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Ultracold Fermi Gas Experimental Group

Laser and evaporative cooling of neutral atoms has paved the way for studies of quantum degenerate gases in which the bosonic or fermionic nature of the particles determines the behaviour. The importance of this field has been recognised by the award of two recent Nobel prizes in physics in 1997 for laser cooling and trapping and in 2001 for the first production and studies of Bose-Einstein Condensates (BECs).

In the cold molecules group at Swinburne, we cool gases of fermionic Li-6 down to temperatures below 100nK and, by tuning the interactions between particles, we can prepare a degenerate system of bosonic molecules or fermionic atoms. The ability to control the interparticle interactions through the use of Feshbach resonance allows unique access to different types of superfluid which span from a BEC of bound molecules to a degenerate Fermi gas of correlated Cooper pairs.

Our goal is to use these cold gases to understand the physical processes at play in these different types of superfluids.

Congratulations to Dr Eva Kuhnle for winning the Bragg Gold Medal for the best Australian PhD in Physics 2012!!

 



The team

Academic and Research Staff

Chris Vale
Peter Hannaford
Sascha Hoinka
Paul Dyke
Mark Kivenen (Technical support)

Students

Tyson Peppler (PhD)
Kristian Fenech (PhD)
Marcus Lingham (PhD)


Former Members

Dr. Eva Kuhnle (PhD)
Dr. Gopi Veeravalli (PhD)
Adj. Prof. Bryan Dalton

Dr. Wayne Rowlands
Dr. Grainne Duffy (postdoc)
Dr. Jurgen Fuchs (PhD)
Dr. Heath Kitson (PhD)
Dr. David Lau (postdoc)
Dr. Chris Ticknor (postdoc)

Dr. Michael Mark
(postoc)
Michael Vanner (R & D)

Li experimental group images (click to enlarge)


Research Highlights

Local pair condensation

We have developed a technique to obtain local measurements of homogeneous parameters in a harmonically trapped quantum gas and used this to make in situ observations of pair condensation in a unitary Fermi gas.

Paper: M. G. Lingham et al., Phys. Rev. Lett. 112 100404 (2014)

 

Precision measurement of the universal contact

We have made a high precision measurement of Tan's universal contact parameter and compared this with the latest QMC results. Our data have now reached a level of accuracy that allows us to distinguish between several of the established theoretical predictions.

Paper: S. Hoinka et al., Phys. Rev. Lett. 110, 055305 (2013)

 

Spin Bragg spectroscopy

A new type of Bragg spectroscopy is presented in which we measure the dynamic spin susceptibility of a strongly interacting Fermi gas. This allows independent measurement of the spin-parallel and spin-antiparallel response functions which become universal at high energy.

Paper: S. Hoinka et al., Phys. Rev. Lett. 109, 050403 (2012)

 

Bragg spectroscopic studies of the Contact parameter

Here we present a detailed review of the contact in strongly interacting Fermi gases using Bragg spectroscopy. Both the interaction and temperature dependence are studied and two methods for Bragg spectrocscopy are described.

Paper: E. Kuhnle et al., New J. Phys. 13, 055010 (2011)

 

Temperature dependence of the Contact parameter

The contact is a recently introduced parameter to characterise universal properties of strongly interacting quantum gases. In a Fermi gas, the contact depends on the strength of the interparticle interactions and the temperature relative to the Fermi temperature. In this paper we present the first measurements of the temperature dependence of the contact in a unitary Fermi gas.

Paper: E. Kuhnle et al., Phys. Rev. Lett. 106, 170402 (2011)

 

Crossover from 2D to 3D

A weakly interacting Fermi gas is prepared in an oblate trapping potential to study the 2D-3D dimensional crossover. Shell structure, associated with the filling of discrete transverse states becomes apparent in the density profile of the gas.

Paper: P. Dyke et al., Phys. Rev. Lett. 106, 105304 (2011)

 

Dynamic structure factor (Exp vs Theory)

The dynamic structure factor of a strongly interacting Fermi gas was measured and compared to theoretical calculations based on the Random Phase Approximation (RPA). Surprisingly, these show very good agreement with experimental findings.

Paper: P. Zou et al., Phys. Rev .A. 82, 061605 (2010)

Universal pairing in a strongly interacting Fermi gas

Pairing in strongly interacting Fermi gases is shown to follow Tan's universal law and is verified through the measurement of the static structure factor using Bragg spectroscopy.

Paper: E. Kuhnle et al., Phys. Rev. Lett. 105, 070402 (2010)

Molecular BEC

Bragg spectroscopy of a strongly interacting Fermi gas

We have performed a comprehensive study of the Bose-Einstein condensate to Bardeen-Cooper-Schrieffer (BEC-BCS) crossover in fermionic 6Li using Bragg spectroscopy. A smooth transition from molecular to atomic spectra was observed with a clear signature of pairing at and above unitarity. These spectra probe the dynamic and static structure factors of the gas and provide a direct link to two-body correlations. We have characterised these correlations and measured their density dependence across the broad Feshbach resonance at 834 G.

Paper: G. Veeravalli et al., Phys. Rev. Lett. 101, 250403 (2008) arXiv:0809.2145

BEC, degenerate Fermi gas

p-wave Feshbach Molecules

We have measured the binding energies of 6Li2 p-wave Feshbach molecules using magneto-association spectroscopy. These allow us to determine the magnetic moment of the molecules and our values are in good agreement with theoretical predictions. We have also investigated the size of p-wave molecules which we find to be much smaller than s-wave molecules even within a few mG of the Feshbach resonance.

Paper: J. Fuchs et al., Phys. Rev. A 77, 053616 (2008).

Molecular BEC

BEC of 6Li2 Molecules

We have produced Bose-Einstein condensates of Li-6 molecules in a low power crossed optical dipole trap. Our experimental procedures for generating condensates and degenerate Fermi Gases are described in the paper below. More recently we have introduced a 100 W fibre laser to our experiments, now we can produce near pure molecular condensates containing over 200,000 molecules.

Paper: J Fuchs et al., J.Phys.B 40 4109 (2007)


BEC, degenerate Fermi gas

Degenerate Fermi Gas of 6Li

With our system we can readily compare degenerate Fermi gases with Bosonic molecular gases. The image to the left shows a gas of bosonic molecules and Fermionic atoms prepared in the same trap at the same temperature, but at different magnetic fields. Much higher densities are observed in the bosonic cloud due to the formation of a BEC while Fermi pressure decreases the atomic density of the fermionic gas to below what would be expected for a classical gas at the same temperature.

EIA

EIT and EIA in 6Li

We have also performed measurements of electromagnetically induced transparency and absorption using a Li-6 atomic beam line.

Papers:
J. Fuchs et al., J. Phys. B: At. Mol. Opt. Phys. 40, 1117-1129 (2007)
J. Fuchs et al., J. Phys. B: At. Mol. Opt. Phys. 39, 3479–3489 (2006)

RbK

Scattering of Polar Molecules (Theory)

Polar molecules represent an exciting new direction in ultra-cold gas research. Currently we have a theoretical program investigating the scattering properties of polar molecules which will form a basis for understanding the many body physics of dipolar quatum gases.

Papers:
C. Ticknor, Phys. Rev. Lett. 100, 133202 (2008)
C. Ticknor, Phys. Rev. A 76, 052703 (2007)



Grants

ARC DECRA, Dr Paul Dyke (2014-2016)
ARC Discovery Project, "Collective dynamics in Fermi superfluids" (2013-2016)
ARC Future Fellowship, A/Prof Chris Vale (2012-2016)
ARC Centre of Excellence for Quantum-Atom Optics, project "Molecular BEC" (2003-2010)
ARC Australian Postdoctoral Fellowship (2008-2010) Dr. C. Ticknor
ARC LIEF grant "Advanced microwave facility for quantum atom optics" (2006)
ARC LIEF grant "Quatum limited single atom detectors" (2008)


Research Podcast
Towards Absoltue Zero on an Atom Chip · Windows Media Player · QuickTime