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Optical and Quantum Electronics

Confinement-induced phase transitions and quantum optical signatures in optically trapped quantum rod systems

Maurice Tiotsop, Polynice Diffo Nguedia, A. J. Fotué

Published Aug 8, 2026• 0 citations
Publisher

Abstract

Abstract We present a theoretical study of confinement-induced quantum phase transitions in optically trapped atomic gases confined to rod-shaped quasi-one-dimensional geometries, and identify their signatures in experimentally accessible optical observables. The rod aspect ratio η = L/(2R) provides a geometric knob for tuning the effective one-dimensional coupling parameter γ ∝ η: increasing the rod length at fixed atom number reduces the linear density and drives the system from the weakly interacting regime through the Tonks–Girardeau (TG) fermionization limit for bosons, and from a Fermi liquid into a strongly correlated Tomonaga–Luttinger liquid (TLL) phase for fermions. Using the Lieb–Liniger model extended to finite temperature via Yang–Yang thermodynamics, and TLL bosonisation for fermions, we compute the phase diagram in the (γ, T/TF ) plane, first- and second-order spatial correlation functions g (1) and g (2) , the isothermal compressibility κ(μ, T ), and the local density fluctuations ⟨(δˆn) 2 ⟩. We show that the zero-distance second-order coherence g (2) (0) provides an unambiguous optical signal distinguishing all relevant phases: g (2) (0) ≈ 1 for the quasicondensate, g (2) (0) ≈ 0 in the TG limit, and g(2)(0) ≈ K c (γ)/2 < 1/2 in the TLL regime, where K c is the Luttinger parameter. These predictions are measurable via Hanbury Brown–Twiss atom interferometry, single-atom fluorescence imaging, and matter-wave interference in optical rod traps. Our results establish the rod aspect ratio as a design parameter for engineering correlated quantum phases and demonstrate optical coherence as their natural experimental readout.

DOI
10.1007/s11082-026-09033-6

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Abstract We present a theoretical study of confinement-induced quantum phase transitions in optically trapped atomic gases confined to rod-shaped quasi-one-dimensional geometries, and identify their signatures in experimentally accessible optical observables. The rod aspect ratio η = L/(2R) provides a geometric knob for tuning the effective one-dimensional coupling parameter γ ∝ η: increasing the rod length at fixed atom number reduces the linear density and drives the system from the weakly interacting regime through the Tonks–Girardeau (TG) fermionization limit for bosons, and from a Fermi liquid into a strongly correlated Tomonaga–Luttinger liquid (TLL) phase for fermions. Using the Lieb–Liniger model extended to finite temperature via Yang–Yang thermodynamics, and TLL bosonisation for fermions, we compute the phase diagram in the (γ, T/TF ) plane, first- and second-order spatial correlation functions g (1) and g (2) , the isothermal compressibility κ(μ, T ), and the local density fluctuations ⟨(δˆn) 2 ⟩. We show that the zero-distance second-order coherence g (2) (0) provides an unambiguous optical signal distinguishing all relevant phases: g (2) (0) ≈ 1 for the quasicondensate, g (2) (0) ≈ 0 in the TG limit, and g(2)(0) ≈ K c (γ)/2 < 1/2 in the TLL regime, where K c is the Luttinger parameter. These predictions are measurable via Hanbury Brown–Twiss atom interferometry, single-atom fluorescence imaging, and matter-wave interference in optical rod traps. Our results establish the rod aspect ratio as a design parameter for engineering correlated quantum phases and demonstrate optical coherence as their natural experimental readout.
Ultracold bosonic and fermionic quantum gases confined to quasi-one-dimensional (1D) geometry are promising candidates for probing fundamental concepts of Luttinger liquid (LL) physics. They can also be exploited for devising applications in quantum information processing and precision measurements. Here, we focus on 1D dipolar Bose gases, where evidence of super-strong coupling behavior has been demonstrated by analyzing the low-energy static and dynamical structures of the fluid at zero temperature by a combined reptation quantum Monte Carlo (RQMC) and bosonization approach. Fingerprints of LL behavior emerge in the whole crossover from the already strongly interacting Tonks–Girardeau at low density to a dipolar density wave regime at high density. We have also shown that a LL framework can be effectively set up and utilized to describe this strongly correlated crossover physics in the case of confined 1D geometries after using the results for the homogeneous system in LL hydrodynamic equations within a local density approximation. This leads to the prediction of observable quantities such as the frequencies of the collective modes of the trapped dipolar gas under the more realistic conditions that could be found in ongoing experiments. The present paper provides a description of the theoretical framework in which the above results have been worked out, making available all the detailed derivations of the hydrodynamic Luttinger equations for the inhomogeneous trapped gas and of the correlation functions for the homogeneous system.
Using two-dimensional optical lattices, it has recently become possible to split up clouds of ultra cold atoms into collections of parallel one-dimensional tubes in which the dynamics is restricted to the direction along their axis. Since the Feshbach-resonance interactions among atoms are short ranged, they can be approximated as contact interactions. It is thus possible to realize optical-trap versions of some classic integrable systems first studied by Lieb, Gaudin, Yang and others. Such systems can display exotic physics like spin-charge separation and unusual paired states. We focus on the study of systems of fermions in one-dimension. At finite temperatures, regimes such as the spin-coherent and the spin-incoherent Luttinger liquids can be realized by tuning the inter-atomic interaction strength and trap parameters. We identify the noise correlation of density fluctuations as a robust observable to discriminate between these two regimes. We also address the effects of spin imbalance for the case of attractive interactions, and study the one-dimensional analog of spatially modulated (FFLO) superconducting states. Our focus is on how the temperature affects the density profiles that are being measured in the experiments. The theoretical study of temperature effects in these systems is of experimental relevance for the problem of thermometry.
The coherence of a quantum system is one of the key signatures of many-body physics. While a three-dimensional Bose-Einstein condensate (BEC) exhibits true long-range coherence, a Bose gas confined in a one-dimensional (1D) potential displays quasi-long-range order. Experimentally, these coherence properties are typically probed via the momentum distribution, which corresponds to the Fourier transform of the one-body correlation function. In two-dimensional (2D) systems, for example, momentum distribution enables the observation of the Berezinskii-Kosterlitz-Thouless (BKT) transition from a thermal cloud to a superfluid. With trimodal distribution fitting, an intermediate quasicondensate phase is also revealed. In 1D systems, the momentum distribution of a quenched Tonks-Girardeau (TG) Bose gas can be mapped to that of a non-interacting Fermi gas at a specific evolution time, achieving the rapidity distribution. On the other hand, spin-orbit coupling (SOC) has emerged as one of the central topics in ultracold atomic physics. In additional to creating an artificial electric or magnetic field, researchers also observe negative-mass dynamics in a spin-orbit-coupled BEC by engineering the curvature of the band dispersion. This phenomenon is directly observable in the density distribution of the system. In 1D systems, SOC also introduces rich physical phenomena; while low-energy 1D quantum systems are conventionally described by Tomonaga-Luttinger liquid (TLL) theory, SOC notably alters the energy-momentum dispersion. This modification gives rise to unconventional Luttinger liquids whose properties are governed by the spin-orbit coupling strength.In this work, we first investigate the qualitative properties of a 1D Bose gas and demonstrate the temperature dependence of the momentum distribution, revealing how the thermal fluctuations destroy the quasi-long-range order. In the second part, we probe the effects of 1D SOC on a 1D Bose gas, focusing on how SOC modifies the energy-momentum dispersion and the resulting momentum distribution. We demonstrate that within a specific parameter regime, the momentum distribution and its associated coherence properties undergo distinct qualitative changes. The occupied higher momentum states induce phase fluctuations and destroy the coherence of the system, showing signals of the non-Luttinger liquid physics. We probe the coherence properties of a 1D system along these two parameter axes.
This dissertation serves as a summary of my Ph.D. work numerically studying equilibrium and non-equilibrium properties of strongly-interacting one-dimensional (1D) boson systems. This work is motivated by the fact that 1D systems are realizable and highly controllable with ultracold atoms in optical lattice and atom chip experiments. We apply a recent worm algorithmic Monte Carlo approach developed for 1D continuous systems to study their equilibrium properties, both with and without an underlying lattice. We also apply an exact lattice approach based on the Bose-Fermi mapping to check our Monte Carlo results in the Tonks-Girardeau limit, and more importantly, to study far-from-equilibrium expansion dynamics of the systems. We first study the scaling of one-particle correlations of the harmonically trapped Lieb-Liniger gas with changing temperature and interaction strength. Based on the universal behaviors of the density and momentum profiles, we are able to determine the effective parameters needed to fully characterize the system. We also find that the Tonks-Girardeau limit at low temperatures is the ideal regime for the experimental observation of the 1/k41/k^41/k4 momentum tail. An extra periodic lattice can drive the transition from superfluid to Mott insulator states. Exact and complete phase diagrams for such transitions are available only in the weak interacting and deep lattice limit, in which the system can be described using one-band Bose-Hubbard model. Beyond this limit, we use the worm algorithm in continuous space to map out the phase diagrams at various interaction strengths. We compare our phase diagrams with one-band Bose-Hubbard predictions and identify the regime where the one-band description breaks down. We introduce an inverse confined scattering solution to obtain effective Hubbard parameters, with which the Bose-Hubbard model provides correct results for strong interactions and deep lattices at unit filling. In addition to the equilibrium properties, we also study the expansion dynamics of ultracold atoms in the hard-core limit. Experimentally, this is usually achieved by turning off confining potentials and letting atoms expand in optical lattices. Theoretical studies from initial ground states predicted the occurrence of fermionization of the momentum distribution after long expansion times. In addition, quasicondensation at finite momenta emerges when expanding from Mott insulating domains. Here, we develop a finite-temperature extension of the lattice approach for dynamics. We find the dynamical ferminoization of the momentum distributions at all temperatures. For expansion from initial Mott domains, we observe enhanced correlations reminiscent of dynamical quasicondensation. Surprisingly, we find the systems appear to cool down during the melting of the Mott domains. We use an emergent local Hamiltonian to understand these emergent phenomena.
Rydberg and Rydberg-dressed atomic gases have recently emerged as a promising quantum simulator for a variety of models in condensed matter physics. Here we investigate one-dimensional bosons with soft-core Rydberg-dressed interactions using exact path-integral quantum Monte Carlo simulations. The finite-range and the negative Fourier component of the interaction potential generate a roton mode at finite momentum, while particle-hole backscattering processes enhance the susceptibility of one-dimensional systems at twice the Fermi momentum. The competition between the corresponding length scales yields a rich phase diagram, featuring a conventional Tomonaga-Luttinger liquid (TLL) regime, a beyond-TLL regime, and commensurate cluster phases. In the TLL regime, the system transitions from Lieb-Liniger-like bnehavior with Luttinger parameter K>1K>1K>1 to hard-rod-like behavior with K<1K<1K<1, with a quasi-supersolid phase emerging for K<1/2K < 1/2K<1/2. For strong interactions and high densities, deviations from TLL theory appear as precursors for the onset of cluster phases, where particles aggregate into stable clusters of several particles. The properties of each phase is discussed in detail.
Ultracold gases are a versatile platform to simulate condensed matter physics, as virtually any parameter is experimentally tunable. In particular, highly anisotropic traps allow the realization of low-dimensional systems, where the role of quantum fluctuations is enhanced. I investigate the paradigmatic model of a one-dimensional Bose gas with contact interactions, a.k.a the Lieb-Liniger model, using powerful analytical tools such as Bethe Ansatz, Conformal Field Theory and the Tomonaga-Luttinger liquid formalism, as well as the Bose-Fermi mapping. The effect of an additional harmonic trap in the longitudinal direction is studied within the Local Density Approximation. These methods allow to investigate with high accuracy its ground-state thermodynamics of the gas, its excitation spectra and correlation functions, from the k-body local correlations to the momentum distribution and the dynamical structure factor. These quantities characterize the coherence and superfluidity of the gas. In particular, I study the drag force exerted by a weak gaussian barrier stirred into the gas within linear response theory. Then, I investigate the appearance of a multimode structure obtained by releasing a transverse trapping, and the dimensional crossover in the limit of an infinite number of transverse modes.