arXiv subjects

Emilia Witkowska

Publications and source records attributed to Emilia Witkowska.

At least 19 recordsLinked to original sources

arXiv2026

Limit of spin squeezing in trapped Bose-Einstein condensates

The evolution of an interacting two-component Bose-Einstein condensate from an initial phase state leads to a spin squeezed state that may be used in atomic clocks to increase the signal-to-noise ratio, opening the way to quantum metrology. The efficiency of spin squeezing is limited by the finite temperature of the gas, as was shown theoretically in a spatially homogeneous system. Here we determine the limit of spin squeezing in the realistic trapped case, with classical field simulations, and with a completely analytical treatment that includes the quantum case.

quant-ph↗

arXiv2026

Universal spin-squeezing dynamics in spinor condensates

The production of large-scale entangled states is one of the main goals of next-generation quantum technologies, with an immediate potential for applications in the context of entanglement-assisted quantum sensing. A very promising platform to achieve this goal is offered by ultracold spinor gases, made of atoms with a large internal spin sensitive to magnetic fields. Here we show that the native spin-changing collisions in a spinor Bose-Einstein condensate, combined with an arbitrary quadratic Zeeman shift, can generate scalable spin squeezing in the collective spin of the ensemble, following the universal paradigm of the celebrated one-axis-twisting model. Squeezing dynamics is driven by the quadratic Zeeman shift when this shift is small; and by the spin-changing collisions for large shifts, in the form of stroboscopic squeezing. Turning off the Zeeman shift freezes out the collective-spin dynamics, so that the ensuing collective spin dynamics can be uniquely governed by an external field to be sensed. Our theoretical results pave the way for the use of spinor Bose gases with a large spin in fundamental studies of entanglement, as well as in advanced metrological applications.

quant-ph↗

arXiv2026

Degeneracy-governed spin squeezing in high-spin Fermi-Hubbard systems weakly coupled to light

High-spin alkaline-earth fermions in optical lattices are promising platforms for spin squeezing beyond the spin-1/2 paradigm. We show that spin-squeezing dynamics is qualitatively modified by degeneracies inherent to the extended internal spin structure. We identify these degeneracies as the microscopic origin of the breakdown of the conventional maximal-spin description and develop an effective population-eigenstate framework that quantitatively reproduces the spin-squeezing dynamics. Our results establish degeneracy as a generic driver of collective spin dynamics in high-spin systems.

cond-mat.quant-gas↗

arXiv2026

Population eigenstates of the SU(d) spin-exchange model for high-spin fermions in optical lattices

We investigate the $\mathrm{SU}(d)$ spin exchange model describing ultra-cold fermionic atoms with spin $s\ge 1$ in a one-dimensional optical lattice. The model emerges from the Fermi-Hubbard model in the strongly interacting regime with one atom in each lattice site. The central result of this work is the systematic construction of eigenstates in terms of magnetic sub-level populations, which we call population eigenstates. Exploiting this framework, we derive effective light-induced Hamiltonians via a second-order Schrieffer-Wolff transformation projected onto the population eigenstates. The resulting models reveal a qualitative difference between spin-1/2 and higher-spin systems: whereas spin-1/2 dynamics remains confined to the maximal-spin Dicke manifold, the extensive $\mathrm{SU}(d)$ degeneracies for $s\ge 1$ allow coherent population transfer across sectors of different collective spin length, generating unconventional spin dynamics that cannot be captured by any fixed-spin-manifold description. Agreement with exact Fermi-Hubbard dynamics confirms the framework as a practical foundation for quantum-enhanced correlations and metrological protocols in high-spin fermionic systems.

cond-mat.quant-gas↗

arXiv2026

Dynamic structure factor of quantum hard rods from exact form-factors

We study the quantum hard-rods model and obtain compact analytical expressions for density form factors, and a semi-analytical treatment for dynamic and static structure factors calculations, greatly reducing computational complexity. We identify conditions under which these form factors vanish and analyze real-space correlations, confirming the model's Tomonaga-Luttinger liquid behavior. The results reveal universal features of low energy physics of a gapless quantum fluid and its relation to Luttinger liquid theory, providing precise benchmarks for numerical simulations. This work establishes quantum hard rods as an important testbed for theories of strongly correlated one-dimensional systems.

cond-mat.quant-gas↗

arXiv2025

Multisetting protocol for Bell correlated states detection with spin-$f$ systems

We propose a multisetting protocol for the detection of two-body Bell correlations, and apply it to spin-nematic squeezed states realized in $f$ pairs of SU(2) subsystems within spin-$f$ atomic Bose-Einstein condensates. Experimental data for $f=1$, alongside with numerical simulations using the truncated Wigner method for $f=1,\,2,\,3$, demonstrate the effectiveness of the proposed protocol. Our findings extend the reach of multisetting Bell tests in ultracold atomic system, paving the way for extended quantum information processing in high-spin ensemble platforms.

cond-mat.quant-gas↗

arXiv2025

Magnetization-induced reordering of ground states phase diagram in a two-component Bose-Hubbard model

We investigate the influence of non-zero magnetization on the ground-state phase diagram of the two-component Bose-Hubbard model. Employing a mean-field theoretical framework, both analytically and numerically, we demonstrate that positions and sizes of specific phases on the diagram are magnetization dependent. In particular, non-zero magnetization introduces different Mott insulator phase boundaries for each of the two components. This effect leads to the emergence of a hybrid phase characterized by the coexistence of superfluid in one of the components and Mott insulator in the another one. Our findings highlight the important role of a conserved quantities, which is magnetization here, in reshaping the phase landscape, significantly influencing the stability and emergence of distinct quantum phases.

cond-mat.quant-gas↗

arXiv2025

On the fluctuations of the number of atoms in the condensate

Bose-Einstein condensation represents a remarkable phase transition, characterized by the formation of a single quantum subsystem. As a result, the statistical properties of the condensate are highly unique. In the case of a Bose gas, while the mean number of condensed atoms is independent of the choice of statistical ensemble, the microcanonical, canonical, or grand canonical variances differ significantly among these ensembles. In this paper, we review the progress made over the past 30 years in studying the statistical fluctuations of Bose-Einstein condensates. Focusing primarily on the ideal Bose gas, we emphasize the inequivalence of the Gibbs statistical ensembles and examine various approaches to this problem. These approaches include explicit analytic results for primarily one-dimensional systems, methods based on recurrence relations, asymptotic results for large numbers of particles, techniques derived from laser theory, and methods involving the construction of statistical ensembles via stochastic processes, such as the Metropolis algorithm. We also discuss the less thoroughly resolved problem of the statistical behavior of weakly interacting Bose gases. In particular, we elaborate on our stochastic approach, known as the hybrid sampling method. The experimental aspect of this field has gained renewed interest, especially following groundbreaking recent measurements of condensate fluctuations. These advancements were enabled by unprecedented control over the total number of atoms in each experimental realization. Additionally, we discuss the fluctuations in photonic condensates as an illustrative example of grand canonical fluctuations. Finally, we briefly consider the future directions for research in the field of condensate statistics.

cond-mat.quant-gas↗

arXiv2024

Effective light-induced Hamiltonian for atoms with large nuclear spin

Ultra-cold fermionic atoms, having two valence electrons, exhibit a distinctive internal state structure, wherein the nuclear spin becomes decoupled from the electronic degrees of freedom in the ground electronic state. Consequently, the nuclear spin states are well isolated from the environment, rendering these atomic systems an opportune platform for quantum computation and quantum simulations. Coupling with off-resonance light is an essential tool to selectively and coherently manipulate the nuclear spin states. In this paper, we present a systematic derivation of the effective Hamiltonian for the nuclear spin states of ultra-cold fermionic atoms due to such an off-resonance light. We obtain compact expressions for the scalar, vector and tensor light shifts taking into account both linear and quadratic contributions to the hyperfine splitting. The analysis has been carried out using the Green operator approach and solving the corresponding Dyson equation. Finally, we analyze different scenarios of light configurations which lead to the vector- and tensor-light shifts, as well as the pure spin-orbit coupling for the nuclear spin.

quant-ph↗

arXiv2024

Bounds on detection of Bell correlations with entangled ultra-cold atoms in optical lattices under occupation defects

Bell non-locality stems from quantum correlations effectively identified using inequalities. Spin chains, simulated with ultra-cold atoms in optical lattices, Rydberg atoms in tweezer arrays, trapped ions, or molecules, allow single-spin control and measurement. Therefore, they are suitable for studying fundamental aspects of these correlations and non-locality. Occupation defects, such as vacancies or multiple atoms occupying a single site due to imperfect system preparation, limit the detection of Bell correlations. We study their effects with the help of a simplified toy model parameterised by the probability $p$ of having a single occupation for a given site. Within this model, and for entangled systems obtained by one-axis twisting evolution from an initial factorised state, we derive two Bell inequalities, one based on many-site correlations and the other on two-site correlations, and identify the smallest probability $p$ that allows the Bell inequalities violation to be detected. We then consider two physical realizations using entangled ultra-cold atoms in optical lattices where the parameter $p$ is related to a non-unitary filling factor and non-zero temperature. We test the predictions of the toy model against exact numerical results.

cond-mat.quant-gas↗

arXiv2024

Magnetically generated spin-orbit coupling for ultracold atoms with slowly varying periodic driving

The spin-orbit coupling (SOC) affecting the center of mass of ultracold atoms can be simulated using a properly chosen periodic sequence of magnetic pulses. Yet such a method is generally accompanied by micro-motion which hinders a precise control of atomic dynamics and thus complicating practical applications. Here we show how to by-pass the micro-motion emerging in the magnetically induced SOC by switching on and off properly the oscillating magnetic fields at the initial and final times. We consider the exact dynamics of the system and demonstrate that the overall dynamics can be immune to the micro-motion. The exact dynamics is shown to agree well with the evolution of the system described by slowly changing effective Floquet Hamiltonian including the SOC term. The agreement is shown to be the best when the phase of the periodic driving takes a specific value for which the effect of the spin-orbit coupling is maximum.

cond-mat.quant-gas↗

arXiv2024

Exploring spin-squeezing in the Mott insulating regime: role of anisotropy, inhomogeneity and hole doping

Spin-squeezing in systems with single-particle control is a well-established resource of modern quantum technology. Applied in an optical lattice clock can reduce the statistical uncertainty of spectroscopic measurements. Here, we consider dynamic generation of spin-squeezing with ultra-cold bosonic atoms with two internal states loaded into an optical lattice in the strongly interacting regime as realized with state-of-the-art experiments using a quantum gas microscope. We show that anisotropic interactions and inhomogeneous magnetic fields generate scalable spin-squeezing if their magnitudes are sufficiently small, but not negligible. The effect of non-uniform filling caused by hole doping, non-zero temperature and external confinement is studied at a microscopic level demonstrating their limiting role in the dynamics and scaling of spin squeezing.

cond-mat.quant-gas↗

arXiv2023

Generation of scalable many-body Bell correlations in spin chains with short-range two-body interactions

Dynamical generation of strong and scalable quantum resources, like many-body entanglement and Bell correlations, in spin-$1/2$ chains, is possible with all-to-all interactions, either for constant interaction strength realizing one-axis twisting protocol or for power-law decaying potentials. We show, however, that such quantum resources can also be dynamically generated with a finite range of interactions. We identify a necessary critical range and indicate a critical time when scalable quantum correlations appear. Finally, we show that the certification of generated states is accessible in the modern quantum simulator platforms.

quant-ph↗

arXiv2023

Spin squeezing in open Heisenberg spin chains

Spin squeezing protocols successfully generate entangled many-body quantum states, the key pillars of the second quantum revolution. In our recent work [Phys. Rev. Lett. 129, 090403 (2022)] we showed that spin squeezing described by the one-axis twisting model could be generated in the Heisenberg spin-1/2 chain with periodic boundary conditions when accompanied by a position-dependent spin-flip coupling induced by a single laser field. This work shows analytically that the change of boundary conditions from the periodic to the open ones significantly modifies spin squeezing dynamics. A broad family of twisting models can be simulated by the system in the weak coupling regime, including the one- and two-axis twisting under specific conditions, providing the Heisenberg level of squeezing and acceleration of the dynamics. Full numerical simulations confirm our analytical findings.

quant-ph↗

arXiv2022

Critical quantum thermometry and its feasibility in spin systems

In this work, we study temperature sensing with finite-sized strongly correlated systems exhibiting quantum phase transitions. We use the quantum Fisher information (QFI) approach to quantify the sensitivity in the temperature estimation, and apply a finite-size scaling framework to link this sensitivity to critical exponents of the system around critical points. We numerically calculate the QFI around the critical points for two experimentally-realizable systems: the spin-1 Bose-Einstein condensate and the spin-chain Heisenberg XX model in the presence of an external magnetic field. Our results confirm finite-size scaling properties of the QFI. Furthermore, we discuss experimentally-accessible observables that (nearly) saturate the QFI at the critical points for these two systems.

quant-ph↗

arXiv2022

Accelerating many-body entanglement generation by dipolar interactions in the Bose-Hubbard model

The spin squeezing protocols allow the dynamical generation of massively correlated quantum many-body states, which can be utilized in entanglement-enhanced metrology and technologies. We study a quantum simulator generating twisting dynamics realized in a two-component Bose-Hubbard model with dipolar interactions. We show that the interplay of contact and long-range dipolar interactions between atoms in the superfluid phase activates the anisotropic two-axis counter-twisting mechanism, accelerating the spin squeezing dynamics and allowing the Heisenberg-limited accuracy in spectroscopic measurements.

cond-mat.quant-gas↗

arXiv2022

One-axis twisting as a method of generating many-body Bell correlations

We demonstrate that the one-axis twisting (OAT), a versatile method of creating non-classical states of bosonic qubits, is a powerful source of many-body Bell correlations. We develop a fully analytical and universal treatment of the process, which allows us to identify the critical time at which the Bell correlations emerge, and predict the depth of Bell correlations at all subsequent times. Our findings are illustrated with a highly non-trivial example of the OAT dynamics generated using the Bose-Hubbard model.

quant-ph↗

arXiv2020

Multifaceted phase ordering kinetics of an antiferromagnetic spin-1 condensate

We study phase domain coarsening in the long time limit after a quench of magnetic field in a quasi one-dimensional spin-1 antiferromagnetic condensate. We observe that the growth of correlation length obeys scaling laws predicted by the two different models of phase ordering kinetics, namely the binary mixture and vector field. We derive regimes of clear realization for both of them. We demonstrate appearance of atypical scaling laws, which emerge in intermediate regions.

cond-mat.quant-gas↗