OEAW   Start Prize   IQOQI


Creating the first strontium quantum gases (2008 - 2013)

In September 2009, our research group created the first strontium Bose-Einstein condensate (BEC) in the world. Now we are exploring the exciting new fields of research opened up by this break-through.

Strontium is an alkaline-earth element and has two electrons in its outer shell. This gives it many unique properties not existing in conventional alkali systems. Those comprise a ground state free of electronic magnetic moment, the existence of metastable states, and optical transitions with small linewidth. These properties have been used since a few years to built optical clocks and are at the heart of recent proposals in fields as diverse as the variation of fundamental constants, mHz linewidth lasers, and quantum simulation and computation. Our goal is to create and investigate novel quantum systems that are beyond the reach of alkali quantum gases. The guiding topics of our research will be quantum computation and quantum simulation of many-body systems. The fundamental idea behind quantum computation with Sr is that the fermionic isotope 87Sr has a nuclear spin, which can be used to store quantum information in a well-protected way. At the same time, the complex electronic structure of Sr allows to manipulate the information.

The interest in quantum simulation of lattice many-body systems comes from the fact that those systems are very difficult or even impossible to describe using classical computers. Even very fundamental and relevant models have not yet been solved. A quantum simulator is a special task quantum computer that emulates the physics of the system of interest. Cold atom systems can serve as quantum simulators since they are very well controlled. Strontium with its unique properties enables the simulation of systems not accessible with the simpler alkali atoms used so far.

We will also study mixtures of strontium with rubidium with the goal to create RbSr ground state molecules. Heteronuclear bi-alkali ground state molecules possess a permanent electric dipole moment, which gives rise to directional, long-range interactions. They are currently a hot topic and have just recently been produced for the first time. The difference between bi-alkali and alkali/alkaline-earth ground state molecules is that the latter possess an unpaired outer shell electron. This provides them with a magnetic dipole moment in addition to the electric dipole moment. The properties of the molecules can thus be tuned with electric and magnetic fields. This can for example be used to engineer spin-dependent, tunable, long-range interactions, which can be used for quantum simulation. These research avenues for quantum-degenerate strontium gases are very rich and promise new insights into physics ranging from molecules over novel quantum computation approaches to quantum many-body systems.


The achievements

Bose-Einstein Condensation of Strontium

In September 2009, just 17 months after starting the project in an empty lab, we were the first to attain Bose-Einstein condensation of strontium. We used the 84Sr isotope, which has a low natural abundance but offers excellent scattering properties for evaporative cooling. We obtained pure condensates containing 1.5 x 105 atoms, which puts 84Sr in a prime position for future experiments on quantum-degenerate gases of atomic two-electron systems. more info


The phase-transition from a thermal sample to a pure BEC of 84Sr.


Double-degenerate Bose-Fermi Mixture of Strontium

87Sr is the only fermionic isotope of strontium and the only isotope possessing a nuclear spin, which is at the heart of nearly all schemes of quantum computation and simulation with strontium. We have prepared 87Sr in a single internal state and cooled it to quantum degeneracy using 84Sr. more info


A pure BEC of 84Sr and a Fermi sea of 87Sr 15 ms after release from the trap.


Bose-Einstein condensation of 86Sr

86Sr has a large scattering length of +800 Bohr, leading to strong three-body loss. We show that it is all the same possible to perform evaporative cooling and obtain a BEC. Arrow more info


86Sr BEC phase transition image

Evaporative cooling from a thermal sample to a pure BEC of 86Sr.


Detection and manipulation of nuclear spin states in fermionic strontium

87Sr has a large nuclear spin, which has many applications in quantum simulation and computation. We detect and manipulate the spin-state distribution, as required for those applications. Arrow more info


optical Stern-Gerlach separation

The ten mF states of 87Sr after optical Stern-Gerlach state separation.


Mott-insulator of 84Sr

We have observed the superfluid to Mott-insulator transition with 84Sr, by loading a BEC into an optical lattice. In quantum simulations, the optical lattice simulates the crystaline lattice of a solid.


Mott insulator transition

The superfluid to Mott-insulator transition (click image to enlarge). A BEC (left, back) is subjected to an optical lattice of increasing depth. For small lattice depths, diffraction peaks appear. For large depths, the Mott-insulater phase is reached: atoms are localized on lattice sites and coherence is lost (front). The process is adiabatic and a BEC reappears when reducing the lattice depth again (right, back).


Creation of ultracold Sr2 molecules in the electronic ground state

We have created Sr2 molecules from pairs of atoms on the sites of an optical lattice. Our work demonstrates a new method for the creation of ultracold molecules that can be applied to species that do not possess a suitable magnetic Feshbach resonance, necessary for the traditional magnetoassociation approach. Arrow more info


Sr2 molecules after dissociation

Sr2 molecules are detected by conversion into repulsively bound atom pairs, which fly appart in opposite directions on this image.


Production of quantum-degenerate strontium gases: Larger, better, faster, colder

We have improved our scheme to generate Bose-Einstein condensates and degenerate Fermi gases of strontium. This scheme allows us to create quantum gases with higher atom number, a shorter time of the experimental cycle, or deeper quantum degeneracy than before. Arrow more info


84Sr monster BEC

Phase transition from a thermal gas to a 84Sr BEC with 107 atoms.


Laser cooling to quantum degeneracy

We are able to create Bose-Einstein condensates (BEC) using laser cooling as the only cooling mechanism, not relying on evaporation. This work follows one of the early goals of laser cooling, and might find applications in the construction of a continuous atom laser. Arrow more info


optical BEC

Absorption images of atomic Sr clouds released from a sophisticated trapping potential. The BEC is formed only in a central region of the trap, where the density is increased by a dimple, and where atoms are protected from photon scattering. Atoms in the dimple are in thermal contact with a surrounding reservoir, which is constantly laser-cooled to dissipate heat. The BEC forms after 60ms of hold in this trapping geometry (center image), and imaging only the dimple region lets the BEC stand out clearly (right image).


Quantum degenerate mixtures of strontium and rubidium atoms

We have created Sr-Rb double BECs. A crucial stage in our scheme is sympathetic narrow-line laser cooling of Rb by Sr. These quantum gas mixtures constitute an important step towards the production of a quantum gas of polar, open-shell RbSr molecules. Arrow more info


Sr-Rb double BEC

Time-of-flight absorption images of 84Sr and 87Rb during evaporative cooling.


Reservoir spectroscopy of repumping transitions in strontium

We performed extensive studies on two repumping transitions in strontium, using light at 403 and 497 nm, respectively. We employ a scheme named "reservoir spectroscopy", which we use to determine the absolute frequencies of 70 individual lines. Arrow more info


Sr repump spectra

A typical spectroscopy scan, showing the resonances of the three bosonic isotopes (left), as well as the hyperfine structure of the fermionic isotope (right).


Further reading


The Team

Simon Stellmer, Meng Khoon Tey, Bo Huang, Florian Vogl, Mark Parigger, Alex Bayerle, Slava Tzanova, Benjamin Pasquiou, Rudolf Grimm and Florian Schreck



This project was hosted by the Austrian Academy of Sciences at the Institute for Quantum Optics and Quantum Information. Funding was provided by a START prize of the FWF and the BMWF and the iSense FET-Open grant of the European Commission.


last change: 22.04.2015 by FS
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