arXiv subjects

Ravindra W. Chhajlany

Publications and source records attributed to Ravindra W. Chhajlany.

At least 19 recordsLinked to original sources

arXiv2026

Dynamical Signatures and Kibble-Zurek Scaling of Localization in Tilted Bose-Einstein Condensates

We study nonequilibrium signatures of tilt-induced localization in a one-dimensional Bose-Einstein condensate loaded in a shallow optical lattice. The tilt strength acts as a control parameter for the localization-delocalization crossover. We also consider the effects of repulsive interactions, which tend to delocalize the condensate. We first characterize localized and delocalized regimes through sudden quenches of the interaction strength and the external tilt. The resulting dynamics is analyzed using the survival probability and its power spectral density. Localized condensates exhibit strong memory retention, pronounced revivals, regular dynamics and a narrow spectral response, whereas delocalized condensates show suppressed recurrences, irregular dynamics and a broader distribution of spectral weight over many frequencies. We then investigate finite-rate ramps of the tilt strength across the localization threshold. Using the localization length and the Bogoliubov excitation gap, we extract the relevant critical exponents and perform Kibble-Zurek scaling analysis in the driven dynamics. Our results establish quench response and finite-rate scaling as complementary dynamical probes of localization in interacting Bose gases, with direct relevance to cold-atom experiments in tilted optical lattices.

cond-mat.quant-gas↗

arXiv2026

Precision assessment in non-Hermitian systems: a comparative study of three formalisms

Quantifying measurement precision in quantum systems is vital for advancing quantum technologies such as sensing, communication, and computation. The quantum Fisher information (QFI) sets the ultimate precision bound in Hermitian systems; however, extending this concept to non-Hermitian systems, even those with real spectra, poses conceptual challenges due to their non-unitary dynamics. We compare three probability-conserving approaches for evaluating QFI in such systems: (i) simple normalization, (ii) metric formalism, and (iii) master-equation framework. Although all three ensure probability conservation, they differ in physical interpretation and in how they quantify estimation precision. Our study is particularly motivated by previous studies that have shown that the simple normalization method for non-Hermitian Hamiltonian generated dynamics may lead to misleading or even unphysical conclusions for certain quantum information theoretic tasks. We emphasize, in this article, that the metric formalism naturally enables the use of standard Hermitian metrology tools in cases where it provides a coherent and physically consistent framework for non-Hermitian systems.

quant-ph↗

arXiv2025

Revealing the anisotropic charge-density-wave order of TiSe$_2$ through high harmonic generation

Titanium diselenide (TiSe$_{2}$) is a transition-metal dichalcogenide material that undergoes a charge-density-wave (CDW) transition at $T_{c}\approx 200\,\mathrm{K}$. In a recent experiment [I. Tyulnev {\it et al.}, Commun. Mater. 6, 152 (2025)], the high harmonic generation (HHG) spectra of this material has been studied, which exhibits asymmetric behavior with respect to the polarization angle of the incident light and provides a new perspective to the CDW phase transition. In this work, we work out a theoretical explanation for the experimentally observed behavior of HHG spectra. We propose a simplified phenomenological mean-field model for this material, based on which the HHG spectra is calculated through the time-dependent Schr{\" o}dinger equation. This model correctly describes the measured intensity distribution of the third-, fifth-, and seventh-order harmonic generation as a function of polarization direction and reveals a strong asymmetry due to the anisotropic CDW order in the low temperature phase. Our work provides a basis for applying high harmonic spectroscopy to reveal a new perspective on the nature of CDW orders.

cond-mat.str-el↗

arXiv2025

Anomalous fluctuations of Bose-Einstein condensates in optical lattices

Fluctuations are fundamental in physics and important for understanding and characterizing phase transitions. In this spirit, the phase transition to the Bose-Einstein condensate (BEC) is of specific importance. Whereas fluctuations of the condensate particle number in atomic BECs have been studied in continuous systems, experimental and theoretical studies for lattice systems were so far missing. Here, we explore the condensate particle number fluctuations in an optical lattice BEC across the phase transition in a combined experimental and theoretical study. We present both experimental data using ultracold $^{87}$Rb atoms and numerical simulations based on a hybrid approach combining the Bogoliubov quasiparticle framework with a master equation analysis for modeling the system. We find strongly anomalous fluctuations, where the variance of the condensate number $δN_{\rm BEC}^2$ scales with the total atom number as $N^{1+γ}$ with an exponent around $γ_{\rm theo}=0.74$ and $γ_{\rm exp}=0.62$, which we attribute to the 2D/3D crossover geometry and the interactions. Our study highlights the importance of the trap geometry on the character of fluctuations and on fundamental quantum mechanical properties.

cond-mat.quant-gas↗

arXiv2025

Enhanced quantum sensing with hybrid exceptional-diabolic singularities

We report an enhanced sensitivity for detecting linear perturbations near hybrid (doubly degenerated) exceptional-diabolic (HED) singular points in a four mode bosonic system. The sensitivity enhancement is attributed to a singular response function, with the pole order determining the scaling of estimation error. At HED singular points, the error scaling exhibits a twofold improvement over non-HED singular points. The ultimate bound on estimation error is derived via quantum Fisher information, with heterodyne detection identified as the measurement achieving this optimal scaling.

quant-ph↗

arXiv2025

Contemporary tensor network approaches to gapless and topological phases in an extended Bose-Hubbard ladder

The development of numerically efficient computational methods has facilitated in depth studies of various correlated phases of matter including critical and topological phases. A quantum Monte-Carlo study of an extended Bose-Hubbard ladder has recently been used to identify an exotic phase with hidden order, where superfluid correlations coexist with string order, dubbed a Haldane superfluid (HSF). However, finite-size methods can struggle to uniquely determine the boundaries of quasi-long-range ordered states with nonlocal, e.g. string-like, correlations. In the present Letter, we revisit the HSF scenario using tensor network algorithms specialized for finite/infinite (quasi-)1D systems, \textit{i.e.} the well-governed finite-size density matrix renormalization group (DMRG), and the state-of-the-art infinite-size variational uniform matrix product state (VUMPS) methods. While DMRG results extrapolated to the thermodynamic limit are compatible with a putative HSF, the results from the VUMPS calculations provide sharper phase boundaries that leave no room for such a topological superfluid. Our results demonstrate the crucial advantage of the VUMPS in characterizing topological and critical interacting phases providing the precise phase boundaries.

cond-mat.quant-gas↗

arXiv2024

Competing order in two-band Bose-Hubbard chains with extended-range interactions

Motivated by the recent progress in realizing and controlling extended Bose-Hubbard systems using excitonic or atomic devices, the present Letter theoretically investigates the case of a two-band Bose-Hubbard chain with nearest-neighbor interactions. Specifically, this study concentrates on the scenario where, due to the interactions, one band supports a density wave phase, i.e. a correlated insulating phase with spontaneous breaking of translational symmetry in the lattice, while the other band supports superfluid behavior. Using the density matrix renormalization group method, we show that supersolid order can emerge from such a combination, that is, an elusive quantum state that combines crystalline order with long-range phase coherence. Depending on the filling of the bands and the interband interaction strength, the supersolid phase competes with phase-separation, superfluid order, or Mott insulating density-wave order. As a possible setup to observe supersolidity, we propose the combination of a lower band supporting density-wave order and a thermally excited band that supports superfluidity due to weaker lattice confinement.

cond-mat.quant-gas↗

arXiv2024

High harmonic spectroscopy reveals anisotropy of the Mott to Charge-Density-Wave phase transition in TiSe$_2$

This work explores the use of polarization-resolved high harmonic generation (HHG) spectroscopy to investigate the quantum phases and transitions in the correlated charge density wave (CDW) phase of TiSe$_2$. Unlike previous studies focusing on crystallographic changes, the research examines the reordering that occurs within the CDW phase as the material is cooled from room temperature to 14 K. By linking ultrafast field-driven dynamics to the material's potential landscape, the study demonstrates how HHG is sensitive to quantum phase transitions. The findings reveal an anisotropic component below the CDW transition temperature, providing new insights into the nature of this phase. The investigation highlights the interplay between linear and nonlinear optical responses and their departure from simple perturbative dynamics, offering a fresh perspective on correlated quantum phases in condensed matter systems.

cond-mat.str-el↗

arXiv2024

Observing quantum many-body scars in random quantum circuits

The Schwinger model describes quantum electrodynamics in 1+1-dimensions, it is a prototype for quantum chromodynamics, and its lattice version allows for a quantum link model description that can be simulated using modern quantum devices. In this work, we devise quantum simulations to investigate the dynamics of this model in its low dimensional form, where the gauge field degrees of freedom are described by spin 1/2 operators. We apply trotterization to write quantum circuits that effectively generate the evolution under the Schwinger model Hamiltonian. We consider both sequential circuits, with a fixed gate sequence, and randomized ones. Utilizing the correspondence between the Schwinger model and the PXP model, known for its quantum scars, we investigate the presence of quantum scar states in the Schwinger model by identifying states exhibiting extended thermalization times in our circuit evolutions. Our comparison of sequential and randomized circuit dynamics shows that the non-thermal sector of the Hilbert space, including the scars, are more sensitive to randomization.

quant-ph↗

arXiv2024

High Harmonic Tracking of Ultrafast Electron Dynamics across the Mott to Charge Density Wave Phase Transition

Different insulator phases compete with each other in strongly correlated materials with simultaneous local and non-local interactions. It is known that the homogeneous Mott insulator converts into a charge density wave (CDW) phase when the non-local interactions are increased, but there is ongoing debate on whether and in which parameter regimes this transition is of first order, or of second order with an intermediate bond-order wave phase. Here we show that strong-field optics applied to an extended Fermi-Hubbard system can serve as a powerful tool to reveal the nature of the quantum phase transition. Specifically, we show that in the strongly interacting regime characteristic excitations such as excitons, biexcitons, excitonic strings, and charge droplets can be tracked by the non-linear optical response to an ultrafast and intense laser pulse. Subcycle analysis of high harmonic spectra unravels the ultrafast dynamics of these increasingly complex objects, which partially escape the scrutiny of linear optics. Their appearance in the high harmonic spectrum provides striking evidence of a first-order transition into the CDW phase, and makes a strong case for using strong-field optics as a powerful tool to reveal the nature of quasiparticles in strongly correlated matter, and to track the electron dynamics during a first-order quantum phase transition.

cond-mat.str-el↗

arXiv2024

Decoupled few-femtosecond phase transitions in vanadium dioxide

The nature of the insulator-to-metal phase transition in vanadium dioxide (VO2) is one of the longest-standing problems in condensed-matter physics. Ultrafast spectroscopy has long promised to determine whether the transition is primarily driven by the electronic or structural degree of freedom, but measurements to date have been stymied by their sensitivity to only one of these components and/or their limited temporal resolution. Here we use ultra-broadband few-femtosecond pump-probe spectroscopy to resolve the electronic and structural phase transitions in VO2 at their fundamental time scales. We find that the system transforms into a bad-metallic phase within 10 fs after photoexcitation, but requires another 100 fs to complete the transition, during which we observe electronic oscillations and a partial re-opening of the bandgap, signalling a transient semi-metallic state. Comparisons with tensor-network simulations and density-functional theory calculations show these features originate from oscillations around the equilibrium high-symmetry atomic positions during an unprecedentedly fast structural transition, in which the vanadium dimers separate and untwist with two different timescales. Our results resolve the complete structural and electronic nature of the light-induced phase transition in VO2 and establish ultra-broadband few-femtosecond spectroscopy as a powerful new tool for studying quantum materials out of equilibrium.

cond-mat.str-el↗

arXiv2024

Light-induced phase transitions in vanadium dioxide: a tensor network study

Nonequilibrium phase transitions driven by light pulses represent a rapidly developing field in condensed matter physics. As one of the archetypal strongly correlated materials, vanadium dioxide (VO2) undergoes a structural phase transition (SPT) from a monoclinic (M1) to rutile (R) structure and an insulator-to-metal transition (IMT) either when heated above 340 K or when excited by an ultrafast laser pulse. Here, we present a tensor network study of the light-induced phase transitions in VO2 based on a quasi-one-dimensional model with all the important ingredients -- multi-orbital character, electron-lattice coupling, and electron-electron correlations -- being included. We show that this model qualitatively captures the equilibrium properties of VO2 by calculating the ground state phase diagram and finite-temperature phase transitions. A hybrid quantum-classical tensor-network method is used to simulate the dynamics following photoexcitation. We find that the structure can transform faster than the harmonic phonon modes of M1 phase, suggesting lattice nonlinearity is key in the SPT. We also find separate timescales for the evolution of dimerization and tilt distortions in the lattice dynamics, as well as the loss and subsequent partial restoration behavior of the displacements, which can provide an explanation for the complex dynamics observed in recent experiments [C. Brahms et al., arXiv:XXXX.XXXXX]. Moreover, decoupled SPT and IMT dynamics are observed in the numerical simulations: while the initial M1 structure transforms to the R one in tens of femtoseconds, the IMT occurs quasi-instantaneously, consistent with recent experimental findings. Our theoretical studies provide insight into the light-induced phase transitions of VO2, revealing unexpected non-monotonic transformation pathways and paving the way for future studies of non-thermal phase transformations.

cond-mat.str-el↗

arXiv2023

Synthetic dimensions for topological and quantum phases: Perspective

In this Perspective article we report on recent progress on studies of synthetic dimensions, mostly, but not only, based on the research realized around the Barcelona groups (ICFO, UAB), Donostia (DIPC), Poznań (UAM), Kraków (UJ), and Allahabad (HRI). The concept of synthetic dimensions works particularly well in atomic physics, quantum optics, and photonics, where the internal degrees of freedom (Zeeman sublevels of the ground state, metastable excited states, or motional states for atoms, and angular momentum states or transverse modes for photons) provide the synthetic space. We describe our attempts to design quantum simulators with synthetic dimensions, to mimic curved spaces, artificial gauge fields, lattice gauge theories, twistronics, quantum random walks, and more.

quant-ph↗

arXiv2023

Multiparameter estimation perspective on non-Hermitian singularity-enhanced sensing

Describing the evolution of quantum systems by means of non-Hermitian generators opens a new avenue to explore the dynamical properties naturally emerging in such a picture, e.g. operation at the so-called exceptional points, preservation of parity-time symmetry, or capitalising on the singular behaviour of the dynamics. In this work, we focus on the possibility of achieving unbounded sensitivity when using the system to sense linear perturbations away from a singular point. By combining multiparameter estimation theory of Gaussian quantum systems with the one of singular-matrix perturbations, we introduce the necessary tools to study the ultimate limits on the precision attained by such singularity-tuned sensors. We identify under what conditions and at what rate can the resulting sensitivity indeed diverge, in order to show that nuisance parameters should be generally included in the analysis, as their presence may alter the scaling of the error with the estimated parameter.

quant-ph↗

arXiv2020

Clustered Superfluids in the One Dimensional Bose-Hubbard model with extended correlated hopping

Bosonic lattice systems with non-trivial interactions represent an intriguing platform to study exotic phases of matter. Here, we study the effects of extended correlated hopping processes in a system of bosons trapped in a lattice geometry. The interplay between single particle tunneling terms, correlated hopping processes and on-site repulsion is studied by means of a combination of exact diagonalization, strong coupling expansion and cluster mean field theory. We identify a rich ground state phase diagram where, apart the usual Mott and superfluid states, superfluid phases with interesting clustering properties occur.

cond-mat.quant-gas↗

arXiv2020

Effects of extended correlated hopping in a bose-bose mixture

We study the effects of assisted tunneling or correlated hopping between next nearest neighbours in a two species Bose-Hubbard system. The system is the bosonic analong of the fermionic system studied in Phys. Rev. Lett. {\bf 116}, 225303 (2016). Using a combination of cluster mean field theory, exact diagonlization and analytical results, a rich phase diagram is determined including a pair superfluid phase as well as a superfluid quantum droplet phase. The former is the result of the interplay between single particle and correlated hopping, while the latter is the effect of large correlated hopping.

cond-mat.quant-gas↗

arXiv2020

Simulating twistronics without a twist

Rotational misalignment or twisting of two mono-layers of graphene strongly influences its electronic properties. Structurally, twisting leads to large periodic supercell structures, which in turn can support intriguing strongly correlated behaviour. Here, we propose a highly tunable scheme to synthetically emulate twisted bilayer systems with ultracold atoms trapped in an optical lattice. In our scheme, neither a physical bilayer nor twist is directly realized. Instead, two synthetic layers are produced exploiting coherently-coupled internal atomic states, and a supercell structure is generated \emph{via} a spatially-dependent Raman coupling. To illustrate this concept, we focus on a synthetic square bilayer lattice and show that it leads to tunable quasi-flatbands and Dirac cone spectra under certain magic supercell periodicities. The appearance of these features are explained using a perturbative analysis. Our proposal can be implemented using available state-of-the-art experimental techniques, and opens the route towards the controlled study of strongly-correlated flat band accompanied by hybridization physics akin to magic angle bilayer graphene in cold atom quantum simulators.

cond-mat.quant-gas↗

arXiv2020

Quantum anomalous Hall phase in synthetic bilayers via twistless twistronics

We recently proposed quantum simulators of "twistronic-like" physics based on ultracold atoms and syntheticdimensions [Phys. Rev. Lett. 125, 030504 (2020)]. Conceptually, the scheme is based on the idea that aphysical monolayer optical lattice of desired geometry is upgraded to a synthetic bilayer system by identifyingthe internal states of the trapped atoms with synthetic spatial dimensions. The couplings between the internalstates, i.e. between sites on the two layers, can be exquisitely controlled by laser induced Raman transitions.By spatially modulating the interlayer coupling, Moiré-like patterns can be directly imprinted on the latticewithout the need of a physical twist of the layers. This scheme leads practically to a uniform pattern across thelattice with the added advantage of widely tunable interlayer coupling strengths. The latter feature facilitates theengineering of flat bands at larger "magic" angles, or more directly, for smaller unit cells than in conventionaltwisted materials. In this paper we extend these ideas and demonstrate that our system exhibits topologicalband structures under appropriate conditions. To achieve non-trivial band topology we consider imanaginarynext-to-nearest neighbor tunnelings that drive the system into a quantum anomalous Hall phase. In particular,we focus on three groups of bands, whose their Chern numbers triplet can be associated to a trivial insulator(0,0,0), a standard non-trivial (-1,0,1) and a non-standard non-trivial (-1,1,0). We identify regimes of parameterswhere these three situations occur. We show the presence of an anomalous Hall phase and the appearance oftopological edge states. Our works open the path for experiments on topological effects in twistronics without atwist

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