arXiv subjects

K. Sengupta

Publications and source records attributed to K. Sengupta.

At least 19 recordsLinked to original sources

arXiv2026

Resonant pair production and flat band melting in a driven Rydberg chain

We study a Rydberg chain subjected to a two-tone periodic drive of the detuning $Δ(t)$ and Rabi frequency $Ω(t)$ at frequencies $ω_1$ and $ω_2=νω_1$ respectively with $ν\in \mathbf{Z}$. In the strong Rydberg blockade limit, such a system reduces to the PXP model and exhibits exact Floquet flat band. Here we determine, using a time-dependent Schrieffer-Wolff transformation, the domain of validity of such PXP dynamics once the blockade-violating processes are taken into account. We study the melting of such flat bands in the presence of strong, but finite, blockade strength. Using Floquet perturbation theory (FPT), we identify two mechanisms for such melting; the first occurs due to the presence of many-body resonances involving resonant production of nearest-neighbor Rydberg excitation pairs while the second involves virtual processes that access intermediate states with such pairs. Our analytical FPT results match with those obtained using exact diagonalization on a finite chain. We study the dynamics for $|{\rm vac}\rangle$ (Fock state with no Rydberg excitation), $|{\mathbb {Z}}_2\rangle$, and $|{\mathbb {Z}}_3\rangle$ (Fock states with Rydberg excitations on alternate and every third site respectively) initial states. We find that $|{\rm vac}\rangle$ shows exceptional stability against resonant pair formation while $|{\mathbb {Z}}_3\rangle$ exhibits flat band melting via virtual processes away from the direct resonances. We find oscillatory dynamics with large time period for the $|{\mathbb {Z}}_2\rangle$ initial state and the presence of Floquet scars that have large overlap with $|{\mathbb {Z}}_3\rangle$ state; these phenomena occur due to processes that are absent in the PXP limit. We propose measurement of nearest neighbor pair correlation functions using Rydberg atom quantum simulator platforms as an accessible experimental probe for these effects.

quant-ph↗

arXiv2026

Dressed Floquet scars from protected zero modes in a Rydberg chain

We show that a symmetry-protected Floquet nullspace can host anomalous many-body eigenstates that are obtained by projecting structured parent states onto an exact zero-quasienergy sector. We demonstrate this in a periodically driven PXP chain with an exponentially large (in system size) protected nullspace. Over the accessible system sizes, special parent states show an exponentially increasing enhancement of their nullspace weight relative to the typical value, even as their absolute overlaps decrease. The resulting Floquet scars can therefore become globally orthogonal to their parents while retaining anomalous correlations inherited from them. We demonstrate this mechanism for two contrasting parents: a locally rotated volume-entangled Ivanov-Motrunich scar and the unentangled Rydberg vacuum. Floquet perturbation theory captures the weak-dressing regime, while exact diagonalization shows persistence into nonperturbative regimes where the Floquet Hamiltonian generically contains increasingly nonlocal terms. Our results identify projection into a protected zero-quasienergy sector as a route to Floquet quantum many-body scars.

cond-mat.quant-gas↗

arXiv2026

A projection operator approach for ultracold bosons coupled to a cavity

We employ a projection operator technique to study the equilibrium phases and the non-equilibrium quench and ramp dynamics of ultracold bosons described by a two-dimensional (2D) Bose-Hubbard model coupled to a high-finesse cavity with competing local short-range and cavity-mediated long-range interactions. Our analysis relies on a systematic elimination of high-energy virtual hopping processes; this enables us to capture the effects of short-range quantum fluctuations on the phase diagram of the model while treating the cavity-induced long-range interaction within mean-field theory. We find that the phase boundaries between the superfluid (SF), supersolid (SS), charge-density-wave (CDW) and Mott insulating (MI) phases are significantly modified by the presence of these fluctuations. We use this approach to study the dynamics of bosons following a quench/ramp from the CDW to the SS and SF phases; our analysis indicates the presence of oscillatory dynamics characterized by a single frequency due to a quench which takes the system near the critical point. In contrast, for a quench deep inside the SF phase, the dynamics involves multiple frequencies. An explicit comparison with Gutzwiller mean-field theory shows the importance of quantum fluctuations in shaping such post-quench dynamics; this feature may be tested experimentally as we discuss. We also study the ramp dynamics of these bosons and identify a bound on the ramp rate above which the present method is expected to produce accurate results.

cond-mat.quant-gas↗

arXiv2026

Generating pairwise entanglement in periodically driven quantum spin chains with stochastic resetting

We show that stochastic resetting may lead to finite entanglement between individual, spatially separated spins (pairwise entanglement) in the steady state of the spin chains driven periodically with frequency $ω_D$. We find the presence of a critical resetting rate $r_c$ below which the steady state pairwise entanglement, measured via concurrence $C$, vanishes. We also identify an optimal resetting rate $r_m$ at which $C$ becomes maximum. These critical and optimal rates exhibit a non-monotonic dependence on $ω_D$. Our analysis demonstrates the existence of special drive frequencies at which $r_c$ vanishes and $r_m$ attains minima. We compute $C$ in the presence of stochastic resetting using exact diagonalization for both the integrable XY model and non-integrable Rydberg spin chains, which demonstrate these features. Our numerical results match perturbative analytical expressions for the special drive frequencies in the large drive amplitude regime.

quant-ph↗

arXiv2026

Emergent prethermal Bethe integrability in a periodically driven Rydberg chain

We study a chain of periodically driven Rydberg atoms and identify a class of drive protocols for which the system exhibits emergent prethermal Bethe integrability at special drive frequencies. We provide a perturbative analytic expression of its Floquet Hamiltonian in the large drive amplitude regime. We demonstrate integrability of the leading term of this Floquet Hamiltonian at special drive frequencies, which we identify, by mapping it to the Hamiltonian of the paradigmatic spin-$1/2$ ${\rm XXZ}$ chain. We support our analytical results by exact diagonalization studies on finite chains. Our numerical results on level statistics, half-chain entanglement entropy, and longitudinal magnetization of the driven chain brings out its emergent integrable nature at the special drive frequencies which persists up to a large prethermal timescale.

quant-ph↗

arXiv2026

Floquet scars and prethermal fragmentation in a driven spin-one chain

We study the periodic dynamics of a spin-one chain driven using a square-pulse protocol with amplitude $Q_0$ and frequency $ω_D$. The Hamiltonian of the spin chain hosts a thermodynamically large number of $Z_2$-valued conserved quantities $W_{\ell}$ on the links $\ell$. This allows us to study the Floquet dynamics of this chain within a given sector with fixed values of $W_{\ell}$. For the sector with all $W_{\ell}=1$, we find signatures of quantum many-body scar states for $\hbar ω_D \gg Q_0$; they lead to oscillatory dynamics and fidelity revival for specific initial states. Upon lowering $ω_D$, we find an ergodic regime exhibiting fast thermalization consistent with the prediction of the (Floquet) eigenstate thermalization hypothesis. In addition, we identify special drive frequencies $ω_n^{\ast}= Q_0/(2n \hbar)$ (where $n = 1, 2, 3, \cdots$) at which the Floquet Hamiltonian exhibits prethermal strong Hilbert space fragmentation (HSF) with the largest fragment being ergodic; in contrast, a weak HSF is found at $ω'_n= Q_0/[\hbar(2n+1)]$ (where $n = 0, 1, 2, \cdots$). We also study the sector with $W_{\ell} =\{\cdots 1,1,-1,1,1,-1 \cdots \}$ which shows strong HSF at $ω_n^{\ast}$ but no fragmentation at $ω'_n$. Our analysis indicates that the strong HSF in this sector harbors an integrable largest fragment. We provide numerical support for our analytical and perturbative results using exact-diagonalization (ED) studies on finite chains of length $L\le 24$. Our numerical results for entanglement entropy, fidelity, and correlation functions of the driven chain provide definitive signatures of prethermal strong HSF for both sectors.

cond-mat.stat-mech↗

arXiv2025

Index-theoretic route to the subgap Andreev bands and topological response in Josephson junctions

We demonstrate that the subgap Andreev bound states in a transparent Josephson junction, comprising of either chiral or non-chiral superconductors, can be viewed as a consequence of the index theorem in supersymmetric quantum mechanics. We provide an exact solution for these states starting from the Bogoliubov-de Gennes (BdG) equations describing quasiparticles in such junctions. We demonstrate that the dispersion of these subgap states depends only on the asymptotic properties of the pair-potential and not on its local spatial variation. Our study reveals the crucial distinction between junctions of non-chiral $p$-wave superconductors and those of $s$-wave or chiral superconductors by analyzing the wavefunction of their subgap bound states. We find a stable topological response leading to the well-known $4π$ periodic Josephson effect protected against weak disorder potential for the non-chiral $p$-wave junctions; no such protection is found for junctions of $s$-wave or chiral superconductors. We supplement our analytic results with numerical computation of the Josephson currents in such junctions using exact numerical Green functions and starting from a lattice model of an itinerant altermagnet which is expected to host triplet $p$-wave superconductivity with equal-spin-pairing. We also discuss the implications of our results for Josephson junctions away from the transparent limit.

cond-mat.mes-hall↗

arXiv2025

Destructive Interference induced constraints in Floquet systems

We introduce the paradigm of destructive many-body interference between quantum trajectories as a means to systematically generate prethermal kinetically constrained dynamics in Floquet systems driven at special frequencies. Depending on the processes that are suppressed by interference, the constraint may or may not be associated with an emergent global conservation; the latter kind having no mechanism of generation in time-independent settings. As an example, we construct an one-dimensional interacting spin model exhibiting strong Hilbert space fragmentation with and without dipole moment conservation, depending on the drive frequency. By probing the spatiotemporal profile of the out-of-time-ordered correlator, we show that this model, in particular, has initial states in which quantum information can be spatially localized - a useful feature in the field of quantum technologies. Our paradigm unifies various types of Hilbert space fragmentation that can be realized in driven systems.

cond-mat.str-el↗

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

Entanglement asymmetry in periodically driven quantum systems

We study the dynamics of entanglement asymmetry in periodically driven quantum systems. Using a periodically driven XY chain as a model for a driven integrable quantum system, we provide semi-analytic results for the behavior of the dynamics of the entanglement asymmetry, $ΔS$, as a function of the drive frequency. Our analysis identifies special drive frequencies at which the driven XY chain exhibits dynamic symmetry restoration and displays quantum Mpemba effect over a long timescale; we identify an emergent approximate symmetry in its Floquet Hamiltonian which plays a crucial role for realization of both these phenomena. We follow these results by numerical computation of $ΔS$ for the non-integrable driven Rydberg atom chain and obtain similar emergent-symmetry-induced symmetry restoration and quantum Mpemba effect in the prethermal regime for such a system. Finally, we provide an exact analytic computation of the entanglement asymmetry for a periodically driven conformal field theory (CFT) on a strip. Such a driven CFT, depending on the drive amplitude and frequency, exhibits two distinct phases, heating and non-heating, that are separated by a critical line. Our results show that for $m$ cycles of a periodic drive with time period $T$, $ΔS \sim \ln mT$ [$\ln (\ln mT)$] in the heating phase [on the critical line] for a generic CFT; in contrast, in the non-heating phase, $ΔS$ displays small amplitude oscillations around it's initial value as a function of $mT$. We provide a phase diagram for the behavior of $ΔS$ for such driven CFTs as a function of the drive frequency and amplitude.

quant-ph↗

arXiv2025

Emergent criticality in a constrained boson model

We show, via explicit computation on a constrained bosonic model, that the presence of subsystem symmetries can lead to a quantum phase transition (QPT) where the critical point exhibits an emergent enhanced symmetry. Such a transition separates a unique gapped ground state from a gapless one; the latter phase exhibits a broken $Z_2$ symmetry which we tie to the presence of the subsystem symmetries in the model. The intermediate critical point separating these phases exhibits an additional emergent $Z_2$ symmetry which we identify. This emergence leads to a critical theory which seems to be different from those in the Ising universality class. Instead, within the data obtained from finite-size scaling analysis, we find the critical theory to be not inconsistent with Ashkin-Teller universality in the sense that the transitions of the model reproduces a critical line with variable correlation length exponent $ν$ but constant central charge $c$ close to unity. We verify this scenario via explicit exact-diagonalization computations, provide an effective Landau-Ginzburg theory for such a transition, and discuss the connection of our model to the PXP model describing Rydberg atom arrays.

cond-mat.str-el↗

arXiv2025

Floquet realization of prethermal Meissner phase in a two-leg flux ladder

We show that a periodically driven two-leg flux ladder hosting interacting hardcore bosons exhibits a prethermal Meissner phase for large drive amplitudes and at special drive frequencies. Such a prethermal Meissner phase is characterized by a finite time-averaged chiral current. We find an analytic expression of these frequencies using Floquet perturbation theory. Our analysis reveals that the presence of the prethermal Meissner phase is tied to the emergence of strong Hilbert space fragmentation in these driven ladders. We support our analytical results by numerical study of finite-size flux ladders using exact diagonalization and discuss experiments using ultracold dipolar atom platforms that may test our theory.

cond-mat.quant-gas↗

arXiv2025

Scar-induced imbalance in staggered Rydberg ladders

We demonstrate that the kinematically-constrained model of Rydberg atoms on a two-leg ladder with staggered detuning, $Δ\in [0,1]$, has quantum many-body scars (QMBS) in its spectrum and represents a non-perturbative generalization of the paradigmatic PXP model defined on a chain. We show that these QMBS result in coherent many-body revivals and site-dependent magnetization dynamics for both Néel and Rydberg vacuum initial states around $Δ=1$. The latter feature leads to eigenstate thermalization hypothesis (ETH)-violating finite imbalance at long times in a disorder-free system. This is further demonstrated by constructing appropriate local imbalance operators that display nonzero long-time averages for Néel and vacuum initial states. We also study the fidelity and Shannon entropy for such dynamics which, along with the presence of long-time finite imbalance, brings out the qualitatively different nature of QMBS in PXP ladders with $Δ\sim 1$ from those in the PXP chain. Finally, we identify additional exact mid-spectrum zero modes that stay unchanged as a function of $Δ$ and violate ETH.

quant-ph↗

arXiv2025

Quantum dynamics of a spin model with an extensive degeneracy

We study the role played by extensive degeneracy in shaping the nature of the quantum dynamics of a one-dimensional spin model for both ramp and periodic drive protocols. The model displays an extensive degenerate manifold of states for a specific value of one of the parameters of its Hamiltonian. We study a linear ramp which takes the spin model through this degenerate point and show that it leads to a deviation from the usual Kibble-Zurek behavior. We also study the Stückelberg oscillations in such a model for a ramp which passes twice through the degenerate point. Our study indicates that such oscillations are strongly suppressed leading to a distinct behavior compared to those arising from double passage through a quantum critical point. Finally, we study the periodic dynamics of the model and show, for a large drive amplitude, the existence of special drive frequencies at which the system exhibits an approximate emergent $U(1)$ symmetry. We study the effect of this emergent symmetry on the correlators of the driven system and demonstrate the existence of dynamic symmetry restoration at these frequencies. We study the fate of the emergent symmetry when the drive amplitude is decreased and discuss possible experiments to test our theory.

quant-ph↗

arXiv2024

Emergent symmetries in prethermal phases of periodically driven quantum systems

Periodically driven closed quantum systems are expected to eventually heat up to infinite temperature reaching a steady state described by a circular orthogonal ensemble (COE). However, such finite driven systems may exhibit sufficiently long prethermal regimes; their properties in these regimes are qualitatively different from that in their infinite temperature steady states. These, often experimentally relevant, prethermal regimes host a wide range of phenomena; they may exhibit dynamical localization and freezing, host Floquet scars, display signatures of Hilbert space fragmentation, and exhibit time crystalline phases. Such phenomena are often accompanied by emergent approximate dynamical symmetries which have no analogue in equilibrium systems. In this review, we provide a pedagogical introduction to the origin and nature of these symmetries and discuss their role in shaping the prethermal phases of a class of periodically driven closed quantum systems.

quant-ph↗

arXiv2024

Emergence of a quasi-ergodic steady state in a dissipative Tavis-Cummings array

In an atom-photon interacting system described by Tavis Cummings Hubbard (TCH) model, we demonstrate the emergence of a quasi-steady state in a dissipative environment that exhibits intriguing ergodic behavior. The TCH model undergoes a dissipative transition from normal to superradiant phase hosting a gapped Higgs and gapless Goldstone modes. However, in a large region of the phase diagram, the instability of the Goldstone mode leads to the disappearance of the stable superradiant phase. In this regime, the decorrelator dynamics reveals light cone spreading of the perturbations and positive Lyapunov exponent, indicating enhanced fluctuations. Remarkably, a quasi-steady state emerges under quench dynamics in this unstable regime; in this state, a class of collective quantities such as site averaged photon number and atomic excitations approach a steady value, in spite of large temporal fluctuations in corresponding microscopic variables. This quasi-steady state describes an incoherent fluid of photons with significant phase fluctuation. The phase space dynamics reveals a fascinating ergodic behavior in presence of dissipation, leading to the characterization of the dynamical variables into two distinct classes. The first class includes site-averaged photon numbers and atomic excitations; these exhibit a stationary distribution regardless of the initial condition indicating ergodic behavior. The second class of variables, particularly those related to phase in contrast, retain information about the initial conditions, resulting in a violation of ergodicity for finite size system. Additionally, the dynamical variables of the ergodic class exhibit fascinating collective scarring phenomenon as the peak of their distribution is attracted towards the unstable steady state, analogous to the single particle quantum scar. We discuss the relevance of our findings in the current experiments.

cond-mat.stat-mech↗

arXiv2024

Variational wave-functions for correlated metals

We study a set of many-body wave-functions of Fermions that are naturally written using momentum space basis and allow for quantum superposition of Fermion occupancy, $\{n_{\bf k}\}$. This {enables} us to capture the fluctuations of the Fermi-surface {(FS)} -- the singularly most important signature of a metal. We bench-mark our results in one spatial dimensions (1D) to show that these wave-functions allow for quantitative understanding of the Tomonaga-Luttinger liquid (TLL); computations of certain correlators using them can in fact be extended to larger systems sizes compared to conventional exact diagonalization (ED) allowing for a more systematic comparison with bosonization techniques. Finally we show that this basis may be useful for obtaining fixed-point wave-function for strongly correlated metals {in dimensions greater that one}. In particular, we study the case of coherent (equal) superposition of elliptical FS {in continuum (2D) and on a} square lattice{. In case of the former, our variational wave-function systematically interpolates between the phenomenology of the Fermi liquid ground state, i.e., finite single-Fermion residue at a sharp FS, to a non-Fermi liquid (NFL) with zero residue. In the NFL the jump in $\langle n_{\bf k}\rangle$ at the FS is replaced by a point of inflection (similar to a 1D TLL) whose contour is consistent with the Luttinger Theorem. In case of the square lattice, we} find highly anisotropic distribution of the quasi-particle residue, which, at finite resolution has an uncanny resemblance to the Fermi-arcs{, albeit at zero temperature,} seen in the pseudo-gap state of the cuprates.

cond-mat.str-el↗

arXiv2024

Hilbert space fragmentation imposed real spectrum of non-Hermitian systems

We show that constraints imposed by strong Hilbert space fragmentation (HSF) along with the presence of certain global symmetries can ensure the reality of eigenspectra of non-Hermitian quantum systems; such a reality cannot be guaranteed by global symmetries alone. We demonstrate this insight for two interacting finite chains, namely the fermionic Nelson-Hatano and the Su-Schrieffer-Heeger models, none of which has a $\mathcal{PT}$ symmetry. We show analytically that strong HSF and real spectrum are both consequences of the same dynamical constraints in the limit of large interaction, provided the systems have sufficient global symmetries. We also show that a local equal-time correlation function can detect the many-body exceptional point at a finite critical interaction strength above which the eigenspectrum is real.

cond-mat.str-el↗