Publications

Order by: publication date number of citations

• Ground state topology of a four-terminal superconducting double quantum dot

  Lev Teshler, Hannes Weisbrich, Jonathan Sturm, Raffael L. Klees, Gianluca Rastelli, Wolfgang Belzig

  SciPost Phys. 15, 214 (2023) · published 29 November 2023 | Toggle abstract · pdf

  In recent years, various classes of systems were proposed to realize topological states of matter. One of them are multiterminal Josephson junctions where topological Andreev bound states are constructed in the synthetic space of superconducting phases. Crucially, the topology in these systems results in a quantized transconductance between two of its terminals comparable to the quantum Hall effect. In this work, we study a double quantum dot with four superconducting terminals and show that it has an experimentally accessible topological regime in which the non-trivial topology can be measured. We also include Coulomb repulsion between electrons which is usually present in experiments and show how the topological region can be maximized in parameter space.

  1 citation

• A convenient Keldysh contour for thermodynamically consistent perturbative and semiclassical expansions

  Vasco Cavina, Sadeq S. Kadijani, Massimiliano Esposito, Thomas L. Schmidt

  SciPost Phys. 15, 209 (2023) · published 27 November 2023 | Toggle abstract · pdf

  The work fluctuation theorem (FT) is a symmetry connecting the moment generating functions (MGFs) of the work extracted in a given process and in its time-reversed counterpart. We show that, equivalently, the FT for work in isolated quantum systems can be expressed as an invariance property of a modified Keldysh contour. Modified contours can be used as starting points of perturbative and path integral approaches to quantum thermodynamics, as recently pointed out in the literature. After reviewing the derivation of the contour-based perturbation theory, we use the symmetry of the modified contour to show that the theory satisfies the FT at every order. Furthermore, we extend textbook diagrammatic techniques to the computation of work MGFs, showing that the contributions of the different Feynman diagrams can be added to obtain a general expression of the work statistics in terms of a sum of independent rescaled Poisson processes. In this context, the FT takes the form of a detailed balance condition linking every Feynman diagram with its time-reversed variant. In the second part, we study path integral approaches to the calculation of the MGF, and discuss how the arbitrariness in the choice of the contour impacts the final form of the path integral action. In particular, we show how using a symmetrized contour makes it possible to easily generalize the Keldysh rotation in the context of work statistics, a procedure paving the way to a semiclassical expansion of the work MGF. Furthermore, we use our results to discuss a generalization of the detailed balance conditions at the level of the quantum trajectories.

  1 citation

• Estimation of the geometric measure of entanglement with Wehrl moments through artificial neural networks

  Jérôme Denis, François Damanet, John Martin

  SciPost Phys. 15, 208 (2023) · published 27 November 2023 | Toggle abstract · pdf

  In recent years, artificial neural networks (ANNs) have become an increasingly popular tool for studying problems in quantum theory, and in particular entanglement theory. In this work, we analyse to what extent ANNs can accurately predict the geometric measure of entanglement of symmetric multiqubit states using only a limited number of Wehrl moments (moments of the Husimi function of the state) as input, which represents partial information about the state. We consider both pure and mixed quantum states. We compare the results we obtain by training ANNs with the informed use of convergence acceleration methods. We find that even some of the most powerful convergence acceleration algorithms do not compete with ANNs when given the same input data, provided that enough data is available to train these ANNs. We also provide an experimental protocol for measuring Wehrl moments, which is state-independent. More generally, this work opens up perspectives for the estimation of entanglement measures and other SU(2)-invariant quantities, such as the Wehrl entropy, in a way that is more accessible in experiments than by means of full state tomography.

• Quantum simulation of colour in perturbative quantum chromodynamics

  Herschel A. Chawdhry, Mathieu Pellen

  SciPost Phys. 15, 205 (2023) · published 24 November 2023 | Toggle abstract · pdf

  Quantum computers are expected to give major speed-ups for the simulation of quantum systems. In this work, we present quantum gates that simulate the colour part of the interactions of quarks and gluons in perturbative quantum chromodynamics (QCD). As a first application, we implement these circuits on a simulated noiseless quantum computer and use them to calculate colour factors for various examples of Feynman diagrams. This work constitutes a first key step towards a quantum simulation of generic scattering processes in perturbative QCD.

• Interpolating between the gauge and Schrödinger pictures of quantum dynamics

  Sayak Guha Roy, Kevin Slagle

  SciPost Phys. Core 6, 081 (2023) · published 24 November 2023 | Toggle abstract · pdf

  Although spatial locality is explicit in the Heisenberg picture of quantum dynamics, spatial locality is not explicit in the Schrödinger picture equations of motion. The gauge picture is a modification of Schrödinger's picture such that locality is explicit in the equations of motion. In order to achieve this explicit locality, the gauge picture utilizes (1) a distinct wavefunction associated with each patch of space, and (2) time-dependent unitary connections to relate the Hilbert spaces associated with nearby patches. In this work, we show that by adding an additional spatially-local term to the gauge picture equations of motion, we can effectively interpolate between the gauge and Schrödinger pictures, such that when this additional term has a large coefficient, all of the gauge picture wavefunctions approach the Schrödginer picture wavefunction (and the connections approach the identity).

• Group theoretical derivation of consistent particle theories

  Giuseppe Nisticò

  SciPost Phys. Proc. 14, 040 (2023) · published 24 November 2023 | Toggle abstract · pdf

  Current quantum theories of an elementary free particle assume unitary space inversion and anti-unitary time reversal operators. In so doing robust classes of possible theories are discarded. The present work shows that consistent theories can be derived through a strictly deductive development from the principle of relativistic invariance and position covariance, also with anti-unitary space inversion and unitary time reversal operators. In doing so the class of possible consistent theories is extended for positive but also zero mass particles. In particular, consistent theories for a Klein-Gordon particle are derived and the non-localizability theorem for a non zero helicity massless particle is extended.

• Wigner function analysis of finite matter-radiation systems

  Eduardo Nahmad-Achar, Ramón López-Peña, Sergio Cordero, Octavio Castaños Garza

  SciPost Phys. Proc. 14, 039 (2023) · published 24 November 2023 | Toggle abstract · pdf

  We show that the behaviour in phase space of the Wigner function associated to the electromagnetic modes carries the information of both, the entanglement properties between matter and field, and the regions in parameter space where quantum phase transitions take place. A finer classification for the continuous phase transitions is obtained through the computation of the surface of minimum fidelity.

• Mixed permutation symmetry quantum phase transitions of critical three-level atom models

  Alberto Mayorgas, J. Guerrero, Manuel Calixto

  SciPost Phys. Proc. 14, 036 (2023) · published 24 November 2023 | Toggle abstract · pdf

  We define the concept of Mixed Symmetry Quantum Phase Transition (MSQPT), considering each permutation symmetry sector $\mu$ of an identical particles system, as singularities in the properties of the lowest-energy state into each $\mu$ when shifting a Hamiltonian control parameter $\lambda$. A three-level Lipkin-Meshkov-Glick (LMG) model is chosen to typify our construction. Firstly, we analyze the finite number $N$ of particles case, proving the presence of MSQPT precursors. Then, in the thermodynamic limit $N\to∞$, we calculate the lowest-energy density inside each sector $\mu$, augmenting the control parameter space by $\mu$, and showing a phase diagram with four different quantum phases.

• Linking ladder operators for the Rosen-Morse and Pöschl-Teller systems

  Simon Garneau-Desroches, Véronique Hussin

  SciPost Phys. Proc. 14, 026 (2023) · published 24 November 2023 | Toggle abstract · pdf

  An analysis of the realizations of the ladder operators for the Rosen-Morse and Pöschl-Teller quantum systems is carried out. The failure of the algebraic method of construction in the general Rosen-Morse case is exposed and explained. We present the reduction of a recently obtained set of $(2n ± 1)$-th-order Rosen-Morse ladder operators to the usual first-order realization for the Pöschl-Teller case known in the literature.

• On old relations of Lie theory, classical geometry and gauge theory

  Rolf Dahm

  SciPost Phys. Proc. 14, 025 (2023) · published 24 November 2023 | Toggle abstract · pdf

  Having been led by hadron interactions and low-energy photoproduction to SU(4) and non-compact SU*(4) symmetry, the general background turned out to be projective geometry (PG) of $P^3$, or when considering line and Complex geometry to include gauge theory, aspects of $P^5$. Point calculus and its dual completion by planes introduced quaternary (quadratic) "invariants" $x_{\mu}x^{\mu}=0$ and $p_{\mu}p^{\mu}=0$, and put focus on the intermediary form $(xu)$ and its treatment. Here, the major result is the identification of the symmetric 20 of SU(4) comprising nucleon and Delta states as related to the quaternary cubic forms discussed by Hilbert in his work on full invariant systems. So PG determines geometrically the scene by representations (reps) and invariant theory without having to force affine restrictions and additional (spinorial or gauge) rep theory.