Publications Emil KIRILOV

2025

Article

Z. Ye, A. Canali, C. Wong, M. Kreyer, E. Kirilov, R. Grimm Dipole-Mode Spectrum and Hydrodynamic Crossover in a Resonantly Interacting Two-Species Fermion Mixture, (2025-02-07), (ID: 721466)

2023

Articles

C. Baroni, B. Huang, I. Fritsche, E. Dobler, G. Anich, E. Kirilov, R. Grimm, M. A. Bastarrachea-Magnani, P. Massignan, G. Bruun Mediated interactions between Fermi polarons and the role of impurity quantum statistics, Nature Phys. (2023-10-26), http://dx.doi.org/10.1038/s41567-023-02248-4 doi:10.1038/s41567-023-02248-4 (ID: 721140) Toggle Abstract
The notion of quasi-particles is essential for understanding the behaviour of complex many-body systems. A prototypical example of a quasi-particle is a polaron, formed by an impurity strongly interacting with a surrounding medium. Fermi polarons, created in a Fermi sea, provide a paradigmatic realization of this concept. Importantly, such quasi-particles interact with each other via the modulation of the medium. However, although quantum simulation experiments with ultracold atoms have substantially improved our understanding of individual polarons, the detection of their interactions has so far remained elusive. Here we report the observation of mediated interactions between Fermi polarons consisting of K impurities embedded in a Fermi sea of Li atoms. Our results confirm two predictions of Landau’s Fermi-liquid theory: the shift in polaron energy due to mediated interactions, which is linear in the concentration of impurities; and its sign inversion with impurity quantum statistics. For weak-to-moderate interactions between the impurities and the medium, our results agree with the static predictions of Fermi-liquid theory. For stronger impurity–medium interactions, we show that the observed behaviour at negative energies can be explained by a more refined many-body treatment including retardation and dressed molecule formation.
C. Baroni, B. Huang, I. Fritsche, E. Dobler, G. Anich, E. Kirilov, R. Grimm, M. A. Bastarrachea-Magnani, P. Massignan, G. Bruun Mediated interactions between Fermi polarons and the role of impurity quantum statistics, Nature Phys. 20 (2023-10-26), http://dx.doi.org/10.1038/s41567-023-02248-4 doi:10.1038/s41567-023-02248-4 (ID: 721077) Toggle Abstract
The notion of quasi-particles is essential for understanding the behaviour of complex many-body systems. A prototypical example of a quasi-particle, a polaron, is an impurity strongly interacting with a surrounding medium. Fermi polarons, created in a Fermi sea, provide a paradigmatic realization of this concept. As an inherent and important property such quasi-particles interact with each other via modulation of the medium. While quantum simulation experiments with ultracold atoms have significantly improved our understanding of individual polarons, the detection of their interactions has remained elusive in these systems. Here, we report the unambiguous observation of mediated interactions between Fermi polarons consisting of K impurities embedded in a Fermi sea of Li atoms. Our results confirm two landmark predictions of Landau's Fermi-liquid theory: the shift of the polaron energy due to mediated interactions, linear in the concentration of impurities, and its sign inversion with impurity quantum statistics. For weak to moderate interactions between the impurities and the medium, we find excellent agreement with the static (zero-momentum and energy) predictions of Fermi-liquid theory. For stronger impurity-medium interactions, we show that the observed behaviour at negative energies can be explained by a more refined many-body treatment including retardation and molecule formation
E. Soave, A. Canali, Z. Ye, M. Kreyer, E. Kirilov, R. Grimm Optically trapped Feshbach molecules of fermionic 161Dy and 40K, Phys. Rev. Research 5 33117 (2023-04-17), http://dx.doi.org/10.1103/PhysRevResearch.5.033117 doi:10.1103/PhysRevResearch.5.033117 (ID: 721079) Toggle Abstract
We report on the preparation of a pure ultracold sample of bosonic DyK Feshbach molecules, composed of the fermionic isotopes 161Dy and 40K. By sweeping a magnetic field across a resonance located near 7.3 G, we generate up to 5000 molecules at a temperature of approximately 50 nK. To purify the sample from remaining atoms, we employ a Stern-Gerlach technique that relies on magnetic levitation of the molecules in a very weak optical dipole trap. With the trapped molecules we finally reach a high phase-space density of about 0.1. We investigate the magnetic field dependence of the molecular binding energy and the magnetic moment, refining our knowledge of the resonance parameters. We also demonstrate a peculiar anisotropic expansion effect observed when the molecules are released from the trap and expand freely in the magnetic levitation field. Furthermore, we identify an important lifetime limitation that is imposed by the 1064-nm infrared trap light itself and not by inelastic collisions. The light-induced decay rate is found to be proportional to the trap light intensity and the closed-channel fraction of the Feshbach molecule. These observations suggest a one-photon coupling to electronically excited states to limit the lifetime and point to the prospect of loss suppression by optimizing the wavelength of the trapping light. Our results offer crucial insights and experimental progress towards achieving quantum-degenerate samples of DyK molecules and novel superfluids based on mass-imbalanced fermion mixtures.

Preprint

G. Anich, R. Grimm, E. Kirilov Comprehensive Characterization of a State-of-the-Art Apparatus for Cold Electromagnetic Dysprosium Dipoles, (2023-04-25), arXiv:2304.12844 arXiv:2304.12844 (ID: 721078) Toggle Abstract
We developed a new advanced ultra-cold Dysprosium (Dy) apparatus, which incorporates a quantum gas microscope (QGM) with a resolution of a quarter micrometer. The QGM and the cooling and trapping regions are within the same vacuum glass vessel assuring simple atom transport between them. We demonstrate the essential experimental steps of laser and evaporative cooling, lattice loading, transporting and precise positioning of a cloud of the bosonic isotope 164 Dy at the QGM focal plane. Preliminary basic characterization of the QGM and future plans in enabling its full capacity are outlined. We also present a feasible platform for simulating complex spin models of quantum magnetism, such as XYZ model, by exploiting a set of closely spaced opposite parity levels in Dy with a large magnetic and electric dipole moment. We isolate a degenerate isospin-1/2 system, which possesses both magnetic and electric dipole-dipole coupling, containing Ising, exchange and spin-orbit terms. The last gives rise to a spin model with asymmetric tunable rates, dependable on the lattice geometry.

2022

Articles

Zhuxiong Ye, Alberto Canali, Elisa Soave, Marian Kreyer, Yaakov Yudkin, Cornelis Ravensbergen, Emil Kirilov, Rudolf Grimm Observation of low-field Feshbach resonances between 161Dy and 40K, Phys. Rev. A 106 PhysRevA.106.043314 (2022-10-17), http://dx.doi.org/10.1103/PhysRevA.106.043314 doi:10.1103/PhysRevA.106.043314 (ID: 720848) Toggle Abstract
We report on the observation of Feshbach resonances at low magnetic field strength (below 10 G) in the Fermi-Fermi mixture of 161Dy and 40K. We characterize five resonances by measurements of interspecies thermalization rates and molecular binding energies. As a case of particular interest for applications, we consider a resonance near 7.29 G, which combines accurate magnetic tunability and access to the universal regime of interactions with experimental simplicity. We show that lifetimes of a few 100 ms can be achieved for the optically trapped, resonantly interacting mixture. We also demonstrate the hydrodynamic expansion of the mixture in the strongly interacting regime and the formation of DyK Feshbach molecules. Our work opens up new experimental possibilities in view of mass-imbalanced superfluids and related phenomena.
Elisa Soave, Vincent Corre, Cornelis Ravensbergen, Jeong Ho Han, Marian Kreyer, Emil Kirilov, Rudolf Grimm Low-field Feshbach resonances and three-body losses in a fermionic quantum gas of 161Dy, Ukr. J. Phys. 67 334 (2022-08-29), http://dx.doi.org/10.15407/ujpe67.5.334 doi:10.15407/ujpe67.5.334 (ID: 720847) Toggle Abstract
We report on high-resolution Feshbach spectroscopy on a degenerate, spin-polarized Fermi gas of 161Dy atoms, measuring three-body recombination losses at low magnetic field. For field strength up to 1\,G, we identify as much as 44 resonance features and observe plateaus of very low losses. For four selected typical resonances, we study the dependence of the three-body recombination rate coefficient on the magnetic resonance detuning and on the temperature. We observe a strong suppression of losses with decreasing temperature already for small detunings from resonance. The characterization of complex behavior of three-body losses of fermionic 161Dy is important for future applications of this peculiar species in research on atomic quantum gases.

2021

Articles

M. Kreyer, J. H. Han, C. Ravensbergen, V. Corre, E. Soave, E. Kirilov, R. Grimm Measurement of the dynamic polarizability of Dy atoms near the 626-nm intercombination line, Phys. Rev. A 104 33106 (2021-09-10), http://dx.doi.org/10.1103/PhysRevA.104.033106 doi:10.1103/PhysRevA.104.033106 (ID: 720639) Toggle Abstract
We report on measurements of the anisotropic dynamical polarizability of Dy near the 626-nm intercombination line, employing modulation spectroscopy in a one-dimensional optical lattice. To eliminate large systematic uncertainties resulting from the limited knowledge of the spatial intensity distribution, we use K as a reference species with accurately known polarizability. This method can be applied independently of the sign of the polarizability, i.e. for both attractive and repulsive optical fields on both sides of a resonance. By variation of the laser polarization we extract the scalar and the tensorial part. To characterize the strength of the transition, we also derive the natural linewidth. We find our result in excellent agreement with literature values, which provide a sensitive benchmark for the accuracy of our method. In addition we demonstrate optical dipole trapping on the intercombination line, confirming the expected long lifetimes and low heating rates. This provides an additional tool to tailor optical potentials for Dy atoms and for the species-specific manipulation of atoms in the Dy-K mixture.
I. Fritsche, C. Baroni, E. Dobler, E. Kirilov, B. Huang, R. Grimm, G. Bruun, P. Massignan Stability and breakdown of Fermi polarons in a strongly interacting Fermi-Bose mixture, Phys. Rev. A 103 53314 (2021-05-17), http://dx.doi.org/10.1103/PhysRevA.103.053314 doi:10.1103/PhysRevA.103.053314 (ID: 720640) Toggle Abstract
We investigate the properties of a strongly interacting imbalanced mixture of bosonic 41K impurities immersed in a Fermi sea of ultracold 6Li atoms. This enables us to explore the Fermi polaron for large impurity concentrations including the case where they form a Bose-Einstein condensate. The system is characterized by means of radio-frequency injection spectroscopy for tunable interactions using an interspecies Feshbach resonance. We find that the energy of the Fermi polarons formed in the thermal fraction of the impurity cloud remains rather insensitive to the impurity concentration, even as we approach equal densities for both species. The apparent insensitivity to high concentration is consistent with the theoretical prediction, based on Landau's quasiparticle theory, of a weak effective interaction between the polarons. The condensed fraction of the bosonic 41K gas is much denser than its thermal component, which leads to a break-down of the Fermi polaron description. Instead, we observe a new branch in the radio-frequency spectrum with a small energy shift, which is consistent with the presence of Bose polarons formed by 6Li fermions inside the 41K condensate. A closer investigation of the behavior of the condensate by means of Rabi oscillation measurements support this observation, indicating that we have realized Fermi and Bose polarons, two fundamentally different quasiparticles, in one cloud.

2020

Article

C. Ravensbergen, E. Soave, V. Corre, M. Kreyer, B. Huang, E. Kirilov, R. Grimm Resonantly Interacting Fermi-Fermi Mixture of 161Dy and 40K, Phys. Rev. Lett. 124 203402 (2020-05-22), http://dx.doi.org/10.1103/PhysRevLett.124.203402 doi:10.1103/PhysRevLett.124.203402 (ID: 720519)

2019

Article

B. Huang, I. Fritsche, R. Lous, C. Baroni, J. T. Walraven, E. Kirilov, R. Grimm Breathing Mode of a BEC Repulsively Interacting with a Fermionic Reservoir, Phys. Rev. A 99 41602 (2019-04-03), http://dx.doi.org/10.1103/PhysRevA.99.041602 doi:10.1103/PhysRevA.99.041602 (ID: 720106) Toggle Abstract
We investigate the fundamental breathing mode of a small-sized elongated Bose-Einstein condensate coupled to a large Fermi sea of atoms. Our observations show a dramatic shift of the breathing frequency when the mixture undergoes phase separation at strong interspecies repulsion. We find that the maximum frequency shift in the full phase-separation limit depends essentially on the atom number ratio of the components. We interpret the experimental observations within a model that assumes an adiabatic response of the Fermi sea, or within another model that considers single fermion trajectories for a fully phase-separated mixture. These two complementary models capture the observed features over the full range of interest.

2018

Articles

C. Ravensbergen, V. Corre, E. Soave, M. Kreyer, E. Kirilov, R. Grimm Production of a degenerate Fermi-Fermi mixture of dysprosium and potassium atoms, Phys. Rev. A 98 63624 (2018-12-20), http://dx.doi.org/10.1103/PhysRevA.98.063624 doi:10.1103/PhysRevA.98.063624 (ID: 720087) Toggle Abstract
We report on the realization of a mixture of fermionic 161^{161}Dy and fermionic 40^{40}K where both species are deep in the quantum-degenerate regime. Both components are spin-polarized in their absolute ground states, and the low temperatures are achieved by means of evaporative cooling of the dipolar dysprosium atoms together with sympathetic cooling of the potassium atoms. We describe the trapping and cooling methods, in particular the final evaporation stage, which leads to Fermi degeneracy of both species. Analyzing cross-species thermalization we obtain an estimate of the magnitude of the inter-species ss-wave scattering length at low magnetic field. We demonstrate magnetic levitation of the mixture as a tool to ensure spatial overlap of the two components. The properties of the Dy-K mixture make it a very promising candidate to explore the physics of strongly interacting mass-imbalanced Fermi-Fermi mixtures.
R. Lous, I. Fritsche, M. Jag, F. Lehmann, E. Kirilov, B. Huang, R. Grimm Probing the Interface of a Phase-Separated State in a Repulsive Bose-Fermi Mixture, Phys. Rev. Lett. 120 243403 (2018-02-06), http://dx.doi.org/10.1103/PhysRevLett.120.243403 doi:10.1103/PhysRevLett.120.243403 (ID: 719979) Toggle Abstract
We probe the interface between a phase-separated Bose-Fermi mixture consisting of a small BEC of 41K residing in a large Fermi sea of 6Li. We quantify the residual spatial overlap between the two components by measuring three-body recombination losses for variable strength of the interspecies repulsion. A comparison with a numerical mean-field model highlights the importance of the kinetic energy term for the condensed bosons in maintaining the thin interface far into the phase-separated regime. Our results demonstrate a corresponding smoothing of the phase transition in a system of finite size.
C. Ravensbergen, V. Corre, E. Soave, M. Kreyer, S. Tzanova, E. Kirilov, R. Grimm Accurate Determination of the Dynamical Polarizability of Dysprosium, Phys. Rev. Lett. 120 223001 (2018-01-18), http://dx.doi.org/10.1103/PhysRevLett.120.223001 doi:10.1103/PhysRevLett.120.223001 (ID: 719964) Toggle Abstract
We report a measurement of the dynamical polarizability of dysprosium atoms in their electronic ground state at the optical wavelength of 1064 nm, which is of particular interest for laser trapping experiments. Our method is based on collective oscillations in an optical dipole trap, and reaches unprecedented accuracy and precision by comparison with an alkali atom (potassium) as a reference species. We obtain values of 184.4(2.4) a.u. and 1.7(6) a.u. for the scalar and tensor polarizability, respectively. Our experiments have reached a level that permits meaningful tests of current theo- retical descriptions and provides valuable information for future experiments utilizing the intriguing properties of heavy lanthanide atoms.

2015

Article

E. Kirilov, M. J. Mark, M. Segl, H. Nägerl Compact, robust, and spectrally pure diode-laser system with a filtered output and a tunable copy for absolute referencing, Appl. Phys. B Las. Opt. 119/2 0946-2171 (2015-03-07), http://dx.doi.org/10.1007/s00340-015-6049-5 doi:10.1007/s00340-015-6049-5 (ID: 719196) Toggle Abstract
We report on a design of a compact laser system composed of an extended-cavity diode laser with high passive stability and a pre-filter Fabry–Perot cavity. The laser is frequency-stabilized relative to the cavity using a serrodyne technique with a correction bandwidth of ≥6 MHz and a dynamic range of ≥700 MHz. The free-running laser system has a power spectral density (PSD) ≤100 Hz2/Hz centered mainly in the acoustic frequency range. A highly tunable, 0.5–1.3 GHz copy of the spectrally pure output beam is provided, which can be used for further stabilization of the laser system to an ultra-stable reference. We demonstrate a simple one-channel lock to such a reference that brings down the PSD to the sub-Hz level. The tuning, frequency stabilization, and sideband imprinting are achieved by a minimum number of key elements comprising a fibered electro-optic modulator, acousto-optic modulator, and a nonlinear transmission line. The system is easy to operate, scalable, and highly applicable to atomic/molecular experiments demanding high spectral purity, long-term stability, and robustness.

2014

Articles

F. Meinert, M. J. Mark, E. Kirilov, K. Lauber, P. Weinmann, M. Gröbner, A. J. Daley, H. Nägerl Observation of many-body dynamics in long-range tunneling after a quantum quench, Science 344 1262 (2014-06-13), http://dx.doi.org/10.1126/science.1248402 doi:10.1126/science.1248402 (ID: 719039) Toggle Abstract
Quantum tunneling is at the heart of many low-temperature phenomena. In strongly correlated lattice systems, tunneling is responsible for inducing effective interactions, and long-range tunneling substantially alters many-body properties in and out of equilibrium. We observe resonantly enhanced long-range quantum tunneling in one-dimensional Mott-insulating Hubbard chains that are suddenly quenched into a tilted configuration. Higher-order tunneling processes over up to five lattice sites are observed as resonances in the number of doubly occupied sites when the tilt per site is tuned to integer fractions of the Mott gap. This forms a basis for a controlled study of many-body dynamics driven by higher-order tunneling and demonstrates that when some degrees of freedom are frozen out, phenomena that are driven by small-amplitude tunneling terms can still be observed.
F. Meinert, M. J. Mark, E. Kirilov, K. Lauber, P. Weinmann, M. Gröbner, H. Nägerl Interaction-Induced Quantum Phase Revivals and Evidence for the Transition to the Quantum Chaotic Regime in 1D Atomic Bloch Oscillations, Phys. Rev. Lett. 112 193003 (2014-05-15), http://dx.doi.org/10.1103/PhysRevLett.112.193003 doi:10.1103/PhysRevLett.112.193003 (ID: 719073) Toggle Abstract
We study atomic Bloch oscillations in an ensemble of one-dimensional tilted superfluids in the Bose-Hubbard regime. For large values of the tilt, we observe interaction-induced coherent decay and matter-wave quantum phase revivals of the Bloch oscillating ensemble. We analyze the revival period dependence on interactions by means of a Feshbach resonance. When reducing the value of the tilt, we observe the disappearance of the quasiperiodic phase revival signature towards an irreversible decay of Bloch oscillations, indicating the transition from regular to quantum chaotic dynamics.

2013

Article

F. Meinert, M. J. Mark, E. Kirilov, K. Lauber, P. Weinmann, A. J. Daley, H. Nägerl Quantum Quench in an Atomic One-Dimensional Ising Chain, Phys. Rev. Lett. 111 053003 (2013-07-31), http://dx.doi.org/10.1103/PhysRevLett.111.053003 doi:10.1103/PhysRevLett.111.053003 (ID: 718496) Toggle Abstract
We study nonequilibrium dynamics for an ensemble of tilted one-dimensional atomic Bose-Hubbard chains after a sudden quench to the vicinity of the transition point of the Ising paramagnetic to antiferromagnetic quantum phase transition. The quench results in coherent oscillations for the orientation of effective Ising spins, detected via oscillations in the number of doubly occupied lattice sites. We characterize the quench by varying the system parameters. We report significant modification of the tunneling rate induced by interactions and show clear evidence for collective effects in the oscillatory response.