LPTMC Seminars

Quantum to classical computability transition via negative Markov chains formalism

In this talk I will present a recently introduced representation of quantum dynamics based on negative Markov chain processes. By introducing particles and antiparticles, this formalism enables the mapping of generic quantum dynamics onto a Markov process defined over an exponentially large configuration space. Within this framework, quantum complexity arises from the proliferation of stochastic particles, which ultimately renders classical simulation intractable beyond a certain timescale. In the presence of noise, we demonstrate that for any unitary evolution generated by a linear combination of local or pairwise interactions, there exists at least one noise channel that effectively classicalizes the system by suppressing the growth of stochastic particles. As a corollary, we show that for this class of unitaries, the dynamics of an open quantum spin chain subject to depolarizing noise undergoes an exact transition to classical simulability once the noise strength exceeds a critical threshold.

Majorana or not? A closer look at Fe(Se,Te)

The search for Majorana fermions in condensed matter systems has
resulted in a number of putative claims of their discovery. If true,
these exotic particles that are their own anti-particle could be
exploited for error-free quantum computing, turning a fundamental
curiosity into a billion dollar business. Unambiguous proof, however,
is thus far lacking and challenging to provide. A recently proposed
method to distinguish Majorana bound states from more conventional
Andreev-, and Yu-Shiba-Rusinov states is to measure their shot noise
[1]. Using our MHz enabled scanning tunnelling microscope [2], we set
out to investigate three possible Majorana sightings in Fe(Se,Te):
zero energy bound states at single Fe impurities [3] and other native
impurities, linear sub-gap density of states at 1D defects [4] and
vortex cores. In this talk I will discuss our findings.

[1] Phys. Rev. B 104, L121406 (2021)
[2] Rev. Sci. Instrum. 89, 093708 (2018)
[3] Nature Communications 15, 8526 (2024)
[4] Nature Communications 15, 3774 (2024)