arXiv subjects

Matthis Lehmkuehler

Publications and source records attributed to Matthis Lehmkuehler.

5 recordsLinked to original sources

arXiv2024

Parity questions in critical planar Brownian loop-soups (or "where did the free planar bosons go?")

The critical two-dimensional Brownian loop-soup is an infinite collection of non-interacting Brownian loops in a planar domain that possesses some combinatorial features related to the notion of indistinguishability of bosons. The properly renormalized occupation time field of this collection of loops is known to be distributed like the properly defined square of a Gaussian free field. In the present paper, we investigate aspects of the question about how much information these fields provide about the loop-soup. Among other things, we show that the exact set of points that are actually visited by some loops in the loop-soup is not determined by these fields. We further prove that given the fields, a dense family of special points will each have a conditional probability 1/2 of being part of the loop-soup. We also exhibit another instance where the possible decompositions (given the field) into individual loops and excursions can be grouped into two clearly different groups, each having a conditional probability 1/2 of occurring.

math.PR↗

arXiv2023

Provable bounds for noise-free expectation values computed from noisy samples

In this paper, we explore the impact of noise on quantum computing, particularly focusing on the challenges when sampling bit strings from noisy quantum computers as well as the implications for optimization and machine learning applications. We formally quantify the sampling overhead to extract good samples from noisy quantum computers and relate it to the layer fidelity, a metric to determine the performance of noisy quantum processors. Further, we show how this allows us to use the Conditional Value at Risk of noisy samples to determine provable bounds on noise-free expectation values. We discuss how to leverage these bounds for different algorithms and demonstrate our findings through experiments on a real quantum computer involving up to 127 qubits. The results show a strong alignment with theoretical predictions.

quant-ph↗

arXiv2022

The fuzzy Potts model in the plane: Scaling limits and arm exponents

We consider a critical Fortuin-Kasteleyn (FK) percolation with cluster weight $q \in [1,4)$ in the plane, and color its clusters in red (respectively blue) with probability $r \in (0,1)$ (respectively $1-r$), independently of each other. We study the resulting fuzzy Potts model, which corresponds to the critical Ising model in the special case $q=2$ and $r=1/2$. We show that under the assumption that the critical FK percolation converges to a conformally invariant scaling limit (which is known to hold for the FK-Ising model, i.e. $q=2$), the obtained coloring converges to variants of Conformal Loop Ensembles constructed, described and studied by Miller, Sheffield and Werner. Based on discrete considerations, we also show that the arm exponents for this coloring in the discrete model are identical to the ones of the continuum model. Using the values of these arm exponents in the continuum, we determine the arm exponents for the fuzzy Potts model.

math.PR↗

arXiv2022

Liouville quantum gravity weighted by conformal loop ensemble nesting statistics

We study Liouville quantum gravity (LQG) surfaces whose law has been reweighted according to nesting statistics for a conformal loop ensemble (CLE) relative to $n\in \mathbb{N}_0$ marked points $z_1,\dots,z_n$. The idea is to consider a reweighting by $\prod_{B\subseteq \{1,\dots,n\}} e^{\sigma_B N_B}$, where $\sigma_B\in\mathbb{R}$ and $N_B$ is the number of CLE loops surrounding the points $z_i$ for $i\in B$. This is made precise via an approximation procedure where as part of the proof we derive strong spatial independence results for CLE. The reweighting induces logarithmic singularities for the Liouville field at $z_1,\dots,z_n$ with a magnitude depending explicitly on $\sigma_1,\dots,\sigma_n$. We define the partition function of the surface, compute it for $n\in\{0,1\}$, and derive a recursive formula expressing the $n>1$ point partition function in terms of lower-order partition functions. The proof of the latter result is based on a continuum peeling process previously studied by Miller, Sheffield and Werner in the case $n=0$, and we derive an explicit formula for the generator of a boundary length process that can be associated with the exploration for general $n$. We use the recursive formula to partly characterize for which values of $(\sigma_B\colon B\subseteq \{1,\dots,n\})$ the partition function is finite. Finally, we give a new proof for the law of the conformal radius of CLE, which was originally established by Schramm, Sheffield, and Wilson.

math.PR↗

arXiv2021

The trunks of CLE(4) explorations

A natural class of conformally invariant ways for discovering the loops of a conformal loop ensemble $\text{CLE}_4$ is given by a certain family of $\text{SLE}_4^{\langle\mu\rangle}(-2)$ exploration processes for real $\mu$. Such an exploration consists of one simple continuous path called the trunk of the exploration that discovers $\text{CLE}_4$ loops along the way. The parameter $\mu$ appears in the Loewner chain description of the path that traces the trunk and all CLE loops encountered by the trunk in chronological order. These explorations can also be interpreted in terms of level lines of a Gaussian free field. It has been shown by Miller, Sheffield and Werner that the trunk of such an exploration is an $\text{SLE}_4(\rho,-2-\rho)$ process for some (unknown) value of $\rho \in (-2, 0)$. The main result of the present paper is to establish the relation between $\mu$ and $\rho$, more specifically to show that $\mu = -\pi\cot(\pi\rho/2)$. The crux of the paper is to show how explorations of $\text{CLE}_4$ can be approximated by explorations of $\text{CLE}_\kappa$ for $\kappa \uparrow 4$, which then makes it possible to use recent results by Miller, Sheffield and Werner about the trunks of $\text{CLE}_\kappa$ explorations for $\kappa < 4$.

math.PR↗