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Evaporative cooling of atomic chromium

Jonathan D. Weinstein, Robert deCarvalho, Cindy Hancox, John M. Doyle

Published May 1, 2001• 0 citations
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Abstract

We report the magnetic trapping and evaporative cooling of bosonic and fermionic isotopes of atomic chromium. Using a cryogenic helium buffer gas, 10 1 2 chromium atoms are trapped at an initial temperature of ∼ 1 K. The chromium atoms are then cooled adiabatically and evaporatively to temperatures as low as ∼10 mK. Elastic and inelastic 5 2 Cr collisional cross sections are measured over this temperature range. Prospects for simultaneously creating a 5 2 Cr Bose-Einstein condensate and 5 3 Cr Fermi degenerate gas will he discussed.

Related papers

Evaporative cooling of atomic chromium

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We report the magnetic trapping and evaporative cooling of bosonic and fermionic isotopes of atomic chromium. Using a cryogenic helium buffer gas, ${10}^{12}$ chromium atoms are trapped at an initial temperature of \ensuremath{\sim}1 K. The chromium atoms are then cooled adiabatically and evaporatively to temperatures as low as \ensuremath{\sim}10 mK. Elastic and inelastic ${}^{52}\mathrm{Cr}$ collisional cross sections are measured over this temperature range. Prospects for simultaneously creating a ${}^{52}\mathrm{Cr}$ Bose-Einstein condensate and ${}^{53}\mathrm{Cr}$ Fermi degenerate gas will be discussed.

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Scattering properties of ultra-cold chromium atoms

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In this work a gas of ultra-cold chromium atoms in a magnetic trap has been prepared and its elastic and inelastic scattering properties have been investigated with regard to Bose-Einstein condensation. Bose-Einstein condensation of dilute atomic gases is achieved using different cooling and trapping techniques. Deviating from the standard way we were able to devise a continuous loading mechanism for a magnetic trap as a result of the spectroscopic properties and the large magnetic dipole moment of chromium. It not only allows us to trap more atoms but also facilitates the subsequent preparation steps. After loading, the magnetic trap is compressed to increase the atomic density. In doing so we also increases the temperature of the atomic cloud. Doppler cooling of the atoms in the compressed trap reduces the temperature and increases the density. Subsequent evaporative cooling further reduces the temperature. The efficiency of the cooling mechanism is determined by the elastic and inelastic scattering properties of atomic species. Ultra-cold collisions between ground state atoms are dominated by s-wave collisions. They are characterized by a single parameter, the scattering length a. We were able to determine the temperature dependence of the elastic collision rate for the two bosonic isotopes 52Cr and 50Cr in a relaxation experiment. Comparing our results with the effective range theory allowed us not only to extract the magnitude of the scattering length, but also its sign. The sign of the scattering length is important since only condensates with a positive scattering length are stable. Our efforts to achieve Bose-Einstein condensation in chromium by evaporative cooling in a magnetic trap resulted in a maximum phase space density of 0.04. Further cooling reduced the phase space density due to increased loss of atoms from dipolar relaxation. We were able to obtain preliminary results on the magnetic field dependence of dipolar relaxation. Our experimental data is in excellent agreement with theory. These experiments confirm the theoretical prediction that the dipolar relaxation rate is independent from the details of the interaction potential but rather scales with the magnetic offset field and the magnetic dipole moment of the atoms. The findings of this work mark an important step towards the realization of a Bose-Einstein condensate with chromium atoms. Especially the knowledge of the elastic and inelastic scattering properties allow now to devise a successful strategy.

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We have magnetically trapped and evaporatively cooled atomic molybdenum. Using a cryogenic helium buffer gas, $2\ifmmode\times\else\texttimes\fi{}{10}^{10}$ molybdenum atoms are loaded into a magnetic trap at an initial temperature of 500 mK. The molybdenum atoms undergo two-body decay with a measured inelastic rate constant of ${g}_{\mathrm{in}}=(1.2\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ for the initial conditions after loading. The molybdenum atoms are cooled by adiabatic expansion and forced evaporation to 200 mK.

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In the perspective of studying dipole-dipole interactions in quantum degenerate Bose and Fermi gases, we have built an experimental setup for cooling and trapping Chromium (Cr) atoms. In this respect, one needs to use appropriate technological solution, such as a high-temperature oven (∼ 1500◦C) and a laser system providing high laser power at 425nm. Our experiments allow us to obtain magneto-optical traps for the two main isotopes of Cr (i.e. the bosonic 52Cr and the fermionic 53Cr). These traps are characterized by large light-assisted collisional loss rates, which have been studied both experimentally and theoretically. The existence of radiative decay channels towards two long-lived metastable states allows the magnetic trapping and accumulation of fairly large atom numbers of cold Cr atoms. We have studied the possibility of modifying the shape of these traps with the use of RF magnetic fields, which allowed the study of collisional properties of Cr atoms in the metastable states. Finally, we have demonstrated a new continuous loading technique of an optical dipole trap with more than one million metastable 52Cr atoms, at 100μK. The transfer into the ground state and the polarization in the lowest-energy Zeeman sublevel opens the perspective for reaching the Bose-Einstein condensation of Cr through evaporative cooling.

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Temperature dependence of the cross-section for elastic ground-state collisions of /sup 52/Cr- and /sup 50/Cr-atoms using the method of cross-dimensional relaxation, respectively is determined. The corresponding absolute values of the scattering length are on the order of 100a/sub 0/ for /sup 52/Cr and /sup 50/a/sub 0/ for /sup 50/Cr, respectively. However, to use evaporative cooling to increase the phase space density and achieve Bose-Einstein condensation with chromium atoms, have been limited to values below 10/sup -2/. Unusuually high inelastic collisio rates due to dipolar relaxation cause atom loss and heating of the sample. These losses are investigated and found similar magnetic-field-dependent rate coefficients (/spl ap/10/sup -11/ cm/sup 3//s @ 50 G offset field) for the two isotopes /sup 50/Cr and /sup 52/Cr indicating that the strongly enhanced dipolar relaxation is a generic feature of the dipole-dipole interaction. This conclusion is supported by theoretical calculations. To circumvent these losses, an optical dipole trap is currently being set up. Here, chromium can be trapped in the energetically lowest state where dipolar relaxation is absent.

European Quantum Electronics Conference (EQEC)•Jan 1, 2003•0 citations

Magnetic trapping of an atomic Mn55−Cr52 mixture

Scott V. Nguyen, Joel S. Helton, Kenneth Maussang +2 more

Atomic manganese $^{55}\mathrm{Mn}$ and chromium $^{52}\mathrm{Cr}$ are simultaneously loaded and confined in a magnetic trap. Using a cryogenic $^{3}\mathrm{He}$ buffer gas, ${10}^{11}$ manganese and ${10}^{12}$ chromium atoms are trapped at an initial temperature of 600 mK. The buffer gas is then pumped away, thermally isolating the sample. The Mn-Cr interspecies inelastic rate constant is measured to be ${\ensuremath{\Gamma}}_{\mathrm{Mn},\mathrm{Cr}}=1.5(\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{3}∕\mathrm{s}$.

Physical Review A•Feb 18, 2005•10 citations
We report the magnetic trapping and evaporative cooling of bosonic and fermionic isotopes of atomic chromium. Using a cryogenic helium buffer gas, 10 1 2 chromium atoms are trapped at an initial temperature of ∼ 1 K. The chromium atoms are then cooled adiabatically and evaporatively to temperatures as low as ∼10 mK. Elastic and inelastic 5 2 Cr collisional cross sections are measured over this temperature range. Prospects for simultaneously creating a 5 2 Cr Bose-Einstein condensate and 5 3 Cr Fermi degenerate gas will he discussed.
We report the magnetic trapping and evaporative cooling of bosonic and fermionic isotopes of atomic chromium. Using a cryogenic helium buffer gas, 1012{10}^{12}1012 chromium atoms are trapped at an initial temperature of \ensuremath{\sim}1 K. The chromium atoms are then cooled adiabatically and evaporatively to temperatures as low as \ensuremath{\sim}10 mK. Elastic and inelastic 52Cr{}^{52}\mathrm{Cr}52Cr collisional cross sections are measured over this temperature range. Prospects for simultaneously creating a 52Cr{}^{52}\mathrm{Cr}52Cr Bose-Einstein condensate and 53Cr{}^{53}\mathrm{Cr}53Cr Fermi degenerate gas will be discussed.
In this work a gas of ultra-cold chromium atoms in a magnetic trap has been prepared and its elastic and inelastic scattering properties have been investigated with regard to Bose-Einstein condensation. Bose-Einstein condensation of dilute atomic gases is achieved using different cooling and trapping techniques. Deviating from the standard way we were able to devise a continuous loading mechanism for a magnetic trap as a result of the spectroscopic properties and the large magnetic dipole moment of chromium. It not only allows us to trap more atoms but also facilitates the subsequent preparation steps. After loading, the magnetic trap is compressed to increase the atomic density. In doing so we also increases the temperature of the atomic cloud. Doppler cooling of the atoms in the compressed trap reduces the temperature and increases the density. Subsequent evaporative cooling further reduces the temperature. The efficiency of the cooling mechanism is determined by the elastic and inelastic scattering properties of atomic species. Ultra-cold collisions between ground state atoms are dominated by s-wave collisions. They are characterized by a single parameter, the scattering length a. We were able to determine the temperature dependence of the elastic collision rate for the two bosonic isotopes 52Cr and 50Cr in a relaxation experiment. Comparing our results with the effective range theory allowed us not only to extract the magnitude of the scattering length, but also its sign. The sign of the scattering length is important since only condensates with a positive scattering length are stable. Our efforts to achieve Bose-Einstein condensation in chromium by evaporative cooling in a magnetic trap resulted in a maximum phase space density of 0.04. Further cooling reduced the phase space density due to increased loss of atoms from dipolar relaxation. We were able to obtain preliminary results on the magnetic field dependence of dipolar relaxation. Our experimental data is in excellent agreement with theory. These experiments confirm the theoretical prediction that the dipolar relaxation rate is independent from the details of the interaction potential but rather scales with the magnetic offset field and the magnetic dipole moment of the atoms. The findings of this work mark an important step towards the realization of a Bose-Einstein condensate with chromium atoms. Especially the knowledge of the elastic and inelastic scattering properties allow now to devise a successful strategy.
We have magnetically trapped and evaporatively cooled atomic molybdenum. Using a cryogenic helium buffer gas, $2\ifmmode\times\else\texttimes\fi{}{10}^{10}$ molybdenum atoms are loaded into a magnetic trap at an initial temperature of 500 mK. The molybdenum atoms undergo two-body decay with a measured inelastic rate constant of ${g}_{\mathrm{in}}=(1.2\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ for the initial conditions after loading. The molybdenum atoms are cooled by adiabatic expansion and forced evaporation to 200 mK.
In the perspective of studying dipole-dipole interactions in quantum degenerate Bose and Fermi gases, we have built an experimental setup for cooling and trapping Chromium (Cr) atoms. In this respect, one needs to use appropriate technological solution, such as a high-temperature oven (∼ 1500◦C) and a laser system providing high laser power at 425nm. Our experiments allow us to obtain magneto-optical traps for the two main isotopes of Cr (i.e. the bosonic 52Cr and the fermionic 53Cr). These traps are characterized by large light-assisted collisional loss rates, which have been studied both experimentally and theoretically. The existence of radiative decay channels towards two long-lived metastable states allows the magnetic trapping and accumulation of fairly large atom numbers of cold Cr atoms. We have studied the possibility of modifying the shape of these traps with the use of RF magnetic fields, which allowed the study of collisional properties of Cr atoms in the metastable states. Finally, we have demonstrated a new continuous loading technique of an optical dipole trap with more than one million metastable 52Cr atoms, at 100μK. The transfer into the ground state and the polarization in the lowest-energy Zeeman sublevel opens the perspective for reaching the Bose-Einstein condensation of Cr through evaporative cooling.
Temperature dependence of the cross-section for elastic ground-state collisions of /sup 52/Cr- and /sup 50/Cr-atoms using the method of cross-dimensional relaxation, respectively is determined. The corresponding absolute values of the scattering length are on the order of 100a/sub 0/ for /sup 52/Cr and /sup 50/a/sub 0/ for /sup 50/Cr, respectively. However, to use evaporative cooling to increase the phase space density and achieve Bose-Einstein condensation with chromium atoms, have been limited to values below 10/sup -2/. Unusuually high inelastic collisio rates due to dipolar relaxation cause atom loss and heating of the sample. These losses are investigated and found similar magnetic-field-dependent rate coefficients (/spl ap/10/sup -11/ cm/sup 3//s @ 50 G offset field) for the two isotopes /sup 50/Cr and /sup 52/Cr indicating that the strongly enhanced dipolar relaxation is a generic feature of the dipole-dipole interaction. This conclusion is supported by theoretical calculations. To circumvent these losses, an optical dipole trap is currently being set up. Here, chromium can be trapped in the energetically lowest state where dipolar relaxation is absent.
Atomic manganese 55Mn^{55}\mathrm{Mn}55Mn and chromium 52Cr^{52}\mathrm{Cr}52Cr are simultaneously loaded and confined in a magnetic trap. Using a cryogenic 3He^{3}\mathrm{He}3He buffer gas, 1011{10}^{11}1011 manganese and 1012{10}^{12}1012 chromium atoms are trapped at an initial temperature of 600 mK. The buffer gas is then pumped away, thermally isolating the sample. The Mn-Cr interspecies inelastic rate constant is measured to be ${\ensuremath{\Gamma}}_{\mathrm{Mn},\mathrm{Cr}}=1.5(\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{3}∕\mathrm{s}$.