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Ultracold Bosonic Gases

In our laboratory we produce some of coldest atomic ensembles (temperatures below 100 nK above absolute zero) and focus on precision measurements of coherence, quantum noise and atomic interactions in Bose-Einstein condensates of dilute atomic gases. In particular, we study how interference of matter waves is affected by quantum noise and atomic interactions, how coherence can survive and be re-created and how ultracold molecules can be associated in Bose-Einstein condensates. In a new development we plan to use ultracold atoms for simulating the early Universe inflation during the Big Bang.


The team

Academic and Research Staff
Andrei Sidorov
Peter Hannaford
Brenton Hall
Russell McLean
Bryan Dalton

Mark Kivenen (Technical support)
Students
Alessandro Brolis (PhD)
Campbell Biggs (Honours)


Former students
Dr Russell Anderson
Dr Michael Egorov
Dr Valentin Ivannikov
Dr Iurii Mordovin
Dr Bogdan Opanchuk
Dr Falk Scharnberg
Dr Shannon Whitlock
Dr Holger Wolff
Simon Cunningham (Honours)
Stephen McDonald (Honours)
Michael Pullen (Honours)
Alan Leigh (R&D)
Tim Mapperson (R&D)
Sven Teichmann
Sung Jong Park (Postdoc)

3D Figure 1 PRA

Research
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The false vacuum decay: towards realization with ultra-cold atoms

Quantum decay of a relativistic scalar field from a false vacuum is relevant to the inflationary model of the early Universe where the nucleation and the exponential growth of bubbles of the true vacuum gives rise to the creation of matter. We propose a realistic test of this model using the pseudo-spin 1/2 bosonic gas. The relative phase of two spin components serves as a scalar quantum field and the modulating RF coupling creates the metastable state of the false vacuum. Using the stochastic numerical simulations we clearly demonstrate the seeding and growth of the bubbles in the relative phase domain.

Paper: O. Fialko et al, Europhysics Letters 110, 56001 (2015) arXiv: 1408.1163

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Radio-frequency induced association and spectroscopy of cold 87Rb2 molecules

We employ powerful RF radiation to couple atomic scattering states |1, -1> and |2,+1> in 87Rb to several weakly bound molecular states. Using the simultaneous imaging of two states we monitor the appearance of molecular resonance by detecting the sudden loss of the atoms in state |2,+1> when RF frequency is tuned into the resonance. We discover five families of molecular states by monitoring their Zeeman dependence. The conversion efficiency can reach 90% for the RF field of 4 G and an interaction time of 420 ms

3D Figure 1 PRA

Precision measurement of s-wave scattering lengths in a two-component Bose-Einstein condensate

We use collective oscillations of a two-component Bose-Einstein condensate of 87Rb atoms prepared in the internal states |1> = |1,-1> and |2> = |2, 1> for the precision measurement of the interspecies scattering length a12 with a relative uncertainty of 1.6 x 10-4. By fitting numerical simulations of the coupled Gross-Pitaevskii equations to the recorded temporal evolution of the axial width we obtain the values a12 = 98.006a0 and a22 = 95.44a0. We characterize two-body losses for component 2 and obtain the loss coefficients γ12 = 1.51 x 10-14 cm3/s and γ22 = 8.1 x 10-14 cm3/s.

Paper: M. Egorov et al, Phys. Rev. A 87, 053614 (2013)
Paper: M.Egorov et al, Phys. Rev. A 84, 021605(R) (2011)

3D Figure 1 PRA

Quantum noise in three-dimensional BEC interferometry

We develop a theory of quantum fluctuations and squeezing in a three-dimensional Bose-Einstein condensate atom interferometer with nonlinear losses. We use stochastic equations in a truncated Wigner representation to treat quantum noise. Our approach includes the multi-mode spatial evolution of spinor components and describes the many-body dynamics of a mesoscopic quantum system.

Paper: B. Opanchuk et al, Europhysics Letters, 97, 50003 (2012)

3D Figure 1 PRA

Long-lived periodic revivals of coherence in an interacting Bose-Einstein condensate

We observe the coherence of an interacting two-component Bose-Einstein condensate (BEC) surviving for seconds in a trapped Ramsey interferometer. Mean-field-driven collective oscillations of two components lead to periodic dephasing and rephasing of condensate wave functions with a slow decay of the interference fringe visibility. We apply spin echo synchronous with the self-rephasing of the condensate to reduce the influence of state-dependent atom losses, significantly enhancing the visibility up to 0.75 at the evolution time of 1.5 s. Mean-field theory consistently predicts higher visibility than experimentally observed values. We quantify the effects of classical and quantum noise and infer a coherence time of 2.8 s for a trapped condensate of 55,000 interacting atoms.

Paper: M.Egorov et al Phys. Rev. A 84, 021605(R) (2011)

   
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Radio-frequency dressing of multiple Feshbach resonances

We demonstrate and theoretically analyze the dressing of several proximate Feshbach resonances in 87Rb using radio-frequency (rf) radiation. We present accurate measurements and characterizations of the resonances, and the dramatic changes in scattering properties that can arise through the rf dressing. Our scattering theory analysis yields quantitative agreement with the experimental data. We also present a simple interpretation of our results in terms of rf-coupled bound states interacting with the collision threshold.

Paper: A. M. Kaufman et al, Phys. Rev. A 80, 050701(R) (2009)

absorption images

Spatially inhomogeneous phase evolution of a two-component Bose-Einstein condensate

We investigate the spatial evolution of the relative phase of an elongated two-component Bose-Einstein condensate. The pseudo-spin 1/2 system is comprised of the F = 1; mF = -1 and F = 2; mF = 1 hyperfine ground states of 87Rb , which we magnetically trap and interrogate with radio-frequency and microwave fields. By performing Ramsey interferometry with the BEC we probe the relative phase evolution, and observe temporal decay of the interferometric contrast which is well described by a mean-field formalism. We show that this decay is due to inhomogeneous growth of the collective relative phase, where decoherence and phase diffusion are not dominant mechanisms in the loss of interferometric contrast

Paper: R. P. Anderson et al, Phys. Rev. A 80, 023603 (2009)

laser ablated grooves

Fabricating atom chips with femtosecond laser ablation

We fabricate atom chips using femtosecond laser ablation to sculpt conductive metal films and permanent magnetic materials. The ablation process is investigated by extracting the power spectral density of the edge roughness from composite SEM images and by using a MR-microscope. A sculptured wire which produces arrays of tunable double wells is fabricated for near-surface force sensing with a BEC.

Paper: C. H. Wolff et al, J. Phys. B: At. Mol. Opt. Phys. 42, 085301 (2009), Europhysics News Highlights 40/3 2009:

MR microscope schematic

Scanning magnetoresistance microscopy of atom chips

Surface based geometries of microfabricated wires or patterned magnetic films can be used to magnetically trap and manipulate ultracold neutral atoms or Bose-Einstein condensates. We have investigated the magnetic properties of such atom chips using a newly developed scanning magnetoresistive MR microscope with offers high spatial resolution (10μm) and high field sensitivity (100nT). This provides a convenient and powerful technique for precisely characterizing the magnetic fields near the surface of atom chips in atmosphere.

Paper: M. Volk et al, Rev. Sci. Instrum. 79, 023702 (2008)

double well sensor

Double well sensor of asymmetry in trapping potentials

Adiabatic splitting of a rubidium condensate in an asymmetric double-well potential leads to an unequal distribution of atoms in the wells. Precise measurements of the number of atoms in each well provide information about an asymmetrical component of the trapping potential. This non-interferometric method can be used for the construction of a new sensor which is sensitive to small-scale variations of a gravitational force.

Paper: B.V. Hall et al, Phys. Rev. Lett. 98, 030402 (2007)

multiple becs

Disordered potential and multiple BECs

We have found that cold rubidium clouds are fragmented above the magnetic film atom chip. Inhomogeneity in the film magnetisation generates a corrugation term in the axial trapping potential. Using a combination of RF spectroscopy, optical imaging and the truncated Boltzmann distribution we qualitatively characterised spatial variations of the disordered magnetic field. Our model produces analytical expressions consistent with the observations and predicts a y-2 decay of the random field amplitude. Ten independent condensates were produced via evaporative cooling of a highly elongated cloud.

Paper: S. Whitlock et al, Phys. Rev. A 75, 043602 (2007)

atom chip

Bose-Einstein condensation on a magnetic film atom chip

We have developed a hybrid atom chip comprising a magnetic film TbGdFeCo with perpendicular magnetisation and current-carrying silver wires.  This design allows us to combine the stability of permanent magnetic fields with versatility of time-variable magnetic fields.  We observe the heating rate of 3 nK/s when the atoms are confined in the magnetic film trap.  We produce a Bose condensate of 200,000 rubidium atoms using either a Z-shaped current or the magnetic film trap.

Papers:
B.V. Hall et al, Journal of Physics B 39, 27 (2006)         (Highlights of J. Physics B for 2006)
B.V. Hall et al, Laser Spectroscopy XVII, 275 (2005) 
S. Whitlock et al, Australian Physics 43, No.1, 8 (2006)

hysteresis loop

Magnetic films for atom optics

Permanent magnetic materials can produce sophisticated magnetic field configurations and offer an alternative to current-carrying wires for the construction of miniature optical elements on atom chips. Gd10Tb6Fe80Co4 magneto-optical films have large perpendicular magnetic anisotropy and are suitable for the production of periodically grooved, micron-scale structures. We studied the deposition process and properties of GdTbFeCo films that make them suitable for the applications on the atom chips.

Paper: J. Wang et al , Journal of Physics D 38, 4015 (2005)

double well interferometer

Asymmetric potential in double-well atom interferometry

We examine the effect of asymmetry on a splitting process in a double-well interferometer. The interferometer involves a measurement of the first excited state population as a sensitive measure of the asymmetric potential. The Bloch vector model and multi-mode numerical simulations account for the effect of asymmetry and the role of adiabaticity throughout the interferometric process and allow appropriate time scales to be chosenfor the splitting and merging stages.

Paper: A.I. Sidorov et al, Phys. Rev A 74, 023612 (2006)

two mode bec

Theory of decoherence in BEC interferometry

A theory of BEC interferometry in an unsymmetrical double-well trap has been developed for small boson numbers, based on the two-mode approximation. The bosons are initially in the lowest mode of a single well trap, which is split into a double well and then recombined. Possible fragmentations into separate BEC states in each well during the splitting and recombination processes are allowed for. Self-consistent sets of equations for the amplitudes of the fragmented states and for the two single boson mode functions are obtained. The latter are coupled Gross-Pitaevskii equations. Interferometric effects may be measured via boson numbers in the first excited mode.
A full treatment of decoherence and dephasing effects in BEC interferometry is being developed, based on using quantum correlation functions for describing interferometric effects. The BEC is described via a phase space distribution functional of the Wigner type for the condensate modes and the positive P type for the non-condensate modes. Ito equations for stochastic condensate and non-condensate field functions replace the functional Fokker-Planck equation for the distribution functional and stochastic averages of field function products determine the quantum correlation functions.

Papers: B.J. Dalton, J. Mod. Opt. 54, 615 (2007); J. Phys: Conference Series 67 012059 (2007)


Grants
Swinburne Strategic Initiative grant “Atom Optics” (2001-2005)
ARC Discovery-Project grant “Integrated Atom Optics” (2002)
ARC Centre of Excellence for Quantum-Atom Optics, project “Coherence of BEC on an atom chip” (2003-2010)
ARC LIEF grant “Advanced microwave facility for quantum atom optics (2006)”
ARC LIEF grant “Quantum limited single atom detectors (2008)"

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