Talks

Abstract: We present a fluctuating hydrodynamic description of an active lattice gas model with excluded volume interactions that exhibits motility-induced phase separation under appropriate conditions. For quasi-one dimension and higher, stability analysis of the noiseless hydrodynamics gives quantitative bounds on the phase boundary of the motility-induced phase separation in terms of spinodal and binodal. Inclusion of the multiplicative noise in the fluctuating hydrodynamics describes the exponentially decaying two-point correlations in the stationary-state homogeneous phase. Our hydrodynamic description and theoretical predictions based on it are in excellent agreement with our Monte Carlo simulations and pseudospectral iteration of the hydrodynamics equations.

Abstract:  Mpemba effect occurs when a system, residing far away from the steady state, relaxes faster than a relatively nearer state. We look for the presence of this highly counterintuitive effect in the relaxation dynamics of the operators within the open quantum system setting. Since the operators evolve under a non-trace preserving map, trace distance measure can not serve as a reliable measure for detecting the Mpemba effect in operator dynamics. We circumvent this problem by defining a dressed distance between operators that decays monotonically with time, enabling a generalized framework to explore the Mpemba-like effect for operators. Applying the formalism to various open quantum system settings, we find that, interestingly, in the single qubit case, only accelerated relaxation of operators is possible, while genuine Mpemba-like effects emerge in higher-dimensional systems such as qutrits and beyond.

Abstract:  We explore here the interacting two types of polar active matter, charaterizing by their different aligning and propulsion strengths. We study in field theory framework and find the parameters region where they have their oriented flocking as parallel or antiparallel, ordered rotating phase coherently meaning chiral phase. Our study characterizes the scaling properties of the different phases.

Abstract: Suspensions of active particles can exhibit striking collective behaviors, including spontaneous flow generation and complex pattern formation. These dynamics arise from the ability of individual particles to convert chemical energy into mechanical work on their surroundings. We now know that a Stokesian flock of such active particles are rendered unstable due to a ‘generic’ alignment instability arising from dipolar stresses. Here, we investigate a distinct class of suspensions composed of elongated, chiral particles that are driven along their long axis by an external torque. Due to their chirality, this drive induces self-propulsion via translation–rotation coupling. Crucially, unlike classical force dipoles, these particles act as torque monopoles, generating an antisymmetric polar stress at the continuum level. Using a continuum model and numerical analysis, we demonstrate that this mode of driving gives rise to a new class of hydrodynamic instability, fundamentally rooted in the self- propulsion mechanism itself. This contrasts with the well-known alignment instability, where self-propulsion plays no essential role and may even have stabilizing effects. To understand the origin of the instability, we perform a linear stability analysis of aligned and isotropic states—both of which are shown to be unstable in the presence of chiral torque-driven particles. Our 3D simulations further reveal that these instabilities can lead to the emergence of turbulence-like flow structures and large-scale concentration fluctuations. Together, our results uncover a novel instability mechanism in torque-driven chiral suspensions, with implications for their rheological behavior and the design of active materials.

Abstract: We investigate the dynamics of a run-and-tumble particle in a double-well potential and demonstrate that, in stark contrast to Brownian particles, active dynamics can lead to ergodicity breaking. When the barrier height exceeds a critical threshold, the long-time position distribution depends crucially on the initial condition: if the particle starts within the basin of attraction of one well, it remains trapped there, while if it begins between the two basins, it can reach either well with a finite probability, which we compute exactly via hitting probabilities. Below the critical barrier height, ergodicity is restored and the system converges to a unique stationary distribution, which we derive analytically. Moreover, we estimate the relaxation time to reach this stationary state and show that it diverges near the transition following a Vogel-Fulcher-Tammann-like form with exponent 1/2.

Abstract: We investigate the dynamics of two harmonically coupled active Brownian particles (ABPs) in presence of thermal fluctuations. Existence of harmonic trap and the bounded nature of the active noise ensure that the relative distance r between the particles will eventually reach a stationary state. However, the interplay between the relaxation time-scale in the harmonic trap and the active time-scale gives rise to three limiting scenarios—strong-coupling, moderate-coupling, and weak-coupling regimes. In the absence of thermal fluctuations, we have analytically shown that in the strong coupling regime, an effective short-range repulsion emerges between the pair of particles when their self-propulsion speeds are not equal. The short-range repulsion is also present when the ABPs are subjected to a generic long-range attractive potential. We have  demonstrated that the emergence of repulsion is robust as it emerges in presence of thermal fluctuations also. Analytical results as well as numerical simulation confirm that in the moderate-coupling regime, there exists short-range repulsion between the ABPs.

Ref:

[1] Emergent short-range repulsion for attractively coupled active particles, Ritwick Sarkar, and Urna Basu, Soft Matter, 21 (18), 3595-3603.

[2] From Attraction to Repulsion: Emergent Interactions in Harmonically Coupled Active Binary System, Ritwick Sarkar, Sreya Chatterjee and Urna Basu, Journal of Physics A: Mathematical and Theoretical, 58, 415001.

Abstract: Replicating efficient and adaptable microbial navigation strategies, such as run and tumble (RnT) and tactic motions to synthetic active agents has been an enduring quest. To this end, we introduce a stochastic orientational reset (SOR) protocol, in which the propulsion direction of an active Brownian particle (ABP) is reassigned to a random orientation within a defined reset-cone. When the reset-cone is aligned with the instantaneous propulsion direction, ABPs reproduce the RnT dynamics of E. coli; when set along an attractant gradient, they exhibit taxis - with extensive adaptability in persistence through the angular width of the reset-cone and reset rate. We establish the robustness of this protocol across a broad range of swimming speeds using experiments, simulations, and analytical theory.

Abstract:  Nonequilibrium processes driven by nonequilibrium baths is an important yet less explored area. Here we shall discuss some results for a run-and-tumble particle (RTP) moving in a one dimensional heated channel connected to active particle reservoirs at the boundaries. This elementary setup exhibits rich behaviour: The distribution has 'kinetic boundary layers' and can be nonmonotonous; current is carried in absence of a density gradient, current reversal and optimisation takes place on tuning dynamical parameters only; and in particular a new 'global' transport effect, very loosely resembling a Seebeck-like effect, emerges. In the dynamics we find a crossover in relaxation related to band mixing, and the large time distribution in an absorbing boundary setup deviates strongly from the passive one. Time permitting we shall also outline a curious universality in the distribution and appearance of a Milne length. Reservoir driven active processes in presence of thermal diffusion may thus provide a common physical ground for diverse and novel nonequilibrium phenomena.

Abstract: We investigate, using Langevin dynamics simulations, the Rouse-type dynamics of active fractal bead-spring networks constructed using the critical bond percolation clusters of the square lattice. Two types of active stochastic forces, modelled as random telegraph process with finite decorrelation time, are considered: force monopoles, acting on individual nodes in random directions, and force dipoles, where extensile or contractile forces act between pairs of connected nodes. For force monopoles, a dynamical steady state is reached where the network is dynamically swollen and the mean square displacement (MSD) shows sub-diffusive behavior determined solely by the spectral dimension of the underlying fractal network, in accord with a previously proposed general analytic theory [Singh and Granek, Chaos, 34(113107), 2024]. In contrast, dipolar forces require diverging times to reach the steady state and lead to network shrinkage.  We find a continuous crossover to a collapsed state for the non-diluted square lattice, resulting from its marginal stability. The MSD is found to saturate at the same temporal regime, followed by ballistic-like and/or diffusive behaviors. We further extend our study of dipolar forces to diluted regimes above the “rigidity percolation” threshold  for triangular lattices [D. Majumdar et al., J. Chem. Phys., 163(114902), 2025]. Here, weak dipolar forces do not shrink the network in the steady state. Moreover, for the triangular lattice, an incipient discontinuous collapse transition occurs above a critical force amplitude. Importantly, we find that the inclusion of active force dipoles rigidify the triangular network even below the regular rigidity percolation point, which can be exactly identified by including the extra constraints from the active dipole links in the Maxwell constraint counting method. Finally, we will show how spatial correlation in these active dipoles affect the dynamics of the active network.

Abstract:  Resetting has been shown to reduce the mean completion time for a stochastic process. The time between two consecutive resetting events is drawn from a waiting time distribution that defines the resetting strategy or protocol. Previously, it has been shown that deterministic resetting process with a constant time period, referred to as sharp restart, can minimize the mean time to reach a fixed target. We consider the more realistic problem of a target positioned at a random distance from the resetting site, selected from a given target distribution 1 . We introduce the notion of a conjugate target distribution to a given waiting time distribution. For this conjugate target distribution, the waiting time distribution extremizes the mean time to locate the target. In the case of diffusion we derive an explicit expression for the conjugate target distribution to a given waiting time distribuition that holds in arbitrary spatial dimension. Our results show that stochastic resetting prevails over sharp restart for target distributions with exponential or heavier tails.

Ref: Evans, M. and Ray, S. Physical Review Letters, 2025, 134, 247102.

Abstract: From protein-DNA interaction to the foraging of whales in ocean, target search is one of the ubiquitous phenomena observed across multiple scales in nature. Despite of various search strategies, intermittent restart holds most striking mechanism for optimizing the mean search time. Falling back to the initial location in random intervals can discard errant or diverging trajectories, results minimization in mean search time. Moreover, it’s quite significant to ask when and how resetting can do such optimization. In such scenario, CV0>1-criterion serves a foundational role, where CV0=σ(T0 )/⟨T0⟩. This criterion puts a strong bound on the system parameters for instance, the initial location of the particle or volume of the domain. However, in the presence of multiple targets, this classical criterion fails to capture the full story based on the preferential and non-preferential outcomes. In our work, we examine scenarios involving multiple absorbing boundaries, that results the following criterion: CV0σ >Λ0σ, where σ denotes the set of boundaries. The respective criterion emerges as a necessary condition in this context, and that results the mean search time to be biased in terms of the efficiency of resetting. 

Abstract: In the last decades, interacting particle systems in a fluctuating heat bath, such as colloids, have been established as excellent models for phase transitions and dynamics of condensed matter at room temperature. A large body of research nowadays targets out-of-equilibrium situations. Besides external driving, much attention has recently been devoted to systems that are intrinsically out of equilibrium, a prime example being “active” particles, but also systems involving nonreciprocal (e.g., prey-predator) interactions or time delay. In this talk I will give an overview of our recent activities in this area, with a focus on systems with non-reciprocal couplings. We employ a spectrum of methods from statistical physics, including particle-based simulations, hydrodynamic approaches, and elements from stochastic thermodynamics.

Abstract: Non-reciprocal interactions are ubiquitous in nature, and their impact on the collective behavior of many-body systems has attracted significant research attention. We incorporate non-reciprocal (NR) coupling into the two-species Vicsek model (TSVM), such that species A aligns with species B, whereas B anti-aligns with A. This coupling gives rise to a time-dependent phase characterized by macroscopic chiral motion, consistent with observations from earlier studies. Here, I will discuss the effects on the flocking dynamics of the NRTSVM through varying parameters such as particle density, velocity, system size and the interaction strength. I will further demonstrate how population and motility imbalances between species affect the alignment behavior and the emergence of chiral motion in the system.

Abstract: We investigate the critical properties of a recently discovered finite-time dynamical phase transition in the non-equilibrium relaxation of the magnetization in Ising magnets during an instantaneous temperature quench. The transition is characterized by a sudden switch in the relaxation dynamics and it occurs at a sharp critical time. While previous works have focused either on mean-field interactions or on investigating the critical time, we analyze the critical fluctuations at the phase transition in the nearest-neighbor Ising model on a square lattice using Monte Carlo simulations. By means of a finite-size scaling analysis, we extract the critical exponents for the transition. We find that in two spatial dimensions when the system is initially in the vicinity of the critical point, the critical exponents are consistent with those of the two-dimensional Ising universality class. For initial temperatures below the critical one, however, the critical exponents differ from the Ising-exponents, indicating a distinct dynamical critical phenomenon. In addition to the magnetization, we also analyze the finite-time dynamics of the released heat after the quench, and identify a finite-time dynamical phase transition analogous to that observed for magnetization. Our preliminary results indicate that the associated critical properties are, again, similar to those of the two-dimensional Ising universality class, thus providing intriguing parallels between the equilibrium and the finite-time dynamical critical phenomena.

Abstract: Here, we present an improved version of the 2-site per amino-acid resolution Self-Organized polymer (SOP) model for studying Intrinsically Disordered Proteins (IDPs). The original SOP-IDP model works well for chains in good solvent that adopt expanded conformations. However, we found that SOP-IDP force-field parameters need to be recalibrated for IDP chains whose Flory exponents deviate from the random-coil limit. For example, for hnRNPA1-LCD, the SOP-IDP force field produces conformational ensembles (with mean Rg of 3.26 nm) that are much more expanded as compared to the experiments (with mean Rg of 2.70 nm) with an error of ~20%. Other examples also suggest that there is scope for refining the SOP-IDP model. Most condensate forming IDPs are rich in residues that can form π - π and cation-π interactions, which are weak but important interactions for biomolecular recognition and also for driving and stabilizing biomolecular condensates. In our improved model, we reparametrize the SOP-IDP force field to account for π - π and cation - π interaction at a very coarse-grained level such that the simulations provide experimentally-consistent behaviours for IDPs both in terms of single-chain and bulk properties – especially those rich in cation and aromatic residues. Our method addresses an immediate need in the IDP-biophysics and biomolecular condensate simulations community by providing a well-grounded prescription to simulate and generate faithful conformations of IDPs that are better suited to recapitulate experimental realities.

Abstract: We demonstrate how finite relaxation of key dynamical variables like velocity or heading direction impacts the behavior of active particles far beyond the corresponding relaxation times. We illustrate this effect using inertial active Brownian particles and flocking models with finite orientational relaxation times. Through exact calculations and simulations, we reveal how relaxation processes shape late-time and steady-state behavior and obtain the associated phase diagrams.

Abstract: Collective cell movement drives crucial biological processes, including cancer invasion, wound healing, and embryo development. Yet, how tissue-scale material properties emerge from cellular interactions remains unclear. In this talk, I will introduce an active particle-spring model of tissue monolayers that captures the transition from a fluid-like to a solid-like state with increasing cell-cell adhesion, as reported in experiments. Close to the liquid–solid transition, the system exhibits glassy dynamics marked by subdiffusive motion, swirling velocity fields, and non-Gaussian displacement statistics, indicating a dynamically heterogeneous tissue. By eliminating many ad hoc assumptions of previous frameworks, our model offers a unified and minimal description of tissue mechanics, with broad implications for understanding collective cell behaviors.

Abstract: Essential morphological processes like cell growth, division and sporulation in bacteria require remodelling of its cell wall.  We focus on the formation of endospore in gram positive bacteria and show how the ordered structure of its cell wall systematically cracks to produce a new wall for the spore. Unlike crack propagation in passive materials, where stress is induced externally, stress is generated internally in the cell wall, due to synthesis of new material. Our phenomenological model shows that the crack propagation speed is determined by the shear modulus of the constituent peptidoglycan (PG) lattice, PG synthesis rate and osmotic pressure of genetic material which translocates into the endospore from the mother cell.

Abstract: When exposed to a time-periodic chemical signal, an E. coli cell responds by modulating its receptor activity in a similar time-periodic manner. However, there is a phase lag between the applied signal and the activity response. We study the variation of activity amplitude and phase lag as a function of the applied frequency ω, using numerical simulations. The amplitude increases with ω, reaches a plateau, and then decreases again for large ω. The phase lag increases monotonically with ω and finally saturates to 3π/2 when ω is large. The activity is no longer a single-valued function of the attractant signal, and plotting activity versus attractant concentration over one complete time period generates a loop. We monitor the loop area as a function of ω and find two peaks for small and large ω, with a sharp minimum at intermediate ω values. We explain these results as arising from an interplay between the timescales associated with adaptation, activity switching, and applied signal variation. In particular, for very large ω, the quasi-equilibrium approximation for activity dynamics breaks down, a regime that has not been explored in earlier studies. We perform analytical calculations in this limit and find good agreement with our simulation results.

Abstract: 

Abstract: Individual behaviors strongly influence environmental change, which subsequently may feedback on how individual behaviors evolve. The feedback between behavior and environment can be studied using the model of evolutionary dynamics with game transitions, wherein an individual’s strategic choice in one game determines the subsequent game to be played. Recent studies suggest that game transitions can promote cooperation significantly in a structured population. In particular, if mutual cooperation is rewarded by transitioning to a valuable state, while any defection is punished by leading to a less valuable game, cooperation can be favored by selection—even if it is disadvantaged within each game. Knowledge of the environmental (game) state, when available, can often modulate individual choices to act altruistically or selfishly in a particular state. However, previous studies in structured populations failed to capture the effect of state-dependent behavior predicated on environmental knowledge. Here, we investigate how knowledge about environmental states can affect the evolution of cooperation in a structured population and compare this with scenarios where such knowledge is scarce. Using both theoretical analysis and numerical simulations, we systematically analyze all types of game transitions of a given complexity class to assess whether environmental knowledge promotes cooperation. For a broad class of game transition rules, we find that knowledge of the environmental state can reduce the critical threshold of benefit-to-cost ratio for the evolution of cooperation. However, surprisingly, we also find that greater knowledge about the environmental state doesn’t always confer an advantage for the evolution of cooperation; instead, for a few game transitions, it can yield neutral or even detrimental effects on the collective cooperation level. By comprehensively analyzing all game transition patterns and strategy update rules, our study delineates when environmental knowledge facilitates the spread of cooperation in structured populations—or, paradoxically, gives rise to a “knowledge curse”.

Abstract: We investigate a 1D lattice model that allows multiple particle occupancy on each site. The particles undergo stochastic nearest-neighbour jumps influenced by both a directional bias and on-site repulsive Bose-Hubbard interactions Un(n-1)/2 where n is the site occupancy. With periodic boundary conditions, we find a non-monotonic dependence of inter-site correlation functions on the interaction strength. The large U state is interesting: an emergent ASEP riding on stacks of quiescent particles. As a result, the particle current exhibits a periodic dependence on density. By contrast, with open boundary conditions, the system displays step-like density profiles reminiscent of those in tilted quantum Bose-Hubbard systems.

Abstract: We show that, on a d-dimensional hypercubic lattice with d > 1, conserved-mass transport processes, with multidirectional hopping that respects all symmetries of the lattice, exhibit power-law correlations for generic parameter values -- even far from phase transition point, if any. The key idea for generating the algebraic decay is the notion of multidirectional hopping, which means that several chunks of mass, or several particles, can hop out simultaneously from a lattice site in multiple directions, thus violating detailed balance. Using hydrodynamic and exact microscopic theory, we show that, for spatial dimensions d > 1, the steady-state (static) density-density correlation functions generically decay as a power-law, where the exponent depends on dimensions, symmetries and conservation laws. In particular, our theory provides an explanation for the emergence of long-ranged correlations in systems governed by center-of-mass–conserving dynamics, which have recently been used to model disordered hyperuniform states of matter. In such systems, we show that unequal-time current–current correlations also exhibit power-law, though with a faster decay than that observed in diffusive systems having only a single conserved quantity (density).

References: 

[1] Animesh Hazra, Tanmoy Chakraborty, Anirban Mukherjee, and Punyabrata

Pradhan, PHYSICAL REVIEW E 112, 044130 (2025); 

[2] Animesh Hazra, Anirban Mukherjee, and Punyabrata Pradhan, J. Stat. Mech. (2025) 023201.

Abstract: In 2D Ising Model, without Magnetic Field, Equillibrium Phase Transition occurs at Tc=2.269. We usually proceed numerically by Metropolis Algorithm. If we modify the Transition Probability rate of Metropolis, through a added parameter E0, we see that dynamics changes, with Tc changing, and it could be non-equillibrium phase transition. We study the Dynamical Critical Exponent, z : which indicates how fast or slow convergence numerically is , near Tc, towards steady state. We use Autocorrelation and Integrated Autocorrelation Graphs of Energy and Magnetisation to obtain z. We see how z changes as E0 change.

Abstract: Intermittency in turbulence appears as intense, localized fluctuations in energy dissipation and deviations from normal scaling laws, effectively described by the multifractal cascade framework. From the Eulerian viewpoint, these fluctuations are spatially distributed, while the Lagrangian perspective captures them along tracer particle trajectories. In this work, we investigate the statistical signatures of both Eulerian and Lagrangian intermittency, revisiting bridge relations that link their respective fluctuations. Through causal analysis and comparison with numerical simulations, we demonstrate that Lagrangian fluctuations follow a large-deviation principle governing time-averaged dissipation along tracer paths. Our results offer new insight into the interplay between spatially concentrated dissipation and intermittent particle dynamics, highlighting both the strengths and limitations of existing models. This analysis lays the groundwork for a more unified framework bridging Eulerian and Lagrangian descriptions of turbulence.

Abstract: Dilute polymer suspensions can exhibit a chaotic flow state known as elastic turbulence, arising from contractile stresses generated as polymers relax toward their equilibrium configurations. Similarly, chaotic flow patterns emerge in active nematics, driven by active stresses that destabilize nematic order. In this talk, I will compare these two examples of low–Reynolds-number turbulence, and discuss implications for the chaotic dynamical states observed in epithelial cell monolayers.

Abstract: The male fruit fly produces ∼1.8 mm long sperm, thousands of which can be stored until mating in a ∼200 ⁢ μm sac, the seminal vesicle. While the evolutionary pressures driving such extreme sperm (flagellar) lengths have long been investigated, the physical consequences of their gigantism are unstudied. Through high-resolution three-dimensional reconstructions of in vivo sperm morphologies and rapid live imaging, we discovered that stored sperm are organized into a dense and highly aligned state. The packed flagella exhibit system-wide collective ‘material’ flows, with persistent and slow-moving topological defects; individual sperm, despite their extraordinary lengths, propagate rapidly through the flagellar material, moving in either direction along material director lines. To understand how these collective behaviors arise from the constituents’ nonequilibrium dynamics, we conceptualize the motion of individual sperm as topologically confined to a reptation-like tube formed by its neighbors. Therein, sperm propagate through observed amplitude-constrained and internally driven flagellar bending waves, pushing off counter-propagating neighbors. From this conception, we derive a continuum theory that produces an extensile material stress that can sustain an aligned flagellar material. Experimental perturbations and simulations of active elastic filaments verify our theoretical predictions. Our findings suggest that active stresses in the flagellar material maintain the sperm in an unentangled, hence functional state, in both sexes, and establish giant sperm in their native habitat as a novel and physiologically relevant active matter system.

Abstract: We present a hydrodynamic theory of anisotropic and inversion-asymmetric moving active permeable fluid membranes. These are described by an anisotropic Kardar-Parisi-Zhang equation. Depending upon the anisotropy parameters, the membrane is either effectively isotropic and algebraically rough with translational short, but orientational long range order, or unstable, suggestive of membrane crumpling.

Abstract: Active back-and-forth beating of cilia and flagella, plays a crucial role in enabling coordinated motions which is essential for efficient fluid transport as in mucociliary clearance and locomotion of microorganisms in a fluid medium. While past studies have been done on cilia beating in Newtonian fluids, however many biological fluids where cilia are embedded in such as mucus are viscoelastic. In this work, we model the medium as the Jeffrey fluid which can be characterized by two parameters memory time λ,  and memory strength β. We observe that these parameters have interesting effect on the time period of cilia and precision for oscillation.

Abstract: Response of many driven systems to driving is not smooth and occurs in bursts called avalanches. Random field Ising model under quasi-static driving has been successful in explaining the origin and properties of this burst-like activity. The dynamics in the case of the random field Ising model is abelian. We will discuss a spin -1 model with  three states, the random field Blume-Emery-Griffiths model.  The dynamics of relaxation for this model loses its abelian property in the  presence of repulsive biquadratic coupling. This lack of abelian nature of the dynamics leaves a clear fingerprint in the form of a discontinuity in the distribution of the field increments between successive avalanches. The statistics of the avalanches on the two sides of the discontinuity is found to be different. We provide analytical arguments that locate the onset of this discontinuity in excellent agreement with the numerical simulations. 

Abstract: We have investigated the reversal of magnetisation in a quasi-one-dimensional (needle-like) Blume–Capel ferromagnetic system using extensive Monte Carlo simulations, focusing on the decay of the metastable volume fraction under a weak magnetic field. The system, characterised by an elongated geometry along one direction relative to its cross-section, exhibits pronounced anisotropy effects on the metastable lifetime. By analysing the time evolution of the metastable volume fraction β(t), we uncover multiple dynamical regimes during the decay. The crossover times between these regimes are determined, and the corresponding Avrami exponents 'n' are estimated, revealing multiple dynamical crossovers in the magnetisation reversal process.

Abstract: We introduce a time-energy uncertainty relation within the context of monitored quantum dynamics [1] . Previous studies have established that the mean recurrence time, which represents the time taken to return to the initial state, is quantized as an integer multiple of the sampling time, displaying point-wise discontinuous transitions at resonances. Our findings demonstrate that the natural utilization of the restart mechanism in laboratory experiments [2], driven by finite data collection time spans, leads to a broadening effect on the transitions of the mean recurrence time. Our proposed uncertainty relation captures the underlying essence of these phenomena, by connecting the broadening of the mean hitting time near resonances, to the intrinsic energies of the quantum system and to the fluctuations of recurrence time. Our uncertainty relation has also been validated through remote experiments conducted on an International Business Machines Corporation (IBM) quantum computer. We then discuss fractional quantization of the recurrence time for interacting spin systems using sub-space measurements [3].

References:

[1] R. Yin, Q. Wang, S. Tornow, and E. Barkai, Restart uncertainty relation for monitored quantum dynamics, Proceedings of the National Academy of Sciences 122 (1) e2402912121, (2025).

[2] R. Yin, E. Barkai,  Restart expedites quantum walk hitting times Phys, Rev. Lett. 130, 050802 (2023).

[3] Q. Liu, S. Tornow, D. Kessler, and E. Barkai, Properties of Fractionally Quantized Recurrence Times for Interacting Spin Models, arXiv:2401.09810 [cond-mat.stat-mech] (submitted)

Abstract:

Abstract: We investigate quantum melting in a two-dimensional system of interacting “Boltzmann” particles in both clean and randomly pinned environments at zero temperature. The melting transition is driven by varying the particle density. As the density increases from low values, the system evolves from a disordered liquid to an ordered solid and subsequently melts back into a disordered liquid at a higher critical density, exhibiting re-entrant quantum melting. The melting is tracked by the onset of positional and bond-orientational orders and from the peaks of their corresponding susceptibilities. In the clean system, the emergence of positional and bond-orientational orders occurs simultaneously, ruling out the appearance of any intermediate hexatic phase. In contrast, in the disordered (pinned) system, the two order parameters decouple at distinct densities, indicating the possibility of a hexatic-like phase stabilized by quantum fluctuations. The presence of disorder thus renders the melting behavior inhomogeneous and intriguing, highlighting a rich interplay between disorder and quantum effects. The implication of our findings in light of recent experiments are being investigated.

Abstract: Many-body quantum systems, under suitable conditions, exhibit time-translation symmetry breaking and settle in a discrete time crystalline (DTC) phase – an out-of-equilibrium quantum phase of matter. The defining feature of DTC is a robust subharmonic response. However, the DTC phase is fragile in the presence of environmental dissipation. Here, we propose and exemplify a DTC phase in a noninteracting system that owes its stability to environmental dissipation. The lifetime of this DTC is independent of initial conditions and the size of the system, though it depends on the frequency of the external driver. We experimentally demonstrate this realization of DTC using Nuclear Magnetic Resonance spectroscopy.

Abstract: Quantum many-body scars are atypical high-energy eigenstates that do not follow the eigenstate thermalization hypothesis, appearing even in models where interactions normally lead to thermalization. We present exact construction of a class of such states in several paradigmatic models both in one and higher dimensions. Many of these scars can be viewed as atypical zero modes of the interacting model when its parameters are tuned to special values where an exponentially large number (in system size) of mid-spectrum zero modes emerge due to an index theorem.

Abstract: Recently, the Sachdev-Ye-Kitaev (SYK) model and its variants have breathed new life into our efforts to understand quasiparticle-less matter. The model offers a rare example of a highly interacting system that is non-integrable and yet remains exactly solvable analytically. In a short time, it has gained prominence across various areas of physics due to its deep connections to black-hole physics, quantum many-body chaos, operator complexity, non-Fermi liquids, and other strongly correlated phenomena. In this talk, I will highlight some of our attempts to use the SYK model and its related variations to understand entanglement and non-equilibrium relaxation mechanisms in quantum many-body systems. In particular, I will discuss a new field-theoretic interpretation that we developed while addressing questions of entanglement in SYK-type models. This interpretation can also be coupled to existing numerical approaches to quantify entanglement in more general systems. While most studies have focused on the SYK model in equilibrium--owing to the exact solvability of its partition function--I will show that non-equilibrium formulations of the model are also analytically tractable. I will present a few recent results that use such formulations to demonstrate phenomena including dynamical transitions under quenches and universal dynamical behavior.

Abstract: Unlike conventional magnets, highly frustrated quantum spin systems may not exhibit long-range magnetic order. Instead, they can host exotic ground states such as quantum spin liquids. These phases are particularly likely to emerge in two-dimensional spin-1/2 systems, where strong quantum fluctuations play a crucial role. One promising platform for exploring such physics is the trellis lattice—a highly frustrated two-dimensional geometry introduced in [1]. In this work, we perform a fully symmetric Projective Symmetry Group (PSG) classification on the trellis lattice, following the framework developed by Wen [2]. Using the Abrikosov fermion representation for spin operators, we identify 256 algebraic PSGs associated with a Z2 invariant gauge group (IGG), along with 448 distinct U(1) PSGs. By restricting our analysis to the three nearest neighbor symmetry-inequivalent bonds, we further narrow down the possibilities. We find 7 U(1) and 25 Z2 short-ranged Ansatze, uncovering both gapped and Dirac QSLs as well as a novel semi-Dirac spin liquid, in which the spinon dispersion is linear along one momentum direction but quadratic along the orthogonal one. Moreover, by optimizing over all mean-field states, we map out a phase diagram – featuring six distinct phases – of the nearest-neighbor Heisenberg Hamiltonian on the trellis lattice  and analyze their dynamical spin structure factors as experimentally relevant fingerprints.[3]

References: 

[1] Phase diagram of the 𝑆=1/2 frustrated coupled ladder system, Phys. Rev. B 56, R5736 (1997).

[2] Quantum orders and symmetric spin liquids, Phys. Rev. B 65, 165113 (2002).

[3] Semi-Dirac spin liquids and frustrated quantum magnetism on the trellis lattice, arXiv:2509.02663 (2025)

Abstract: Understanding quantum entanglement through its various measures, such as von Neumann entanglement entropy, negativity, concurrence etc. is a crucial task to grab the knowledge about quantum states. In this talk, I will present our recent work on quantum entanglement where we have studied the entanglement entropy dynamics in fermionic lattice in presence of different quantum trajectory protocols. We have considered a filled fermionic system coupled with an infinite reservoir where both the system and the reservoir are subjected to dissipative effects in the form of (i) onsite stochastic noise and (ii) continuous quantum measurement of local densities. I will begin by briefly discussing the physical intuition and motivation behind these two kinds of trajectory protocol and then I will discuss the dynamics of entanglement entropy in the quantum trajectories. I will present our key findings on the temporal growth and eventual decay of entanglement and its connection with the particle transport. Finally, I will conclude with the summary and future outlooks.

Abstract: Stochastic thermodynamics and associated fluctuation theorems have long been of interest to the statistical mechanics community. The studies in this domain have mostly focussed on the calculation of the distribution functions of various thermodynamic variables, such as work, heat and entropy for microscopic systems. In this talk, I will discuss our recent work on this topic, specifically, the calculation of the work and heat distributions of a colloid in a non-linear flow that also experiences an external Ornstein-Uhlenbeck noise [1]. I will then also discuss our extension to this work where the flow field now becomes time dependent [2]. All of these distributions obey the fluctuation theorem individually and also have implications for the other thermodynamic quantities.

References:

[1] Work distribution of a colloid in an elongational flow field and under Ornstein-Uhlenbeck noise. Debasish Saha and Rati Sharma*. Physical Review E 109, 014111 (2024).
[2] Stochastic thermodynamics of a colloid in a time dependent flow field. Debasish Saha, Yasheel Sharma and Rati Sharma* (Manuscript in preparation).

Abstract: Microtubules exhibit a remarkable ability to alternate stochastically between phases of growth and shrinkage—a phenomenon known as dynamic instability. This intrinsic dynamism plays a central role in enabling efficient intracellular search and the spatial organization of many functional architectures within the cell. For example, during mitotic spindle formation in prometaphase, Golgi reassembly following cell division, or the establishment of contacts between T cells and antigen-presenting cells during immune responses, dynamic microtubules guide complex and targeted exploration of the subcellular space. In this talk, I will present our modeling efforts that capture the emergent dynamics underlying these processes, revealing how microtubules optimize search efficiency and drive self-organization across diverse cellular contexts.

Abstract: We determine  the exact entropy per site of full covering of the kagome lattice by straight rigid trimers in the thermodynamic limit.  We relate the transfer matrix of this model to the full covering of the hexagonal lattice by dimers.  The entropy of covering the kagome lattice per trimer  is exactly equal to the entropy per dimer of the dimer covering of the hexagonal lattice. These  results are verified by exact  numerical diagonalization of the transfer matrix for the model   for cylinders of circumference L, for L = 4,6,8.... 18.

Abstract: In two-dimensional models of critical non-intersecting loops, we conjecture an exact formula for three-point functions of fields that insert legs (open loop segments) and can have nonzero conformal spin. The conjecture extends a previous result for diagonal fields, recently proved by Ang-Cai-Sun-Wu, who also proved our conjecture for three spinless two-leg operators. We discuss in details the supporting evidence for our general conjecture coming from transfer-matrix computations using the unoriented Jones-Temperley-Lieb algebra.

Abstract: We consider a one-dimensional gas of hard rods, one of the simplest examples of an interacting integrable model. It is well known that the hydrodynamics of such integrable models can be understood by viewing the system as a gas of quasiparticles. Here, we explore the dynamics of individual quasiparticles for a variety of initial conditions of the background gas. The mean, variance, and two-time correlations are computed exactly and lead to a picture of quasiparticles as drifting Brownian particles. For the case of a homogeneous background, we show that the motion of two tagged quasiparticles is strongly correlated, and they move like a rigid rod at late times. Apart from a microscopic derivation based on the mapping to point particles, we provide an alternate derivation which emphasizes that quasiparticle fluctuations are related to initial phase-space fluctuations, which are carried over in time by Euler scale dynamics. For the homogeneous state, we use the Brownian motion picture to develop a fluctuating hydrodynamic theory, formally having the same structure as that derived recently by Ferrari and Olla.

Abstract: We use the relaxation time approximation to study GHD-Boltzmann equation, compute the transport coefficients and compare dynamics of the GHD-Boltzmann with the emergent Navier-Stokes equations.

Abstract: Run-and-tumble (RT) motion in flagellated microswimmers, such as the biflagellated alga Chlamydomonas reinhardtii, arises from synchronous and asynchronous flagellar beating. The role of hydrodynamic interactions has long been associated with such a swimming behaviour. However, the role of contractile stress fibers, which mechanically couple these flagella, is relatively less well known. To investigate this, we take a unique experimental approach. We create a robotic analog of Chlamydomonas using two dry, centimeters-sized, self-propelled robots linked by a rigid rod, with adjustable attachment points to mimic fiber contractility. Each robot is programmed to perform overdamped active Brownian motion, capturing low–Reynolds–number conditions. We show that our robotic model produces clear RT-like dynamics, including sharp direction-reversing tumbles and exponentially distributed run times. Moreover, we demonstrate that the tumbling frequency can be tuned through experimental parameters, providing valuable insights into how such a motion might be regulated in a real organism.

Abstract: The Overhauser effect (OE) and the Solid effect (SE) are two Dynamic Nuclear Polarization techniques. These two-spin techniques are widely used to create nonequilibrium nuclear spin states having polarization far beyond its equilibrium value. OE is commonly encountered in liquids, and SE is a solid-state technique. Here, we report a single framework, built from first principles, to explain both OE and SE. To this end, we use a fluctuation-regularized quantum master equation that predicts dipolar relaxation and drive-induced dissipation, in addition to the standard environmental dissipation channels. Furthermore, our framework predicts the existence of optimal microwave drive amplitudes that maximize the OE and SE enhancements. We envisage the work to have immediate applications in the broad field of hyperpolarization.

Abstract: In this work, we study two scalar field driven dark energy models characterized by the axion potential and the inverse power-law potential, each coupled to dark matter through momentum exchange. By formulating the dynamics as an autonomous system, we identify the equilibrium points and analyze their stability. To constrain these models, we utilize observational data from Pantheon Plus Type Ia Supernovae, DES Y5, DESI DR2 BAO, and Planck 2018 CMB compressed likelihood, employing Markov Chain Monte Carlo (MCMC) methods. Both potentials exhibit weak to strong preference over the Λ-CDM model, with a particularly strong preference for the momentum-coupled scenario when Supernova data are included in the analysis. Furthermore, we find the coupling parameter to be negative, with no lower bound, for both potentials. This finding agrees with previous studies and suggests that momentum-exchange coupling between the dark sectors cannot be ruled out. From the stability analysis, we observe that for both potentials, the late-time attractor corresponds to a dark energy–dominated phase, and the scalar field can behave as a stiff fluid during the early epoch. According to the model selection criteria, the inverse power-law potential is favoured over the axion potential.