Publications

Order by: publication date number of citations

• Impurity Knight shift in quantum dot Josephson junctions

  Luka Pavešić, Marta Pita Vidal, Arno Bargerbos, Rok Žitko

  SciPost Phys. 15, 070 (2023) · published 24 August 2023 | Toggle abstract · pdf

  Spectroscopy of a Josephson junction device with an embedded quantum dot reveals the presence of a contribution to level splitting in external magnetic field that is proportional to $\cos\phi$, where $\phi$ is the gauge-invariant phase difference across the junction. To elucidate the origin of this unanticipated effect, we systematically study the Zeeman splitting of spinful subgap states in the superconducting Anderson impurity model. The magnitude of the splitting is renormalized by the exchange interaction between the local moment and the continuum of Bogoliubov quasiparticles in a variant of the Knight shift phenomenon. The leading term in the shift is linear in the hybridisation strength $\Gamma$ (quadratic in electron hopping), while the subleading term is quadratic in $\Gamma$ (quartic in electron hopping) and depends on $\phi$ due to spin-polarization-dependent corrections to the Josephson energy of the device. The amplitude of the $\phi$-dependent part is largest for experimentally relevant parameters beyond the perturbative regime where it is investigated using numerical renormalization group calculations. Such magnetic-field-tunable coupling between the quantum dot spin and the Josephson current could find wide use in superconducting spintronics.

  6 citations

• One-form symmetries in $\mathcal{N} = 3$ S-folds

  Antonio Amariti, Davide Morgante, Antoine Pasternak, Simone Rota, Valdo Tatitscheff

  SciPost Phys. 15, 132 (2023) · published 3 October 2023 | Toggle abstract · pdf

  We classify the global one-form symmetries for non-Lagrangian $\mathcal{N}=3$ SCFTs that arise by the action of $S$-fold projections on D3-branes. Such a classification is dictated, on a generic point of the Coulomb branch, by probing the charge spectrum of $(p, q)$-strings in the brane setup. The charge lattice of lines is then obtained by finding the ones that are genuine modulo screening by dynamical particles. The one-form symmetries are then extracted from the maximal sub-lattices of mutually local lines. We further comment on the existence of non-invertible symmetries for some of these $\mathcal{N}=3$ SCFTs.

  6 citations

• Quantum Monte Carlo detection of SU(2) symmetry breaking in the participation entropies of line subsystems

  David J. Luitz, Nicolas Laflorencie

  SciPost Phys. 2, 011 (2017) · published 24 March 2017 | Toggle abstract · pdf

  Using quantum Monte Carlo simulations, we compute the participation (Shannon-R\'enyi) entropies for groundstate wave functions of Heisenberg antiferromagnets for one-dimensional (line) subsystems of length $L$ embedded in two-dimensional ($L\times L$) square lattices. We also study the line entropy at finite temperature, i.e. of the diagonal elements of the density matrix, for three-dimensional ($L\times L\times L$) cubic lattices. The breaking of SU(2) symmetry is clearly captured by a universal logarithmic scaling term $l_q\ln L$ in the R\'enyi entropies, in good agreement with the recent field-theory results of Misguish, Pasquier and Oshikawa [arXiv:1607.02465]. We also study the dependence of the log prefactor $l_q$ on the R\'enyi index $q$ for which a transition is detected at $q_c\simeq 1$.

  6 citations

• State diagrams to determine tree tensor network operators

  Richard M. Milbradt, Qunsheng Huang, Christian B. Mendl

  SciPost Phys. Core 7, 036 (2024) · published 19 June 2024 | Toggle abstract · pdf

  This work is concerned with tree tensor network operators (TTNOs) for representing quantum Hamiltonians. We first establish a mathematical framework connecting tree topologies with state diagrams. Based on these, we devise an algorithm for constructing a TTNO given a Hamiltonian. The algorithm exploits the tensor product structure of the Hamiltonian to add paths to a state diagram, while combining local operators if possible. We test the capabilities of our algorithm on random Hamiltonians for a given tree structure. Additionally, we construct explicit TTNOs for nearest neighbour interactions on a tree topology. Furthermore, we derive a bound on the bond dimension of tensor operators representing arbitrary interactions on trees. Finally, we consider an open quantum system in the form of a Heisenberg spin chain coupled to bosonic bath sites as a concrete example. We find that tree structures allow for lower bond dimensions of the Hamiltonian tensor network representation compared to a matrix product operator structure. This reduction is large enough to reduce the number of total tensor elements required as soon as the number of baths per spin reaches 3.

  6 citations

• Gauging non-invertible symmetries on the lattice

  Sahand Seifnashri, Shu-Heng Shao, Xinping Yang

  SciPost Phys. 19, 063 (2025) · published 29 August 2025 | Toggle abstract · pdf

  We provide a general prescription for gauging finite non-invertible symmetries in 1+1d lattice Hamiltonian systems. Our primary example is the Rep(D$_8$) fusion category generated by the Kennedy-Tasaki transformation, which is the simplest anomaly-free non-invertible symmetry on a spin chain of qubits. We explicitly compute its lattice F-symbols and illustrate our prescription for a particular (non-maximal) gauging of this symmetry. In our gauging procedure, we introduce two qubits around each link, playing the role of "gauge fields" for the non-invertible symmetry, and impose novel Gauss's laws. Similar to the Kramers-Wannier transformation for gauging an ordinary $\mathbb{Z}_2$, our gauging can be summarized by a gauging map, which is part of a larger, continuous non-invertible cosine symmetry.

  6 citations

• Picking the low-hanging fruit: testing new physics at scale with active learning

  Juan Rocamonde, Louie Corpe, Gustavs Zilgalvis, Maria Avramidou, Jon Butterworth

  SciPost Phys. 13, 002 (2022) · published 15 July 2022 | Toggle abstract · pdf

  Since the discovery of the Higgs boson, testing the many possible extensions to the Standard Model has become a key challenge in particle physics. This paper discusses a new method for predicting the compatibility of new physics theories with existing experimental data from particle colliders. By using machine learning, the technique obtained comparable results to previous methods (>90% precision and recall) with only a fraction of their computing resources (<10%). This makes it possible to test models that were impossible to probe before, and allows for large-scale testing of new physics theories.

  6 citations

• Massive holographic QFTs in de Sitter

  José Manuel Penín, Kostas Skenderis, Benjamin Withers

  SciPost Phys. 12, 182 (2022) · published 1 June 2022 | Toggle abstract · pdf

  We study strongly coupled mass-deformed-CFT on a fixed de Sitter spacetime in three dimensions via holography. We elucidate the global causal structure of the four-dimensional spacetime dual to the de Sitter invariant vacuum state. The conformal boundaries of de Sitter appear as spacelike defects sourced by the mass deformation, which extend into the bulk as curvature singularities in AdS. We compute all one- and two-point functions of the deformed-CFT stress tensor and a scalar operator order-by-order in the mass deformation for a simple holographic model. These correlation functions admit a spectral representation as a sum of simple poles corresponding to normalisable modes in the bulk.

  6 citations

• Electric conductivity in non-Hermitian holography

  Zhuo-Yu Xian, David Rodríguez Fernández, Zhaohui Chen, Yang Liu, René Meyer

  SciPost Phys. 16, 004 (2024) · published 5 January 2024 | Toggle abstract · pdf

  We study the phase structure and charge transport at finite temperature and chemical potential in the non-Hermitian $\mathcal{PT}$-symmetric holographic model of [SciPost Phys. 9, 032 (2020)]. The non-Hermitian $\mathcal{PT}$-symmetric deformation is realized by promoting the parameter of a global U(1) symmetry to a complex number. Depending on the strength of the deformation, we find three phases: stable $\mathcal{PT}$-symmetric phase, unstable $\mathcal{PT}$-symmetric phase, and an unstable $\mathcal{PT}$-symmetry broken phase. In the three phases, the square of the condensate and also the spectral weight of the AC conductivity at zero frequency are, respectively, positive, negative, and complex. We check that the Ferrell-Glover-Tinkham sum rule for the AC conductivity holds in all the three phases. We also investigate a complexified U(1) rotor model with $\mathcal{PT}$-symmetric deformation, derive its phase structure and condensation pattern, and find a zero frequency spectral weight analogous to the holographic model.

  6 citations

• Traffic models and traffic-jam transition in quantum (N+1)-level systems

  Andrea Nava, Domenico Giuliano, Alessandro Papa, Marco Rossi

  SciPost Phys. Core 5, 022 (2022) · published 19 April 2022 | Toggle abstract · pdf

  We propose a model to implement and simulate different traffic-flow conditions in terms of quantum graphs hosting an (N+1)-level dot at each site, which allows us to keep track of the type and of the destination of each vehicle. By implementing proper Lindbladian local dissipators, we derive the master equations that describe the traffic flow in our system. To show the versatility and the reliability of our technique, we employ it to model different types of traffic flow (the symmetric three-way roundabout and the three-road intersection). Eventually, we successfully compare our predictions with results from classical models.

  6 citations

• Symmetry energy in holographic QCD

  Lorenzo Bartolini, Sven Bjarke Gudnason

  SciPost Phys. 16, 156 (2024) · published 19 June 2024 | Toggle abstract · pdf

  We study the symmetry energy (SE), an important quantity in nuclear physics, in the Witten-Sakai-Sugimoto model and in a much simpler hard-wall model of holographic QCD. The SE is the energy contribution to the nucleus due to having an unequal number of neutrons and protons. Using a homogeneous Ansatz representing smeared instantons and quantizing their isospin, we extract the SE and the proton fraction assuming charge neutrality and beta-equilibrium, using quantization of the isospin zeromode. We also show the equivalence between our method adapted from solitons and the usual way of the isospin controlled by a chemical potential at the holographic boundary. We find that the SE can be well described in the WSS model if we allow for a larger 't Hooft coupling and lower Kaluza-Klein scale than is normally used in phenomenological fits.

  6 citations