Postdoc in the group of Ignacio Cirac at the Max Planck Institute of Quantum Optics in Garching, working on the development, implementation and application of tensor network algorithms.
I will be leaving academia in February 2020. If you have any questions, queries etc., please get in touch.
Research is focused on numerical and algorithmic improvements to tensor network methods in condensed matter physics and related fields such as quantum chemistry. Examples include advances in convergence helpers for ground-state search methods, a detailled study of the errors in time evolution methods and the implementation of automatic differenation as used in gradient descent methods in tensor network applications. When applied to physical problem settings, both one-dimensional matrix-product state methods and two-dimensional infinite projected entangled pair states are utilised.
Work on the tensor network toolkit SyTen has been ongoing since 2015. SyTen is capable of handling arbitrary-rank tensors with named indices, non-abelian symmetries such as SU(2)-Spin, automatic handling of fermionic contractions and automatic differentiation while providing a high degree of parallelisation. Standard MPS and iPEPS tools are implemented atop of the basic tensor library, with further extensions facilitated by Python bindings and well-documented code.
Imaginary-time matrix product state impurity solver in a real material
calculation: Spin-orbit coupling in Sr2RuO4
Nils-Oliver Linden, Manuel Zingl, Claudius Hubig, Olivier Parcollet, Ulrich Schollwöck
MPS-based DMFT impurity solver applied to a real material while incorporating spin-orbit coupling at zero temperature, a regime where previous CTQMC-based solvers encountered a severe sign problem.arXiv:1909.02503
Use and implementation of autodifferentiation in
tensor network methods with complex scalars
Sketches the implementation of reverse-mode autodifferentiation in tensor network toolkits and stresses that this is not too hard. Also discusses the extension of autodifferentiation to calculations based on complex scalars.arXiv:1907.13422
Gaussian TDVP for the Bose-Hubbard model
Tommaso Guaita, Lucas Hackl, Tao Shi, Claudius Hubig, Eugene Demler, J. Ignacio Cirac
We systematically extend Bogoliubov theory beyond the mean field approximation of the Bose- Hubbard model in the superfluid phase. Our approach is based on the time dependent variational principle applied to the family of all Gaussian states (i.e., Gaussian TDVP). First, we find the best ground state approximation within our variational class using imaginary time evolution in 1d, 2d and 3d. We benchmark our results by comparing to Bogoliubov theory and DMRG in 1d. Second, we compute the approximate 1- and 2-particle excitation spectrum as eigenvalues of the linearized projected equations of motion (linearized TDVP).arXiv:1907.04837
Dynamical topological quantum phase transitions in nonintegrable models
Imre Hagymási, Claudius Hubig, Örs Legeza, Ulrich Schollwöck
We consider sudden quenches across quantum phase transitions in the S=1 XXZ model starting from the Haldane phase. We demonstrate that dynamical phase transitions may occur during these quenches that are identified by nonanalyticities in the rate function for the return probability. In addition, we show that the temporal behavior of the string order parameter is intimately related to the subsequent dynamical phase transitions. We furthermore find that the dynamical quantum phase transitions can be accompanied by enhanced two-site entanglement.arXiv:1904.00867 Phys. Rev Lett. 122, 250601
Quantum phases and topological properties of interacting fermions in one-dimensional
Leo Stenzel, Andrew L. C. Hayward, Claudius Hubig, Ulrich Schollwöck, Fabian Heidrich-Meisner
A very extensive study of the possible quantum phases of interacting fermions in effective one-dimensional superlattices. For certain filling factors, a series of different phases from band insulators to other topological insulators is observed, which are then linked to topological transitions observed in the Chern numbers of the whole manifold of states.arXiv:1903.06108 Phys. Rev A 99, 053614
Time-evolution methods for matrix-product states
Sebastian Paeckel, Thomas Köhler, Andreas Swoboda, Salvatore R. Manmana, Ulrich Schollwöck and Claudius Hubig
An overview and detailed description of time-evolution methods for matrix-product states. Discusses TEBD, the MPO WI,II method, the global Krylov method, the local Krylov method and the time-dependent variational principle. Also includes a lengthy section on additional tricks to be combined with any of the methods described before and four exemplary applications in condensed-matter physics to test the advantages of each individual method.
Exemplary excitation spectrum of a near-Ising XXZ Heisenberg chain obtained using two-site TDVP-MPS within a few CPU hours.arXiv:1901.05824
Time-dependent study of disordered models with infinite projected entangled pair
Claudius Hubig and J. Ignacio Cirac
Applies real-time evolution operators to an infinite projected entangled pair state to study the averaged dynamics of the Néel state under the Heisenberg square lattice Hamiltonian with bi-valued disorder. The primary challenge was in the stability of the corner transfer matrix used for the more precise full update mechanism here. However, careful choice of parameters allowed us to obtain times beyond the reach of MPS, ED or NLCE calculations and demonstrate a slow-down of the dynamics in the presence of the bi-valued disorder field (implemented by an ancilla Hilbert space).
Evolution of ⟨sz(t)⟩ starting from a Néel state under the Heisenberg XXX Hamiltonian with increasingly stronger disorder. MPS calculations at zero disorder exhibit finite-size effects at t < 1, while time-dependent iPEPS can obtain longer times and also demonstrate a marked slowdown of dynamics, which may hint towards many-body localisation.arXiv:1812.03801 SciPost Phys. 6, 031
Thermal control of spin excitations in the coupled Ising-chain material
Mattia Mena, Nora Hänni, Simon Ward, Eva Hirtenlechner, Robert Bewley, Claudius Hubig, Ulrich Schollwöck, Bruce Normand, Karl W. Krämer, Des F. McMorrow and Christian Rüegg
Using finite-temperature time-dependent MPS (with TDVP), it was possible to obtain excitation spectra which precisely matched the data experimentally obtained using neutron scattering. In particular, it was possible to derive the temperature-dependence of the spectrum essentially from first principles and model it without free parameters, which allows for the understanding of two phase transitions present in the material at finite temperature.arXiv:1811.07178
Density-matrix embedding theory study of the one-dimensional Hubbard-Holstein model
Teresa E. Reinhard, Uliana Mordovina, Claudius Hubig, Joshua S. Kretchmer, Ulrich Schollwöck, Heiko Appel, Michael A. Sentef and Angel Rubio
An extension of DMET to handle mixed bosonic/fermionic systems and solution of the subsequent impurity problem using DMRG. The resulting method is applied to the Hubbard-Holstein model in one dimension to obtain the single-particle excitation gap and by extension allow for a study of the phase diagram.arXiv:1811.00048 J. Chem. Theory Comput. 15 (4), 2221
Interaction quench and thermalization in a one-dimensional topological Kondo
Imre Hagymási, Claudius Hubig and Ulrich Schollwöck
Study of the non-equilibrium dynamics of a one-dimensional topological Kondo insulator modelled by a p-wave Anderson lattice model. The system is quenched by a change of the on-site interaction strength and topological quantities are studied in a real-time evolution after the quench. Up to a certain interaction strength, observables can be captured well by a thermal ensemble while topological properties are preserved.arXiv:1810.09799 Phys. Rev. B 99, 075145
Abelian and non-abelian symmetries in infinite projected entangled pair
Studies the effect of implementing (that is, preserving exactly) abelian and non-abelian symmetries at the example of the Heisenberg model on the square and kagome lattice in two dimensions.
It was found that (a) using the symmetries greatly improves computational efficiency and (b) implementing the symmetry also tends to force the state into a symmetry-preserving phase, leading to marked signals in the variational energy obtained at phase transitions.
Symmetry breaking detection in the XXZ Heisenberg model: at Δ<1, the U(1) symmetry is spontaneously broken, resulting in a higher energy if tensors are restricted to be U(1) symmetric (“U(1)”) while the energy curve for unrestricted tensors (“None”) shows a kink. Monte Carlo data at Δ=1 is given as a reference (“MC”).1808.10804 SciPost Phys. 5, 047
Error estimates for extrapolations with matrix-product states
Claudius Hubig, Jutho Haegeman and Ulrich Schollwöck
Using single-site DMRG, much larger bond dimensions and much lower variational energies are obtainable than with two-site DMRG due to computational speed-ups and reduced memory usage. However, single-site DMRG does not produce a reliable error measure which allows extrapolation of the observables to the zero-error limit. Here, we introduce an approximation to the variance ⟨H²⟩-⟨H⟩² which allows for the calculation of a reliable error measure even in challenging cases and hence extrapolation of the variational data to the zero-error case.
Comparison of the two-site variance approximation introduced here and the 2DMRG truncation error applied to a 10×4 Hubbard cylinder. Both methods can extrapolate their variational results to comparable ground-state energy estimates, but by using single-site DMRG and only the two-site variance, much larger bond dimensions and hence lower variational energies are obtained.1711.01104 Phys. Rev. B 97, 045125
Doped Kondo chain, a heavy Luttinger liquid
Ilia Khait, Patrick Azaria, Claudius Hubig, Ulrich Schollwöck and Assa Auerbach
A study of the one-dimensional Kondo lattice model using SU(2)-symmetric DMRG beyond the analytically approachable large-N limit. Found signatures of a heavy Tomonaga-Luttinger liquid phase and divergent charge and spin susceptibilities at the large Fermi surface.1710.04847 PNAS 115, 5140
Spinon confinement in a quasi one dimensional anisotropic Heisenberg magnet
Anup K. Bera, Bella Lake, Fabian H. L. Essler, Laurens Vanderstraeten, Claudius Hubig, Ulrich Schollwöck, A. T. M. Nazmul Islam, Astrid Schneidewind and Diana L. Quintero-Castro
A study of spinon confinement in a one-dimensional Heisenberg chain in the presence of an external staggered field. Ground-state DMRG results on the gap and time-dependent MPS data of the spectrum could qualitatively reproduce the experimental results with quantitative agreement upon use of a tangent-space MPS method.1705.01259 Phys. Rev. B 96, 054423
Generic construction of efficient matrix product operators
Claudius Hubig, Ian P. McCulloch and Ulrich Schollwöck
Presents a generic method to algorithmically construct efficient matrix-product operators (MPOs). Apart from established MPO arithmetic, two lossless MPO compression schemes are presented which ensure optimality of the resulting MPO. Furthermore may serve as an introduction to the use of MPOs with some explicit examples provided.1611.02498 Phys. Rev. B 95, 035129
Universal long-time behavior of stochastically driven interacting quantum systems
Zi Cai, Claudius Hubig and Ulrich Schollwöck
A systematic study the long-time relaxation dynamics in stochastically driven interacting quantum systems. Finds that even though the stochastic forces will inevitably drive the systems into a featureless infinite temperature state, the way to approach the steady state can be highly nontrivial and exhibit rich universal dynamical behavior determined by the interplay between the stochastic driving and quantum many-body effects.1609.08518 Phys. Rev. B 96, 054303
Strictly single-site DMRG algorithm with subspace expansion
Claudius Hubig, Ian P. McCulloch, Ulrich Schollwöck and F. Alexander Wolf
Introduces a strictly single-site DMRG variant using subspace expansion to adapt the auxiliary basis size. This variant improves both runtime and memory usage over the traditional two-site DMRG and the centermatrix wavefunction approach.
The DMRG method introduced in this paper is the default method used in the SyTen toolkit.
Error in energy and runtime for the introduced single site variant (“DMRG3S”) and the previous centermatrix wavefunction approach (“CWF“) when applied to free fermions on a one-dimensional chain. Traditional two-site DMRG was slower by a factor of two to three than the CWF approach shown here.1501.05504 Phys. Rev. B 91, 155115
Symmetry-Protected Tensor Networks
PhD thesis submitted to the Faculty of Physics at LMU Munich on 2017-08-31, supervised by Ulrich Schollwöck
A detailed introduction to symmetry-protection in tensor networks in Chapter 2, which is useful as an overview for the SyTen tensor network implementation and also discusses matrix-product states and operators (cf.~paper 3 above). Subsequent chapters 3 and 4 summarise papers 1 and 4 above. The last chapter 5 sums up my work on the two-dimensional Hubbard model using MPS-DMRG on finite-size cylinders.Direct link at LMU
Spin Dynamics in Adiabatic Transport through Quantum Dots
BSc thesis submitted to the Faculty of Physics at TU Berlin on 2012-07-03, supervised by Anja Metelmann and Tobias Brandes
01/2019: Quantum Emitters in
Non-Conventional Baths at MPQ Garching
together with Ignacio Cirac and Eduardo Sanchez-Burillo
06/2016: First International ExQM Workshop at Lake Chiemsee
with the ExQM graduate school
- Arnold Sommerfeld PhD Prize, 2017
Arnold Sommerfeld Center, LMU Munich
- Deutschlandstipendium, 2011/2012
TU Berlin & Robert Bosch GmbH
- 05/2019: Exploring Quantum Matter Seminar, Garching: Recent developments in tensor networks
- 04/2019: Condensed Matter Theory Seminar, Paul Scherrer Institut, Villigen: Recent developments in tensor networks
- 03/2019: Physics Seminar, University of Queensland, Brisbane: Recent developments in tensor networks
- 11/2018: Tensor Network based approaches to Quantum Many-Body Systems, Dresden: Symmetries in Tensor Networks [slides]
- 11/2018: Arnold Sommerfeld Center, Munich: PhD Prize Colloquium: Evolving Tensor Networks
- 07/2018: Seminar of the Institute for Theoretical Physics, Göttingen: Applications of Tensor Networks in Ab Initio Methods
- 06/2018: Optimising, Renormalising, Evolving and Quantising Tensor Networks, Dresden: Symmetry implementations in tensor networks
- 03/2018: Frühjahrstagung der DPG, Berlin: Error estimates for extrapolations with matrix-product states
- 02/2018: Entanglement in Strongly Correlated Systems, Benasque: Error estimates for extrapolations with matrix-product states [slides] [local copy]
- 11/2017: 2017 School, European Tensor Network, Ghent: Tutorial on DMRG with Subspace Expansion; Python script
- 09/2017: Arnold Sommerfeld School on Numerical Methods for Strongly Correlated Many-Body Systems, Munich: Tutorial & Hands-On Session on DMRG and MPS Methods
- 04/2017: Theory Department, Max-Planck-Institut für Struktur und Dynamik, Hamburg: Time Evolution with Matrix-Product States
- 03/2017: Theory Seminar, Max-Planck-Institut für Quantenoptik, Garching: DMRG with Subspace Expansion on Symmetry-Protected Tensor Networks
- 03/2017: Frühjahrstagung der DPG, Dresden: Generic Construction of Efficient Matrix Product Operators
- 01/2017: Theory Winter School, MagLab, Tallahassee: Tutorial & Hands-On Session on DMRG and DMRG+DMFT
- 03/2016: Frühjahrstagung der DPG, Regensburg: DMRG on Binary Tree Tensor Networks
- 01/2016: Fritz Haber Institut der Max-Planck-Gesellschaft, Berlin: Symmetries in Tensor Networks and Subspace Expansion with DMRG
- 09/2017: Tensor Computation Workshop, Flatiron Institute, New York City: Claudius Hubig and Ulrich Schollwöck: Error Estimates for Matrix-Product States
- 09/2017: Arnold Sommerfeld School on Numerical Methods for Strongly Correlated Many-Body Systems, Munich: Claudius Hubig and Ulrich Schollwöck: Error Estimates for Matrix-Product States
- 05/2017: Munich Quantum Center Workshop: Andreas Swoboda, Nils-Oliver Linden, Ulrich Schollwöck and Claudius Hubig: Time Evolution for Matrix-Product States with the Krylov Subspace Method
- 10/2016: Munich Quantum Symposium: Claudius Hubig, Ian P. McCulloch and Ulrich Schollwöck: A Generic Algorithm for the Construction of Efficient Matrix Product Operators
- 06/2016: International Summer School on Computational Quantum Materials, Sherbrooke: Claudius Hubig, Ian P. McCulloch and Ulrich Schollwöck: Generic Algorithm for the Construction of Efficient Matrix Product Operators
- 02/2016: Entanglement in Strongly Correlated Systems, Benasque: Claudius Hubig and Ulrich Schollwöck: DMRG on Binary Tree Tensor Networks
- 11/2015: WEH-Workshop Isolated Quantum Many-Body Systems out of Equilibrium, Bad Honnef: Claudius Hubig, Fabian H. L. Essler and Ulrich Schollwöck: Spin Excitations in a Staggered Magnetic Field
- 02/2015: Advanced Computational Methods for Strongly Correlated Quantum Systems, Würzburg: Claudius Hubig, Ian P. McCulloch, Ulrich Schollwöck and F. Alexander Wolf: A Strictly Single-Site DMRG Algorithm with Subspace Expansion
- 09/2014: Munich Quantum Day: Claudius Hubig, F. Alexander Wolf and Ulrich Schollwöck: Algorithmic Advancements in DMRG
- Summer 2017
- Supervision of Bachelor thesis project: Study of a Sampling Method for Singular Values
- Summer 2016
- Tutoring for Numerische Mathematik (BSc, LMU)
- Summer 2016
- Supervision of Bachelor thesis project: Parallelized Time Evolution on the Heisenberg Spin Chain with Matrix Product States
- Summer 2015
- Tutoring for Advanced Statistical Physics (MSc, LMU)
- Winter 2014/15
- Tutoring for Thermodynamik und Statistische Physik (BSc, LMU)
- Summer 2014
- Tutoring for Advanced Statistical Physics (MSc, LMU)
- since 11/2017
- Postdoc in the group of Ignacio Cirac, Max Planck Institute of Quantum Optics, Garching, Germany.
- 10/2013 - 10/2017
- PhD Student at the Chair of Uli Schollwöck, LMU Munich, Germany. Funded by the ExQM Graduate School and the Nanosystems Initiative Munich. Title of thesis: Symmetry-Protected Tensor Networks.
- 09/2012 - 07/2013
- Read Applied Mathematics (Part III) at Hughes Hall, University of Cambridge, United Kingdom. Graduated with Masters of Advanced Studies in Applied Mathematics.
- 10/2009 - 07/2012
- Read Physics at TU Berlin. Graduated with Bachelor of Science in Physics, title of thesis: Spin Dynamics in Adiabatic Transport through Quantum Dots. Partially funded by Deutschlandstipendium in 2011/2012.