ELFT HÍRADÓ

[Janos Asboth <asboth.janos AT ttk.bme.hu>]

Meghívó

BME Elméleti Fizika Szeminárium,

okt. 3. péntek 10h15,

1111 Budapest, Budafoki út 8., BME F III. magasföldszint 01.,
Elméleti Fizika Tanszék szemináriumi szoba

Angelo Valli (BME Elm. Fiz. Tanszék)
Efficient Computation of Cumulant Evolution and Full Counting Statistics:
Application to Infinite Temperature Quantum Spin Chains

We introduce a novel tensor-network approach to extract the time-dependent
cumulants of the full counting statistics up to unprecedently long times
[1], which we apply to the investigation of spin-transfer in quantum spin
chains. The archetypal S=1/2 anisotropic Heisenberg model exhibits
ballistic and diffusive transport regimes, separated (at the isotropic
point) by a superdiffusive regime — which has proven to be ubiquitous in
integrable quantum and classical models with non-abelian symmetries.
Surprisingly, aspects of the superdiffusive spin dynamics are reminiscent
of classical interface growth phenomena. On the one hand, spatio-temporal
spin correlations display a scaling behavior with a characteristic
dynamical exponent (z=3/2), and superdiffusive spin transfer was
conjectured to fall within the Kardar-Parisi-Zhang (KPZ) universality class
[2] despite lacking key features of KPZ physics. The hypothesis has been
falsified by recent experiments on Google's Sycamore quantum processor [3].
Our results are in quantitative agreement with the experimental data and
extend to timescales far beyond the coherence time of the superconducting
qubits architecture, thus providing unambiguous evidence that spin transfer
in integrable quantum spin chains is incompatible with KPZ universality.
On the other hand, we show that, for the quantum analogue of surface
roughness, the subsystem and temporal fluctuation are well-described by the
self-similar Family-Vicsek scaling behavior. We verify that the relation z
= \alpha / \beta between the roughness (\alpha), growth (\beta), and
dynamical exponents holds in all spin transport regimes and across models
with SU(N) symmetry [4]. Our results shed light on how classical universal
scaling laws extend to the quantum many-body realm.

[1] A. Valli, C. P. Moca, M. A. Werner, M. Kormos, Ž. Krajnik, T. Prosen,
and G. Zaránd, Phys. Rev. Lett. 135, 100401 (2025)
[2] M. Ljubotina, M. Žnidarič, and T. Prosen, Phys. Rev. Lett. 122, 210602
(2019)
[3] E. Rosenberg et al., Science 384, 48-53 (2024)
[4] C. P. Moca, B. Dóra, D. Sticlet, A. Valli, T. Prosen, and G. Zaránd,
arXiv:2503.21454 (2024)

Minden érdeklődőt szeretettel várunk.
Asbóth János,
 szemináriumi koordinátor