Publications

Order by: publication date number of citations

• Quantifying uncertainties in crystal electric field Hamiltonian fits to neutron data

  Allen Scheie

  SciPost Phys. Core 5, 018 (2022) · published 31 March 2022 | Toggle abstract · pdf

  We systematically examine uncertainties from fitting rare earth single-ion crystal electric field (CEF) Hamiltonians to inelastic neutron scattering data. Using pyrochlore and delafossite structures as test cases, we find that uncertainty in CEF parameters can be large despite visually excellent fits. These results show Yb$^{3+}$ compounds have particularly large $g$-tensor uncertainty because of the few available peaks. In such cases, additional constraints are necessary for meaningful fits.

  2 citations

• Universality and the thermoelectric transport properties of a double quantum dot system: Seeking for conditions that improve the thermoelectric efficiency

  Ronald Santiago Cortes-Santamaria , J. A. Landazabal-Rodríguez, Jereson Silva-Valencia , Edwin Ramos , M. S. Figueira, Roberto Franco Peñaloza.

  SciPost Phys. Core 7, 058 (2024) · published 23 August 2024 | Toggle abstract · pdf

  Employing universal relations for the Onsager coefficients in the linear regime at the symmetric point of the single impurity Anderson model, we calculate the conditions under which the quantum scattering phase shift should satisfy to produce the asymptotic Carnot’s limit for the thermoelectric efficiency. We show that a single quantum dot connected by metallic leads at the Kondo regime cannot achieve the best thermoelectric efficiency. We study a serial double quantum dot system, with the quantum dots immersed in ballistic conduction channels. Each QD exhibits a strong but finite local electronic correlation U. We show that maintaining one dot in the electron-hole symmetric point and allowing charge fluctuations in the other QD makes it possible to drive the system to the maximum thermoelectric efficiency. We also show that the bound states in the continuum (BICs) and quasi-BICs associated with the quantum scattering interference process drive the DQD to the maximum thermoelectric efficiency. We identify two types of quasi-BICs that occur at low and high temperatures: The first is associated with single Fano resonances, and the last is with several Fano processes. We also discussed possible temperature values and conditions that could be linked with the experimental realization of the results.

  2 citations

• Quasi-characters in $\widehat{su(2)}$ current algebra at fractional levels

  Sachin Grover

  SciPost Phys. Core 6, 068 (2023) · published 18 October 2023 | Toggle abstract · pdf

  We study the even characters of $\widehat{su(2)}$ conformal field theories (CFTs) at admissible fractional levels obtained from the difference of the highest weight characters in the unflavoured limit. We show that admissible even character vectors arise only in three special classes of admissible fractional levels which include the threshold levels, the positive half-odd integer levels, and the isolated level at -$5/4$. Among them, we show that the even characters of the half-odd integer levels map to the difference of characters of $\widehat{su(2)}_{4N+4}$, with $N\in\mathbb{Z}_{>0}$, although we prove that they do not correspond to rational CFTs. The isolated level characters maps to characters of two subsectors with $\widehat{so(5)}_1$ and $\widehat{su(2)}_1$ current algebras. Furthermore, for the $\widehat{su(2)}_1$ subsector of the isolated level, we introduce discrete flavour fugacities. The threshold levels saturate the admissibility bound and their even characters have previously been shown to be proportional to the unflavoured characters of integrable representations in $\widehat{su(2)}_{4N}$ CFTs, where $N\in\mathbb{Z}_{> 0}$ and we reaffirm this result. Except at the three classes of fractional levels, we find special inadmissible characters called quasi-characters which are nice vector valued modular functions but with $q$-series coefficients violating positivity but not integrality.

  2 citations

• Chaotic correlation functions with complex fermions

  Ritabrata Bhattacharya, Dileep P. Jatkar, Arnab Kundu

  SciPost Phys. Core 4, 018 (2021) · published 9 June 2021 | Toggle abstract · pdf

  We study correlation functions in the complex fermion SYK model. We focus, specifically, on the h = 2 mode which explicitly breaks conformal invariance and exhibits the chaotic behaviour. We explicitly compute fermion six-point function and extract the corresponding six-point OTOC which exhibits an exponential growth with maximal chaos. Following the program of Gross-Rosenhaus, this correlator contains information of the bulk cubic coupling, at the conformal point as well as perturbatively away from it. Unlike the conformal modes with high values of h, the h = 2 mode has contact interaction dominating over the planar in the large q limit.

  2 citations

• On the quantum simulation of complex networks

  Duarte Magano, João Moutinho, Bruno Coutinho

  SciPost Phys. Core 6, 058 (2023) · published 24 August 2023 | Toggle abstract · pdf

  Quantum walks provide a natural framework to approach graph problems with quantum computers, exhibiting speedups over their classical counterparts for tasks such as the search for marked nodes or the prediction of missing links. Continuous-time quantum walk algorithms assume that we can simulate the dynamics of quantum systems where the Hamiltonian is given by the adjacency matrix of the graph. It is known that such can be simulated efficiently if the underlying graph is row-sparse and efficiently row-computable. While this is sufficient for many applications, it limits the applicability for this class of algorithms to study real world complex networks, which, among other properties, are characterized by the existence of a few densely connected nodes, called hubs. In other words, complex networks are typically not row-sparse, even though the average connectivity over all nodes can be very small. In this work, we extend the state-of-the-art results on quantum simulation to graphs that contain a small number of hubs, but that are otherwise sparse. Hopefully, our results may lead to new applications of quantum computing to network science.

  2 citations

• Cover your bases: asymptotic distributions of the profile likelihood ratio when constraining effective field theories in high-energy physics

  Florian Urs Bernlochner, Daniel C. Fry, Stephen Burns Menary, Eric Persson

  SciPost Phys. Core 6, 013 (2023) · published 16 March 2023 | Toggle abstract · pdf

  We investigate the asymptotic distribution of the profile likelihood ratio (PLR) when constraining effective field theories (EFTs) and show that Wilks' theorem is often violated, meaning that we should not assume the PLR to follow a $\chi^2$-distribution. We derive the correct asymptotic distributions when either one or two real EFT couplings modulate observable cross sections with a purely linear or quadratic dependence. We then discover that when both the linear and quadratic terms contribute, the PLR distribution does not have a simple form. In this case we provide a partly-numerical solution for the one-parameter case. Using a novel approach, we find that the constants which define our asymptotic distributions may be obtained experimentally using a profile of the Asimov likelihood contour. Our results may be immediately used to obtain the correct coverage when deriving real-world EFT constraints using the PLR as a test-statistic.

  2 citations

• A simple coin for a 2d entangled walk

  Ahmadullah Zahed, Kallol Sen

  SciPost Phys. Core 6, 048 (2023) · published 10 July 2023 | Toggle abstract · pdf

  We analyze the effect of a simple coin operator, built out of Bell pairs, in a 2d Discrete Quantum Random Walk (DQRW) problem. The specific form of the coin enables us to find analytical and closed form solutions to the recursion relations of the DQRW. The coin induces entanglement between the spin and position degrees of freedom, which oscillates with time and reaches a constant value asymptotically. We probe the entangling properties of the coin operator further, by two different measures. First, by integrating over the space of initial tensor product states, we determine the Entangling Power of the coin operator. Secondly, we compute the Generalized Relative Rényi Entropy between the corresponding density matrices for the entangled state and the initial pure unentangled state. Both the Entangling Power and Generalized Relative Rényi Entropy behaves similar to the entanglement with time. Finally, in the continuum limit, the specific coin operator reduces the 2d DQRW into two 1d massive fermions coupled to synthetic gauge fields, where both the mass term and the gauge fields are built out of the coin parameters.

  2 citations

• A solvable model for graph state decoherence dynamics

  Jérôme Houdayer, Haggai Landa, Grégoire Misguich

  SciPost Phys. Core 7, 009 (2024) · published 23 February 2024 | Toggle abstract · pdf

  We present an exactly solvable toy model for the continuous dissipative dynamics of permutation-invariant graph states of $N$ qubits. Such states are locally equivalent to an $N$-qubit Greenberger-Horne-Zeilinger (GHZ) state, a fundamental resource in many quantum information processing setups. We focus on the time evolution of the state governed by a Lindblad master equation with the three standard single-qubit jump operators, the Hamiltonian part being set to zero. Deriving analytic expressions for the expectation values of observables expanded in the Pauli basis at all times, we analyze the nontrivial intermediate-time dynamics. Using a numerical solver based on matrix product operators, we simulate the time evolution for systems with up to 64 qubits and verify a numerically exact agreement with the analytical results. We find that the evolution of the operator space entanglement entropy of a bipartition of the system manifests a plateau whose duration increases logarithmically with the number of qubits, whereas all Pauli-operator products have expectation values decaying at most in constant time.

  2 citations

• Photoemission "experiments" on holographic lattices

  Filip Herček, Vladan Gecin, Mihailo Čubrović

  SciPost Phys. Core 6, 027 (2023) · published 6 April 2023 | Toggle abstract · pdf

  We construct a 2D holographic ionic lattice with hyperscaling-violating infrared geometry and study single-electron spectral functions ("ARPES photoemission curves") on this background. The spectra typically show a three-peak structure, where the central peak undergoes a crossover from a sharp but not Fermi-liquid-like quasiparticle to a wide incoherent maximum, and the broad side peaks resemble the Hubbard bands. These findings are partially explained by a perturbative near-horizon analysis of the bulk Dirac equation. Comparing the holographic Green functions in imaginary frequency with the Green functions of the Hubbard model obtained from quantum Monte Carlo, we find that the holographic model provides a very good fit to the Hubbard Green function. However, the information loss when transposing the holographic Green functions to imaginary frequencies implies that a deeper connection to Hubbard-like models remains questionable.

  2 citations

• Improved Hilbert space exploration algorithms for finite temperature calculations

  Albertus J. J. M. de Klerk, Jean-Sébastien Caux

  SciPost Phys. Core 6, 039 (2023) · published 23 May 2023 | Toggle abstract · pdf

  Computing correlation functions in strongly-interacting quantum systems is one of the most important challenges of modern condensed matter theory, due to their importance in the description of many physical observables. Simultaneously, this challenge is one of the most difficult to address, due to the inapplicability of traditional perturbative methods or the few-body limitations of numerical approaches. For special cases, where the model is integrable, methods based on the Bethe Ansatz have succeeded in computing the spectrum and given us analytical expressions for the matrix elements of physically important operators. However, leveraging these results to compute correlation functions generally requires the numerical evaluation of summations over eigenstates. To perform these summations efficiently, Hilbert space exploration algorithms have been developed which has resulted most notably in the ABACUS library. While this performs quite well for correlations on ground states or low-entropy states, the case of high entropy states (most importantly at finite temperatures or after a quantum quench) is more difficult, and leaves room for improvement. In this work, we develop a new Hilbert space exploration algorithm for the Lieb-Liniger model, specially tailored to optimize the computational order on finite-entropy states for correlations of density-related operators.

  2 citations