Luis Santos – Institut für Theoretische Physik

Willkommen in der theoretischen Arbeitsgruppe der ultrakalten Quantengase!

Ultrakalte Quantengase stellen ein sich schnell entwickelndes und spannendes Gebiet dar, das in den letzten Jahren ein großes interdisziplinäres Interesse auf sich gezogen hat, das von der AMO-Physik bis zur Quantenoptik, der nichtlinearen Physik, der Physik der kondensierten Materie und darüber hinaus reicht.

Wenn Atome oder Moleküle extrem kalt werden, entwickeln sie faszinierende Eigenschaften, wie zum Beispiel die Bose-Einstein-Kondensation. Darüber hinaus können kalte Gase sehr präzise manipuliert und kontrolliert werden, was viele Möglichkeiten für die Quantenemulation oder Simulation von Vielteilchen-Quantensystemen eröffnet.

Unsere Theoriegruppe arbeitet in einer Vielzahl von Themen in diesem spannenden Feld.

Wir bieten ein offenes Masterprojekt!

Interessensgebiete

Aktuelle Forschung in unserer Gruppe umfasst

Dipolar Bose-Einstein condensates

Cold gases experiments have typically dealt with atoms interacting via contact interactions. A new generation of experiments is starting to reveal the fascinating physics of dipolar gases, formed by particles with large magnetic or electric dipole moments. The presence of the dipole-dipole interaction changes radically the properties of the gases, resulting in an intriguing qualitatively new physics.

Some of our recent results in this topic include the study of roton excitations [1], ultra-dilute quantum droplets [2], low-dimensional dipolar gases [3], doubly-dipolar condensates [4], dipolar supersolids [5], and dipolar mixtures [6]. Many of these results were obtained in close collaboration with experimental groups, including G. Modugno's group (Pisa) and, very especially, F. Ferlaino's group (Innsbruck).

_{[1] L. Chomaz et al., Nature Phys. 14, 442 (2018).}

_{[2] F. Wächtler and L. Santos, Phys. Rev. A 93, 061603(R) (2016); F. Wächtler and L. Santos, Phys. Rev. A 94, 043618 (2016); L. Chomaz et al., Phys. Rev. X 6, 041039 (2016).}

_{[3] D. Edler et al., Phys. Rev. Lett. 119, 050403 (2017).}

_{[4] C. Mishra, L. Santos, and R. Nath, Phys. Rev. Lett. 124, 073402 (2020).}

_{[5] L. Tanzi et al., Phys. Rev. Lett. 122, 130405 (2019); M. A. Norcia et al., Nature 596, 357 (2021).}

_{[6] R. N. Bisset, L. A. Peña Ardila, and L. Santos, Phys. Rev. Lett. 126, 025301 (2021).}

1D-to-2D transition in a dipolar supersolid [M. Norcia et al., Nature 596, 357 (2021)]
Spinor gases

Quantum gases of atoms with various available internal state offer a lot of possibilities ranging from their use for the quantum simulation of spin models in optical lattices to the rich ground-state properties of spinor Bose-Einstein condensates. Interestingly, spinor condensates offer, due to the so-called spin-changing collisions, the possibility of generating highly-entangled many-body states of interest for precision measurements.

In our group, we explore this interesting physics in close collaboration with the experimental group of C. Klempt (Institute of Quantum Optics, Hannover). Some of our recent results include the study of the interferometric sensitivity and entanglement generation by scanning through quantum phase transitions in a spinor condensate [1], the proposal for heralded generation of macroscopic quantum states [2], the proposal for an interferometric order parameter for excited-state quantum phase transitions [3], the generation of momentum entanglement for atom interferometry [4],

_{[1] P. Feldmann et al., Phys. Rev. A 97, 032339 (2018).}

_{[2] L. Pezze et al., Phys. Rev. Lett. 123, 260403 (2019).}

_{[3] P. Feldmann et al., Phys. Rev. Lett. 126, 230602 (2021).}

_{[4] F. Anders et al., Phys. Rev. Lett. 127, 140402 (2021).}

Classical phase space and trajectories for different excited-state phases in an F=1 spinor condensate [P. Feldmann et al., Phys. Rev. Lett. 126, 230602 (2021)]
Synthetic magnetism

Although neutral atoms experience by definition no Lorenz force, it is possible, by different means, to impose to the atoms a synthetic gauge field, both in a condensate and for atom in an optical lattice. However, typically, the gauge field is not affected by the atoms. In our group we have recently explored different scenarios in which the synthetic fields are affected by the atoms.

Recent works in our group include: the study of density dependent gauge fields [1], anyon-Hubbards models [2], and quantum link models [3].

_{[1] S. Greschner, G. Sun, D. Poletti, and L. Santos, Phys. Rev. Lett. 113, 215303 (2014); S. Greschner, D. Huerga, G. Sun, D. Poletti, and L. Santos, Phys. Rev. B 92, 115120 (2015).}

_{[2] S. Greschner and L. Santos, Phys. Rev. Lett. 115, 053002 (2015); L. Cardarelli, S. Greschner, and L. Santos, Phys. Rev. A 94, 023615 (2016); S. Greschner, L. Cardarelli, and L. Santos, Phys. Rev. A 97, 053605 (2018) (Editor's suggestion).}

_{[3] L. Cardarelli, S. Greschner, and L. Santos, Phys. Rev. Lett. 119, 180402 (2017); L. Cardarelli, S. Greschner, and L. Santos, Phys. Rev. Lett. 124, 123601 (2020).}

Sketch of the quantum link model on a 2D lattice [L. Cardarelli et al., Phys. Rev. Lett. 124, 123601 (2020)]
Dynamics of quantum gases in optical lattices

Ultra-cold gases in optical lattices offer fascinating possibilities for the study of the dynamics of out-of-equilibrium quantum many-body systems, including the study of quantum ergodicity, thermalization, prethermalization, and localization in the presence or even in the absence of quenched disorder.

Our group has recently focused on the dynamics of systems with power-law interactions.

On one hand we have studied dynamics and localization in spin models with power-law exchange and Ising interactions. Recent contributions include the study of multifractality of spin excitations in pinned polar molecules in an optical lattice [1], of algebraic localization in 1D disordered systems with power-law hopping [2], of mobility and ergodic edges in 1D quasicrystals with power-law hopping [3], and of universal algebraic growth of entanglement entropy in many-body localized systems with power-law interactions [4].

On the other hand, we have analyzed the dynamics of Hubbard models with dipolar interactions, which may be generated e.g. by lanthanide atoms, polar molecules, or Rydberg atoms. Recent results include the study of quasi-localization via dimer clusterization and self-bound lattice droplets [5], of multi-scaled dynamics and effective flat-band physics of dimers in 2D polar lattice gases [6], and of Hilbert-space shattering and disorder-free localization in 1D polar lattice gases [7].

_{[1] X. Deng, B. L. Altshuler, G. V. Shlyapnikov, and L. Santos, Phys. Rev. Lett. 117, 020401 (2016).}

_{[2] X. Deng, V. E. Kravtsov, G. V. Shlyapnikov, and L. Santos, Phys. Rev. Lett. 120, 110602 (2018).}

_{[3] X. Deng, S. Ray, S. Sinha, G. V. Shlyapnikov, and L. Santos, Phys. Rev. Lett. 123, 025301 (2019).}

_{[4] X. Deng, G. Masella, G. Pupillo, and L. Santos, Phys. Rev. Lett. 125, 010401 (2020).}

_{[5] W. Li, A. Dhar, X. Deng, K. Kasamatsu, L. Barbiero, and L. Santos, Phys. Rev. Lett. 124, 010404 (2020).}

_{[6] W. Li, A. Dhar, X. Deng, and L. Santos, Phys. Rev. A 103, 043331 (2021).}

_{[7] W. Li, X. Deng, and L. Santos, arXiv:2103.13780.}

Singlon trapped between two dimers results in the strengthen formation of a dimer-dimer cluster [W. Li et al., Phys. Rev. Lett. 124, 010404 (2020)]
Ion Coulomb crystals

Coulomb crystals of trapped ions constitute an excellent system to probe energy transport with atomic resolution both in the classical and in the quantum regime. Interestingly, these systems allow for the realization of different versions of the Frenkel-Kontorova model, a key model for the study of nanofriction.

Our group has recently collaborated with the experimental group of T. Mehlstäubler (PTB, Braunschweig) and the theory group of H. Weimer (ITP, Hannover) in the analysis of excitation transport. Recent results include the study of energy localization [1], and quantum nanofriction [2] in Coulomb ion crystals with a topological soliton.

_{[1] L. Timm, H. Weimer, L. Santos, and T. E. Mehlstäubler, Phys. Rev. Research 2, 033198 (2020)}

_{[2] L. Timm, L. A. Rüffert, H. Weimer, L. Santos, and T. E. Mehlstäubler, Phys. Rev. Research (accepted 2021; arXiv:2108.07635)}

Ion chain with a central zigzag region with a kink in the sliding phase, experimental picture from T. Mehlstäubler’s group. Computed equilibrium positions for the sliding phase, pinned phase, and odd-kink phase (from top to bottom) [L. Timm et al., Phys. Rev. Research 2, 033198 (2020)]
Quantum simulation of many-body quantum systems using NISQ devices

Last years have witnessed spectacular developments in the field of quantum computers, although nowadays we are still living in the so-called NISQ era, characterized by quantum computers of few and noisy qubits. Our group has started very recently a collaboration with the group of T. Osborne (ITP, Hannover) in the frame of the Quantum Valley Lower Saxony initiative, concerning the study of quantum many-body models using NISQ devices.