# FERMIX Lab

Principal Investigator: Frédéric Chevy**Permanent Members**

Frédéric Chevy (PI)

Christophe Salomon

Tarik Yefsah

**Senior scientists **

Julian Struck

**Postdocs and PhD Students **

Thomas Reimann

**Selected Publications**

**2.4-watts second-harmonic generation in ppZnO:LN ridge waveguide for lithium laser cooling**

Norman Kretzschmar, Ulrich Eismann, Franz Sievers, Frédéric Chevy, and Christophe Salomon

Optics Express **25**, 13, 14840-14855 (2017)

**Analog simulation of Weyl particles with cold atoms**

Daniel Suchet, Mihail Rabinovic, Thomas Reimann, Norman Kretschmar, Franz Sievers, Christophe Salomon, Johnathan Lau, Olga Goulko, Carlos Lobo, Frédéric Chevy

EPL **114**, 26005 (2016), arXiv:1507.02106

**Sub-Doppler laser cooling of fermionic 40K atoms in three-dimensional gray optical molasses**

D. Rio Fernandes, F. Sievers, N. Kretzschmar, S. Wu, C. Salomon, F. Chevy

EPL** 100**, 63001 (2012), arXiv:1210.1310

**An all-solid-state laser source at 671 nm for cold atom experiments with lithium**

U. Eismann, F. Gerbier, C. Canalias, A. Zukauskas, G. Trénec, J. Vigué, F. Chevy, C. Salomon

Appl. Phys. B **106**, 25 (2012), arXiv:1103.5841

## Mixtures of ultracold Fermi gases

BCS theory predicts that superconductivity arises from pairing of particles with same masses and opposite spins. But what happens when the system is constituted of fermions with different masses? Our mixture experiment permits to study mixtures of fermions with two different atomic species, Lithium 6 and Potassium 40, and explore this question. The difference in mass and atomic transitions between these two atomic species, allows one to create trapping potentials which are species-dependent and create a variety of fascinating physical situations.

Our experimental setup was designed around a two species magneto-optical trap capturing lithium and potassium atoms from a Zeeman slower (Lithium) and a two-dimensional magneto-optical trap (Potassium). Using this scheme up to 5×10^9 Lithium atoms and 8×10^9 potassium atoms can be captured.

Using a magnetic conveyor belt, cold atoms are then transfered towards the science cell, an auxiliary chamber where, after evaporative cooling is performed, we obtain degenerate fermi gases.

## News from the lab

December 13th, 2018 — Thomas Reimann PhD Thesis Defense

Congratulations to Thomas for defending his PhD thesis entitled “Resonant spin dynamics and 3D-1D dimensional crossovers in ultracold Fermi gases”!

June 26th, 2017 — Our work `2.4-watts second-harmonic generation in ppZnO:LN ridge waveguide for lithium laser cooling’ got published in Optics Express

Norman Kretzschmar, Ulrich Eismann, Franz Sievers, Frédéric Chevy, and Christophe Salomon

Optics Express **25**, 13, 14840-14855 (2017)

We present a simple all-solid-state laser source emitting 2.4 W of single-frequency light at 671 nm for laser cooling of lithium atoms. It is based on a diode-pumped solid-state laser, which is frequency doubled in a ppZnO:LN ridge waveguide with an internal doubling efficiency of 54%. We develop a simple theory for the thermal effects we observed at elevated fundamental powers, and compare the setup to a more efficient but more complex one with an external resonant frequency doubling cavity providing 5.2 W at 671 nm.

May 11th, 2017 — Mihail Rabinovic PhD Thesis Defense

Congratulations to Mihail for defending his PhD thesis entitled “Quasi-thermalization of Fermions in a Quadrupole Potential and Evaporative Cooling of 40K to Quantum Degeneracy”!

April 28th, 2017 — Our work `Long-range mediated interactions in a mixed-dimensional system‘ got published in Physical Review A

*in collaboration with Zhigang Wu, and Georg M. Bruun*

Daniel Suchet, Zhigang Wu, Frédéric Chevy, and Georg M. Bruun

Phys. Rev. A **95**, 043643 (2017) — arXiv:1702.08129

We present a mixed-dimensional atomic gas system to unambiguously detect and systematically probe mediated interactions. In our scheme, fermionic atoms are confined in two parallel planes and interact via exchange of elementary excitations in a three-dimensional background gas. This interaction gives rise to a frequency shift of the out-of-phase dipole oscillations of the two clouds, which we calculate using a strong-coupling theory taking the two-body mixed-dimensional scattering into account exactly. The shift is shown to be easily measurable for strong interactions and can be used as a probe for mediated interactions.

July 8th, 2016 — Daniel Suchet PhD Thesis Defense

Congratulations to Daniel for defending his PhD thesis entitled “Simulating the dynamics of harmonically trapped Weyl particles with cold atoms”!

May 18th, 2016 — Our work `Analog simulation of Weyl particles with cold atoms‘ got published as editor’s choice in Europhysics Letters

*in collaboration with J. Lau, O. Goulko and C. Lobo*

Daniel Suchet, Mihail Rabinovic, Thomas Reimann, Norman Kretzschmar, Franz Sievers, Christophe Salomon, Johnathan Lau, Olga Goulko, Carlos Lobo and Frédéric Chevy

EPL **114**, 2 (2016) — arXiv:1507.02106

In this letter we report on a novel approach to study the dynamics of harmonically confined Weyl particles using magnetically trapped fermionic atoms. We find that after a kick of its center of mass, the system relaxes towards a steady state even in the absence of interactions, in stark contrast with massive particles which would oscillate without damping. Remarkably, the equilibrium distribution is non-Boltzmann, exhibiting a strong anisotropy which we study both numerically and experimentally.

June 26th, 2015 — Norman Kretzschmar PhD Thesis Defense

Congratulations to Norman for defending his PhD thesis entitled “Experiments with Ultracold Fermi Gases: Quantum Degeneracy

of Potassium-40 and All-solid-state Laser Sources for Lithium”!

February 23rd, 2015 — Our work `Simultaneous sub-Doppler laser cooling of fermionic 6Li and 40K on the D1 line: Theory and Experiment’ gets published in Physical Review A

Franz Sievers, Norman Kretzschmar, Diogo Rio Fernandes, Daniel Suchet, Michael Rabinovic, Saijun Wu, Colin V. Parker, Lev Khaykovich, Christophe Salomon, and Frédéric Chevy

Phys. Rev. A **91**, 023426 (2015) — arXiv:1410.8545

We report on simultaneous sub-Doppler laser cooling of fermionic 6Li and 40K using the D1 optical transitions. We compare experimental results to a numerical simulation of the cooling process applying a semi-classical Monte Carlo wavefunction method. The simulation takes into account the three dimensional optical molasses setup and the dipole interaction between atoms and the bichromatic light field driving the D1 transitions. We discuss the physical mechanisms at play, we identify the important role of coherences between the ground state hyperfine levels and compare D1 and D2 sub-Doppler cooling. In 5 ms, the D1 molasses phase largely reduces the temperature for both 6Li and 40K at the same time, with a final temperature of 44 μK and 11 μK, respectively. For both species this leads to a phase-space density close to 10−4. These conditions are well suited to directly load an optical or magnetic trap for efficient evaporative cooling to quantum degeneracy.

September 29th, 2014 — Diogo Rio Fernandes PhD Thesis Defense

Congratulations to Diogo for defending his PhD thesis entitled “Trapping and cooling of fermionic alkali atoms to quantum degeneracy. Sub-Doppler cooling of Potassium-40 and Lithium-6 in gray molasses.”!

July 21st, 2014 — Franz Sievers PhD Thesis Defense

Congratulations to Franz for defending his PhD thesis entitled “Ultracold Fermi mixtures and simultaneous sub-Doppler laser cooling of fermionic 6Li and 40K.”!

December 31st, 2012 — Our work `Sub-Doppler laser cooling of fermionic 40K atoms in three-dimensional gray optical molasses’ gets published in Europhysics Letters

D. Rio Fernandes, F. Sievers, N. Kretzschmar, S. Wu, C. Salomon and F. Chevy

EPL **100**, 6 (2012) — arXiv:1210.1310

We demonstrate sub-Doppler cooling of 40K on the D1 atomic transition. Using a gray-molasses scheme, we efficiently cool a compressed cloud of 6.5 × 108 atoms from ~4 mK to 20 μK in 8 ms. After transfer to a quadrupole magnetic trap, we measure a phase space density of ~10−5. This technique offers a promising route for fast evaporation of fermionic 40K.

September 2nd, 2012– Our work `An all-solid-state laser source at 671 nm for cold-atom experiments with lithium’ gets published in Applied Physics B

U. Eismann, F. Gerbier, C. Canalias, A. Zukauskas, G. Trénec, J. Vigué, F. Chevy, C. Salomon

Appl. Phys. B **106**, 25 (2012) — arXiv:1103.5841

We present an all-solid-state narrow-linewidth laser source emitting 670 mW output power at 671 nm delivered in a diffraction-limited beam. The source is based on a frequency-doubled diode-end-pumped ring laser operating on the 4F3/2→4I13/2 transition in Nd:YVO4. By using periodically poled potassium titanyl phosphate (ppKTP) in an external buildup cavity, doubling efficiencies of up to 86% are obtained. Tunability of the source over 100 GHz is accomplished. We demonstrate the suitability of this robust frequency-stabilized light source for laser cooling of lithium atoms. Finally, a simplified design based on intra-cavity doubling is described and first results are presented.

October 14th, 2011 — Our work `Photoassociative creation of ultracold heteronuclear 6Li40K* molecules’ gets published in Europhysics Letters

A. Ridinger, S. Chaudhuri, T. Salez, D. R. Fernandes, N. Bouloufa, O. Dulieu, C. Salomon and F. Chevy

EPL **96**, 33001 (2011) — arXiv:1108.0618

We investigate the formation of weakly bound, electronically excited, heteronuclear 6Li40K* molecules by single-photon photoassociation in a magneto-optical trap. We performed trap loss spectroscopy within a range of 325 GHz below the Li(2S1/2)+K(4P3/2) and Li(2S1/2)+K(4P1/2) asymptotic states and observed more than 60 resonances, which we identify as rovibrational levels of 7 of 8 attractive long-range molecular potentials. The long-range dispersion coefficients and rotational constants are derived. We find large molecule formation rates of up to ~3.5×107s− 1, which are shown to be comparable to those for homonuclear 40K2*. Using a theoretical model we infer decay rates to the deeply bound electronic ground-state vibrational level X1Σ+(v’=3) of ~5×104 s−1. Our results pave the way for the production of ultracold bosonic ground-state 6Li40K molecules which exhibit a large intrinsic permanent electric dipole moment.

September 27th, 2011 — Thomas Salez PhD Thesis Defense

Congratulations to Thomas for defending his PhD thesis entitled “Towards quantum degenerate atomic Fermi mixtures.”!

April 29th, 2011 — Armin Ridinger PhD Thesis Defense

Congratulations to Armin for defending his PhD thesis entitled “Towards quantum degenerate Fermi mixtures: Photoassociation of weakly bound 6Li40K molecules.”!

April 27th, 2011 — Our work `Large atom number dual-species magneto-optical trap for fermionic 6Li and 40K atoms’ published in The European Physical Journal D

A. Ridinger, S. Chaudhuri, T. Salez, U. Eismann, D. R. Fernandes, K. Magalhães, D. Wilkowski, C. Salomon, F. Chevy

Eur. Phys. J. D **65**, 223 (2011) — arXiv:1103.0637

We present the design, implementation and characterization of a dual-species magneto-optical trap (MOT) for fermionic ^{6}Li and ^{40}K atoms with large atom numbers. The MOT simultaneously contains 5.2 × 10^{9 }^{6}Li-atoms and 8.0 × 10^{9 }^{40}K-atoms, which are continuously loaded by a Zeeman slower for ^{6}Li and a 2D-MOT for ^{40}K. The atom sources induce capture rates of 1.2 × 10^{9 }^{6}Li-atoms/s and 1.4 × 10^{9 }^{40}K-atoms/s. Trap losses due to light-induced interspecies collisions of ∼65% were observed and could be minimized to ∼10% by using low magnetic field gradients and low light powers in the repumping light of both atomic species. The described system represents the starting point for the production of a large-atom number quantum degenerate Fermi-Fermi mixture.