Research
My research is driven by a central question: why does the observable universe contain almost no antimatter? Addressing this requires fully nonperturbative, real-time simulations of quantum field theories — a regime where conventional methods such as perturbative expansions or Euclidean lattice Monte Carlo break down entirely. Quantum computers and tensor networks, uniquely suited to simulating real-time quantum evolution, offer a path forward.
My work develops the QFT formalism, quantum and classical algorithms, and qubit embeddings needed to make real-time simulations a practical tool for fundamental physics. Baryogenesis serves as a concrete and demanding target that shapes these formal and algorithmic developments, while the resulting tools apply broadly to strongly coupled QFTs and physics beyond the Standard Model.
A complete list of publications is on iNSPIRE and Google Scholar.
Chiral Fermions and Lattice Chiral Gauge Theories
Realistic simulations of the Standard Model require careful handling of chiral symmetry. There are two related problems on the lattice, both of which I am actively working on.
The “easy” problem concerns theories like QCD with an anomalous global chiral symmetry. While the Ginsparg-Wilson (GW) relation provides a precise understanding of chiral symmetry on the Euclidean lattice, its implementation in Hamiltonian frameworks needed for real-time quantum simulation has remained incomplete. I have developed Hamiltonian generalizations of GW fermions that yield lattice theories with exact chiral symmetry and correct anomaly structure.
The “hard” problem concerns gauging the chiral symmetry, as in the electroweak sector of the Standard Model. This problem remains open even on Euclidean lattices and stands among the deepest unsolved problems in theoretical physics. I am using tensor-network formulations to study anomaly cancellation in 1+1-dimensional chiral gauge theories, providing nonperturbative checks of consistency conditions that are otherwise difficult to probe, while also enabling concrete numerical investigations.
M. Clancy, D. B. Kaplan, H. Singh
Phys. Rev. D · 2024 · arXiv:2309.08542
We give a general derivation of Ginsparg-Wilson relations for both Dirac and Majorana fermions in any dimension. These relations encode continuous and discrete chiral, parity and time reversal anomalies and will apply to the various classes of free fermion topological insulators and superconductors (in the framework of a relativistic quantum field theory in Euclidean spacetime). We show how to formulate the exact symmetries of the lattice action and the relevant index theorems for the anomalies.
@article{Clancy:2023ino,
author = "Clancy, Michael and Kaplan, David B. and Singh, Hersh",
title = "{Generalized Ginsparg-Wilson relations}",
eprint = "2309.08542",
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reportNumber = "INT-PUB-23-024, IQuS@UW-21-064, FERMILAB-PUB-23-541-T",
doi = "10.1103/PhysRevD.109.014502",
journal = "Phys. Rev. D",
volume = "109",
number = "1",
pages = "014502",
year = "2024"
}H. Singh
arXiv:2505.20419
We construct a family of Ginsparg-Wilson Hamiltonians with improved chiral properties, starting from a construction of Creutz-Horvath-Neuberger that provides a doubler-free Hamiltonian lattice regularization for Dirac fermions in even spacetime dimensions. We use a higher-order generalization of the Ginsparg-Wilson relation due to Fujikawa, which yields an order- Hamiltonian overlap operator for each integer , with an exactly conserved but nonquantized chiral charge that becomes quantized as . Our construction provides physical insight into how Fujikawa's higher-order Ginsparg-Wilson relation improves chiral symmetry while reproducing the anomaly, highlighting the trade-offs inherent in any Hamiltonian lattice realization of an anomalous chiral symmetry. This class of Hamiltonian lattice regularizations, with their tunable chiral symmetry properties, offers potential advantages for quantum and tensor-network simulations.
@article{Singh:2025sye,
author = "Singh, Hersh",
title = "{Ginsparg-Wilson Hamiltonians with Improved Chiral Symmetry}",
eprint = "2505.20419",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
reportNumber = "FERMILAB-PUB-25-0330-T",
month = "5",
year = "2025"
}M. Nguyen, H. Singh
arXiv:2405.11348
We consider the Atiyah-Patodi-Singer (APS) index theorem corresponding to the chiral symmetry of a continuum formulation of staggered fermions called Kähler-Dirac fermions, which have been recently investigated as an ingredient in lattice constructions of chiral gauge theories. We point out that there are two notions of chiral symmetry for Kähler-Dirac fermions, both having a mixed perturbative anomaly with gravity leading to index theorems on closed manifolds. By formulating these theories on a manifold with boundary, we find the APS index theorems corresponding to each of these symmetries, necessary for a complete picture of anomaly inflow, using a recently discovered physics-motivated proof. We comment on a fundamental difference between the nature of these two symmetries by showing that a sensible local, symmetric boundary condition only exists for one of the two symmetries. This sheds light on how these symmetries behave under lattice discretization, and in particular on their use for recent symmetric-mass generation proposals.
@article{Nguyen:2024wck,
author = "Nguyen, Mendel and Singh, Hersh",
title = "{Chiral symmetry and Atiyah-Patodi-Singer index theorem for staggered fermions}",
eprint = "2405.11348",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
reportNumber = "FERMILAB-PUB-24-0262-T",
month = "5",
year = "2024"
}H. Singh
PoS · 2025 · arXiv:2503.05900
The Ginsparg-Wilson (GW) relation elegantly captures how the anomalous chiral symmetry of a Dirac fermion manifests on the lattice. In this talk, we discuss how the GW relation and its closed-form solution, the overlap operator, can be generalized to Majorana or Dirac fermions in any dimension for finite symmetry transformations (continuous or discrete). We find an exact symmetry which reproduces both perturbative and global anomalies on the lattice. These generalized GW fermions are boundary theories of various bulk symmetry-protected topological phases and thus provide an explicit lattice realization of the fermionic bulk-boundary correspondence central to recent proposals for chiral gauge theories on the lattice.
@article{Singh:2025wet,
author = "Singh, Hersh",
title = "{Generalized Ginsparg-Wilson relations: Fermionic anomalies on the lattice}",
eprint = "2503.05900",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
reportNumber = "FERMILAB-CONF-25-0062-T",
doi = "10.22323/1.466.0375",
journal = "PoS",
volume = "LATTICE2024",
pages = "375",
year = "2025"
}H. Lamm, A. Roggero, H. Singh, L. Spagnoli
arXiv:2607.28524
We give a quantum signal processing (QSP) algorithm for the overlap fermion Hamiltonian which preserves the Ginsparg-Wilson relation up to a controllable error . Quantum simulations of Dirac fermions with exact chiral symmetry are thus nearly free: applying the overlap Hamiltonian costs only a factor logarithmic in more than the Wilson-Dirac Hamiltonian. Comparing to domain-wall fermions, a mild overhead is found in circuit complexity while reducing qubit costs. We show how QSP effectively constructs an extra dimension when simulating the overlap operator, illustrating that the scaling of quantum algorithms reflects the deeper physics of overlap fermions arising at the boundary of domain-wall fermions.
@article{Lamm:2026xqk,
author = "Lamm, Henry and Roggero, Alessandro and Singh, Hersh and Spagnoli, Luca",
title = "{Exact chiral symmetry with quantum signal processing}",
eprint = "2607.28524",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
reportNumber = "FERMILAB-PUB-26-0223-T",
month = "7",
year = "2026"
}Real-Time Simulations for Baryogenesis
Baryogenesis — the dynamical process that generated the observed matter-antimatter asymmetry in the early universe — requires out-of-equilibrium, nonperturbative dynamics that are inaccessible to perturbative methods and Euclidean lattice simulations. Real-time Hamiltonian simulations, using tensor networks and quantum computers, offer a path to studying these phenomena from first principles.
I am developing a systematic program I call the baryogenesis ladder: a sequence of increasingly realistic models progressing from 1+1 dimensions toward fully dynamical gauge and Higgs fields in 3+1 dimensions, with quantum algorithms developed at each stage. As the first rung, I used tensor-network simulations with dynamical fermions to study charge-asymmetry generation during fermion–bubble scattering across a first-order phase transition. These simulations introduced new real-time observables that directly quantify asymmetry production — quantities inaccessible to perturbative treatments and Euclidean lattice methods.
M. Carena, Y.-Y. Li, T. Ou, H. Singh
Phys. Rev. D · 2026 · arXiv:2412.10365
Motivated by the out-of-equilibrium dynamics during an early-universe first-order phase transition, we perform real-time simulations of fermion-bubble scattering in dimensions. This nonequilibrium process can generate a charge-conjugation asymmetry outside the bubble wall, induced by the complex fermion mass profile. The resulting asymmetry is the 1+1-dimensional analog of the asymmetry in 3+1 dimensions, a key ingredient in baryon asymmetry generation at the electroweak scale. Using tensor network methods, we track the real-time evolution of the asymmetry in the charge density as the fermion interacts with the bubble wall, a regime inaccessible to analytic calculations. We further introduce two observables to quantify the asymmetry in the asymptotic region where reflected particles are well separated from the scattering point: one based on the net charge outside the bubble wall, and the other on the spatial displacement between the reflected particle and antiparticle wavepackets. Our study represents a first step toward nonperturbative, real-time computations of asymmetry in 3+1 dimensions for electroweak baryogenesis.
@article{Carena:2024peb,
author = "Carena, Marcela and Li, Ying-Ying and Ou, Tong and Singh, Hersh",
title = "{Real-time simulation of asymmetry generation in fermion-bubble collisions}",
eprint = "2412.10365",
archivePrefix = "arXiv",
primaryClass = "hep-ph",
reportNumber = "FERMILAB-PUB-24-0931-T",
doi = "10.1103/nphy-2y8q",
journal = "Phys. Rev. D",
volume = "113",
number = "1",
pages = "014502",
year = "2026"
}Qubit Regularization of Quantum Field Theories
A central question in lattice field theory is whether a quantum field theory can be regularized using a system with a finite-dimensional local Hilbert space — a “qubit model”. This is both a foundational question about the nature of QFTs and a practical one: quantum computers natively operate on finite-dimensional systems, so such a regularization is a prerequisite for quantum simulation.
My work on qubit regularization has focused on asymptotically free sigma models as prototypes for non-Abelian gauge theories. I showed that the (1+1)D O(3) nonlinear sigma model — including its asymptotic freedom and topological θ vacua — can be reproduced by a simple spin-1/2 Hamiltonian with two qubits per site. A key challenge was constructing sign-problem-free formulations to enable classical Monte Carlo verification of the qubit models, which we achieved using worldline methods. More recently, the program has been extended to O(N) models and to understanding how anomaly matching constrains which qubit regularizations are possible.
H. Singh, S. Chandrasekharan
Phys. Rev. D · 2019 · arXiv:1905.13204
We construct a qubit regularization of the non-linear sigma model in two and three spatial dimensions using a quantum Hamiltonian with two qubits per lattice site. Using a worldline formulation and worm algorithms, we show that in two spatial dimensions our model has a quantum critical point where the well-known scale-invariant physics of the three-dimensional Wilson-Fisher fixed point is reproduced. In three spatial dimensions, we recover mean-field critical exponents at a similar quantum critical point. These results show that our qubit Hamiltonian is in the same universality class as the traditional classical lattice model close to the critical points. Simple modifications to our model also allow us to study the physics of traditional lattice models with and symmetries close to the corresponding critical points.
@article{Singh:2019uwd,
author = "Singh, Hersh and Chandrasekharan, Shailesh",
title = "{Qubit regularization of the $O(3)$ sigma model}",
eprint = "1905.13204",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
doi = "10.1103/PhysRevD.100.054505",
journal = "Phys. Rev. D",
volume = "100",
number = "5",
pages = "054505",
year = "2019"
}T. Bhattacharya, A. J. Buser, S. Chandrasekharan, R. Gupta, H. Singh
Phys. Rev. Lett. · 2021 · arXiv:2012.02153
We provide strong evidence that the asymptotically free (1+1)-dimensional non-linear O(3) sigma model can be regularized using a quantum lattice Hamiltonian, referred to as the "Heisenberg-comb", that acts on a Hilbert space with only two qubits per spatial lattice site. The Heisenberg-comb consists of a spin-half anti-ferromagnetic Heisenberg-chain coupled anti-ferromagnetically to a second local spin-half particle at every lattice site. Using a world-line Monte Carlo method we show that the model reproduces the universal step-scaling function of the traditional model up to correlation lengths of 200,000 in lattice units and argue how the continuum limit could emerge. We provide a quantum circuit description of time-evolution of the model and argue that near-term quantum computers may suffice to demonstrate asymptotic freedom.
@article{Bhattacharya:2020gpm,
author = "Bhattacharya, Tanmoy and Buser, Alexander J. and Chandrasekharan, Shailesh and Gupta, Rajan and Singh, Hersh",
title = "{Qubit regularization of asymptotic freedom}",
eprint = "2012.02153",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
reportNumber = "LA-UR-20-29558",
doi = "10.1103/PhysRevLett.126.172001",
journal = "Phys. Rev. Lett.",
volume = "126",
number = "17",
pages = "172001",
year = "2021"
}H. Singh
Phys. Rev. D · 2022 · arXiv:1911.12353
Motivated by the prospect of quantum simulation of quantum field theories, we formulate the nonlinear sigma model as a "qubit" model with an -dimensional local Hilbert space at each lattice site. Using an efficient worm algorithm in the worldline formulation, we demonstrate that the model has a second-order critical point in dimensions, where the continuum physics of the nontrivial Wilson-Fisher fixed point is reproduced. We compute the critical exponents and for the qubit models up to , and find excellent agreement with known results in literature from various analytic and numerical techniques for the Wilson-Fisher universality class. Our models are suited for studying nonlinear sigma models on quantum computers up to in spatial dimensions.
@article{Singh:2019jog,
author = "Singh, Hersh",
title = "{Qubit regularized O(N) nonlinear sigma models}",
eprint = "1911.12353",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
reportNumber = "IQuS@UW-21-021,INT-PUB-22-007",
doi = "10.1103/PhysRevD.105.114509",
journal = "Phys. Rev. D",
volume = "105",
number = "11",
pages = "114509",
year = "2022"
}S. Caspar, H. Singh
Phys. Rev. Lett. · 2022 · arXiv:2203.15766
Conventional lattice formulations of vacua in the -dimensional nonlinear sigma model suffer from a sign problem. Here, we construct the first sign-problem-free regularization for arbitrary . Using efficient lattice Monte Carlo algorithms, we demonstrate how a Hamiltonian model of spin- degrees of freedom on a 2-dimensional spatial lattice reproduces both the infrared sector for arbitrary , as well as the ultraviolet physics of asymptotic freedom. Furthermore, as a model of qubits on a two-dimensional square lattice with only nearest-neighbor interactions, it is naturally suited for studying the physics of vacua and asymptotic freedom on near-term quantum devices. Our construction generalizes to vacua in all models, solving a long standing sign problem.
@article{Caspar:2022llo,
author = "Caspar, Stephan and Singh, Hersh",
title = "{From Asymptotic Freedom to {\ensuremath{\theta}} Vacua: Qubit Embeddings of the O(3) Nonlinear {\ensuremath{\sigma}} Model}",
eprint = "2203.15766",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
reportNumber = "IQuS@UW-21-025, INT-PUB-22-012",
doi = "10.1103/PhysRevLett.129.022003",
journal = "Phys. Rev. Lett.",
volume = "129",
number = "2",
pages = "022003",
year = "2022"
}H. Singh
arXiv:2203.00059
Recent work using a large-charge expansion for the Wilson-Fisher conformal field theory has shown that the anomalous dimensions of large-charge operators can be expressed in terms of a few low-energy constants (LECs) of a large-charge effective field theory (EFT). By performing lattice Monte Carlo computations at the Wilson-Fisher fixed point, we compute the anomalous dimensions of large-charge operators up to and charge , and extract the leading and subleading LECs of the large-charge EFT. To alleviate the signal-to-noise ratio problem present in the large-charge sector of conventional lattice formulations of the theory, we employ a recently developed qubit formulation of the nonlinear sigma models with a worm algorithm. This enables us to test the validity of the large-charge expansion and the recent large- predictions for the coefficients of the large-charge EFT.
@article{Singh:2022akp,
author = "Singh, Hersh",
title = "{Large-charge conformal dimensions at the $O(N)$ Wilson-Fisher fixed point}",
eprint = "2203.00059",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
reportNumber = "IQuS@UW-21-022,INT-PUB-22-008",
month = "2",
year = "2022"
}J. Zhou, H. Singh, T. Bhattacharya, S. Chandrasekharan, R. Gupta
Phys. Rev. D · 2022 · arXiv:2111.13780
We explore if space-time symmetric lattice field theory models with a finite Hilbert space per lattice site can reproduce asymptotic freedom in the two-dimensional model. We focus on a simple class of such models with a five dimensional local Hilbert space. We demonstrate how even the simplest model reproduces asymptotic freedom within the D-theory formalism but at the cost of increasing the size of the Hilbert space through coupling several layers of a two-dimensional lattice. We then argue that qubit regularization can be viewed as an effective field theory (EFT) even if the continuum limit cannot be reached, as long as we can tune the model close enough to the continuum limit where perturbation theory, or other analytical techniques, become viable. We construct a simple lattice model on a single layer with a four dimensional local Hilbert space that acts like an excellent EFT of the original theory.
@article{Zhou:2021qpm,
author = "Zhou, Junzhe and Singh, Hersh and Bhattacharya, Tanmoy and Chandrasekharan, Shailesh and Gupta, Rajan",
title = "{Spacetime symmetric qubit regularization of the asymptotically free two-dimensional O(4) model}",
eprint = "2111.13780",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
reportNumber = "LA-UR-21-30982, INT-PUB-21-023, IQuS@UW-21-013",
doi = "10.1103/PhysRevD.105.054510",
journal = "Phys. Rev. D",
volume = "105",
number = "5",
pages = "054510",
year = "2022"
}M. Nguyen, H. Singh
Phys. Rev. D · 2023 · arXiv:2209.12630
Anomalies are a powerful way to gain insight into possible lattice regularizations of a quantum field theory. In this work, we argue that the continuum anomaly for a given symmetry can be matched by a manifestly-symmetric, local, lattice regularization in the same spacetime dimensionality only if (i) the symmetry action is offsite, or (ii) if the continuum anomaly is reproduced exactly on the lattice. We consider lattice regularizations of a class of prototype models of QCD: the (1+1)-dimensional asymptotically-free Grassmannian nonlinear sigma models (NLSMs) with a term. Using the Grassmannian NLSMs as a case study, we provide examples of lattice regularizations in which both possibilities are realized. For possibility (i), we argue that Grassmannian NLSMs can be obtained from antiferromagnets with a well-defined continuum limit, reproducing both the infrared physics of vacua and the ultraviolet physics of asymptotic freedom. These results enable the application of new classical algorithms to lattice Monte Carlo studies of these quantum field theories, and provide a viable realization suited for their quantum simulation. On the other hand, we show that, perhaps surprisingly, the conventional lattice regularization of vacua due to Berg and Lüscher reproduces the anomaly exactly on the lattice, providing a realization of the second possibility.
@article{Nguyen:2022aaq,
author = "Nguyen, Mendel and Singh, Hersh",
title = "{Lattice regularizations of {\ensuremath{\theta}} vacua: Anomalies and qubit models}",
eprint = "2209.12630",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
reportNumber = "IQuS@UW-21-031, INT-PUB-22-027",
doi = "10.1103/PhysRevD.107.014507",
journal = "Phys. Rev. D",
volume = "107",
number = "1",
pages = "014507",
year = "2023"
}T. Bhattacharya, S. Chandrasekharan, R. Gupta, T. R. Richardson, H. Singh
arXiv:2310.06805
Qubit regularization provides a rich framework to explore quantum field theories. The freedom to choose how the important symmetries of the theory are embedded in the qubit regularization scheme allows us to construct new lattice models with rich phase diagrams. Some of the phases can contain topological terms which lead to critical phases. In this work we introduce and study the SU(3)-F qubit regularization scheme to embed the SO(3) spin-symmetry. We argue that qubit models in this regularization scheme contain several phases including a critical phase which describes the k = 1 Wess-Zumino-Witten (WZW) conformal field theory (CFT) at long distances, and two massive phases one of which is trvially gapped and the other which breaks the lattice translation symmetry. We construct a simple space-time Euclidean lattice model with a single coupling U and study it using the Monte Carlo method. We show the model has a critical phase at small U and a trivially massive phase at large U with a first order transition separating the two. Another feature of our model is that it is symmetric under space-time rotations, which means the temporal and spatial lattice spacing are connected to each other. The unitary time evolution operator obtained by a Wick rotation of the transfer matrix of our model can help us compute the physics of the k = 1 WZW CFT in real time without the need for tuning the temporal lattice spacing to zero. We use this idea to introduce the concept of a relativistic quantum circuit on a discrete space-time lattice.
@article{Bhattacharya:2023wuz,
author = "Bhattacharya, Tanmoy and Chandrasekharan, Shailesh and Gupta, Rajan and Richardson, Thomas R. and Singh, Hersh",
title = "{Topological terms with qubit regularization and relativistic quantum circuits}",
eprint = "2310.06805",
archivePrefix = "arXiv",
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reportNumber = "LA-UR-22-27102, MITP-23-013, FERMILAB-PUB-23-604-T",
month = "10",
year = "2023"
}Quantum Simulation on Near-Term Hardware
Qubit regularization provides a Hamiltonian formulation suitable for implementation on quantum hardware. A separate set of questions then arises: how do we actually reach the continuum limit on a finite quantum device, and what platforms are best suited for simulating specific models?
I have worked on mapping the qubit-regularized O(3) sigma model onto cold-atom platforms, identifying a dimensional reduction strategy that allows one to approach the continuum limit with fewer physical qubits. I have also studied Floquet engineering in Ising models, showing that a strongly driven Ising chain reproduces effective Heisenberg dynamics — providing a practical route to engineering the interactions needed for sigma model simulation on existing hardware.
A. N. Ciavarella, S. Caspar, H. Singh, M. J. Savage
Phys. Rev. A · 2023 · arXiv:2211.07684
The 1+1D O(3) non-linear σ-model is a model system for future quantum lattice simulations of other asymptotically-free theories, such as non-Abelian gauge theories. We find that utilizing dimensional reduction can make efficient use of two-dimensional layouts presently available on cold atom quantum simulators. A new definition of the renormalized coupling is introduced, which is applicable to systems with open boundary conditions and can be measured using analog quantum simulators. Monte Carlo and tensor network calculations are performed to determine the quantum resources required to reproduce perturbative short-distance observables. In particular, we show that a rectangular array of 48 Rydberg atoms with existing quantum hardware capabilities should be able to adiabatically prepare low-energy states of the perturbatively-matched theory. These states can then be used to simulate non-perturbative observables in the continuum limit that lie beyond the reach of classical computers.
@article{Ciavarella:2022qdx,
author = "Ciavarella, Anthony N. and Caspar, Stephan and Singh, Hersh and Savage, Martin J.",
title = "{Preparation for quantum simulation of the (1+1)-dimensional O(3) nonlinear {\ensuremath{\sigma}} model using cold atoms}",
eprint = "2211.07684",
archivePrefix = "arXiv",
primaryClass = "quant-ph",
reportNumber = "IQuS@UW-21-038, INT-PUB-22-051",
doi = "10.1103/PhysRevA.107.042404",
journal = "Phys. Rev. A",
volume = "107",
number = "4",
pages = "042404",
year = "2023"
}A. N. Ciavarella, S. Caspar, H. Singh, M. J. Savage, P. Lougovski
Phys. Rev. A · 2023 · arXiv:2207.09438
The time-evolution of an Ising model with large driving fields over discrete time intervals is shown to be reproduced by an effective XXZ-Heisenberg model at leading order in the inverse field strength. For specific orientations of the drive field, the dynamics of the XXX-Heisenberg model is reproduced. These approximate equivalences, valid above a critical driving field strength set by dynamical phase transitions in the Ising model, are expected to enable quantum devices that natively evolve qubits according to the Ising model to simulate more complex systems.
@article{Ciavarella:2022tvc,
author = "Ciavarella, Anthony N. and Caspar, Stephan and Singh, Hersh and Savage, Martin J. and Lougovski, Pavel",
title = "{Simulating Heisenberg interactions in the Ising model with strong drive fields}",
eprint = "2207.09438",
archivePrefix = "arXiv",
primaryClass = "quant-ph",
reportNumber = "IQuS@UW-21-028, INT-PUB-22-020",
doi = "10.1103/PhysRevA.108.042216",
journal = "Phys. Rev. A",
volume = "108",
number = "4",
pages = "042216",
year = "2023"
}H. Singh, T. Bhattacharya, S. Chandrasekharan, R. Gupta
arXiv:2302.12858
The low-energy states of quantum many body systems, such as spin chains, are entangled. Using tensor network computations, we demonstrate a protocol that distills Bell pairs out of the ground state of the prototypical transverse-field Ising model. We explore the behavior of rate of entanglement distillation in various phases, and possible optimizations of the protocol. Finally, we comment on the protocol as we approach quantum criticality defining a continuum field theory.
@article{Singh:2023jii,
author = "Singh, Hersh and Bhattacharya, Tanmoy and Chandrasekharan, Shailesh and Gupta, Rajan",
title = "{Vacuum Entanglement Harvesting in the Ising Model}",
eprint = "2302.12858",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
reportNumber = "LA-UR-21-26805, IQuS@UW-21-045, INT-PUB-23-004",
month = "2",
year = "2023"
}Few-Body Nuclear Physics and Pionless EFT
During my PhD I worked on non-relativistic few-body systems using pionless effective field theory (EFT) — the systematic low-energy expansion for nuclear systems well below the pion mass. One line of work used the large-N_c expansion to derive relationships among two-nucleon contact couplings that are otherwise independent in the EFT, providing a deeper organizational principle consistent with experiment. I also developed a worldline (spacetime lattice) approach to few-body systems as an alternative to Hamiltonian methods, enabling worm-algorithm Monte Carlo for fixed particle-number sectors.
M. R. Schindler, H. Singh, R. P. Springer
Phys. Rev. C · 2018 · arXiv:1805.06056
We analyze two-derivative two-nucleon interactions in a combined pionless effective field theory and large- expansion. At leading order in the large- expansion, relationships among low-energy constants emerge. We find these to be consistent with experiment. However, it is critical to correctly address the subtraction-point dependence of the low-energy constants. These results provide additional confidence that the dual-expansion procedure is useful for analyzing low-energy few-body observables.
@article{Schindler:2018irz,
author = "Schindler, Matthias R. and Singh, Hersh and Springer, Roxanne P.",
title = "{Large-$N_c$ Relationships Among Two-Derivative Pionless Effective Field Theory Couplings}",
eprint = "1805.06056",
archivePrefix = "arXiv",
primaryClass = "nucl-th",
doi = "10.1103/PhysRevC.98.044001",
journal = "Phys. Rev. C",
volume = "98",
number = "4",
pages = "044001",
year = "2018"
}H. Singh, S. Chandrasekharan
Phys. Rev. D · 2019 · arXiv:1812.05080
We formulate the physics of two species of non-relativistic hard-core bosons with attractive or repulsive delta function interactions on a space-time lattice in the worldline approach. We show that worm algorithms can efficiently sample the worldline configurations in any fixed particle-number sector if the chemical potential is tuned carefully. Since fermions can be treated as hard-core bosons up to a permutation sign, we apply this approach to study non-relativistic fermions. The fermion permutation sign is an observable in this approach and can be used to extract energies in each particle-number sector. In one dimension, non-relativistic fermions can only permute across boundaries, and so our approach does not suffer from sign problems in many cases, unlike the auxiliary field method. Using our approach, we discover limitations of the recently proposed complex Langevin calculations in one spatial dimension for some parameter regimes. In higher dimensions, our method suffers from the usual fermion sign problem. Here we provide evidence that it may be possible to alleviate this problem for few-body physics
@article{Singh:2018mnm,
author = "Singh, Hersh and Chandrasekharan, Shailesh",
title = "{Few-body physics on a spacetime lattice in the worldline approach}",
eprint = "1812.05080",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
doi = "10.1103/PhysRevD.99.074511",
journal = "Phys. Rev. D",
volume = "99",
number = "7",
pages = "074511",
year = "2019"
}X. Lin, H. Singh, R. P. Springer, J. Vanasse
Phys. Rev. C · 2023 · arXiv:2210.15650
We calculate the cold neutron-deuteron () capture cross section, , to next-to-next-to leading order (NNLO) using the model-independent approach of pionless effective field theory (EFT()). At leading order we find mb, while the experimental result is 0.508(15) mb [Jurney, Bendt and Browne in Phys. Rev. C 25, 2810 (1982)] for a laboratory neutron velocity of 2200 m/s. At next-to-leading-order (NLO), we show that is sensitive to the low energy constant (LEC), , of the two-nucleon isovector current appearing at NLO. A fit of at NLO to the triton magnetic moment yields a NLO prediction of mb, where the error comes from propagating the error from the fit. At next-to-next-to-leading-order (NNLO), we find that a new three-nucleon magnetic moment counterterm is required for renormalization group invariance of both and the triton magnetic moment. Fitting the NNLO correction to (denoted ) to cold neutron-proton capture () yields a NNLO prediction of mb, where the error comes from propagating the error from the fit. We also study different fittings of and to , , and/or the triton magnetic moment. For example, fitting simultaneously to , , and the triton magnetic moment at NLO, and fitting simultaneously to and at NNLO, yields mb and mb, respectively, where errors are naively estimated from EFT() power counting. In addition, we discuss how Wigner-SU(4) symmetry may alter the naive EFT() expansion of .
@article{Lin:2022yaf,
author = "Lin, Xincheng and Singh, Hersh and Springer, Roxanne P. and Vanasse, Jared",
title = "{Cold neutron-deuteron capture and Wigner-SU(4) symmetry}",
eprint = "2210.15650",
archivePrefix = "arXiv",
primaryClass = "nucl-th",
reportNumber = "INT-PUB-22-029",
doi = "10.1103/PhysRevC.108.044001",
journal = "Phys. Rev. C",
volume = "108",
number = "4",
pages = "044001",
year = "2023"
}H. Singh
PoS · 2018 · arXiv:1812.02364
We study the physics of two species of non-relativistic hard-core bosons with attractive or repulsive delta function interactions on a spacetime lattice using the worldline formulation. By tuning the chemical potential carefully we show that worm algorithms can efficiently sample the worldline configurations in any fixed particle-number sector. Since fermions can be treated as hard-core bosons up to a permutation sign, we also apply this approach to non-relativistic fermions. The fermion permutation sign is treated as an observable in this approach and can be used to extract energies for each particle-number sector. Since in one dimension non-relativistic fermions can only permute due to boundary effects, unlike the auxiliary field method, in many cases our approach does not suffer from sign problems. Using our method we discover limitations of the recently proposed complex Langevin calculations in one dimension.
@article{Singh:2018pci,
author = "Singh, Hersh",
title = "{Worldline approach to few-body physics on the lattice}",
eprint = "1812.02364",
archivePrefix = "arXiv",
primaryClass = "hep-lat",
doi = "10.22323/1.334.0158",
journal = "PoS",
volume = "LATTICE2018",
pages = "158",
year = "2018"
}