arXiv subjects

J. M. McNamara

Publications and source records attributed to J. M. McNamara.

5 recordsLinked to original sources

arXiv2007

Heteronuclear ionizing collisions between laser-cooled metastable helium atoms

We have investigated cold ionizing heteronuclear collisions in dilute mixtures of metastable (2 3S1) 3He and 4He atoms, extending our previous work on the analogous homonuclear collisions [R. J. W. Stas et al., PRA 73, 032713 (2006)]. A simple theoretical model of such collisions enables us to calculate the heteronuclear ionization rate coefficient, for our quasi-unpolarized gas, in the absence of resonant light (T = 1.2 mK): K34(th) = 2.4*10^-10 cm^3/s. This calculation is supported by a measurement of K34 using magneto-optically trapped mixtures containing about 1*10^8 atoms of each species, K34(exp) = 2.5(8)*10^-10 cm^3/s. Theory and experiment show good agreement.

physics.atom-ph↗

arXiv2006

Production of a degenerate Fermi gas of metastable helium-3 atoms

We give an overview of the experiments at the Laser Centre of the Vrije Universiteit in Amsterdam on ultracold gases of metastable helium-4 and helium-3 as well as mixtures of both isotopes. We describe our experimental setup and discuss our experiments on Bose-Einstein condensation of 4He* (more than 10 million atoms in a BEC) and sympathetic cooling of 3He* towards Fermi degeneracy (more than 1 million atoms below the Fermi temperature as well as realization of a degenerate boson-fermion mixture). We also present our results on Penning Ionization in ~1 mK clouds containing a single isotope (homonuclear losses) or a mixture of both isotopes (heteronuclear losses).

cond-mat.other↗

arXiv2006

A Degenerate Bose-Fermi Mixture of Metastable Atoms

We report the observation of simultaneous quantum degeneracy in a dilute gaseous Bose-Fermi mixture of metastable atoms. Sympathetic cooling of helium-3 (fermion) by helium-4 (boson), both in the lowest triplet state, allows us to produce ensembles containing more than 10^6 atoms of each isotope at temperatures below 1 micro-Kelvin, and achieve a fermionic degeneracy parameter of T/Tf=0.45. Due to their high internal energy, the detection of individual metastable atoms with sub-nanosecond time resolution is possible, permitting the study of bosonic and fermionic quantum gases with unprecedented precision. This may lead to metastable helium becoming the mainstay of quantum atom optics.

cond-mat.stat-mech↗

arXiv2006

Homonuclear ionizing collisions of laser-cooled metastable helium atoms

We present a theoretical and experimental investigation of homonuclear ionizing collisions of laser-cooled metastable helium atoms, considering both the fermionic He-3 and bosonic He-4 isotope. The theoretical description combines quantum threshold behavior, Wigner's spin-conservation rule and quantum statistical symmetry requirements in a single-channel model, complementing a more complete close-coupling theory that has been reported for collisions of metastable He-4 atoms. The model is supported with measurements (in the absence of light fields) of ionization rates in magneto-optically trapped samples, that contain about 3*10^8 atoms of a single isotope. The ionization rates are determined from measurements of trap loss due to light-assisted collisions combined with comparative measurements of the ion production rate in the absence and presence of trapping light. Theory and experiment show good agreement.

cond-mat.other↗

arXiv2005

A Large Atom Number Metastable Helium Bose-Einstein Condensate

We have produced a Bose-Einstein condensate of metastable helium (4He*) containing over 1.5x10^7 atoms, which is a factor of 25 higher than previously achieved. The improved starting conditions for evaporative cooling are obtained by applying one-dimensional Doppler cooling inside a magnetic trap. The same technique is successfully used to cool the spin-polarized fermionic isotope (3He*), for which thermalizing collisions are highly suppressed. Our detection techniques include absorption imaging, time-of-flight measurements on a microchannel plate detector and ion counting to monitor the formation and decay of the condensate.

cond-mat.other↗