RbCs Project: Publications

Dual-species Bose-Einstein condensate of 87Rb and 133Cs
D.J. McCarron, H.W. Cho, D.L. Jenkin, M.P. Koeppinger and S.L. Cornish
Phys. Rev. A 84 011603 (2011); arXiv:1102.1576
We report the formation of a dual-species Bose-Einstein condensate of 87Rb and 133Cs in the same trapping potential. Our method exploits the efficient sympathetic cooling of 133Cs via elastic collisions with 87Rb, initially in a magnetic quadrupole trap and subsequently in a levitated optical trap. The two condensates each contain up to 2x104 atoms and exhibit a striking phase separation, revealing the mixture to be immiscible due to strong repulsive interspecies interactions. Sacrificing all the 87Rb during the cooling, we create single-species 133Cs condensates of up to 6x104 atoms.

A high phase-space density mixture of 87Rb and 133Cs: towards ultracold heteronuclear molecules.
H.W. Cho, D.J. McCarron, D.L. Jenkin, M.P. Koeppinger and S.L. Cornish
Eur. Phys. J. D (DOI: 10.1140/epjd/e2011-10716-1); arXiv1107.5767
We report the production of a high phase-space density mixture of 87Rb and 133Cs atoms in a levitated crossed optical dipole trap as the first step towards the creation of ultracold RbCs molecules via magneto-association. We present a simple and robust experimental setup designed for the sympathetic cooling of 133Cs via interspecies elastic collisions with 87Rb. Working with the |m_F = 1,m_F = +1> and the |3, +3> states of 87Rb and 133Cs respectively, we measure a high interspecies three-body inelastic collision rate ~10-25-10-26 cm6s-1 which hinders the sympathetic cooling. Nevertheless by careful tailoring of the evaporation we can produce phase-space densities near quantum degeneracy for both species simultaneously. In addition we report the observation of an interspecies Feshbach resonance at 181.7(5) G and demonstrate the creation of Cs2 molecules via magneto-association on the 4g(4) resonance at 19.8 G. These results represent important steps towards the creation of ultracold RbCs molecules in our apparatus.

Bose-Einstein condensation of 87Rb in a levitated crossed dipole trap
D.L. Jenkin, D.J. McCarron, M.P. Koeppinger, H.W. Cho, S.A. Hopkins and S.L. Cornish
Eur. Phys. J. D (DOI: 10.1140/epjd/e2011-10720-5); arXiv:1104.0495
We report an apparatus and method capable of producing Bose-Einstein condensates (BECs) of ~1x106 87Rb atoms, and ultimately designed for sympathetic cooling of 133Cs and the creation of ultracold RbCs molecules. The method combines several elements: i) the large recapture of a magnetic quadrupole trap from a magneto-optical trap, ii) efficient forced RF evaporation in such a magnetic trap, iii) the gain in phase-space density obtained when loading the magnetically trapped atoms into a far red-detuned optical dipole trap and iv) efficient evaporation to BEC within the dipole trap. We demonstrate that the system is capable of sympathetically cooling the |F = 1, m_F = -1> and |1,0> sublevels with |1, +1> atoms. Finally we discuss the applicability of the method to sympathetic cooling of 133Cs with 87Rb.

Magnetic trapping of an ultracold Rb-Cs atomic mixture
P. Tierney
PhD Thesis (2009)
This thesis reports on the realisation and characterisation of a magnetically trapped ultracold atomic mixture of 87Rb and 133Cs in the F = 1, mF = -1 and F = 3, mF = -3 hyperfine states respectively. A compact two-species double magneto-optical trapping (MOT) apparatus is constructed in which a pyramid MOT acts to provide an independentflux of both atomic species for capture in the ultra-high vacuum science region of the apparatus. For the two-species science MOT in which this atom flux is captured, interspecies light assisted inelastic collisions are found to be a highly signicant loss mechanism. A novel optical pressure spatial displacement technique is developed to minimise such losses, allowing near independent simultaneous loading of up to 8 x 10^8 87Rb and 3 x 10^8 133Cs atoms into an Ioffe-Pritchard `baseball' magnetic trap at magnetic bias fields of 166.70(6) and 165.50(6) G respectively. At the loaded 87Rb and 133Cs atom number densities the magnetic trap lifetime of each atomic species is shown to be 100(10)s and independent of the presence of the second atomic species. Radio-frequency evaporative cooling trajectories for 87Rb and 133Cs are separately optimised under single species magnetic trap operation to achieve phase-space densities of 6(1) x 10^-7 and 3(1) x 10^-4 respectively at temperatures of 7.6(1) microK and 520(10) nK. 133Cs Feshbach resonances at 118.06(8) and 133.4(1) G are characterised through the measurement of magnetic field dependent losses at the increased phase-space density. Implementation of simultaneous evaporative cooling following the single species trajectories is found to be ineffective below 10 microK due to the increased thermal load imposed upon the 133Cs atoms as the 87Rb single species elastic collision cross section approaches the low energy limit. Following simultaneous evaporation to 15 microK thermalisation of the mixtures axial and radial temperature components suggests a 87Rb- 133Cs interspecies elastic collision rate 3(1) and 7(1) times greater than the calculated single species 133Cs and 87Rb elastic collision rates respectively. An interspecies Feshbach resonance search is undertaken by measuring the number of atoms of each species remaining in the magnetic trap as a function of applied magnetic field following simultaneous evaporation. The absence of magnetic field dependent losses in conjunction with analysis of the measurement sensitivity demonstrates that no interspecies Feshbach resonances wider than 1 G with two-body inelastic collision rate constants greater than 5 x 10^-10 cm^3s^-1 are present over the magnetic field range 166 < B < 370 G in the trapped states. The sensitivity of this measurement is found to be highly dependent upon the magnetic field induced differential gravitational sag of the mixtures components.

From atoms to molecules (and back)
S. L. Cornish
Physics 1, 24 (2008)
Atoms colliding in a magnetic field can form weakly bound states called Feshbach molecules. These states have now been used in combination with advanced laser techniques to create tightly bound ground-state molecules close to quantum degeneracy.

Modulation transfer spectroscopy in atomic rubidium
D. J. McCarron, S. A. King and S. L. Cornish
Meas. Sci. Technol. 19 105601 (2008)
We report modulation transfer spectroscopy on the D2 transitions in 85Rb and 87Rb using a simple home-built electro-optic modulator (EOM). We show that both the gradient and amplitude of modulation transfer spectroscopy signals, for the 87Rb F = 2 to F' = 3 and the 85Rb F = 3 to F' = 4 transitions, can be significantly enhanced by expanding the beams, improving the signals for laser frequency stabilization. The signal gradient for these transitions is increased by a factor of 3 and the peak to peak amplitude was increased by a factor of 2. The modulation transfer signal for the 85Rb F = 2 to F' transitions is also presented to highlight how this technique can generate a single, clear line for laser frequency stabilization even in cases where there are a number of closely spaced hyperfine transitions.

Realisation of a cold mixture of Rubidium and Caesium
M.L. Harris
PhD Thesis (2008)
This thesis describes a new apparatus designed to study cold, ultracold, and quantum degenerate mixtures of rubidium and caesium atoms. The Rb- Cs mixture is prepared using a double magneto-optical trap (MOT) system in which a two-species pyramid MOT acts as a source of cold atoms for a science MOT. The first results of experiments on the magneto-optically trapped mixture are presented, including measurements of trap loss rates due to single-species and interspecies inelastic collisions. A technique for reducing interspecies loss by spatially separating the MOTs during loading is described. This technique allows 50-50 mixtures of Rb and Cs atoms to be loaded into a magnetic trap at close to their respective maximum singlespecies atom numbers. Alternatively, one species can be loaded with arbitrarily small amounts of the other. The displaced MOT technique is thus an excellent starting point for investigations of interspecies Feshbach resonances and sympathetic cooling of Rb-Cs mixtures in magnetic and optical traps. In addition, a model of polarisation spectroscopy based on numerical integration of population rate equations is described. Theoretical polarisation spectra generated by the model are shown to agree with experimental spectra for the F = I + 1/2 to F' transitions in Rb and Cs. An investigation of the sub-Doppler dichroic atomic vapour laser locking (DAVLL) technique demonstrates how locking signals can be optimised for the Rb D2 transitions. The role of polarisation purity in generating the spectra is discussed, and impurities are modeled using a Jones matrix approach. Comparisons with polarisation spectroscopy and DAVLL are used to enhance understanding of atom-light interactions in spectroscopic systems, and indicate methods for optimising locking signals for use in cold atom experiments.

Magnetic trapping of a cold Rb-Cs atomic mixture
M.L. Harris, P. Tierney and S. L. Cornish
J. Phys. B 41, 035303 (2008)
We present an apparatus for the study of an ultracold gaseous atomic mixture of 133Cs and 87Rb. The mixture is prepared using a double magneto-optical trap (MOT) system in which a two-species pyramid MOT acts as a source of cold atoms for a 'science' MOT. Measurements of the interspecies trap loss rate coefficients beta_RbCs and beta_CsRb in the science MOT are reported. After the initial MOT phase, atoms in the mixture are optically pumped into the magnetically trappable |F = 3, mF = -3 > and |F = 1, mF = -1> states of Cs and Rb (respectively) and loaded into an Ioffe–Pritchard magnetic trap. We demonstrate a novel technique for limiting the interspecies loss rate in the science MOT by spatially separating the two trapped atom clouds, which greatly enhances the number of atoms which can be loaded into the magnetic trap.

A heated vapor cell unit for dichroic atomic vapor laser lock in atomic rubidium
Danny J. McCarron, Ifan G. Hughes, Patrick Tierney and Simon L. Cornish
Rev Sci Inst 78, 093106 (2007)
The design and performance of a compact heated vapor cell unit for realizing a dichroic atomic vapor laser lock (DAVLL) for the D2 transitions in atomic rubidium is described. A 5 cm long vapor cell is placed in a double-solenoid arrangement to produce the required magnetic field; the heat from the solenoid is used to increase the vapor pressure and correspondingly the DAVLL signal. We have characterized experimentally the dependence of important features of the DAVLL signal on magnetic field and cell temperature. For the weaker transitions both the amplitude and gradient of the signal are increased by an order of magnitude.

DAVLL lineshapes in atomic rubidium
Alfred Millett-Sikking, Ifan G. Hughes, Patrick Tierney and Simon L. Cornish
J. Phys. B 40, 187-198 (2007)
A comprehensive study of the dichroic atomic vapour laser lock (DAVLL) lineshapes for the D2 transitions in atomic rubidium is presented. All four Doppler-broadened transitions (87Rb F = 1 to F ', 87Rb F = 2 to F ', 85Rb F = 2 to F ' and 85Rb F = 3 to F ') have been studied. We have characterized experimentally the dependence of the DAVLL lineshape on magnetic field, angle of the quarter-wave plate and cell temperature. We discuss the sensitivity of the spectra to these parameters, and highlight optimal operating parameters for use of DAVLL to generate laser frequency discriminant signals (laser "locking").

Polarization spectroscopy in rubidium and cesium
M.L. Harris, C.S. Adams, S.L. Cornish, I.C. McLeod, E. Tarleton, I.G. Hughes
Phys. Rev. A 73, 062509 (2006)
We develop a theoretical treatment of polarization spectroscopy and use it to make predictions about the general form of polarization spectra in the alkali-metal atoms. Using our model, we generate theoretical spectra for the D2 transitions in 87Rb, 85Rb, and 133Cs. Experiments demonstrate that the model accurately reproduces spectra of transitions from the upper hyperfine level of the ground state only. Among these, the closed transition dominates, with a steep gradient through line center ideally suited for use as a reference in laser locking.