arXiv subjects

Sayan Choudhury

Publications and source records attributed to Sayan Choudhury.

At least 19 recordsLinked to original sources

arXiv2026

Robust continuous symmetry breaking and multiversality in the chiral Dicke model

The Dicke model (DM) serves as a paradigm for understanding collective light-matter interactions. We introduce the chiral Dicke model, a generalization where an atomic ensemble couples to a two-mode cavity via chiral interactions. Unlike the standard DM, the chiral DM is endowed with an inherent continuous $U(1)$ symmetry associated with angular momentum conservation. The ground-state phase diagram and the associated quantum phase transitions are charted out, revealing a $U(1)$-broken superradiant phase that spans a broad parameter space. We demonstrate that the spectrum of quantum fluctuations is highly tunable in both the symmetric and broken phases. Strikingly, our calculations reveal that the system exhibits `multiversality', where distinct universality classes govern the transition between the same two phases. In particular, along a special line in parameter space, the dynamical critical exponent for the normal-superradiant phase transition changes from $z\nu=1$ to $z\nu=1/2$. Our work establishes the chiral Dicke model as a powerful platform to realize novel quantum phases and multiversal critical phenomena in light-matter coupled systems.

quant-ph↗

arXiv2026

Quantum quenches in a spin-1 chain with tunable symmetry

In recent years, the dynamics of interacting quantum systems far from equilibrium have attracted significant research interest. Driven by rapid progress in quantum simulators, various non-equilibrium phenomena have now been realized experimentally. In this work, we use the time-evolving block decimation (TEBD) method to investigate the dynamics of an anisotropic spin-1 Heisenberg chain for a wide range of experimentally accessible initial states. By adjusting the parameter $J_q$ that controls the quadrupolar interaction strength, we can tune the system from a non-integrable SU(2) Heisenberg model to an integrable SU(3) Heisenberg model. We examine the local magnetization, entanglement entropy, and spin correlations, and characterize their dependence on $J_q$. We identify a new conserved quantity at the SU(3) symmetric point and provide a theoretical framework to explain our numerical observations in terms of the number of accessible states permitted by this conservation law. Our results provide a route to realize a rich array of non-equilibrium behavior in spin-1 lattice models, which can be engineered in several experimental platforms such as ultracold atoms in optical lattices.

cond-mat.quant-gas↗

arXiv2026

Non-equilibrium dynamics of the disordered Power of Two model

Motivated by recent experimental realizations of programmable spin models with long-range interactions, we investigate the non-equilibrium dynamics of the Power-of-Two (PWR2) model. This model consists of sparse long-range couplings between spin-$1/2$ objects separated by $d = 2^n$. In the absence of disorder, the system exhibits rapid scrambling and fast thermalization. We explore the impact of disorder in this system by analyzing the time evolution of the survival probability, half-chain entanglement entropy, and out-of-time-ordered correlators (OTOCs). We find that sufficiently strong disorder suppresses information spreading and induces localization. Remarkably, in the strong-disorder regime, the OTOCs display a non-monotonic spatial profile arising from the intrinsic nonlocality of the interactions, signaling qualitatively distinct scrambling dynamics compared to conventional long-range interacting systems. To characterize the localization transition, we extract the critical disorder strength $h_c$ from the spectral statistics and the eigenstate entanglement. We observe that $h_c$ increases with system size. Furthermore, at a fixed disorder strength, the eigenstate-averaged entanglement entropy increases with system size, while the inverse participation ratio decreases, indicating enhanced delocalization at larger sizes. These results collectively suggest that the PWR2 model remains ergodic in the thermodynamic limit for any finite disorder strength.

cond-mat.dis-nn↗

arXiv2026

Quantum metrology with partially accessible chaotic sensors

Most quantum metrology protocols harness highly entangled probe states and globally accessible measurements to surpass the standard quantum limit. However, it is challenging to satisfy these requirements in realistic many-body sensors. We demonstrate that both of these constraints can be overcome in quantum chaotic sensors. Crucially, we establish that even in the presence of partial measurement accessibility, chaotic dynamics enables initial unentangled states to exhibit Heisenberg scaling of the quantum Fisher information, $I_{\alpha}$ with time. In the weakly chaotic regime, we identify spin-coherent states placed at the edge of the regular islands in the mixed classical phase space as optimal initial states for enhanced sensitivity. On the other hand, in the strongly chaotic regime, $I_{\alpha}$ is insensitive to the choice of the initial state. Notably, quantum-enhanced sensitivity is achieved even when a very low fraction ($\sim 5\%$) of the qubits are accessible. These results establish quantum chaos as a robust resource for quantum-enhanced sensing under realistic accessibility constraints on accessibility.

quant-ph↗

arXiv2026

Dissipation stabilizes Dicke Time Quasicrystals

Quasi-periodic driving protocols provide a powerful route to realize novel non-equilibrium phases of matter beyond the Floquet paradigm. However, these protocols inevitably lead to infinite-temperature heat death in isolated systems, which poses a major challenge to their experimental realization. We demonstrate that dissipation can be harnessed to stabilize quasi-periodically driven systems, enabling the realization of robust non-equilibrium phases. Using the paradigmatic open Dicke model, we provide a blueprint for realizing a stable time quasicrystal (TQC) by Fibonacci driving. This TQC is characterized by a robust sub-harmonic quasi-periodic response that is dictated by, but qualitatively distinct from the external Fibonacci drive. By directly analyzing the time evolution in the thermodynamic limit, we establish the existence of TQC order in this system for a wide parameter regime. We trace the origin of the stability of the TQC to the attractor structure induced by dissipation. Strikingly, the TQC order persists in the deep quantum regime with as few as two qubits. We systematically study the dependence of the TQC lifetime, $\tau^{\ast}$, on the number of qubits and demonstrate that $\tau^{\ast}$ increases monotonically with the system size. Crucially, the TQC is not observed in the absence of dissipation. Our work establishes dissipation as a mechanism for stabilizing non-equilibrium phases of matter under quasi-periodic drive.

quant-ph↗

arXiv2025

Coexistence of inequivalent time-crystalline orders in a Floquet collective spin system

We investigate the dynamical phases that emerge in collective spin models subjected to a spatially non-uniform periodic drive. Taking the paradigmatic Lipkin-Meshkov-Glick (LMG) model as a concrete platform, we establish that a rich landscape of dynamical phases emerges when two regions of the system are driven with different field strengths, $h_1$ and $h_2$. Remarkably, despite the `all-to-all' nature of the interactions, the system can be driven into dynamical phases characterized by distinct kinds of discrete time crystal (DTC) orders in different parts of the system. Apart from these coexisting DTCs, tuning the driving field leads to the emergence of phases where DTCs coexist with Floquet-synchronized or oscillatory phases; the former has been dubbed a chimera DTC. Finally, we demonstrate that a tunable set of global DTC phases emerges when $h_1$ and $h_2$ are proximate. Crucially, these dynamical regimes can be observed both for experimentally relevant finite-size systems and in the thermodynamic limit. Our results establish spatially structured driving as a powerful route to realize non-equilibrium phase coexistence in collective spin systems.

quant-ph↗

arXiv2025

Tunably realizing flat-bands and exceptional points in kinetically frustrated systems: An example on the non-Hermitian Creutz ladder

We study a non-Hermitian extension of the Creutz ladder with generic non-reciprocal hopping. By mapping the ladder onto two decoupled non-Hermitian Su--Schrieffer--Heeger (SSH) chains, we uncover a rich structure in parameter space under different boundary conditions. Under periodic boundary conditions, the spectrum admits a fine-tuned line in parameter space with entirely real eigenvalues, while deviations from this line induce a real--complex spectral transition without crossing exceptional points. In contrast, an exact analytical diagonalization under open boundary conditions reveals extended regions in parameter space with purely real or purely imaginary spectra, separated from complex spectral domains by exceptional lines. The intersections of these exceptional lines define triple-junction points where distinct spectral regimes meet, giving rise to a structured phase diagram that is absent under periodic boundary conditions. We further show that flat bands in this system can occur both as Hermitian diabolical points and as non-Hermitian exceptional points, known as exceptional flat bands, where the dynamics is more stringent than in the Hermitian case, leading to distinct spectral and dynamical signatures.

quant-ph↗

arXiv2025

Directional quantum walks of two bosons on the Hatano-Nelson lattice

We theoretically investigate the interplay of interactions and non-Hermiticity in the dynamics of two bosons on the one-dimensional Hatano-Nelson lattice with non-reciprocal tunneling. We find that the non-reciprocity in the tunneling leads to the formation of an asymmetric density cone during the time-evolution of the system; the degree of asymmetry can be tuned by tuning the non-reciprocity parameter, $\delta$. Next, we analyze the dynamics of this system in the presence of a static external force and demonstrate that non-Hermiticity leads to asymmetric two-particle Bloch oscillations. Interestingly, when $F=0$ ($F \ne 0$), strong interactions leads to the formation of an inner density-cone (density-hourglass) structure; this inner structure also becomes asymmetric in the presence of non-Hermiticity. We further analyze the spatial correlations and establish that the system exhibits non-reciprocal bunching (anti-bunching) in the presence of weak (strong) interactions. Finally, we examine the growth of the Quantum Fisher Information, $F_Q$, with time, and demonstrate that $F_Q \propto t^{\alpha}$ where $\alpha \sim 3$. This feature persists for both one- and two-particle walks, thereby demonstrating that this system can be employed as a quantum-enhanced sensor for detecting weak forces.

quant-ph↗

arXiv2025

Topological energy pumping in a quasi-periodically driven four-level system

We investigate a quasi-periodically driven four-level system that serves as a temporal analog of topological phenomena found in four-band models with intertwined spin and orbital degrees of freedom. Under a two-tone drive in the strong-driving regime, the system realizes a two-dimensional synthetic Floquet lattice, thus facilitating the realization of topological energy pumping. For a temporal quantum spin Hall insulator, we find that the rates of emission and absorption of energy between the two drives are not exactly opposite for a given band. However, when contributions from two chiral symmetric partner bands are added, they become exactly opposite. This quantized rate of energy exchange is a direct consequence of propagating edge modes in the real-space model, which we further characterize by computing the spin-Chern number. Interestingly, our analysis yields zero rate of exchange of energy between the drives for a temporal higher-order topological insulator, suggesting the presence of localized corner modes that we characterize by the mid-gap Wannier spectra. {Our findings uncover the role of chiral, particle-hole and time reversal symmetries on the energy dynamics in temporal quantum spin Hall and higher-order topological insulators.} Finally, we demonstrate that the perfect (imperfect) nature of the fidelity during the time-evolution of the system serves as a characteristic signature of a topological (trivial) phase.

cond-mat.mes-hall↗

arXiv2025

Heating suppression via two-rate random and quasiperiodic drive protocols

We study a random and quasiperiodically driven one-dimensional non-integrable PXP spin chain in a magnetic field for two distinct drive protocols. Each of these protocols involves square pulses with two driving frequencies which are integer multiples of each other. For the first class of protocols, the duration of the pulse is changed randomly by an amplitude $dT$ while for the second class we use a random/quasiperiodic dipolar drive, where the quasiperiodicity is implemented using the Thue-Morse (TM) or Fibonacci sequences. For both protocols, we identify parameter regimes for which the thermalization of the driven chain is drastically slowed down due to proximity to a two-rate drive induced exact dynamical freezing. We also study the properties of these driven system moving slightly away from the freezing limit. For the first type of protocols, we show the existence of special value of $dT$ for which the thermalization rate remains small and provide an analytic explanation for such slow thermalization. For the second class of protocols, in contrast to random/quasiperiodic drives involving a single frequency studied earlier, we find that the TM quasiperiodic drive leads to a distinctly slower thermalization than that for drive protocols which are either periodic or follow a random or quasiperiodic Fibonacci sequence. We provide a qualitative semi-analytic understanding of these phenomena either using an exact calculation for small system sizes or carrying out a perturbative analysis in the large drive-amplitude limit. Our analysis brings out the central role of such two-frequency protocols in the reduction of heating in driven quantum systems. We discuss experiments which can test our theory.

quant-ph↗

arXiv2025

Discrete Time Crystals in the spin-s Central Spin Model

We propose periodic driving protocols to realize discrete time crystals (DTCs) in a spin-s central spin model. Interestingly, we identify parameter regimes, where eternal period-doubling and higher-order(HO)-DTCs can be realized, even for finite-sized systems. We have determined the dependence of the DTC order on the number of satellite spins and the central spin value, s. Intriguingly, we find that certain classes of HO-DTCs produce a series of maximally entangled Bell cat and super-cat states during their dynamical evolution. Finally, we demonstrate that the HO-DTCs can be employed for quantum-enhanced multiparameter sensing at the Heisenberg limit.

quant-ph↗

arXiv2025

Proposal for many-body quantum chaos detection with single-site measurements

We demonstrate that the long-time dynamics of an observable associated with a single lattice site is sufficient to determine whether a many-body quantum system exhibits level statistics characteristic of random matrix theory, a widely used diagnostic of quantum chaos. In particular, we focus on the partial survival probability and spin autocorrelation function at a single site, both evolved under a disordered spin-1/2 chain, which is a setup realizable in current experimental platforms. Given the precision and timescales currently achievable, our results indicate that the detection of many-body quantum chaos is feasible, but constrained to small system sizes.

cond-mat.stat-mech↗

arXiv2025

The Floquet central spin model: A platform to realize eternal time crystals, entanglement steering, and multiparameter metrology

We propose and characterize protocols to engineer exact quantum revivals, discrete time crystals (DTCs), and entanglement oscillations in the periodically driven central spin model. While period-doubling DTCs have been observed in this system before, we uncover a unifying interaction-induced echo mechanism underlying several distinct dynamical regimes. We first demonstrate that this echo can enable exact period-doubling revivals when the Ising interaction strength, $\lambda$, between the central spin and the $N_{\rm sat}$ satellite spins is tuned to $2 \pi$. Notably, these revivals persist for arbitrary $N_{\rm sat}$ and transverse field $g$. This not only stabilizes the DTC response over a wide parameter regime, but also leads to a dynamical freezing regime. Furthermore, when $\lambda=(2m+1)\pi$ and $g= (2n+1)\pi/2$ ($\forall \, m,n \in \mathbb{Z}$), this echo induces a Clifford group structure. Consequently, higher-period revivals emerge that naturally steer the system through an entangled manifold of Bell-cat and spin-cat states. We establish that due to parity-dependent phases accumulated during the echo, the recurrence period is $12 T\,\, (24 T)$ for even (odd) $N_{\rm sat}$. Finally, we demonstrate that the multipartite entanglement generated by the Clifford dynamics can be harnessed for multiparameter metrology, with odd $N_{\rm sat}$ enabling Heisenberg-limited sensitivity.

quant-ph↗

arXiv2024

Counterdiabatic Route to Entanglement Steering and Dynamical Freezing in the Floquet Lipkin-Meshkov-Glick Model

Controlling the dynamics of quantum many-body systems is crucial for developing quantum technologies. This work demonstrates that counter-diabatic (CD) driving provides a powerful tool for steering collective spin systems along entangled trajectories for a long time. In particular, CD driving leads to approximate stroboscopic freezing and eternal entanglement oscillations for a large class of initial states in the periodically driven Lipkin-Meshkov-Glick model. Intriguingly, CD driving generates spin squeezing and its associated metrologically useful multipartite entanglement at the mid-point of every drive cycle, when the system is initially prepared in a fully x-polarized state. The CD driving induced non-ergodic dynamics is accompanied by a decrease in the average eigenstate entanglement and inverse participation ratio, thereby signalling greater eigenstate localization. Our work opens a new route to evade Floquet heating and control entanglement generation in collective spin systems.

quant-ph↗

arXiv2024

Many-body Physics of Ultracold Alkaline-Earth atoms with SU($N$)-symmetric interactions

Symmetries play a crucial role in understanding phases of matter and the transitions between them. Theoretical investigations of quantum models with SU($N$) symmetry have provided important insights into many-body phenomena. However, these models have generally remained a theoretical idealization, since it is very difficult to exactly realize the SU($N$) symmetry in conventional quantum materials for large $N$. Intriguingly however, in recent years, ultracold alkaline-earth-atom (AEA) quantum simulators have paved the path to realize SU($N$)-symmetric many-body models, where $N$ is tunable and can be as large as 10. This symmetry emerges due to the closed shell structure of AEAs, thereby leading to a perfect decoupling of the electronic degrees of freedom from the nuclear spin. In this work, we provide a systematic review of recent theoretical and experimental work on the many-body physics of these systems. We first discuss the thermodynamic properties and collective modes of trapped Fermi gases, highlighting the enhanced interaction effects that appear as $N$ increases. We then discuss the properties of the SU($N$) Fermi-Hubbard model, focusing on some of the major experimental achievements in this area. We conclude with a compendium highlighting some of the significant theoretical progress on SU($N$) lattice models and a discussion of some exciting directions for future research.

cond-mat.quant-gas↗

arXiv2024

Prethermalization in the PXP Model under Continuous Quasiperiodic Driving

Motivated by recent experiments realizing long-lived non-equilibrium states in aperiodically driven quantum many-body systems, we investigate the dynamics of a quasiperiodically driven Rydberg atom chain in the strong Rydberg blockage regime. In this regime, the system is kinetically constrained and the `PXP' model describes its dynamics. Even without driving, the PXP model exhibits many-body scarring and resultant persistent oscillations for dynamics originating from the N\'{e}el-ordered initial state. We demonstrate that a rich array of dynamical behaviors emerge when the system is subjected to a continuous drive. In the high-frequency regime, the system exhibits revivals and oscillations for the N\'{e}el ordered initial state both for periodic and quasi-periodic drives. We trace the origin of this non-ergodicity to an effective PXP Hamiltonian for both of these driving protocols in this regime. Furthermore, we demonstrate that the behavior of the fidelity and the entanglement entropy is non-monotonic at low frequencies in the high-amplitude regime. This leads to several re-entrant scarring transitions both for both the N\'{e}el-ordered and the fully polarized initial state. Our results demonstrate that continuous quasi-periodic drive protocols can provide a promising route to realize prethermal phases of matter in kinetically constrained systems.

cond-mat.quant-gas↗

arXiv2024

Prethermalization in aperiodically driven classical spin systems

Periodically driven classical many-body systems can host a rich zoo of prethermal dynamical phases. In this work, we extend the paradigm of classical prethermalization to aperiodically driven systems. We establish the existence of a long-lived prethermal regime in spin systems subjected to random multipolar drives (RMDs). We demonstrate that the thermalization time scales as $(1/T)^{2n+2}$, where $n$ is the multipolar order and $T$ is the intrinsic time-scale associated with the drive. In the $n \rightarrow \infty$ limit, the drive becomes quasi-periodic and the thermalization time becomes exponentially long ($\sim \exp(\beta/T)$). We further establish the robustness of prethermalization by demonstrating that these thermalization time scaling laws hold for a wide range of initial state energy densities. Intriguingly, the thermalization process in these classical systems is parametrically slower than their quantum counterparts, thereby highlighting important differences between classical and quantum prethermalization. Finally, we propose a protocol to harness this classical prethermalization to realize time rondeau crystals.

quant-ph↗

arXiv2024

Exact Floquet flat band and heating suppression via two-rate drive protocols

We demonstrate the existence of exact Floquet flat bands implying strong violation of the eigenstate thermalization hypothesis in a large class of closed quantum many-body systems in the presence of a two-rate drive characterized by frequencies $\Omega_1$ and $\Omega_2=\nu \Omega_1$. We provide the exact analytic condition for this phenomenon to occur for a generic protocol; in particular, $\nu=(2p+1)$, where $p$ is an integer, leads to such flat bands for both square-pulse and cosine drive protocols for arbitrary $\Omega_1$. In the vicinity of these points, heating is suppressed up to very long timescales in such driven systems, leading to a prethermal regime; we demonstrate this by exact numerical studies of distribution and bandwidth of the Floquet eigenstates, spectral form factor, entanglement entropy, and correlation functions of an experimentally realizable finite driven Rydberg chain. The corresponding micromotion exhibits coherent reversal of excitations reminiscent of echoes. Our analysis constitutes a yet unexplored mechanism for heating suppression in driven closed quantum systems.

cond-mat.stat-mech↗