VI COLD ATOM WORKSHOP - Two-day meeting of the Cold Atom Physics community in Spain

Jan 30, 2025, 8:30 AM → Jan 31, 2025, 4:00 PM Europe/Madrid

AUDITORIO DEL CUBO AZUL (Ciudad Politécnica de la Innovación. Universitat Politècnica de València. València )

COLD ATOM WORKSHOP 2025 - Two-day meeting of the Cold Atom Physics community in Spain, València

30 January - 31 January

REGISTRATION IS NOW CLOSED

The VI Cold Atom Workshop (CAW) follows the successful editions in Bilbao (2018), Barcelona (2020), and Granada (2021), Madrid (2022), Barcelona (2024). The meeting gathers members of the RSEF Foro de Física de Átomos Fríos (FFAF) and close scientific communities. It is partially supported by the MICINN network Cold Atom Physics Spain (CAPS). 

List of topics:

• Cold atom systems. 
• Ion trap systems.
• Photonic systems.
• Other physical systems for quantum technologies. 
• Quantum simulators.
• Quantum computing.
• Quantum sensing. 
• Quantum communications.
• Quantum  foundations.

 

Registration is NOW CLOSED. Deadline for submitting abstracts 15/01/2025

Sponsors:

 RED2022-134514-T(MCIU - AEI)

   

 .

 

Thursday, January 30

1 Registration for the COLD ATOM WORKSHOP - Two-day meeting of the Cold Atom Physics community in SPAIN

2 Welcome and Presentation of Cold Atom Workshop

Speakers: CAPS Coordinator Alessio Celi (Universitat Autónoma de Barcelona), FERNANDO SOLS (Universidad Complutense de Madrid), Quantum Spain node PI Carmen García_Almudéver (Universitat Politècnica de València), Vice Chancellor for Students and Entrepreneurship Alberto Conejero (Universitat Politècnica de València), Vice Director IUMPA Lluís García-Raffi (Universitat Politècnica de València)

Session 1.

Convener: CHAIR: Miguel Ángel García-March (IUMPA - Universitat Politècnica de València)

3 Talk 1. Quantum-gas microscopy of SU(N) fermions

Ultracold atoms in optical lattices are a popular platform in quantum science for both studies of quantum simulation and quantum metrology. Alkaline-earth atoms like strontium offer many opportunities, such as a large-spin fermions with SU(N) symmetry as well as narrow or ultranarrow transitions. In particular, strontium-87 presents a nuclear spin of I = 9/2 with no electronic spin in its ground state. This allows the study of the SU(N) Fermi-Hubbard model as well as quantum magnetism with up to 10 spin states.In recent experiments, we have demonstrated single-atom imaging of strontium-87 with spin resolution using the narrow-linewidth 689 nm transition. Through a combination of Zeeman shifts and spin-resolved optical pumping we aim to reliably resolve all 10 spin states.Ultracold atoms in optical lattices are a popular platform in quantum science for both studies of quantum simulation and quantum metrology. Alkaline-earth atoms like strontium offer many opportunities such as a large-spin fermions with SU(N) symmetry as well as narrow or ultranarrow transitions. In particular, strontium-87 presents a nuclear spin of I = 9/2 with no electronic spin in its ground state. This allows the study of the SU(N) Fermi-Hubbard model as well as quantum magnetism with up to 10 spin states. In recent experiments, we have demonstrated single-atom imaging of strontium-87 with spin resolution using the narrow-linewidth 689 nm transition. Through a combination of Zeeman shifts and spin-resolved optical pumping we aim to reliably resolve all 10 spin states.

Speaker: ANTONIO RUBIO ABADAL (ICFO - The Institute of Photonic Sciences)

4 Talk 2. Programmable quantum simulation with fermionic atoms in optical lattices

Fermionic atoms in optical lattices enable the study of strongly-correlated electronic systems in regimes that are numerically inaccessible to classical methods [1]. Traditionally, these quantum simulators are operated in an analogue mode to either emulate the dynamics under their native Fermi-Hubbard Hamiltonian, or to approximate its ground state adiabatically [2,3]. However, challenges such as efficient low-temperature state preparation and extending beyo nd native Fermi-Hubbard models limit their potential, including the long-standing challenge of understanding high-Tc superconductivity [4]. On the other hand, recent experimental advances are enabling these simulators to operate in a programmable manner [5], where Fermi-Hubbard dynamics can be used as resources to generate a set of fermionic quantum gates. In this talk, we will present several protocols that harness this programmability to simulate fermionic Hamiltonians relevant in condensed-matter physics. We will construct variational circuits for ground-state preparation and apply them to local and extended Fermi-Hubbard models with and without doping, showing considerable improvements with respect to adiabatic state preparation. Some of these models are expected to hold d-wave superconducting phases, so our protocols pave a way to explore this physics in current quantum simulators [6]. Furthermore, we will discuss some algorithms that use data gathered from the device to accurately investigate properties of the ground state of Fermi-Hubbard Hamiltonians. Overall, our work opens new avenues for programmable fermionic simulators to make a wider and more efficient exploration of their relevant many-body Hilbert space.

[1] M. Troyer and U. Wiese, Phys. Rev. Lett. 94, 170201 (2005).
[2] J. I. Cirac and P. Zoller, Nature Physics 8, 264 (2012).
[3] I. Bloch, J. Dalibard, and W. Zwerger, Rev. Mod. Phys. 80, 885 (2008).
[4] M. Qin et al., Phys. Rev. X 10, 031016 (2020).
[5] A. J. Daley et al., Nature 607, 667–676 (2022).
[6] H.-C. Jiang and T. P. Devereaux, Science 365, 1424 (2019).

Speaker: CRISTIAN TABRES LÓPEZ (Instituto de Física Fundamental (IFF-CSIC))

5 Talk 3. Fermionic quantum computation and simulation with ultracold atoms in optical lattices

One of the key driving forces in the investigation of quantum computing is the search for quantum advantage in real-world applications.[1] These include the simulation of properties and the determination of ground-state energies in many-body systems relevant to material science, quantum chemistry, and particle physics.[2] Given the fermionic nature of the said tasks, qubit-based simulators incur in extra overheads due to non-local Pauli-operator strings.[3] This demands to look for a new class of quantum simulators where the Fermi statistics is already present at the hardware level as a native resource. In this regard, fermionic cold atoms in optical lattices has recently emerged as a powerful platform to simulate fermions with fermions and to implement the so-called Fermionic Quantum Computation.[4-7]

In this work, we devise a native set of gates which, by operating on both the internal and motional degrees of freedom of spin-1/2 fermionic cold atoms, allow for a universal control in the ground-band double-well of an optical lattice. We provide simple and analytic relations between Bloch-sphere angles and Hamiltonian parameters to achieve the desired wave function in each particle sector. This generalizes the particular values employed to implement specific gates such as SWAP and SWAP^1/2. Importantly, our gates operate with a constant on-site interaction U all time, thus avoiding the slow process of switching U on and off. We show how they can be employed to digitlly simulate non-native processes like long-range tunnelling, and discuss on the independent manipulation of the particle-number sectors as a requirement for a universal control on an extended lattice.

[1] Solving an Industrially Relevant Quantum Chemistry Problem on Quantum Hardware, Ludwig Nützel et al. 2025 Quantum Sci. Technol. 10 015066
[2] Efficient simulation of quantum chemistry problems in an enlarged basis set, M. Luo and I. Cirac arXiv:2407.04432
[3] Qudit-based quantum simulation of fermionic systems, M. Chizzini et al. 2024 Phys. Rev. A 110 062602
[4] Fermionic quantum processing with programmable neutral atom arrays, D. González-Cuadra et al. 2023 PNAS 120(35) e2304294120
[5] Simulating Chemistry with Fermionic Optical Superlattices, F. Gkritsis et al. arXiv:2409.05663
[6] Quantum circuits based on topological pumping in optical lattices, Z. Zhu et al. arXiv:2409.02984
[7] Fermionic Quantum Computation, S. B. Bravyi and A. Y. Kitaev 2002 Ann. Phys. 298(1) 210

Speaker: LUIS ESCALERA MORENO (Technical University of Hamburg (TUHH))

6 Talk 4. Ultrafast exciton fluid flow in an atomically thin MoS2 semiconductor

Excitons (coupled electron–hole pairs) in semiconductors can form collective states that sometimes exhibit spectacular nonlinear properties. Here, we show experimental evidence of a collective state of short-lived excitons in a direct-bandgap, atomically thin MoS2 semiconductor whose propagation resembles that of a classical liquid as suggested by the nearly uniform photoluminescence through the MoS2 monolayer regardless of crystallographic defects and geometric constraints. The exciton fluid flows over ultralong distances (at least 60 μm) at a speed of ~1.8 × 107 m s−1 (~6% the speed of light). The collective phase emerges above a critical laser power, in the absence of free charges and below a critical temperature (usually Tc ≈ 150 K) approaching room temperature in hexagonal-boron-nitride-encapsulated devices. Our theoretical simulations suggest that momentum is conserved and local equilibrium is achieved among excitons; both these features are compatible with a fluid dynamics description of the exciton transport.

Speaker: ANDRES GRANADOS DEL AGUILA (University of Valencia)

11:20 AM Coffee Break

Session 2.

Convener: Chair TBA

7 Talk 5. Physics-informed neural networks for an optimal counterdiabatic quantum computation.

A novel methodology that leverages physics-informed neural networks to optimize quantum circuits in systems with N_Q qubits by addressing the counterdiabatic (CD) protocol is introduced. The primary purpose is to employ physics-inspired deep learning techniques for accurately modeling the time evolution of various physical observables within quantum systems. To achieve this, we integrate essential physical information into an underlying neural network to effectively tackle the problem. Specifically, the imposition of the solution to meet the principle of least action, along with the hermiticity condition on all physical observables, among others, ensuring the acquisition of appropriate CD terms based on underlying physics. This approach provides a reliable alternative to previous methodologies relying on classical numerical approximations, eliminating their inherent constraints. The proposed method offers a versatile framework for optimizing physical observables relevant to the problem, such as the scheduling function, gauge potential, temporal evolution of energy levels, among others. This methodology has been successfully applied to 2-qubit representing H_2 molecule using the STO-3G basis, demonstrating the derivation of a desirable decomposition for non-adiabatic terms through a linear combination of Pauli operators. This attribute confers significant advantages for practical implementation within quantum computing algorithms.

Speaker: ANTONIO FERRER-SÁNCHEZ (Department of Electronic Engineering, ETSE, University of Valencia.)

8 Talk 6. An efficient finite-resource formulation of Abelian and non-Abelian lattice gauge theories beyond one dimension

Gauge theories provide an accurate description of fundamental interactions, as both perturbation theory and quantum Monte Carlo computations in lattice gauge theory, when applicable, show remarkable agreement with experimental data from particle colliders and cosmological observations. Complementing these computations, or combining them with quantum-inspired Hamiltonian lattice computations on quantum machines to improve continuum limit predictions with current quantum resources, is a formidable open challenge. Here, we propose a resource-efficient method to compute the running of the coupling in both Abelian and non-Abelian gauge theories beyond one spatial dimension. We first represent the Hamiltonian on periodic lattices in terms of loop variables and conjugate loop electric fields, exploiting the Gauss law to retain the gauge-independent ones. Then, we identify a local basis for small and large loops variationally to minimize the truncation error while computing the running of the coupling on small tori. Our method enables computations at arbitrary values of the bare coupling and lattice spacing with current quantum computers/simulators and tensor-network calculations, in regimes otherwise inaccessible.

Speaker: MARC MIRANDA RIAZA (Universitat Autònoma de Barcelona)

9 Talk 7. Stable collective charging of ultracold-atom quantum batteries

Quantum technologies need a quantum energy initiative [1]. We should develop new strategies and devices able to store quantum energy for further use in consumption hubs in quantum devices. In this study, one possibility is to create quantum batteries [2], that can store and transfer energy efficiently by taking advantage of quantum mechanical effects [3].

We study a system with a three-site lattice, where each site has a different energy, populated with N particles. We also consider an interaction between particles in the same site. Our model starts with a configuration where all the particles are
in the first site and tries to move all the particles to the third one, using the SAP protocol [4], gaining energy in the process.

We explore the charge at the end of the procedure and find different behaviors depending on the number of particles, the coupling, and the interaction strength. We show that in this kind of battery, the interaction develops a crucial role in the charging process. We have developed a simplified two-mode model that captures the diabatic behavior and reproduces the exact time-dependent numerical simulations. This provides a deeper understanding of the charging procedure.

[1] Alexia Auffèves, PRX Quantum 3, 020101 (2022).
[2] Alan C. Santos, Barış Çakmak, Steve Campbell, and Nikolaj T. Zinner, Phys. Rev. E 100, 032107 (2019).
[3] Ju-Yeon Gyhm, Dominik Šafránek, and Dario Rosa, Phys. Rev. Lett. 128, 140501 (2022).
[4] C. J. Bradly, M. Rab, A. D. Greentree, and A. M. Martin, Physical Review A 85, 053609 (2012).

Speaker: ABEL ROJO FRANCÀS

10 Talk 8. Dipolar magnetostirring protocol for three-well atomtronic circuits

We propose a magnetostirring protocol to create persistent currents on an annular system. Under this protocol, polar bosons confined in a three-well ring circuit reach a state with high average circulation. We model the system with an extended Bose-Hubbard Hamiltonian and show that the protocol can create circulation in an atomtronic circuit for a range of tunable parameters. The performance and robustness of this scheme are examined, in particular considering different interaction regimes. We also present a method for predicting the optimal protocol parameters, which improves the protocol's scalability and enables its application to systems with large numbers of bosons, overcoming computational limitations and paving the way for exploring macroscopic quantum phenomena.

Speaker: HÉCTOR BRIONGOS-MERINO (University of Barcelona)

1:10 PM Lunch Break

Session 3.

Convener: Chair TBA

11 Talk 9. Excited modes in a supersolid spin-orbit-coupled Bose-Einstein condensates

Spin-orbit-coupled Bose-Einstein condensates are a flexible experimental platform to engineer synthetic quantum many-body systems. In particular, they host the so-called stripe phase, an instance of a supersolid state of matter. The peculiar excitation spectrum of the stripe phase, a definite footprint of its supersolidity, has so far remained out of experimental reach. We achieve in situ imaging of the stripes and directly observe both superfluid and crystal excitations. We investigate superfluid hydrodynamics and reveal a stripe compression mode, thus demonstrating that the system possesses a compressible crystalline structure. Through the frequency softening of this mode, we locate the supersolid transition point. Our results establish spin-orbit-coupled supersolids as a platform of choice to investigate supersolidity and its rich dynamics.

Speaker: RÉMY VATRÉ (ICFO - The Institute of Photonic Sciences)

12 Talk 10. Excitation spectrum of a double supersolid in a trapped dipolar Bose mixture

Dipolar Bose-Einstein condensates are excellent platforms for studying supersolidity, characterized by coexisting density modulation and superfluidity. The realization of dipolar mixtures opens intriguing new scenarios, most remarkably the possibility of realizing a double supersolid, composed by two interacting superfluids. We analyze the complex excitation spectrum of a miscible trapped dipolar Bose mixture, showing that it provides key insights about the double supersolid regime. We show that this regime may be readily probed experimentally by monitoring the appearance of a doublet of superfluid compressional modes, linked to the different superfluid character of each component. Additionally, the dipolar supersolid mixture exhibits a non-trivial spin nature of the dipolar rotons, the Higgs excitation, and the low-lying Goldstone modes. Interestingly, the analysis of the lowest-lying modes allows for monitoring the transition of just one of the components into the incoherent droplet regime, whereas the other remains coherent, highlighting their disparate superfluid properties.

Speaker: ALBERTO GALLEMI CAMACHO (Leibniz Universität Hannover)

13 Talk 11. Topological gauge theories and anyonic matter

Topological gauge theories provide an explanation of strongly correlated materials through a weakly-interacting picture.
In this talk, I will discuss an experimental scheme to realize with ultracold atoms a one-dimensional instance of these theories, known as the Chiral BF theory, in a ring geometry. I will first introduce the underlying principles of these theories, and examine them in the example of the Chern-Simons theory. This theory is renowned for explaining Fractional Quantum Hall systems via low-energy excitation with fractional statistics, known as anyons. Then, I will present the Chiral BF theory as the effective theory living on the boundary of Chern-Simons one, and describe the requirements which allowed the realization of the former with a Raman-coupled Bose gas with unbalanced interactions in a linear optical waveguide. I will propose the realization of the Chiral BF theory with Bose gases coupled to light with orbital angular momentum in a ring-shaped geometry. Together with unbalanced interactions, the light coupling induces an emergent magnetic field proportional to the atomic density, thus providing a one-dimensional realization of the flux attachment condition because of the non-trivial topology of space. I propose the change of angular momentum with density and the asymmetric speed of sound as indirect observables of the anyonic statistics. I will also briefly comment on the realization of the lattice counterpart of Chiral BF theory, the linear anyon model, and the observation of the anyonic statistics in Raman coupled gases loaded in optical lattices. The discussed approaches pursuit the realization of topological gauge theories with ultracold atoms, opening up new possibilities for engineering exotic quantum matter in a controllable setup.Topological gauge theories provide an explanation of strongly correlated materials through a weakly-interacting picture.
In this talk, I will discuss an experimental scheme to realize with ultracold atoms a one-dimensional instance of these theories, known as the Chiral BF theory, in a ring geometry. I will first introduce the underlying principles of these theories, and examine them in the example of the Chern-Simons theory. This theory is renowned for explaining Fractional Quantum Hall systems via low-energy excitation with fractional statistics, known as anyons. Then, I will present the Chiral BF theory as the effective theory living on the boundary of Chern-Simons one, and describe the requirements which allowed the realization of the former with a Raman-coupled Bose gas with unbalanced interactions in a linear optical waveguide. I will propose the realization of the Chiral BF theory with Bose gases coupled to light with orbital angular momentum in a ring-shaped geometry. Together with unbalanced interactions, the light coupling induces an emergent magnetic field proportional to the atomic density, thus providing a one-dimensional realization of the flux attachment condition because of the non-trivial topology of space. I propose the change of angular momentum with density and the asymmetric speed of sound as indirect observables of the anyonic statistics. I will also briefly comment on the realization of the lattice counterpart of Chiral BF theory, the linear anyon model, and the observation of the anyonic statistics in Raman coupled gases loaded in optical lattices. The discussed approaches pursuit the realization of topological gauge theories with ultracold atoms, opening up new possibilities for engineering exotic quantum matter in a controllable setup.

Speaker: CLAUDIO IACOVELLI (ICFO - UAB)

14 Talk 12. Excited-state quantum phase transitions in spinor BECs

Excited-state quantum phase transitions (ESQPTs) generalize the concept of quantum phase transitions to include transitions occurring at finite energies, beyond the ground state. These transitions can be induced not only by varying a control parameter but also by changing the energy of the system. In this work, we explore ESQPTs in spinor Bose-Einstein condensates, highlighting their experimentally observable signatures. We identify a topological order parameter that distinguishes between excited-state phases, which can be probed through interferometry. Additionally, we discuss a recent experimental realization of these ideas. Our findings open a way for experimental characterization and exploration of excited-state quantum phases in atomic many-body systems.

Speaker: MANUEL GESSNER (IFIC, Universitat de València)

4:20 PM Coffee Break

Session 4.

Convener: CHAIR: Jose David Martín Guerrero (ETSE, Universitat of València.)

15 Talk 13. Sunlight and the single atom - a photodetector for fundamental physics and daylight communications

Emerging photo-detection applications, including ‘light-shining-through-walls’ experiments to search for new particles and interplanetary optical communications, require photodetection with extremely low dark counts and strong background rejection. Here background rejection means filtering to prevent unwanted light (back- ground light) from reaching the photon counting devices. For many contemporary applications, background light comes mainly from the sun: either directly because the photodetector is exposed to sunlight, or indirectly as in the photodetector does not look directly at the sun but is exposed to skylight, which is sunlight scattered from Earth’s atmosphere. In [1], we demonstrated a narrowband quantum jump photodetector (QJPD) to detect single-photons based on a single cold atom and quantified its experimental quantum efficiency and dark count rate. Here, we present the performance of the QJPD in the presence of strong broadband background [2].

The photon counting capabilities of the QJPD under direct and indirect broadband excitation are tested, by measuring quantum jump rates when the atom is illuminated with direct sunlight, and with light scattered by the atmosphere (skylight). A rate equation model is developed to describe QJ probabilities in the presence of both intense broadband background and weak resonant probe light. This model is used to interpret experiments in which a weak signal beam competes with strong broadband background and validated using sunlight, demonstrating a reliable method to extract probe photon numbers even in the presence of background. Measurements where the atom is illuminated with skylight show no observable background-induced QJs.

References
[1] L. Zarraoa, R. Veyron, T. Lamich, L.C. Bianchet, M.W. Mitchell. Quantum jump photodetector for narrowband photon counting with a single atom. Phys. Rev. Research 6, 033338 (2024)
[2] L. Zarraoa, R. Veyron, T. Lamich, M.W. Mitchell. Single-atom quantum jump photodetector with very strong intrinsic background rejection. In preparation (2025)

Speaker: LAURA ZARRAOA

16 Talk 14. Cold atomic ensembles for quantum communication

Cold atomic ensembles are good candidates for quantum nodes serving as quantum memories, deterministic photon sources, and processing units. I will present the latest results obtained in this context, featuring the generation of indistinguishable photons from independent cold atomic ensembles nodes, and the cavity enhanced generation of non-classical light.

Speaker: FELIX HOFFET (ICFO - the Institute of Photonic Sciences)

17 Talk 15. Optimization of Expansion Time and Cooling Rate via Effective Scaling Approach in Trapped Bose Gases

We propose an effective scaling approach for the controlled expansion of trapped Bose gases, introducing an auxiliary equation for the scaling parameter that seamlessly transitions between the non-interacting and Thomas-Fermi regimes. This method enables the design of shortcuts to adiabaticity (STA) for arbitrary interaction strengths, ensuring rapid and excitation-free state transformations. By constraining the trapping frequency, we apply Pontryagin’s maximum principle to determine time-minimal solutions using a bang-bang protocol. Furthermore, these results reveal exponential bounds on cooling rates in quantum refrigerator cycles, shedding light on the impact of atomic interactions in the pursuit of absolute zero.

Speaker: XI CHEN (ICMM-CSIC)

18 Talk 16. Simplifying the simulation of local Hamiltonian dynamics

Local Hamiltonians, H_{k}, describe non-trivial k-body interactions in quantum many-body systems. In this work, we address the dynamical simulatability of a k-local Hamiltonian by a simpler one, H_{k’}, with k’ < k. First, we work on the Hilbert space that both Hamiltonians act on. When it comes to exact simulation, we build upon methods to derive examples of H_{k} and H_{k’} that simulate the same physics. We also address the most realistic case of approximate simulation. There, we upper-bound the error up to which a Hamiltonian can simulate another one, regardless of their internal structure, and prove, by means of an example, that the accuracy of a (k’=2)-local Hamiltonian to simulate H_{k} with k>2 increases with k. Furthermore, we propose a method to search for the k’-local Hamiltonian that simulates, with the highest possible precision, the short time dynamics of a given H_{k} Hamiltonian.

Speaker: AYAKA USUI (Universitat Autònoma de Barcelona)

Poster Session and Jazz (UPV JAZZ Combo)

19 Effectiveness of Dynamical Decoupling on Quantum Random Telegraph Noise

The reason why mitigations techniques such as dynamical decoupling success or fail in different quantum technological scenarios is not fully understood. The random telegraph noise (RTN) model is often used to describe coherent noise, which is a semiclassical model. In this work we introduce a full quantum model to describe coherent noise on a qubit, where the qubit is coupled to a two level system that is in turn subject to thermalization. We derive a master equation to compute the evolution of the qubit and find that this model recovers the semiclassical RTN model in the high temperature limit. We employ this model to assess the effectiveness of dynamical decoupling on this coherent noise model.

Speaker: ANDREU ANGLÉS CASTILLO (Universitat de València)

20 Probing spontaneously symmetry-broken phases through noise correlation measurements with cold atoms

Spontaneously symmetry-broken (SSB) phases are locally ordered states of matter characterizing a large variety of physical systems. Because of their specific ordering, their presence is usually witnessed by means of local order parameters. Here, we propose an alternative approach based on statistical correlations of noise after the ballistic expansion of an atomic cloud. We indeed demonstrate that probing such noise correlators allows one to discriminate among different SSB phases characterized by spin-charge separation. As a particular example, we test our prediction on a 1D extended Fermi-Hubbard model, where the competition between local and nonlocal couplings gives rise to three different SSB phases: a charge density wave, a bond-ordering wave, and an antiferromagnet. Our numerical analysis shows that this approach can accurately capture the presence of these different SSB phases, thus representing an alternative and powerful strategy to characterize strongly interacting quantum matter.

Speaker: JAVIER ARGÜELLO-LUENGO (Universitat Politècnica de Catalunya)

21 The Pauli Engine: A Quantum Heat Engine Realized with a 1D Anyon Hubbard Model

"We propose a 1D anyon Hubbard model as the foundation for the Pauli Engine, a quantum heat engine rooted in the Pauli exclusion principle. Unlike traditional heat engines, this engine operates cyclically and can perform work even without a thermal bath, driven purely by quantum many-body interactions and anyonic statistics.

Our study explores how the statistical parameter and Hubbard interaction strength affect the engine’s efficiency and power, revealing unique thermodynamic behaviors. This work highlights the potential of anyonic systems to redefine quantum heat engines and advance quantum thermodynamics."

Speaker: MOHIT BERA (IFIC, Universitat de València)

22 Topological Dissipative Phases with Trapped Ions

Trapped-ion systems offer exceptional platforms for investigating topological phase transitions and detecting ultra-weak forces, owing to the high degree of control over their vibrational degrees of freedom. We theoretically simulate topological driven-dissipative phases in a one-dimensional chain consisting of approximately 20–30 sites. The emergence of topological phases, characterized by a nontrivial winding number as a topological invariant, indicates the presence of edge states that amplify external fields and provide robustness against disorder. We explore a cutting-edge application in quantum sensing, demonstrating that topological phases enhance the scalability of the signal-to-noise ratio with the number of ions. Specifically, we achieve sensitivities on the order of 1 yN Hz^{-1/2} by measuring the ion's displacement amplitude via photoluminescence in the micrometer regime.

Speaker: MIGUEL CLAVERO RUBIO (Instituto de Física Fundamental- CSIC)

23 Stock volatility as an anomalous diffusion process

"Anomalous diffusion (AD) describes transport phenomena where the mean-square displacement (MSD) of a particle does not scale linearly with time, deviating from classical diffusion. This behavior, often linked to non-equilibrium phenomena, sheds light on the underlying mechanisms in various systems, including biological and financial domains.

Integrating insights from anomalous diffusion into financial analysis could significantly improve our understanding of market behaviors, similar to their impacts on biological systems. In financial markets, accurately estimating asset volatility—whether historical or implied—is vital for investors.

We introduce a novel methodology to estimate the volatility of stocks and similar assets, combining anomalous diffusion principles with machine learning. Our architecture combines convolutional and recurrent neural networks (bidirectional long short-term memory units). Our model computes the diffusion exponent of a financial time series to measure its volatility and it categorizes market movements into five diffusion models: annealed transit time motion (ATTM), continuous time random walk (CTRW), fractional Brownian motion (FBM), Lévy walk (LW), and scaled Brownian motion (SBM).

Our findings suggest that the diffusion exponent derived from anomalous diffusion processes provides insightful and novel perspectives on stock market volatility. By differentiating between subdiffusion, superdiffusion, and normal diffusion, our methodology offers a more nuanced understanding of market dynamics than traditional volatility metrics. We also comment on the possibilities of using quantum machine learning instead of conventional machine learning in this realm o to analyze this problem."

Speaker: JOSÉ ALBERTO CONEJERO CASARES (IUMPA - Universitat Politècnica de València)

24 Solving deep-learning density functional theory via variational autoencoders

In recent years, machine learning models, chiefly deep neural networks, have revealed suited to learn accurate energy-density functionals from data. However, problematic instabilities have been shown to occur in the search of ground-state density profiles via energy minimization. Indeed, any small noise can lead astray from realistic profiles, causing the failure of the learned functional and, hence, strong violations of the variational property. In this article, we employ variational autoencoders to build a compressed, flexible, and regular representation of the ground-state density profiles of various quantum models. Performing energy minimization in this compressed space allows us to avoid both numerical instabilities and variational biases due to excessive constraints. Our tests are performed on one-dimensional single-particle models from the literature in the field and, notably, on a three-dimensional disordered potential. In all cases, the ground-state energies are estimated with errors below the chemical accuracy and the density profiles are accurately reproduced without numerical artifacts.

Speaker: EMANUELE COSTA (University of Barcelona)

25 Shortcuts to Adiabaticity and Strongly Correlated States in Ultra-Cold Atomic Gases

"Shortcuts to Adiabaticity (STA) enable fast generation of states typically obtained through slow adiabatic processes. We explore the creation of Bell states in multi-component ultra-cold atomic gases using controllable inter-particle interactions via Feshbach resonances. Extending previous work, we demonstrate the feasibility of accessing the symmetric Bell states $|\psi_+\rangle = |R\rangle|R\rangle + |L\rangle|L\rangle$ and $|\phi_+\rangle = |R\rangle|L\rangle + |L\rangle|R\rangle$, and completing the computational basis with $|\psi_-\rangle = |R\rangle|R\rangle - |L\rangle|L\rangle$ and$|\phi_-\rangle = |R\rangle|L\rangle - |L\rangle|R\rangle$.

We also study strongly correlated states in few-species ultra-cold bosons confined in a 1D trap. Using tunable interaction strengths and mass ratios, we show how two distinguishable impurities immersed in a bosonic bath can exhibit non-classical correlations, forming Bell states mediated by the bath. These results offer promising avenues for quantum technologies."

Speaker: SERGIO DE MARÍA GARCÍA (Instituto universitario de ciencia de materiales (ICMUV); Instituto universitario de matemática pura y aplicada (IUMPA).)

26 Diffusive propagation of two-particle correlations in the chaotic phase of the Bose-Hubbard model

We study the dynamical manifestation of the chaotic phase in the time-dependent propagation of experimentally relevant two-particle correlations for one-dimensional interacting bosons by means of a conveniently defined two-particle correlation transport distance l. Our results show that the chaotic phase induces the emergence of an effective diffusive regime in the asymptotic temporal growth of l, characterized by an interaction dependent diffusion coefficient, which we estimate [1]. We investigate the origin of such behaviour by analysing the spatial and temporal evolution of two-particle correlations, where we see a clear correspondence between a general change in their profile and the emergence of the diffusive regime.

Speaker: ÓSCAR DUEÑAS SÁNCHEZ (Departamento Física Fundamental, Universidad de Salamanca)

27 Scalable simulation of compact quantum electrodynamics using Gutzwiller wave functions

"We provide a scalable, resource efficient scheme for the quantum simulation and computation of Abelian lattice gauge theories (LGTs) with continuous gauge groups in the Hamiltonian formulation. We study compact quantum electrodynamics (cQED) in two spatial dimensions, formulated in terms of rotors and strings as introduced in [1]. We complement this formulation with a recent technique for the variational determination of the local basis [2], showing improvements in terms of retained states needed to simulate the theory at arbitrary values of the bare coupling and lattice spacings. Within this approach, we determine variationally the ground state of the theory for small lattices with periodic (for pure gauge) and open (in presence of fermionic matter) boundary conditions, and compute in both cases the expectation value of the plaquette operator. We finally show preliminary results for larger lattices obtained by means of a Gutzwiller ansatz for the wave function of the ground state, indicating that our method is suitable to scale up the system without dramatically increasing the number of required resources.

References:
[1] J. F. Haase, L. Dellantonio, A. Celi et al., Quantum 5, 393 (2021)
[2] P. Fontana, M. Miranda-Riaza, A. Celi, arXiv:2409.04441 (2024)
"

Speaker: PIERPAOLO FONTANA (Universitat Autonoma de Barcelona (UAB))

28 Few ultracold atoms in one dimension: integrability, discrete symmetries, and quantum chaos

We study one-dimensional systems of one or two atoms (bosons, fermions, or distinguishable) interacting via contact delta-like interactions with one or more delta potential barriers. They are assumed to be confined either in parabolic or square well potentials. We show that these systems offer a perfect playground to study the interplay between integrability or quantum chaos, discrete symmetries, fragmentation of the Hilbert space, adiabaticity, and non-trivial topological phases. We also implement theoretically adiabatic loops on control parameters, i.e. interaction strength and barrier heights. We find different kinds of loops:  finite periodic and infinite but subjected to fragmentation ones. These loops have implications both theoretical but also for their use in quantum technologies.

Speaker: MIGUEL ANGEL GARCIA MARCH (IUMPA - Universitat Politècnica de València)

29 Generalized spin squeezing with limited measurements

"An important and widely used tool in quantum metrology is the spin squeezing parameter. Its development was mainly motivated by two applications: the improvement of precision measurements beyond the classical limit and the study of particle correlations and entanglement [1]. In quantum metrology, the spin squeezing parameter determines the sensitivity that can be achieved through the measurement of a fixed, possibly suboptimal observable. It therefore determines a lower bound on the quantum Fisher information, which expresses the maximal sensitivity achievable with an optimal observable.

However, these sensitivities can only be achieved for an asymptotically large number of measurements. Since these are a limited resource, it is crucial to explore precision bounds in the low data regime. Guided by the recently derived hierarchy of quantum metrology bounds [2], we investigate approximations to generalized quantum information functions beyond the Fisher information that are of relevance in the presence of low data. We present a family of generalized bounds that includes the relation between standard spin squeezing and Fisher information as a particular case.

Our generalized spin squeezing type of bounds are analytically derived from averages and variances of arbitrary measurement observables. We study the families of quantum bounds that may involve higher-order derivatives (Bhattacharyya) and others that avoid the use of differentials altogether (Barankin), as well as combinations of both of them (Abel).

We derive analytical expressions for the bounds and for the coefficients that optimize them. For a single qubit, the derived generalized bounds show saturation.
References

[1] L. Pezzè et al., Quantum metrology with nonclassical states of atomic ensembles, Rev. Mod. Phys. 90, 035005 (2018)

[2] M. Gessner and A. Smerzi, Hierarchies of Frequentist Bounds for Quantum Metrology: From Cramér-Rao to Barankin, Phys. Rev. Lett. 130, 260801 (2023)."

Speaker: IRENE GARCÍA MARTÍNEZ (Universitat de València (UV))

30 A strontium quantum-gas microscope for Bose and Fermi Hubbard systems

The combination of quantum-gas microscopy with alkaline-earth atoms offers many exciting prospects for quantum simulation of Hubbard models.

In this contribution, we present the latest results on quantum-gas microscopy from the Strontium Lab at ICFO. In a first set of experiments, we worked with the bosonic isotope 84Sr. We routinely prepared Bose-Einstein condensates of 84Sr, load them in a square optical lattice and realized the Bose-Hubbard model. In recent experiments, we have extended our microscope setup to work with fermionic 87Sr.

This opens the door to studies of exotic quantum magnetism with 𝑁 > 2, which could be characterized through site-resolved spin-sensitive detection.

Speaker: CARLOS GAS (ICFO - The Institute of Photonic Sciences)

31 Towards a geodesic algorithm for the optimization of parameterized quantum circuits

Currently, most optimization algorithms for updating the parameters of variational circuits rely on the gradient descent algorithm. Recently, a modification of this algorithm was proposed to include information about the geometry of the quantum state, known as Quantum Natural Gradient. This method involves using the Quantum Fisher Information Matrix, which can be obtained by taking the real part of the Quantum Geometric Tensor (QGT). Sometimes, the real part of the QGT is referred to as the Fubini-Study metric and serves as a metric in the projected Hilbert space. In our work, we propose using the Fubini-Study metric to follow a geodesic path (more efficient in time) during the optimization of a variational circuit. The result is an algorithm that alternates between gradient descent to choose the direction in which the cost is reduced, using this direction as initial conditions for the velocity in the geodesic part, and geodesic evolution, which reduces the optimization time. The algorithm was successfully tested on a toy model of one qubit to find the ground state of the Hamiltonian sigma-z.

Speaker: RAFAEL GOMEZ LURBE (Universitat de Valencia)

32 "Theory of strongly-correlated one-dimensional systems, beyond Tomonaga-Luttinger liquid. "

"Fermi-Landau liquid theory breaks down in case of one-dimensional interacting fermion systems. Instead a framework of Tomonaga-Luttinger liquid theory can be applied, limited to lowest-energy excitations with strictly linear dispersion around Fermi energy.
Here I present the theory of strongly correlated one-dimensional systems of fermions, or bosons, applicable for excitation energies (or temperatures) smaller than (dominant) interaction scale, without linear dispersion constraint. Comparison with Thermodynamic Bethe Ansatz, when possible, shows that present theory recovers main features of exact thermodynamics. Unexpectedly, this thermodynamics describes non-interacting fermion system with an effective length, which is particle number and interaction dependent. This liquid of fermions has no adiabatic connection to original Hamiltonian, nor local transformation between an effective and real space configurations. Instead thermodynamics is of some "non-Fermi liquid" with fractional exclusion statistics and Fermi surface which is present in thermodynamics but not in original momentum space."

Speaker: PRZEMYSLAW GRZYBOWSKI (ICFO - The Institute of Photonic Sciences)

33 Self-Assembled Chains and Solids of Dipolar Atoms in a Multilayer

We predict that ultracold bosonic dipolar gases, confined within a multilayer geometry, may undergo self-assembling processes, leading to the formation of chain gases and solids. These dipolar chains, with dipoles aligned across different layers, emerge at low densities and resemble phases observed in liquid crystals, such as nematic and smectic phases. We calculate the phase diagram using quantum Monte Carlo methods, introducing a newly devised trial wave function designed for describing the chain gas, where dipoles from different layers form chains without in-plane long-range order. We find gas, solid, and chain phases, along with quantum phase transitions between these states. Specifically, we predict the existence of quantum phase transitions from gaseous to self-ordered phases, as the interlayer distance is decreased. Remarkably, in the self-organized phases, the mean interparticle distance can significantly exceed the characteristic length of the interaction potential, yielding solids and chain gases with densities several orders of magnitude lower than those of conventional quantum solids.

Speaker: GRECIA GUIJARRO (Universitat Politècnica de Catalunya)

34 Towards tuneable "coupling efficiency" of a single atom using a pump-probe scheme

"We propose an experimental implementation of a scheme proposed by Goncalves et al. [1], to produce unusual and tunable photon correlations by interfering resonance fluorescence from a single atom with probe light from a weak laser beam. A number of interesting and potentially useful features are predicted by Goncalves et al., including (under different conditions of pump-probe relative phase and power) the complete extinction of the probe, amplification of the probe, and generation of extremes of anti-bunching and bunching, i.e., g(2) approaching zero or infinity. Interestingly, the expressions for the transmitted power and g(2) can be given in terms of a single parameter, an effective interaction efficiency, suggesting that interference can be used to make up for geometrical and technical limitations on the coupling to single atoms.

We will present the current state of the experimental implementation, using a single atom far-of-resonance trap in a "Maltese cross" geometry system of four high numerical aperture lenses [2] and recent experimental results, as well as the technical considerations we made in order to implement it, including: choice of beam geometry, to minimise the the effects of atomic motion, pump and probe polarisation, optical pumping and coherent population trapping induced by the pump and probe beams, atom heating and methods to mitigate it.

[1] D. Goncalves, M. W. Mitchell, et al.“Unconventional quantum correlations of light emitted by a single atom in free space”. In: Phys. Rev. A 104 (1 2021), p. 013724. doi:10.1103/PhysRevA.104.013724.

[2] L. C. Bianchet, N. Alves, et al. “Manipulating and measuring single atoms in the Maltese
cross geometry”. In: Open Research Europe 1.102 (2022). doi: 10.12688/openreseurope.
13972.2."

Speaker: TOMAS LAMICH (ICFO - The Institute of Photonic Sciences)

35 Quantum Reservoir Computing in atomic lattices

Quantum reservoir computing (QRC) exploits the dynamical properties of quantum systems to perform machine learning tasks. We demonstrate that optimal performance in QRC can be achieved without relying on disordered systems. Systems with all-to-all topologies and random couplings are generally considered to minimize redundancies and enhance performance. In contrast, our work investigates the one-dimensional Bose-Hubbard model with homogeneous couplings, where a chaotic phase arises from the interplay between coupling and interaction terms. Interestingly, we find that performance in different tasks can be enhanced either in the chaotic regime or in the weak interaction limit. Our findings challenge conventional design principles and indicate the potential for simpler and more efficient QRC implementations tailored to specific tasks in Bose-Hubbard lattices.

Speaker: GUILLEM LLODRA BISELLACH (University of Balearic Islands)

36 Exploring operation parallelism vs. ion movement in ion-trapped QCCD architectures

Ion-trapped Quantum Charge-Coupled Device (QCCD) architectures have emerged as a promising alternative to scale single-trap devices by interconnecting multiple traps through ion shuttling, enabling the execution of parallel operations across different traps. While this parallelism enhances computational throughput, it introduces additional operations, raising the following question: do the benefits of parallelism outweigh the potential loss of fidelity due to increased ions movement? This poster analyzes the question by exploring the trade-off between the parallelism of operations and fidelity loss due to movement overhead, comparing sequential execution in single-trap devices with parallel execution in QCCD architectures.

Speaker: ANABEL OVIDE GONZÁLEZ (Universitat Politècnica de València)

37 On polarons and dimerons in the two-dimensional attractive Hubbard model

A two-dimensional spin-up ideal Fermi gas interacting attractively with a spin-down impurity in the continuum undergoes, at zero temperature, a first-order phase transition from a polaron to a dimeron state. Here we study a similar system on a square lattice, by considering the attractive 2D Fermi-Hubbard model with a single spin-down and a finite filling fraction of spin-up fermions. We study polaron and dimeron quasi-particle properties via variational Ansatz up to one particle-hole excitation. Moreover, we develop a determinant diagrammatic Monte Carlo algorithm for this problem based on expansion in bare on-site coupling U. This algorithm turns out to be sign-problem free at any filling of spin-up fermions, allowing one to sample very high diagram order (larger than 200 in our study) and to do simulations for large U/t (we go up to U/t=−20 with t the hopping strength). Both methods give qualitatively consistent results. With variational Ansatz we go to even larger on-site attraction. In contrast with the continuum case, we do not observe any polaron-to-dimeron transition for a range of spin-up filling fractions ρ↑ between 0.1 and 0.4. % (away from the low-filling limit). The polaron state always gives a lower energy and has a finite quasi-particle residue.

Speaker: GERARD PASCUAL LÓPEZ (Universitat Politècnica de Catalunya)

38 Interaction effects on the itinerant ferromagnetism phase transition

We present a thorough study of the transition nature of Fermi gases. The exploration can be done thanks to the use of a third order perturbation formula for the energy system. At this level, there are three scattering parameters in play, those are, the S-wave scattering length 𝑎0, the S-wave effective range r0 and the P -wave scattering length a1. We show that the spin value is not determinant in saying the nature of the phase transition. For any spin value, any kind of phase transition can happen depending on the potential. We show how the different transitions are encountered as a function of r0 and a1, which are in units of 𝑎0, for S = 1/2 up to S = 9/2. Moreover, we then present a model based on Landau theory in order to proof this rich variability.

Speaker: JORDI PERA (Universitat Politècnica de Catalunya)

39 "A Rydberg strontium tweezer-array experiment for the simulation of lattice gauge theories"

"In this work, we are developing a cold strontium-88 platform based on arrays of Rydberg atoms trapped in optical tweezers to perform simulations of lattice gauge theories.

In this poster, we present the progress towards the construction of the apparatus. In particular, we report on the status of the vacuum environment of the experiment and the laser cooling and trapping system that will enable us to trap the atoms in optical tweezers. Additionally, we show how we will excite the atoms to Rydberg states and the system of coils and electrodes which we are setting up for the experiment. Finally, we discuss the type of physics we will explore, namely how we will engineer the so-called plaquette interactions akin to the magnetic interactions in electromagnetism using an hexagonal array of decorated Rydberg atoms and a dual Ising formulation proposed by our theoretical collaborators."

Speaker: ANA PEREZ BARRERA (ICFO - The Institute of Photonic Sciences)

40 Many-body quantum resources of graph states for measurement based quantum computing and quantum metrology

Characterizing the non-classical correlations of a complex many-body system is an important part of quantum technologies. A versatile tool for such a task is one that scales well with the size of the system and which can be both easily computed and measured. In this work we focus on graph states, that are promising platforms for quantum computation, simulation and metrology. We consider four topologies, namely the star graph states with edges, Tur\'an graphs, -ary tree graphs, and square grid cluster states, and provide a method to characterise their quantum content: the many-body Bell correlations, non-separability and entanglement depth for an arbitrary number of qubits. We also relate the strength of these many-body correlations to the usefulness of graph states for quantum sensing. Finally, we characterize many-body entanglement depth in graph states with up to qubits in classes non-equivalent under local transformations and graph isomorphisms. The technique presented is simple and does not make any assumptions about the multi-qubit state, so it could find applications wherever precise knowledge of many-body quantum correlations is required.

Speaker: MARCIN PLODZIEN (ICFO - The Institute of Photonic Sciences)

41 Error Rate Prediction for Noise-Aware Quantum Circuit Compilation

Quantum computing is a promising technology that faces several challenges. Among these limitations, high physical error rates are a significant hurdle to overcome. The limited number of physical qubits in current quantum devices is an obstacle to implementing quantum error correction techniques. However, despite this limitation, it may be possible to address the impact of errors by employing noise-aware quantum mapping and routing techniques during the algorithm compilation process. To effectively implement such techniques, exploring quantum gate error prediction using device calibration data is interesting. This estimation provides valuable insight into the variabiliy of error rates associated with different quantum operations between calibration cycles. This predictions enable informed decisions in the compilation process that may increase the reliability of quantum circuits by estimating the error rates of specific operations in the calibration cycle where the circuit is going to be executed in cloud-based systems.

Speaker: LAURA RODRÍGUEZ SORIANO (Universitat Politècnica de València)

42 Accessing elusive two-dimensional phases of dipolar Bose-Einstein condensates by finite temperature

It has been shown that dipolar Bose-Einstein condensates that are tightly trapped along the polarization direction can feature a rich phase diagram. In this paper we show that finite temperature can assist in accessing parts of the phase diagram that otherwise appear hard to realise due to excessively large densities and number of atoms being required. These include honeycomb and stripe phases both in the thermodynamic limit as well as with a finite extent using both variational and numerical calculations. We account for the effect of thermal fluctuations by means of Bogoliubov theory employing the local density approximation. Furthermore, we exhibit real-time evolution simulations leading to such states. We find that finite temperatures can lead to a significant decrease in the necessary particle number and density that might ultimately pave a route for future experimental realisations.

Speaker: JUAN SÁNCHEZ (Universitat Politècnica de Catalunya)

43 Environment-enhanced single-photon absorption cross-section in a nano-ring of quantum emitters

Decoherence (dissipation and dephasing) is usually considered detrimental in quantum information and quantum optics applications. However, the interplay between environment dynamics and unitary evolution can give rise to interesting quantum many-body phenomena and can even be harnessed to become a useful resource.
As it is well known, in dense atomic arrays coupled to a common radiation mode, collective spontaneous emission takes place leading to the emergence of very subradiant states. Here, we demonstrate how these modes in conjunction with additional decoherence mechanisms such as dephasing or interaction with a thermal phonon bath, can significantly enhance the single-photon absorption cross-section in a nanoring of quantum emitters The nanoring geometry is particularly appealing due to its unique optical properties and its resemblance to natural light-harvesting complexes, which serve as efficient antennas in photosynthesis. Despite the complexity of these biological systems, our findings suggest they may operate based on similar principles, potentially shedding light on fundamental aspects of energy absorption and transfer in nature.

Speaker: ERIC SÁNCHEZ LLORENTE (Universitat de Barcelona)

44 Spinor BEC search for new spin-dependent forces

Due to quantum degeneracy and extraordinarily low inelastic collision rates, a 87Rb BEC is, relative to its volume, the most sensitive magnetometer ever demonstrated, with combined spatial, temporal, and field resolution beyond what is possible with existing sensing approaches. Extreme magnetic sensitivity on small length scales enables searches for short-range spin-dependent forces that may signal dark matter and gravitational quantum effects. These include an exotic monopole-dipole force mediated by virtual axions or axion-like particles in the few-meV range, and spin-gravity couplings. Here, we present a spinor BEC co-magnetometer designed for exploring new spin-dependent forces, contributing to the search for physics beyond the Standard Model.

Speaker: ZOI SARGIANNI (ICFO - The Institute of Photonic Sciences and University of Crete)

45 Doublons in a staggered lattice of ring potentials

Topological systems, a remarkable topic in condensed matter physics, have been implemented in different platforms like ultracold atoms in optical lattices and photonic systems. The properties of single-particle topological systems have been extensively studied, but there is still much to learn about many-body systems. Two-body systems, in which tightly-bound pairs of interacting particles named doublon can be realized, are particularly interesting.
In this work, we study a system of ultracold bosons loaded into the states with orbital angular momentum (OAM) in a one-dimensional staggered lattice of ring potentials. Here we consider the hard-core limit and large near-neighbor interactions, which allows us to explore the behavior of doublons within the energy scale defined by the near-neighbor interactions. In the case with OAM l=1 in all sites, the system can be mapped to a Creutz ladder chain, using the circulation as a synthetic dimension. Moreover, focusing on the manifold of the interaction-induced bound states, we derive an effective model consisting of two Bose-Hubbard chains and two Su-Schrieffer-Heeger (SSH) chains exhibiting opposite topological phases and thus, two-particle edge states for some parameter values. By considering the case of OAM l=0 and l=1 in alternating sites, the system can be mapped into a diamond chain formed by the real dimension and the synthetic dimension spanned by the two circulations. In this case, an effective model can be derived consisting of an extended Creutz ladder with interleg hoppings, which maps into two SSH chains in the dimerized limit with two two-bosons edge states embedded in one of the flat bands of the spectrum.

Speaker: YUNJIA ZHAI (Universitat Autonoma de Barcelona (UAB))

46 Spinor Bose-Einstein condensate as an analog simulator of molecular bending vibrations

We demonstrate that spinor Bose-Einstein condensates (BEC) can be operated as an analog simulator of the two-dimensional vibron model. This algebraic model for the description of bending and streching vibrations of molecules, in the case of a triatomic molecules, exhibits two phases where linear and bent configurations are stabilised. Spinor BECs can be engineered to simulate states that correspond to linear or bent triatomic molecules, with the BEC’s Wigner function encoding information about the molecular configuration. We show how quantum simulations of the bending dynamics of linear molecules can be realized, and how the straightening of a bent molecule leads to a dynamical instability. In the dynamics triggered by the corresponding instability, significant
amount of entanglement is generated, and we characterise the dynamics with squeezing parameter and quantum Fisher information (QFI). The non-Gaussian property is caught by the difference between optimal squeezing parameter and optimal QFI, and it enables us to detect dynamical phase transition.

Speaker: AYAKA USUI (Universitat Autònoma de Barcelona)

47 The Fermionic Tonks-Girardeau gas: composite boson formation and a novel formulation of the ground state wave function

Attractive 𝑝-wave one-dimensional fermions are studied in the fermionic Tonks-Girardeau regime in which the diagonal properties are shared with those of an ideal Bose gas. We study the off-diagonal properties and present analytical expressions for the eigenvalues of the one-body density matrix. One striking aspect is the universality of the occupation numbers which are independent of the external potential. We show that the occupation of natural orbitals occurs in pairs, indicating the formation of composite bosons, each consisting of two attractive fermions. The formation of composite bosons sheds light on the pairing mechanism of the system orbitals, yielding a total density equal to that of a Bose-Einstein condensate. Additionally, we propose an alternative form of the Fermionic Tonks-Girardeau ground state

Speaker: ABEL ROJO FRANCÀS

Friday, January 31

Session 5.

Convener: Chair: Rosario Gonzalez-Ferez (Universidad de Granada)

48 Talk 17. Temporal Nambu-Goldstone modes, Floquet thermodynamics, and the time operator in spontaneous Floquet states

We study symmetry-breaking in spontaneous Floquet states, focusing on atomic condensates. First, we quantize the Nambu-Goldstone (NG) modes for stationary states spontaneously breaking several symmetries by invoking the generalized Gibbs ensemble. The quantization involves a Berry-Gibbs connection, which depends on the macroscopic conserved charges and whose curvature is not invariant under generalized gauge transformations. We extend the formalism to Floquet states, where Floquet-NG (FNG) modes emerge with zero quasi-energy. For spontaneous Floquet states, there is a genuine temporal FNG mode associated with the broken time-translation symmetry and whose quantum amplitude provides a unique example of time operator in Quantum Mechanics. Those states conserve energy and a novel charge, the Floquet charge, whose conjugate is the frequency, while driven systems conserve the Floquet enthalpy. This establishes a new thermodynamic paradigm for Floquet physics. We apply our formalism to the CES state, which breaks U(1) and time-translation symmetries, thus embodying a time supersolid. We numerically compute its density-density correlations, predicted to be dominated by the temporal FNG mode at long times, and observe a remarkable agreement between simulation and theory.

Speaker: FERNANDO SOLS (Universidad Complutense de Madrid)

49 Talk 18. Measuring temporal entanglement in experiments as a hallmark for integrability

We introduce a novel experimental protocol to measure generalized temporal entropies in many-body quantum systems. Our approach involves using local operators as probes to observe out-of-equilibrium dynamics of a replicated system induced by a double quench. We compare the dynamics induced by the quench in the intergable transverse field Ising model and the non-integrable transverse field Ising model and show that the generalized temporal entanglement provides a clear signature of integrability breaking.

This protocol demonstrates the experimental feasibility of measuring temporal entropies and it provides a tool for identifying different dynamical classes in the quantum systems.

Speaker: JAN T. SCHNEIDER (Instituto de Física Fundamental IFF-CSIC)

50 Talk 19. Dynamical localization in nonideal kicked rotors driven by two competing pulsatile modulations

We study dynamical localization [1, 2] in an ultracold atom confined in an optical lattice that is simultaneously shaken by two competing pulsatile modulations with different amplitudes, periods, and waveforms. The effects of finite-width time pulses, modulation waveforms, and commensurable and incommensurable driving periods_x000D_
are investigated. We describe a particularly complex scenario and conclude that dynamical localization can survive, or even increase, when a periodic modulation is replaced by a quasiperiodic one of equal amplitude [3]. Our analytical and numerical results indicate that there exists a strong correlation between the strengths of chaos (stochastic layer width) and dynamical localization (difference between the classical and quantum momentum dispersions) over the entire parameter space, which is maintained regardless of the periodic or quasiperiodic nature of the modulation. This persistent correlation provides a useful guide to optimally control the strength of_x000D_
dynamical localization by tuning the modulation parameters in real-world systems subjected to pulses of finite width.x000D
x000D
[1] I. Guarneri, G. Casati, and V. Karle, Phys. Rev. Lett. 113, 174101 (2014).x000D
[2] J. Chabé, H. Lignier, H. Cavalcante, D. Delande, P. Szriftgiser, and J. C. Garreau, Phys. Rev. Lett. 97, 264101 (2006).x000D
[3] F. Revuelta, R. Chacón, and F. Borondo, Phys. Rev. E 110, 054202 (2024).

Speaker: FABIO REVUELTA (Universidad Politécnica de Madrid)

51 Talk 20. Propagation of two-particle correlations across the chaotic phase for interacting bosons

We analyze the propagation of experimentally relevant two-particle correlations for one-dimensional interacting bosons, and give evidence that many-body chaos induces the emergence of an effective diffusive regime for the fully coherent correlation dynamics, characterized by an interaction dependent diffusion coefficient, which we estimate. This result supports very recent experimental observations, and paves the way towards an efficient description of the dynamical behaviour of non-integrable complex many-body systems. Furthermore, we show that the dynamical features within experimentally accessible time scales of a conveniently defined two-particle correlation transport distance provide a direct and unambiguous characterization of many-body quantum chaos.

Speaker: ALBERTO RODRÍGUEZ GONZÁLEZ (Departamento de Física Fundamental, Universidad de Salamanca)

52 Talk 21. Instabilities of superfluids in optical lattices under Floquet driving

Ultracold atoms held in optical lattice potentials have emerged as promising candidates for quantum simulators and quantum computation. In particular, Floquet engineering, manipulating the system's properties by applying a periodic driving, plays a crucial role in generating artificial gauge fields and exotic topological phases. However, driving-induced heating and the growth of phonon modes limit its applications in interacting many-body systems. Here we study the stability of the driven Gross-Pitaevskii equation over a wide range of driving frequencies. At high frequencies the response of the system is chiefly governed by parametric resonances, while at low frequencies modulational instabilities, similar to those seen in static systems, become important. At intermediate driving frequencies an interesting competition between the two types of instabilities emerges. We experimentally confirm the presence of the instabilities, and probe their properties. Our results allow us to predict stable and unstable parameter regions for minimising heating in future applications of Floquet engineering.

Speaker: CHARLES CREFFIELD (Universidad Complutense de Madrid)

11:10 AM Coffee Break

Session 6.

Convener: Chair TBA

53 Talk 22. Superfluid fraction of interacting bosonic gases

The superfluid fraction f of a quantum fluid is defined in terms of the response of the system to a weak and constant drag. Notably, Leggett long ago derived two simple expressions providing a rigorous upper bound and a heuristic lower bound for f. Here we study the superfluid fraction of bosonic gases in various two-dimensional potentials, such as regular optical lattices and disordered speckles, by solving the Gross-Pitaevskii equation and performing Diffusion Monte Carlo simulations. We show that under conditions relevant for most ultracold experiments the bounds proposed by Leggett provide a surprisingly narrow bracketing of the exact value of the superfluid fraction.

Speaker: GRIGORY ASTRAKHARCHIK (Universitat Politècnica de Catalunya)

54 Talk 23. Bose-Einstein condensation in exotic geometries

Modern quantum engineering techniques allow the synthesis of quantum systems in exotic geometries, including fractal lattices characterized by a self-similar pattern and fractal dimensions, or hyperbolic lattices characterized by negative curvature. These geometries can significantly modify single and many-body quantum behavior. We focus on the properties of Bose-Einstein condensation in such geometries. Due to long-range coherence, the condensate wave function is sensitive to the structure of the lattice, which manifests itself in the momentum distribution. It is found that, while the fractal geometry leads to a significant suppression of the critical temperature, the hyperbolic lattice strongly favors condensation. Unlike other 2D systems, the critical temperature in a hyperbolic lattice increases with the system size, hinting for a stable 2D condensate in the thermodynamic limit and at finite temperature. The influence of both finite size and boundary effects has to be considered, as the studied geometries possess a nonvanishing fraction of boundary sites even in the thermodynamic limit.

Speaker: KAMIL DUTKIEWICZ (University of Warsaw and ICFO)

55 Talk 24. Cold atoms in curved geometries

Although condensed matter systems have been extensively studied in one- and two-dimensional configurations, the impact of spatial confinement beyond mere dimensionality has received comparatively little attention. Interestingly, research into quantum systems with curved geometries has gained significant momentum over the past decade. In the field of ultracold atoms [1], recent experiments were able to confine bosonic gases in curved setups such as thin shells [2-3]. A fundamental model, capturing the interplay between spatial curvature, quantum physics, and nontrivial topology, is a bosonic gas confined on the surface of a sphere.

Following a broad review of my previous findings [4], I will present our study of bosons on a sphere with zero-range attractive interactions [5] and possibly other recent works [6-7]. As a main result of [5], we observe a first-order phase transition from a weakly attractive uniform state to a solitonic state as the sphere's radius increases. We thus find an instance of a system whose few- to many-body physics can be controled via its curved geometry. Looking ahead, the research direction of quantum gases in curved geometries could both lead to technological applications and further foundational understanding of quantum physics.

References:
[1] A. Tononi and L. Salasnich, Low-dimensional quantum gases in curved geometries, Nat. Rev. Phys. 5, 398 (2023).
[2] R. A. Carollo, et al., Observation of ultracold atomic bubbles in orbital microgravity, Nature 606, 281 (2022).
[3] F. Jia, et al., Expansion Dynamics of a Shell-Shaped Bose-Einstein Condensate, Phys. Rev. Lett. 129, 243402 (2022).
[4] A. Tononi and L. Salasnich, Shell-shaped atomic gases, Phys. Rep. 1072, 1 (2024).
[5] A. Tononi, G. Astrakharchik, and D. S. Petrov, Gas-to-soliton transition of attractive bosons on a spherical surface, AVS Quantum Sci. 6, 023201 (2024).
[6] A. Tononi and D. S. Petrov, Dimer spectrum on a spherical surface, in preparation.
[7] A. Tononi, M. Lewenstein, and L. Santos, Phase fluctuations in expanding Bose-Einstein condensate shells, in preparation.

Speaker: ANDREA TONONI (ICFO - The Institute of Photonic Sciences)

56 Talk 25. Thermal fading of the 1/k^4-tail of the momentum distribution induced by the hole anomaly

We study the thermal behavior of correlations in a one-dimensional Bose gas with tunable interaction strength, crossing from weakly-repulsive to Tonks-Girardeau regime [1-2]. A reference temperature in this system is that of the hole anomaly [3], observed as a peak in the specific heat and a maximum in the chemical potential. At the anomaly temperature, the spectral states located below the hole branch are thermally occupied and the breakdown of the quasi-particle description occurs. We find that at large momenta k and temperature above the anomaly threshold, the tail C/k^4 of the momentum distribution (proportional to the Tan contact C) is screened by the 1/|k|^3-term due to a dramatic thermal increase of the internal energy emerging from the thermal occupation of spectral excitation states. The same fading is consistently revealed in the behavior at short distances x of the one-body density matrix (OBDM) where the |x|^3-dependence disappears for temperatures above the anomaly. We obtain a new general analytic tail for the momentum distribution and a minimum k fixing its validity range, both calculated with exact Bethe-Ansatz method and valid in all interaction and thermal regimes, crossing from the quantum to the classical gas limit. Our predictions are confirmed by comparison with ab-initio Path Integral Monte Carlo (PIMC) calculations for the momentum distribution and the OBDM exploring a wide range of interaction strength and temperature. Our results unveil a novel connection between excitations and correlations. We expect them to be of interest to any cold atomic, nuclear, solid-state, electronic and spin system exhibiting an anomaly or a thermal second-order phase transition.

[1] G. De Rosi, R. Rota, G. E. Astrakharchik, and J. Boronat, Correlation properties of a one-dimensional repulsive Bose gas at finite temperature, New J. Phys. 25 043002 (2023)

[2] G. De Rosi, G. E. Astrakharchik, M. Olshanii, and J. Boronat, Thermal fading of the 1/k^4-tail of the momentum distribution induced by the hole anomaly, Phys. Rev. A 109, L031302 (2024)

[3] G. De Rosi, R. Rota, G. E. Astrakharchik, and J. Boronat, Hole-induced anomaly in the thermodynamic behavior of a one-dimensional Bose gas, SciPost Phys. 13, 035 (2022)

Speaker: GIULIA DE ROSI (UPC - Universitat Politècnica de Catalunya)

57 Final remarks

1:20 PM Lunch at la Ferradura starts at 14:30