Probing of cold rubidium atoms with femtosecond pulses

IP-2014-09-7342 Optomehanika uzrokovana frekventnim češljem

Frequency-Comb-induced OptoMechanics (MeCombO)

Duration: September, 1st 2015 – August 31st, 2019. Budget: 989.400,00 HRK.


Project team:
Ticijana Ban (PI), Damir Aumiler, Neven Šantić, Ivor Krešić, Danijel Buhin – Institute of Physics, Zagreb, Croatia
Hrvoje Buljan - Departmen
t of Physics, Faculty of Science, University of Zagreb, Croatia
Aleksandra Foltynowicz - Department of Physics, Umeå University, Sweden
Roman CiuryłoPiotr Masłowski – Nicolaus Copernicus University, Torun, Poland

Our aim is is to explore novel phenomena that arise when the mechanical action on cold atoms is induced by frequency comb (FC) excitation.

October 2018. Domagoj Kovačić participated in Les Houches predoc school on Ultracold Fermions in France. He presented a poster about recent work in lab about Frequency comb laser cooling.

October 2018. Damir Aumiler, Danijel Buhin and Mateo Kruljac participated in the 11th meeting of Croatian physics society in Beli Manastir, Croatia. D. Aumiler had talk about Center for Advanced Laser Techniques, CALT. D. Buhin had a poster presentation about his recent work on laser cooling with frequency comb while M. Kruljac had poster presentation about his work on coherent effects in a cold atomis gas.

October 2018. Ticijana Ban and Ivor Krešić participated in the Workshop on Modern Aspects of Quantum Physics in Zagreb, Croatia. T. Ban participated as an invited speaker and she gave a talk “Cooling of atoms using  an optical frequency comb” about the recent work in the lab, and I. Krešić gave talk about his work on “Spontaneous light-mediated magnetism of cold atoms in the transverse plane”.

September 2018. Danijel Buhin, Domagoj Kovačić and Mateo Kruljac participated on the Frontiers of Matter Wave optics summer school and conference in Crete, Greece. They won the grant from the organizers that covered their expenses. Scope of the school and the conference was atom interferometry, quantum technologies and sensors. D. Buhin presented recent work on the Frequency comb cooling, D. Kovačić on the supercontinuum generation and usage of FC as an optical ruler and M. Kruljac on his work on the coherent effects in a cold atomic gas. They visited the Institute of electronic strucure and laser, FORTH. During this visit they participated on the “Intellectual properties rights” lecture.


July 2018. Ticijana Ban participated in the 26th International Conference on Atomic Physics, ICAP 2018 in Barcelona, Spain. She presented our work on the Frequency comb cooling in a poster presentation.

July 2018. Mateo Kruljac defended his Master thesis “Coherent effects in a cold atomic gas“. At the top left figure there is a lambda configuration used for modeling and measurements. Bottom left figure show time evolution of populations of a lambda system. Right figure shows frequency dependence of the force for different parameters – upper part shows measurements and lower part modeled values.



June 2018. Domagoj Kovačić participated in Bridging experiment and theory in precision spectroscopy, 4th MOLIM Training School in Torun, Poland. He presented work on the Frequency comb cooling of rubidium atoms.

June 2018. Danijel Buhin participated in 24th Young Atom Opticians conference hosted by the University of Strathclyde in Glasgow, Scotland. He presented work on the CW laser cooling in the low intensity regime in a poster presentation.

March 2018. We started to build an optical cavity which will be installed in a vacuum chamber and locked to a Frequency comb.


March 2018. Domagoj Kovačić joined MeCombO team as a PhD student.

February 2018. Neven Šantić defended his PhD thesis “Synthetic Lorentz force for neutral cold atoms“. In his thesis he presented his work on the first and second generation of the MOT experiment in Zagreb, Synthetic Lorentz force in an expanding cold atomic gas and cooling of atoms using a stabilized Frequency comb.

October 2017. Danijel Buhin participated in the Les Houches predoc school on Cold atoms and quantum transport in France. He had a poster presentation about Frequency comb optomechanics in cold atoms.

September 2017. Domagoj Kovačić defended his Master’s thesis “Supercontinuum generation in nonlinear optical fiber“.
At the top left side there is picture of photonic-crystal fiber (PCF) used for supercontinuum generation. Figure at the top right side presents spectral evolution simulation of laser pulse due to self-phase modulation (SPM). Bottom figures presents measured and simulated output spectrum from PCF.


September 2017. Danijel Buhin joined MeCombO team as a PhD student.



September, 2017. Postdoc researcher Ivor Krešić participated in the conference PHOTONICA 2017 held in Belgrade, Serbia with a poster about frequency comb laser cooling of 87Rb atoms below Doppler limit. Ivor also had a Progress report talk about pattern formation in cold atoms, which featured work done during his PhD studies at Strathclyde University in Glasgow, Scotland.



July 2017. D. Buhin, A. Cipriš and D. Kovačić participated in Introductory Course on Quantum Information school in Innsbruck, Austria.

July 2017. Danijel Buhin defended his Master’s thesis “Femtosecond laser optomechanics in cold atoms“.
In the left figure there is  presented displacement of cold rubidium atoms cloud center of mass due to force from one femtosecond laser beam. At the top right figure there is presented a) train of femtosecond pulses in time domain and b) their Fourier transform – Frequency comb in frequency domain. A scan of force vs. detuning is presented on the right figure. These measurements represent femtosecond spectroscopy of 87Rb cloud.



July 2017. A. Cipriš and D. Kovačić participated in New Frontiers in Optical Trapping and Optical Manipulation summer school at Barcelona, Spain, where they presented their posters.


June 2017. Ana Cipriš defended her Master’s thesis “Frequency comb laser cooling“.
At the top left side there are picutes of femtosecond laser used in experiment. Bottom left figure presents sub-Doppler cooling with femtosecond laser interaction. Marked numbers correspond to columns in¸the right figure. (1) frequency far from resonance, (2) frequency with minimum temperature, (3) frequency with maximum temperature.


May 2017. New article has been published: “Synthetic Lorentz force in an expanding cold atomic gas“.

In figure left is scheme for obtaining the synthetic Lorentz force and the resulting force patterns in velocity space.
Measurements (b,e,h) and calculation (c,f,i) of atom density distributions after τ = 4 ms of expansion time for three experimental configurations (a,d,g) are presented on right figure.

clanak1         josab_figure

March 2017. Ivor Krešić will join MeCombO team as a potdoc student.

November 2016. New coils for compensating stray magnetic fields are installed. There are three independent coils pairs for compensating three directions of magnetic field.

September 2016. Neven Šantić has joined MeCombO team as a PhD student.

July 2016. Open PhD position announced. The PhD position will be funded by a Croatian Science Foundation, Career development of young researchers program. PhD student will work on the MeCombO project, with the goal to study  the mechanical action on cold rubidium atoms due to excitation by femtosecond pulses in various experimental geometries.


June 2016. Cold cloud of rubidium atoms is observed. Sub-Doppler temperature of 39 µK is measured using TOF technique.


March 2016. New chamber and locked FC are ready for the force measurement.


February 2016. Piotr Maslowski visited Institute of Physics, Zagreb and participated in the frequency stabilization of Er:dopped frequency comb. Offset frequency, f0, is stabilized using optical heterodyne detection, by mixing the frequency comb with a polarization spectroscopy stabilized cw laser (Moglabs cateye ECDL).  The beat signal FWHM is approximately 60 kHz.


December 2015. Using Rb atomic clock (FS725 Rubidium Frequency Standard) and DDS board (Novatech Instruments, Inc.) repetition frequency of our free running Er:doped frequency comb is locked.


November 2015. Damir Aumiler visited Piotr Maslowski at Nicolaus Copernicus University, Torun, Poland, where he took training in the frequency comb stabilization.


October 2015. New optical table is installed in the lab for Cold Atoms @ Institute of Physics, Zagreb, MeCombO started.



A new project grant – Frequency-Comb-induced OptoMechanics (MeCombO) – funded by the Croatian Science Foundation (hrzz) has been awarded. The project starts on 1st Septembehrzzr 2015 and will last four years. Project team consists of: Ticijana Ban (PI), Damir Aumiler, Nataša Vujičić – Institute of Physics, Zagreb, Croatia; Hrvoje Buljan - Department of Physics, Faculty of Science, University of Zagreb, Croatia; Aleksandra FoltynowiczDepartment of Physics, Umeå University, Sweden; Roman Ciuryło, Piotr Masłowski – Nicolaus Copernicus University, Torun, Poland. One PhD and one post-doc student will be employed on the project.
Project summary:
The aim of the project is to explore novel phenomena that arise when the mechanical action on cold atoms is induced by frequency comb (FC) excitation, including entanglement and decoherence, near-resonance dipole trap, and frequency-comb-induced cavity cooling.
MecomboInducing mechanical action on atoms by continuous wave (cw) laser excitation has been a central topic in atomic physics for decades. Laser cooling and trapping of atoms, a technique that enables production of dense (ultra)cold atomic samples, was made available by employing radiation pressure force. Furthermore, atomic self-organization and collective cavity cooling have been demonstrated in cavity-optomechanical systems with cold atomic ensembles. Yet, in contrast to the rich and fruitful field of cw-laser-induced optomechanics, the investigations of mechanical action on atoms due to interaction with trains of ultrashort laser pulses (i.e. frequency combs) are scarce in literature. Meanwhile, recent results made in our group have indicated that in a specific excitation geometry an entirely new type of FC-atom interaction emerges that calls out for further experimental and theoretical investigation.


Frequency comb induced radiative force on cold rubidium atoms

by G. Kregar, N. Šantić, D. Aumiler, H. Buljan, and T. Ban

The measurements of the radiative force of a femtosecond laser on a cold atomic cloud in an out-of-phase counterpropagating configuration are intriguing and challenge previous models. A potential reason for the discrepancy is that the boundary between quantum and classical worlds is not sharp but rather something like many shades of grey. The coupling of internal (quantum) and center-of-mass (classical) degrees of freedom, which is ubiquitous in any model calculating the laser force in atomic systems, is shown to be nontrivial here. It could depend on how quantum entanglement between internal and center-of-mass coordinates decays, which should be taken into account when we transfer information from the quantum to the classical world.

Quite surprisingly, we point out that a very simple experimental setup could potentially probe the boundary between quantum and classical, and explore fundamental phenomena such as decoherence.