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 Research



 

 


Quantum Atom Optics - Theory

The new interdisciplinary field of quantum-atom optics (QAO) has formed at the intersection of atomic, molecular and optical physics, condensed matter physics, and computational physics. The field is driven by an unprecedented level of experimental control of degenerate Bose and Fermi systems that have a well-defined theoretical basis in quantum many-body theory. This leads to tests of theory that were previously unavailable, and thus the development of fundamental knowledge.

In the theory core of the ARC Centre of Excellence for Quantum-Atom Optics (ACQAO) we pursue cutting-edge developments in quantum many-body physics that lead to our growing understanding of QAO systems. We approach these systems from a microscopic perspective, focusing on how to generate, manipulate and measure many-body correlations.

Our motivating scientific questions are:

  • How are the many-body states of QAO systems best characterised, and
    how is the formation of these states to be understood as a non-equilibrium,
    dynamical process?
  • What kinds of entanglement and many-body correlations can be generated
    in QAO systems, and in what ways can they be efficiently detected?
  • How do QAO systems test fundamental predictions of quantum mechanics
    and many-body theory?
  • What new theoretical and computational methods must be developed to
    provide quantitative answers to these questions?

We provide ideas, simulations and advanced quantitative models for leading experimental groups in Australia and worldwide. As an outcome of ACQAO experimental and theoretical work, we expect practical tools that utilize many-body quantum behaviour of QAO systems, e.g. continuously pumped atom laser, sources of entangled atoms, and matter-wave interferometers for precision measurements.

Our research program is structured around five themes:

1) Atom lasers and formation of quantum degenerate gases

2) Coherent manipulations of matter waves

3) Quantum statistics and pairing correlations in ultracold Bose and Fermi systems

4) Macroscopic correlations, entanglement and fundamental tests

5) Computational physics and theoretical methods for Bose and Fermi systems


Recent project reports

2010

2009

 

 

 

Quantum Atom Optics - Experimental Projects

We currently have several active experimental research projects in the atom optics group, and these are described in brief detail below.  Our primary area of interest in the past few years has been on Bose-Einstein condensation (BEC), and the development of time averaged optical potentials for novel trapping geometries. Particular projects include:

  1. Bose-Einstein condensation on an atom chip and versatile optical traps.
  2. All-optical BEC experiment
  3. Neutral atom quantum processor (NEW PROJECT)

 

Bose-Einstein condensation on an atom chip and versatile optical traps.

Our experiment creates and manipulate Bose-Einstein condensates (BECs) of 87Rb in magnetic traps formed above current carrying wires on an atom chip (see Figure 1). We first form a magnetic optical trap (MOT) of Rb, with ~ 3 ×107 atoms. These are then transferred to the magnetic trap formed by the wires on our chip surface. RF evaporative cooling is then used to cool the atoms below the transition temperature to BEC. With this apparatus, BEC was observed for the first time at UQ on the morning of Friday 20th February 2004.

UQ atom chip
Figure 1. The UQ atom chip for BEC. The chip surface is 125 μm silver foil glued onto a ceramic substrate with epoxy. The silver wires were machined using a CNC mill.
Absorption images of BEC transition
Figure 2. False colour absorption images of the atom cloud after 10 ms of ballistic expansion a) Thermal cloud just above Tc, N=60,000 atoms. (b) Transition to BEC with N=40,000 atoms at Tc=250 nK. (c) Almost pure condensate, N=20,000.

Recent Work

With this setup, we have recently been exploring a number of interesting trapping geometries for BEC, including the dimple trap and the scanning beam trap.

Dimple trap

The dimple trap is a dual trapping geometry, consisting of our magnetic trap, and an overlapping light sheet that traps atoms using the optical dipole force. The light sheet is formed by a laser operating at 840 nm, that is focused in one dimension using a cylindrical lens. The wavelength is chosen to be far detuned from the laser cooling transitions at 780 nm. The optical dipole force arises from the interaction between the induced electric dipole moment of the atoms and the gradient in the light field intensity. For red detuned beams, the atoms are drawn to the maximum intensity of the light field, which is simply the focal point of the light sheet. The light sheet then acts as a "dimple" in the harmonic magnetic trap potential.

Using the dimple trap, we have been looking at the dynamics of BEC condensation. With our magnetic trap, we first use RF evaporative cooling to the cloud to just above the critical temperature for BEC formation. The light sheet is then switched on, and we observe a transition to BEC without further cooling, which is a response to the sudden increase in localized phase space density given by the dimple potential. We have recently published this work in Phys. Rev. A 83, 013630 (2011)


Figure 3. a) Dimple potential, formed form overlapping cigar shaped magnetic potential and dipole trap light sheet. (b) Orientation of dipole trap and magnetic trap (MT) to atom chip surface.


Scanning beam trap
scanning beam trap profile
Figure 4. Image and cross section of atoms in a line potential. The flat region in the cross section is about 80μm in width, with ~ 3% fluctuation in density.

An area of great theoretical and expeimental interest is the study of superfluid behaviour in BEC. A defining feature of superfluidity is the that it has zero viscocity (frictionless flow). The original analysis by Landau [1] for known superfluids, such as liquid helium, predicted the observation of a sharp onset of excitations when a microscopic impurity or obstacle was dragged through the superfluid above a critical velocity. For a dilute gas BEC, this analysis predicts a critical velocity equal to the speed of sound in the condensate.

To date, there have been a number of observations of the critical velocity in a BEC. Raman et al [2] used a blue detuned laser beam, which they scanned through the condensate, with the blue laser beam analogous to a microscopic obstacle dragged through the superfluid. They measured a critical velocity of 1.6 mm/s, a factor of 4 lower than that expected by the critical velocity being equal to the speed of sound in the condensate. With the magnetic trapping potential used, howerver, there was the added problem that the condensate does not have a uniform density, meaning that the local speed of sound varied across the condensate, making a reliable measurement of the critical velocity difficult.

We have been investigating new methods that can provide a uniform trapping potential. The method we employ consists of scanning a red detuned dipole beam in 1D using the RF shift on a acousto optical modulator (AOM), with additional confinement of the cloud using a light sheet. If the scanning dipole beam is scanned at a rate faster than the trapping frequencies of the atoms, then the atoms experience the time average of the light potential, a so called "time averaged potential" [3]. By careful feedback to the intensity of the scanning beam, we can create a uniform potential. This is then used to study the superfluid properties of the trapped BEC.

In order to measure the critical velocity, we will reduce the intensity at a particular point in the scan, generating a barrier, and drag this barrier through the BEC at varying speed, measuring the resulting BEC fraction and temperature for evidence of frictional excitation. The advantages of this method include a very homogeneous potential, which in turn leads to a uniform speed of sound in the experiment and a constant speed of the obstacle that is dragged through the condensate. We are at the preliminary stages of this work, but have successfully demonstrated a line trap (see Figure 4 below).

 

 

All-optical BEC experiment

All optical traps for BEC [4] have many advantages over those of conventional magnetic trap geometries, including simplicity of the setup, insensitivity to the spin state, and high BEC production rate. Furthermore, evaporative cooling is achieved by simply reducing the intensity of the trapping dipole beams.

We have built a second setup to produce an all-optical 87Rb Bose-Einstein condensate experiment. Our system starts with a standard magneto optical trap (MOT) loaded in our vacuum chamber from a rubidium dispenser. After a compression and optical molasses stage, the atoms are loaded into a single beam dipole trap overlapping the MOT, formed by a laser operating at 1064 nm, with a beam waist of 27 μm and power of 14W. Our initial conditions in the single dipole trap are ~3×106 atoms, with a temperature of ~250 μK. A first stage of evaporative cooling is performed by lowering the trapping intensity in the single beam. A secondary orthogonal beam is then ramped up in power, compressing the cloud and increasing the collision rate, before both beams are lowered in intensity.

all optical BEC setup
Figure 5. All optical BEC setup.

Absorption images of dipole trap
Figure 6. Absorption images of 87Rb loaded into the single beam and crossed beam dipole traps

 

Current and Future Work

Ring traps using time averaged optical potentials

Ring traps are an interesting geometry for ultra cold atoms, ideal for studies of superfluidity and persistent flow, the study of topological defects in BEC, and atom interferometry. We propose to use an all optical method to form our ring trap, using time averaged optical potentials. By spatially scanning a focused far detuned laser beam with a 2D AOM, at a frequency much higher than the atoms can respond to (ie their trapping frequency), a 2D ring geometry can be created [3]. Confinement in the perpendicular plane can be achieved with a light sheet (see Figure 7).eration and entanglement mediated by Rydberg interactions and can be used as a universal quantum processor or simulator [6, 7]. The setup is shown in Figure 8 belowƶ㯓Ƶ됀

ring trap using time averaged optical potentials
Figure 7. Ring trap using time averaged optical potentials [3]. The dipole trap beam is scanned with a 2D AOM onto a light sheet formed by far detuned laser, confining the atoms in a ring geometry.

 

Neutral atom quantum processor (NEW PROJECT)

We propose to build a manipulable array of tens of single-atom dipole traps with full control over the depth, position and the internal state of a single atom at each individual trap. This will be accomplished by spatially scanning a single laser dipole potential at high speed in a lattice pattern, resulting in a time-averaged lattice potential. The system allows for additional laser beams to facilitate coherent transitions for single qubit operation and Rydberg state excitation. The versatility of the system allows for single atom preparation, relatively fast gate operation and entanglement mediated by Rydberg interactions and can be used as a universal quantum processor or simulator [6, 7].

Lattice trap for quantum processing
Figure 8. Setup for a time-averaged lattice potential. The trapping light is combined with two phase-locked Raman laser (Ω1 and Ω2) and deflected by a 2D AOM. The laser beams are focussed by an aspherical f-theta lens. The entire 2D lattice is overlapped with a UV laser to facilitate excitations to Rydberg states.

 

References:

[1] I. M. Khalatnikov, translated by P.C. Hohenberg. An introduction to the theory of superfluidity. W. A. Benjamin, Inc, 1965.

[2] C. Raman, M. Köhl, R. Onofrio, D. S. Durfee, C. E. Kuklewicz, Z. Hadzibabic, and W. Ketterle. Phys. Rev. Lett., 83(13), 2502–2505, 1999.

[3] S.K. Schnelle, E.D. van Ooijen, M.J, Davis, N.R. Heckenberg, H. Rubinsztein-Dunlop, Optics Express, Vol. 16, Issue 3, 1405-1412 (2008)

[4] M. Barrett, J. Sauer, M. Chapman, Phys. Rev. Lett. 87 10404 (2001)

[5] K. J. Arnold, M.D Barrett, Optics Communications 284, 13, 3288-3291(2011)

[6] Jaksch et al , Phys. Rev. Lett. 85, 2208-2221 (2000)

[7] T. Wilk et al, Phys. Rev. Lett. 104, 010502 (2010)
Authorised by: Karen Kheruntsyan
Maintained by: Paul Schwenn (schwennphysics.uq.edu.au)
Last Updated - 8 June, 2011