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G. J. Sreejith

Publications and source records attributed to G. J. Sreejith.

At least 19 recordsLinked to original sources
arXiv2026

Running Quantum Computers in Discovery Mode

Using a 36-qubit quantum processor, we demonstrate that, by operating in conjunction with a classical machine learning agent, quantum computers can discover instances of interesting quantum many-body dynamics. The central object in this new mode of use of a quantum device is an "interest function" defined for a given circuit (family) instance that can be evaluated on a quantum computer. The circuit is adapted by the learning agent to maximize interest. We illustrate this approach using two examples and show that, within a sufficiently general circuit family, two simple interest functions based on (i) binary classifiability of evolved states and (ii) spectral properties of the unitary circuit, are maximized by discrete time crystals (DTCs) and dual-unitary circuits, respectively. For the classifiability-based interest function, we implement the protocol on a superconducting quantum processor and find that it indeed discovers DTCs with high probability. For the dual-unitaries, our simulations of the dynamics suggest that an interest-function optimization would have set us close to a discovery of such unitaries. Our results using quantum devices and accompanying simulations suggest that learning agents with access to quantum-computing resources can almost autonomously discover new phenomena in many-body quantum dynamics, and establish the design of good interest functions optimizable in hybrid devices as a paradigm for quantum many-body physics.

quant-ph↗
arXiv2026

Screening-controlled dynamical criticality in the quantum Hall regime

At continuous electronic phase transitions, Coulomb interactions can modify the relation between length, energy, and temperature, but experimentally disentangling their effects on spatial versus dynamical criticality has remained difficult, since finite-temperature scaling alone measures only the combined exponent $κ= 1/(zγ)$. Here, we introduce two advances that resolve this limitation. First, by combining temperature scaling with independent current scaling, we separately extract the dynamical exponent $z$ and the localization-length exponent $γ$ at the quantum Hall plateau transition -- rather than inferring one from an assumed value of the other. Second, using dual-graphite-gated graphene devices in which the effective Coulomb interaction range is tuned geometrically by the ratio of the magnetic length $l_B$ to the graphite-gate distance $d$, we track this separation across both screened and unscreened interaction regimes within the same device platform. Temperature scaling gives $κ\simeq 0.21$ in the screened regime and $κ\simeq 0.41$ in the unscreened regime; combining this with current scaling reveals that screening changes $z$ from $\simeq 1$ in the unscreened regime to $\simeq 2$ in the screened regime. In contrast, $γ$ remains close to $2.4$ throughout. Our results establish that gate-controlled screening selectively modifies the interaction-dependent dynamical sector of the quantum Hall transition, leaving the localization-length exponent $γ$ unchanged within experimental uncertainty. More broadly, this work establishes geometric screening as a versatile tool for controlling interactions and disentangling interaction and disorder effects in correlated two-dimensional systems, including fractional quantum Hall states, moiré materials, and other strongly localized electronic phases.

cond-mat.mes-hall↗
arXiv2026

Localization with Hopping Disorder in Quasi-periodic Synthetic Momentum Lattice

Lattice quasi-periodicity is easily realized with ultracold atoms in optical lattices and has been used to study delocalization-localization transition at low dimensions. Models with true disorder, however, remains largely unrealized in experiments. Here, using Bose-Einstein Condensate of ${^{87}{\text{Rb}}}$ atoms, we realize a Generalized Aubry-André (GAA) chain with added hopping disorder in a Momentum Space Lattice (MSL) via multiple Bragg diffractions. Unlike real space lattice simulators, MSL allows simulations of arbitrary disorder configurations and control over spatial disorder correlations. Uncorrelated hopping disorder added to the AA model enhances localization in all phases, smoothening the transition into a crossover between weakly and strongly localized regimes. On the other hand, numerical analysis shows that, spatially correlated hopping disorder induces partial delocalization of localized states in the vicinity of strong hopping bonds. Over a range of disorder strengths and correlations, the experimental results agree quantitatively with the numerical simulation of the dynamics in MSL. Ability of the platform to resolve correlation-dependent dynamical features in dynamics reflects the precision achieved in the realization. Our results demonstrate MSL as a viable platform for studying general disordered quantum systems beyond quasiperiodic systems.

cond-mat.quant-gas↗
arXiv2025

Partial Quantum Shadow Tomography for Structured Operators and its Experimental Demonstration using NMR

Quantum shadow tomography based on the classical shadow representation provides an efficient way to estimate properties of an unknown quantum state without performing a full quantum state tomography. In scenarios where estimating the expectation values for only certain classes of observables is required, obtaining information about the entire density matrix is unnecessary. We propose a partial quantum shadow tomography protocol that estimates a subset of density matrix elements relevant to the expectation values of structured observables. Specifically, we identify specific subsets of the tomographically complete set ${\mathrm{Cl}}(2)^{\otimes n}$ and a simple pseudo-inverse of the associated channel, which can be used to estimate all elements of the density matrix with the same active order. By restricting the protocol to smaller subsets of single-qubit Pauli measurements, it becomes experimentally more efficient. We demonstrate the advantage over unitary designs, such as the Clifford, full Pauli basis, and methods utilizing mutually unbiased bases, by analytically deriving error bounds and numerically evaluating the protocol on structured operators. We experimentally demonstrate the partial shadow estimation scheme for a wide class of two-qubit states (pure, entangled, and mixed) in the nuclear magnetic resonance (NMR) platform. The full density matrix, reconstructed experimentally by combining different partial estimators, achieves fidelities around 99%.

quant-ph↗
arXiv2025

Projected ensemble in a system with conserved charges with local support

The investigation of ergodicity or lack thereof in isolated quantum many-body systems has conventionally focused on the description of the reduced density matrices of local subsystems in the contexts of thermalization, integrability, and localization. Recent experimental capabilities to measure the full distribution of quantum states in Hilbert space and the emergence of specific state ensembles have extended this to questions of {\textit{deep thermalization}}, by introducing the notion of the {\textit{projected ensemble}} -- ensembles of pure states of a subsystem obtained by projective measurements on its complement. While previous work examined chaotic unitary circuits, Hamiltonian evolution, and systems with global conserved charges, we study the projected ensemble in systems where there are an extensive number of conserved charges all of which have (quasi)local support. We employ a strongly disordered quantum spin chain which shows many-body localized dynamics over long timescales as well as the $\ell$-bit model, a phenomenological archetype of a many-body localized system, with the charges being $1$-local in the latter. In particular, we discuss the dependence of the projected ensemble on the measurement basis. Starting with random direct product states, we find that the projected ensemble constructed from time-evolved states converges to a Scrooge ensemble at late times and in the large system limit except when the measurement operator is close to the conserved charges. This is in contrast to systems with global conserved charges where the ensemble varies continuously with the measurement basis. We relate these observations to the emergence of Porter-Thomas distribution in the probability distribution of bitstring measurement probabilities.

cond-mat.stat-mech↗
arXiv2024

Splitting of Girvin-MacDonald-Platzman density wave and the nature of chiral gravitons in fractional quantum Hall effect

A fundamental manifestation of the nontrivial correlations of an incompressible fractional quantum Hall (FQH) state is that an electron added to it disintegrates into more elementary particles, namely fractionally-charged composite fermions (CFs). We show here that the Girvin-MacDonald-Platzman (GMP) density-wave excitation of the $ν{=}n/(2pn{\pm }1)$ FQH states also splits into more elementary single CF excitons. In particular, the GMP graviton, which refers to the recently observed spin-2 neutral excitation in the vanishing wave vector limit [Liang {\it et al.}, Nature {\bf 628}, 78 (2024)], remains undivided for $ν{=}n/(2n{\pm} 1)$ but splits into two gravitons at $ν{=}n/(4n{\pm} 1)$ with $n{>}1$. A detailed experimental confirmation of the many observable consequences of the splitting of the GMP mode should provide a unique window into the correlations underlying the FQH effect.

cond-mat.str-el↗
arXiv2024

Universality of Quantum Phase Transitions in the Integer and Fractional Quantum Hall Regimes

Fractional quantum Hall (FQH) phases emerge due to strong electronic interactions and are characterized by anyonic quasiparticles, each distinguished by unique topological parameters, fractional charge, and statistics. In contrast, the integer quantum Hall (IQH) effects can be understood from the band topology of non-interacting electrons. We report a surprising super-universality of the critical behavior across all FQH and IQH transitions. Contrary to the anticipated state-dependent critical exponents, our findings reveal the same critical scaling exponent $κ= 0.41 \pm 0.02$ and localization length exponent $γ= 2.4 \pm 0.2$ for fractional and integer quantum Hall transitions. From these, we extract the value of the dynamical exponent $z\approx 1$. We have achieved this in ultra-high mobility trilayer graphene devices with a metallic screening layer close to the conduction channels. The observation of these global critical exponents across various quantum Hall phase transitions was masked in previous studies by significant sample-to-sample variation in the measured values of $κ$ in conventional semiconductor heterostructures, where long-range correlated disorder dominates. We show that the robust scaling exponents are valid in the limit of short-range disorder correlations.

cond-mat.mes-hall↗
arXiv2024

Proposal for bulk measurement of braid statistics in fractional quantum Hall effect

The quasiparticles (QPs) or quasiholes (QHs) of fractional quantum Hall states have been predicted to obey fractional braid statistics, which refers to the Berry phase (in addition to the usual Aharonov-Bohm phase) associated with an exchange of two QPs or two QHs, or equivalently, to half of the phase associated with a QP/QH going around another. Certain phase slips in interference experiments in the fractional quantum Hall regime have been attributed to fractional braid statistics, where the interference probes the Berry phase associated with a closed path which has segments along the edges of the sample as well as through the bulk (where tunneling occurs). Noting that QPs / QHs with sharply quantized fractional charge and fractional statistics do not exist at the edge of a fractional quantum Hall state due to the absence of a gap there, we provide arguments that the existence of composite fermions at the edge is sufficient for understanding the primary experimental observations; composite fermions are known to occur in compressible states without a gap. We further propose that transport through a closed $\textit{tunneling}$ loop contained entirely in the bulk can, in principle, allow measurement of the braid statistics in a way that the braiding object explicitly has a fractionally quantized charge over the entire loop. Optimal parameters for this experimental geometry are determined from quantitative calculations.

cond-mat.str-el↗
arXiv2024

Eliashberg theory for dynamical screening in bilayer exciton condensation

We study the effect of dynamical screening of interactions on the transition temperatures ($T_c$) of exciton condensation in a symmetric bilayer of quadratically dispersing electrons and holes by solving the linearized Eliashberg equations for the anomalous interlayer Green's functions. We find that $T_c$ is finite for the range of density and layer separations studied, decaying exponentially with interlayer separation. $T_c$ is suppressed well below that predicted by a Hartree Fock mean field theory with unscreened Coulomb interaction, but is above the estimates from the statically screened Coulomb interaction. Furthermore, using a diagrammatic framework, we show that the system is always an exciton condensate at zero temperature but $T_c$ is exponentially small for large interlayer separation.

cond-mat.str-el↗
arXiv2024

Conformal field theory approach to parton fractional quantum Hall trial wave functions

We show that all lowest Landau level projected and unprojected chiral parton type fractional quantum Hall ground and edge state trial wave functions, which take the form of products of integer quantum Hall wave functions, can be expressed as conformal field theory (CFT) correlation functions, where we can associate a chiral algebra to each parton state such that the CFT defined by the algebra is the ``smallest'' such CFT that can generate the corresponding ground and edge state trial wave functions. A field-theoretic generalisation of Laughlin's plasma analogy, known as generalised screening, is formulated for these states. If this holds, we argue that the inner products of edge state trial wave functions, for parton states where the ``densest'' trial wave function is unique, can be expressed as matrix elements of an exponentiated local action operator of the CFT, generalising the result of Dubail et al. [PRB 85, 11531 (2012)], which implies the equality between edge state and entanglement level counting to state counting in the corresponding CFT. We numerically test this result in two specific cases. We discuss how Read's arguments [PRB 79, 045308 (2009)] still apply, implying that conformal blocks of the CFT defined by the corresponding chiral algebra are valid quasi-hole trial wave functions, with the adiabatic braiding statistics given by the monodromy of these functions, assuming the existence of a quasi-particle trapping Hamiltonian. Generalisations of these constructions are discussed. It is shown that all chiral composite fermion wave functions can be expressed as CFT correlation functions without explicit symmetrisation or anti-symmetrisation and that the ground, edge, and certain quasi-hole trial wave functions of the $ϕ_n^m$ parton states can be expressed as the conformal blocks of the $U(1) \otimes SU(n)_m$ WZW models.

cond-mat.str-el↗
arXiv2023

STM in the fractional quantum Hall effect: Spectroscopy of composite-fermion bound states

The fractional quantum Hall states are non-Fermi liquids of electrons, in that their ground states and low energy excitations are described not in terms of electrons but in terms of composite fermions which are bound states of electrons and $2p$ quantized vortices. An electron or a hole at filling factor $ν=n/(2pn+1)$, where $p,n$ are integers, is a complex molecule of $2pn+ 1$ quasiparticles (excited composite fermions) or quasiholes (missing composite fermions) and has its own internal excitations. Recent scanning tunneling microscopy experiments have succeeded in measuring the electron spectral functions of these states, which provides valuable information on the nature of these strongly correlated molecules and thereby on the short-distance correlations in the fractional quantum Hall liquids. These experiments exhibit several sharp peaks in the tunneling spectra. Detailed calculations based on the composite-fermion theory demonstrate multiple peaks in the local density of states, and we argue that the separation between the peaks represents interaction-corrected composite-fermion cyclotron energy. We discuss what aspects of experiments are explained by our model and which ones remain to be explained.

cond-mat.str-el↗
arXiv2023

Classical fully-packed loop model with attractive interactions on the square lattice

We study a classical model of fully-packed loops on the square lattice, which interact through attractive loop segment interactions between opposite sides of plaquettes. This study is motivated by effective models of interacting quantum matter arising in frustrated magnets or Rydberg atom arrays, for which loop degrees of freedom appear at low energy. Through a combination of Monte Carlo simulations and an effective height field theory, we find that the critical point known to occur at infinite temperature gives rise to a high-temperature critical phase with floating exponents. At lower temperature, the system transitions via a Kosterlitz-Thouless phase transition to a nematic phase where lattice rotation symmetry is broken. We discuss consequences for the phase diagram of the quantum loop model on the same lattice.

cond-mat.str-el↗
arXiv2023

Candidate local parent Hamiltonian for 3/7 fractional quantum Hall effect

While a parent Hamiltonian for Laughlin $1/3$ wave function has been long known in terms of the Haldane pseudopotentials, no parent Hamiltonians are known for the lowest-Landau-level projected wave functions of the composite fermion theory at $n/(2n+1)$ with $n\geq2$. If one takes the two lowest Landau levels to be degenerate, the Trugman-Kivelson interaction produces the unprojected 2/5 wave function as the unique zero energy solution. If the lowest three Landau levels are assumed to be degenerate, the Trugman-Kivelson interaction produces a large number of zero energy states at $ν=3/7$. We propose that adding an appropriately constructed three-body interaction yields the unprojected $3/7$ wave function as the unique zero energy solution, and report extensive exact diagonalization studies that provide strong support to this proposal.

cond-mat.str-el↗
arXiv2023

Real-space entanglement spectra of parton states in fractional quantum Hall systems

Real-space entanglement spectra (RSES) capture characteristic features of the topological order encoded in the fractional quantum Hall (FQH) states. In this work, we numerically compute, using Monte Carlo methods, the RSES and the counting of edge excitations of non-Abelian FQH states constructed using the parton theory. Efficient numerical computation of RSES of parton states is possible, thanks to their product-of-Slater-determinant structure, allowing us to compute the spectra in systems of up to 80 particles. Specifically, we compute the RSES of the parton states $ϕ_2^2$, $ϕ_2^3$, and $ϕ_3^2$, where $ϕ_n$ is the wave function of $n$ filled Landau levels, in the ground state as well as in the presence of bulk quasihole states. We then explicitly demonstrate a one-to-one correspondence of RSES of the parton states with representations of the Kac-Moody algebras satisfied by their edge currents. We also show that for the lowest Landau level projected version of these parton states, the spectra match with that obtained from the edge current algebra. We also perform a computation of spectra of the overlap matrices corresponding to the edge excitations of the parton states with a constrained number of particles in the different parton Landau levels. Counting in these matches the individual branches present in RSES, providing insight about how different branches are formed.

cond-mat.str-el↗
arXiv2022

Re-entrance effect in the high-temperature critical phase of the quantum dimer model on the square lattice

We present a quantum Monte Carlo investigation of the finite-temperature phase diagram of the quantum dimer model on the square lattice. We use the sweeping cluster algorithm, which allows to implement exactly the dimer constraint, supplemented with a equal-time directed loop move that allows to sample winding sectors. We find a high-temperature critical phase with power-law correlations that extend down to the Rokshar-Kivelson point, in the vicinity of which a re-entrance effect in the lines of constant exponent is found. For small values of the kinetic energy strength, we find finite-temperature transitions to ordered states (columnar and staggered) which match those of interacting classical dimer models.

cond-mat.str-el↗
arXiv2022

Exactly Solvable Hamiltonian for Non-Abelian Quasiparticles

Particles obeying non-Abelian braid statistics have been predicted to emerge in the fractional quantum Hall effect. In particular, a model Hamiltonian with short-range three-body interaction ($\hat{V}^\text{Pf}_3$) between electrons confined to the lowest Landau level provides exact solutions for quasiholes, and thereby allows a proof of principle for the existence of quasiholes obeying non-Abelian braid statistics. We construct, in terms of two- and three- body Haldane pseudopotentials, a model Hamiltonian that can be solved exactly for both quasiholes and quasiparticles, and provide evidence of non-Abelian statistics for the latter as well. The structure of the quasiparticle states of this model is in agreement with that predicted by the bipartite composite-fermion model of quasiparticles with exact lowest Landau level projection. We further demonstrate adiabatic continuity for the ground state, the ordinary neutral excitation, and the topological exciton as we deform our model Hamiltonian continuously into the lowest Landau-level $\hat{V}^\text{Pf}_3$ Hamiltonian.

cond-mat.str-el↗
arXiv2021

Anderson localization in fractional quantum Hall effect

The interplay between interaction and disorder-induced localization is of fundamental interest. This article addresses localization physics in the fractional quantum Hall state, where both interaction and disorder have nonperturbative consequences. We provide compelling theoretical evidence that the localization of a single quasiparticle of the fractional quantum Hall state at filling factor $ν=n/(2n+1)$ has a striking {\it quantitative} correspondence to the localization of a single electron in the $(n+1)$th Landau level. By analogy to the dramatic experimental manifestations of Anderson localization in integer quantum Hall effect, this leads to predictions in the fractional quantum Hall regime regarding the existence of extended states at a critical energy, and the nature of the divergence of the localization length as this energy is approached. Within a mean field approximation these results can be extended to situations where a finite density of quasiparticles is present.

cond-mat.mes-hall↗
arXiv2021

An infectious diseases hazard map for India based on mobility and transportation networks

We propose a risk measure and construct an infectious diseases hazard map for India. Given an outbreak location, a hazard index is assigned to each city using an effective distance that depends on inter-city mobilities instead of geographical distance. We demonstrate its utility using an SIR model augmented with air, rail, and road data between top 446 cities. Simulations show that the effective distance from outbreak location reliably predicts the time of arrival of infection in other cities. The hazard index predictions compare well with the observed spread of SARS-CoV-2. The hazard map can be useful in other outbreaks also.

q-bio.PE↗