About

Christopher Laumann is an Associate Professor of Physics at Boston University, with research interests centered on quantum many-body theory, computation, and sensing. He is a quantum condensed matter theorist focusing on topics at the intersection of quantum many-body theory, defect sensing, and computation. His work includes studying strongly correlated quantum phases, quantum sensing, topological order, spin glasses, many-body localization, synthetic quantum platforms for simulation and computation, quantum satisfiability, and typical-case complexity. Recently, he has concentrated on the dynamics in quantum spin liquids and in disordered ensembles of solid state defects, utilizing such defects to sense materials in high-pressure environments. Laumann collaborates closely with the Yao Group at Harvard on these topics. He holds a Ph.D. in Physics from Princeton University, a CASM in Mathematics with distinction from Cambridge University, and an M.Sc. in Informatics with distinction from the University of Edinburgh. His honors include the Gutzwiller Fellowship in 2025, an NSF Career Award from 2018 to 2023, and a Sloan Research Fellowship from 2016 to 2018.

Research topics

• Physics
• Mathematics
• Quantum mechanics
• Statistical physics
• Condensed matter physics
• Nanotechnology
• Theoretical physics
• Materials science
• Chemical physics
• Optics
• Classical mechanics

Selected publications

• Theta electromagnetism in quantum spin ice: Microscopic analysis of improper symmetries

  Physical review. B./Physical review. B · 2025-05-30

  articleOpen accessSenior author

  $U(1)$ gauge theories, including conventional Maxwell electromagnetism, allow $θ$-terms when parity and time-reversal symmetry are broken. In condensed matter systems, the physics of $θ$ as a magnetoelectric response has been explored extensively within the context of topological insulators and multiferroics. We show how $θ$-terms can arise in the internal dynamics of the emergent electromagnetism in a $U(1)$ quantum spin liquid. In its Coulomb phase, the minimal model of pyrochlore quantum spin ice is governed by a six-spin ring exchange Hamiltonian. We identify the next-order contribution to the microscopic Hamiltonian when parity, time-reversal, and all improper spatial symmetries are broken -- a seven-spin term which leads to a two-parameter lattice gauge theory with a $θ$-electromagnetic phase. We derive how the seven-spin term is generated perturbatively within each of the three symmetry classes of short-range pyrochlore spin ice. Within a complete microscopic symmetry analysis, we find that the most general nearest-neighbor Hamiltonians fail to generate the seven-spin term, and one must include next-nearest-neighbor interactions to obtain an emergent $θ$. Using gauge mean-field theory we compute additional contributions to the $θ$-term from the spinon sector. Finally, we determine the conditions required for an internal $θ$-term to generate a significant external magnetoelectic response.

  PublisherOA PDFDOI

• Elucidating the Inter-system Crossing of the Nitrogen-Vacancy Center up to Megabar Pressures

  ArXiv.org · 2025-11-25

  preprintOpen access

  The integration of Nitrogen-Vacancy color centers into diamond anvil cells has opened the door to quantum sensing at megabar pressures. Despite a multitude of experimental demonstrations and applications ranging from quantum materials to geophysics, a detailed microscopic understanding of how stress affects the NV center remains lacking. In this work, using a combination of first principles calculations as well as high-pressure NV experiments, we develop a complete description of the NV's optical properties under general stress conditions. In particular, our ab initio calculations reveal the complex behavior of the NV's inter-system crossing rates under stresses that both preserve and break the defect's symmetry. Crucially, our proposed framework immediately resolves a number of open questions in the field, including: (i) the microscopic origin of the observed contrast-enhancement in (111)-oriented anvils, and (ii) the surprising observation of NV contrast-inversion in certain high-pressure regimes. Our work lays the foundation for optimizing the performance of NV high-pressure sensors by controlling the local stress environment, and more generally, suggests that symmetry-breaking stresses can be utilized as a novel tuning knob for generic solid-state spin defects.

  PublisherOA PDFDOI

• Optimizing the dynamical preparation of quantum spin lakes on the ruby lattice

  ArXiv.org · 2025-12-09

  preprintOpen access

  Quantum spin liquids are elusive long-range entangled states. Motivated by experiments in Rydberg quantum simulators, recent excitement has centered on the possibility of dynamically preparing a state with quantum spin liquid correlation even when the ground state phase diagram does not exhibit such a topological phase. Understanding the microscopic nature of such quantum spin "lake" states and their relationship to equilibrium spin liquid order remains an essential question. Here, we extend the use of approximately symmetric neural quantum states for real-time evolution and directly simulate the dynamical preparation in systems of up to $N=384$ atoms. We analyze a variety of spin liquid diagnostics as a function of the preparation protocol and optimize the extent of the quantum spin lake thus obtained. In the optimal case, the prepared state shows spin-liquid properties extending over half the system size, with a topological entanglement entropy plateauing close to $γ= \ln 2$. We extract two physical length scales $λ$ and $ξ$ which constrain the extent of the quantum spin lake $\ell$ from above and below.

  PublisherOA PDFDOI

• Uncovering origins of heterogeneous superconductivity in La$_3$Ni$_2$O$_7$ using quantum sensors

  ArXiv.org · 2025-10-02

  preprintOpen access

  The family of nickelate superconductors have long been explored as analogs of the high temperature cuprates. Nonetheless, the recent discovery that certain stoichiometric nickelates superconduct up to high $T_c$ under pressure came as a surprise. The mechanisms underlying the superconducting state remain experimentally unclear. In addition to the practical challenges posed by working in a high pressure environment, typical samples exhibit anomalously weak diamagnetic responses, which have been conjectured to reflect inhomogeneous `filamentary' superconducting states. We perform wide-field, high-pressure, optically detected magnetic resonance spectroscopy to image the local diamagnetic responses of as grown La$_3$Ni$_2$O$_7$ samples \emph{in situ}, using nitrogen vacancy quantum sensors embedded in the diamond anvil cell. These maps confirm significant inhomogeneity of the functional superconducting responses at the few micron scale. By spatially correlating the diamagnetic Meissner response with both the local tensorial stress environment, also imaged \emph{in situ}, and stoichiometric composition, we unravel the dominant mechanisms suppressing and enhancing superconductivity. Our wide-field technique simultaneously provides a broad view of sample behavior and excellent local sensitivity, enabling the rapid construction of multi-parameter phase diagrams from the local structure-function correlations observed at the sub-micron pixel scale.

  PublisherOA PDFDOI

• Approximately Symmetric Neural Networks for Quantum Spin Liquids

  Physical Review Letters · 2025-06-27 · 2 citations

  article

  We propose and analyze a family of approximately symmetric neural networks for quantum spin liquid problems. These tailored architectures are parameter efficient, scalable, and significantly outperform existing symmetry-unaware neural network architectures. Utilizing the mixed-field toric code and PXP Rydberg Hamiltonian models, we demonstrate that our approach is competitive with state-of-the-art tensor network and quantum Monte Carlo methods. Moreover, at the largest system sizes (N=480 for toric code, N=1584 for Rydberg PXP), our method allows us to explore Hamiltonians with sign problems beyond the reach of both quantum Monte Carlo and finite-size matrix-product states. The network comprises an exactly symmetric block following a nonsymmetric block, which we argue learns a transformation of the ground state analogous to quasiadiabatic continuation. Our Letter paves the way toward investigating quantum spin liquid problems within interpretable neural network architectures.

  PublisherDOI

• Unitary <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>k</mml:mi></mml:mrow></mml:math>-Designs from Random Number-Conserving Quantum Circuits

  Physical Review X · 2025-04-21 · 8 citations

  articleOpen accessSenior author

  Local random circuits scramble efficiently and, accordingly, have a range of applications in quantum information and quantum dynamics. With a global U(1) charge, however, the scrambling ability is reduced; for example, such random circuits do not generate the entire group of number-conserving unitaries. We establish two results using the statistical mechanics of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>k</a:mi></a:math>-fold replicated circuits. First, we show that finite moments cannot distinguish the ensemble that local random circuits generate from the Haar ensemble on the entire group of number-conserving unitaries. Specifically, the circuits form a <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>k</c:mi><c:mi>c</c:mi></c:msub></c:math>-design with <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi>k</e:mi><e:mi>c</e:mi></e:msub><e:mo>=</e:mo><e:mi>O</e:mi><e:mo stretchy="false">(</e:mo><e:msup><e:mi>L</e:mi><e:mi>d</e:mi></e:msup><e:mo stretchy="false">)</e:mo></e:math> for a system in <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>d</i:mi></i:math> spatial dimensions with linear dimension <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>L</k:mi></k:math>. Second, for <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>k</m:mi><m:mo>&lt;</m:mo><m:msub><m:mi>k</m:mi><m:mi>c</m:mi></m:msub></m:math>, we derive bounds on the depth <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mi>τ</o:mi></o:math> required for the circuit to converge to an approximate <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mi>k</q:mi></q:math>-design. The depth is lower bounded by diffusion <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mi>k</s:mi><s:msup><s:mi>L</s:mi><s:mn>2</s:mn></s:msup><s:mi>ln</s:mi><s:mo stretchy="false">(</s:mo><s:mi>L</s:mi><s:mo stretchy="false">)</s:mo><s:mo>≲</s:mo><s:mi>τ</s:mi></s:math>. In contrast, without number conservation <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mi>τ</w:mi><w:mo>∼</w:mo><w:mrow><w:mi>poly</w:mi></w:mrow><w:mo stretchy="false">(</w:mo><w:mi>k</w:mi><w:mo stretchy="false">)</w:mo><w:mi>L</w:mi></w:math>. The convergence of the circuit ensemble is controlled by the low-energy properties of a frustration-free quantum statistical model which spontaneously breaks <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"><ab:mi>k</ab:mi></ab:math> U(1) symmetries. We conjecture that the associated Goldstone modes set the spectral gap for arbitrary spatial and qudit dimensions, leading to an upper bound <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML" display="inline"><cb:mi>τ</cb:mi><cb:mo>≲</cb:mo><cb:mi>k</cb:mi><cb:msup><cb:mi>L</cb:mi><cb:mrow><cb:mi>d</cb:mi><cb:mo>+</cb:mo><cb:mn>2</cb:mn></cb:mrow></cb:msup></cb:math>.

  PublisherOA PDFDOI

• Quantum optics of strongly driven optical phonons

  2025-01-01

  article

  We show that ultrafast driving of polar insulators enables creating squeezed states of phonons and entangled states of photons at mid-infrared frequencies. The same effects also allow ultrafast control of the optical response of materials.

  PublisherDOI

• Efficient Local Classical Shadow Tomography with Number Conservation

  Physical Review Letters · 2024-08-07 · 11 citations

  articleSenior author

  Shadow tomography aims to build a classical description of a quantum state from a sequence of simple random measurements. Physical observables are then reconstructed from the resulting classical shadow. Shadow protocols which use single-body random measurements are simple to implement and capture few-body observables efficiently, but do not apply to systems with fundamental number conservation laws, such as ultracold atoms. We address this shortcoming by proposing and analyzing a new local shadow protocol adapted to such systems. The All-Pairs protocol requires one layer of two-body gates and only poly(V) samples to reconstruct arbitrary few body observables. Moreover, by exploiting the permutation symmetry of the protocol, we derive a linear time postprocessing algorithm which applies to both hardcore bosons and spinless fermions in any spatial dimension. We provide a proof-of-principle reference implementation and demonstrate the reconstruction of two- and four-point functions in a paired Luttinger liquid of hardcore bosons.

  PublisherDOI

• A New Proof of Many-Body Localization in Finite Spin Chains

  Journal Club for Condensed Matter Physics · 2024-09-30

  articleOpen access1st authorCorresponding

  Absence of Normal Heat Conduction in Strongly Disordered Interacting Quantum Chains Authors: Wojciech De Roeck, Lydia Giacomin, Francois Huveneers, and Oskar Pro&sacute;niak arXiv:2408.04338, DOI: 10.48550/arXiv.2408.04338 Recommended with a commentary by Chris R. Laumann , Boston University and Max-Planck-Institute for Physics of Complex Systems |View Commentary (pdf)| This commentary may be cited as: DOI: 10.36471/JCCM_September_2024_02 https://doi.org/10.36471/JCCM_September_2024_02

  PublisherOA PDFDOI

• [111]-strained spin ice: Localization of thermodynamically deconfined monopoles

  arXiv (Cornell University) · 2024-06-26

  preprintOpen accessSenior author

  We study classical spin ice under uniaxial strain along the $[111]$ crystallographic axis. Remarkably, such strain preserves the extensive ice degeneracy and the corresponding classical Coulomb phase. The emergent monopole excitations remain thermodynamically deconfined exactly as in the isotropic case. However, their motion under local heat bath dynamics depends qualitatively on the sign of the strain. In the low-temperature limit for negative strain, the monopoles diffuse, while for positive strain, they localize. Introducing additional ring exchange dynamics into the ice background transforms the localized monopoles into sub-dimensional excitations whose motion is restricted to diffusion in the $(111)$-plane. The phenomena we identify are experimentally accessible in rare-earth pyrochlores under uniaxial pressure as well as in tripod kagome materials. The diffusive versus localized nature of the monopoles manifests in characteristic magnetic noise spectra, which we compute.

  PublisherOA PDFDOI

Recent grants

• Quantum Mechanics at the Complexity Frontier

  NSF · $276k · 2016–2018

• Quantum Mechanics at the Complexity Frontier

  NSF · $195k · 2015–2016

• CAREER: Quantum Optimization, Glassiness and Localization

  NSF · $500k · 2018–2023

Frequent coauthors

• Norman Y. Yao

  44 shared

• Antonello Scardicchio

  39 shared

• Anushya Chandran

  Harvard University

  30 shared

• Roderich Moessner

  Max Planck Institute for Physics

  22 shared

• Siddhardh C. Morampudi

  17 shared

• S. L. Sondhi

  University of Oxford

  15 shared

• Michael Knap

  15 shared

• Christopher L. Baldwin

  14 shared

Labs

• PhysicsPI

Education

• Ph.D., Physics

  Princeton University

  2010

• CASM, DAMTP, DPMMS

  University of Cambridge

  2005

• M.Sc., Informatics

  University of Edinburgh

  2004

• AB

  Harvard University

  2003

Awards & honors

• Gutzwiller Fellow 2025
• NSF Career Award 2018-2023
• Sloan Research Fellow 2016-2018