parameters. Our microwave detection floor is about 3 Standard Quantum Limit (SQL) at 6 GHz; the resolution of our fastest acquisition tracks reached about 100 phonons, and is directly related to the rather poor opto-mechanical coupling of the device (g_0/(2pi) = 0.5Hz). This result is deeply in the classical regime, but shall be extended to the quantum case in the future with systems presenting a much larger g_0 (up to 2pi*250 Hz), potentially reaching the resolution of a single mechanical quantum. We believe that it will open a new experimental field: phonon-based quantum stochastic thermodynamics, with fundamental implications for quantum heat transport and macroscopic mechanical quantum coherence.