2pm - 3pm BST
Friday 21 October 2022
Loschmidt echoes evidence of the emergence of irreversibility for quantum many-body dynamics in the thermodynamic limit
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The justification and appropriate description of irreversible macroscopic dynamics of fluids from time reversible mechanics was initiated by Boltzmann, Loschmidt, Einstein and Smoluchowski. However, in spite of the impressive experimental advances and clarification of the statistical assumptions, the possibility of an arrow of time has remained a polemic issue. Nowadays, the focus shifted towards the quantum realm, mainly because quantum information processing handles an increasing number of qubits/spins and AdS/CFT unification of gravitation with quantum mechanics requires quantum chaos in the vicinity of a black hole . Indeed, Einstein observed that discrete quantum level is not compatible with the classical instabilities conceived by Boltzmann in his “stothaltzansatz”.
More than two decades ago we realised [1-3], at Córdoba, that Solid-State Nuclear Magnetic Resonance could serve to identify the quantum signatures in, otherwise “classical”, spin diffusion. Also, it could implement, for a macroscopic number (1023) of interacting spins, a time reversal procedure, just as suggested by Loschmidt to tease Boltzmann. With this purpose we developed a number of experimental strategies, the Loschmidt Echoes . “Just” by changing the sign of the many-body Hamiltonian, one generalises the one-body time-reversal of the Hahn´s Spin Echo. This could allow us to observe decoherence, irreversibility and the emergence of hydrodynamic behaviour.
I will review our experimental and theoretical quest to improve many-body time reversal, whose unsurmountable limitations led us to propose a Central Hypothesis of Irreversibility . According to this, much as zero temperature residual resistance of metals only depends on reversible collisions with impurities, quantum dynamics of many-body systems far from equilibrium becomes intrinsically decoherent, and hence irreversible, in a time-scale fixed by the reversible interactions. This requires the thermodynamic limit in which, the time scales of the interaction with the environment, TS, are much longer than the time scales of the reversed Hamiltonian, T2. Thus, our most recent experiments, [7,8] indicate that hydrodynamical behaviour and the arrow of time should result as an emergent property of reversible quantum dynamics.
Watch the talk
 - Will the Universe Remember Us after We´re Gone?, John Horgan, Scientific American (2021).
 - Quantum Dynamical Echoes in Spin Diffusion on Mesoscopic Systems. HM Pastawski, PR Levstein and G Usaj, Phys. Rev. Lett. 75, 4310-4313 (1995).
 - Attenuation of polarization echoes in nuclear magnetic resonance: A study of the emergence of dynamical irreversibility in many-body quantum systems PR Levstein, G Usaj and HMPastawski, J. Chem. Phys. 108, 2718 (1998).
 - A Nuclear Magnetic Resonance answer to the Boltzmann-Loschmidt controversy? HM Pastawski , G Usaj , PR Levstein, J Raya and J Hirschinger, Physica A 283 166 (2000).
 - Loschmidt Echo, A Goussev, RA Jalabert, HM Pastawski and DA Wisniacki. Scholarpedia, 7(8):11687 (2012).
 - Loschmidt echo in many-spin systems: a quest for intrinsic decoherence and emergent irreversibility. PR Zangara and HM Pastawski, Phys. Scr. 92 033001(2017).
 - Perturbation independent decay of the Loschmidt echo in a many-spin system studied through scaled dipolar dynamics CM Sánchez, AK Chattah, KX Wei, L Buljubasich, P Cappellaro, and HM Pastawski, Phys. Rev. Lett. 124, 030601 (2020).
 - Emergent decoherence induced by quantum chaos in a many-body system: A Loschmidt echo observation through NMR, CM Sánchez, AK Chattah, and HM Pastawski. Phys.Rev.A, 105, 052232 (2022), J. Magn. Res. (submitted).
Professor Horacio M. Pastawski
Professor of Physics, National University of Córdoba and Senior Researcher, Enrique Gaviola Institute of Physics
Horacio M. Pastawski is a professor of physics at the National University of Córdoba and senior researcher at the Enrique Gaviola Institute of Physics. He graduated from the Instituto Balseiro in Argentina and worked at the Massachusetts Institute of Technology, before joining Córdoba in 1993. He is also a member of the Argentine National Academy of Sciences.
He has wide expertise in quantum aspects of molecular electronics and nuclear magnetic resonance, both with an emphasis in decoherence and time reversibility. His interests include sonic lasers (SASERs) and adiabatic quantum motors. He develops time-reversal experiments, such as Loschmidt echoes and acoustic time-reversal mirrors, as sensors for quantum dynamical phase transitions, many-body localization, and heterogeneous catalysis.