VERS UNE MEMOIRE QUANTIQUE AVEC DES IONS PIEGES

tel-00430795, version 1 - 9 Nov 2009
122 CHAPITRE 5. CHARGEMENT DU PIÈGE
Photoionisation loading of large Sr + ion clouds with ultrafast pulses. 5
1000 trapped ions. In fact, with TPPI an optimum of
4 × 10 4 trapped Sr + ions is obtained for Vrf = 125 V.
In the case of EB it is impossible to explore the same
range for the parameter Vrf without affecting the sample
purity, since too many spurious ionic species, of mass
lower than Sr, are produced and trapped at low Vrf .
This effect is clearly visible in the mass spectra of Figure
6b obtained with the two techniques in an intermediate
regime (Vrf = 350 V). Therefore, in our case of a noncrystalized
regime, the possibility to work at a low Vrf
is crucial in order to obtain a large number of trapped
ions. We interpret this result in terms of rf-heating process
that increases with Vrf [30]. In this situation, and
in the presence of laser-cooling, the ion-density depends
on the balance between cooling and heating. A maximum
density is then achievable by minimizing Vrf (i.e.
the rf heating) within the stability region. But the total
number of trapped ions also depends on the volume
of the trap that is imposed by Vrf and Vec. Numerical
simulations carried-out using Simion R○ software[26] allowed
us to estimate the trap volume as a function of
Vrf for Vec = 500 V. An optimum (maximum) volume
is obtained for Vrf = 180 V, slightly larger than the
experimental value that maximizes the total number of
trapped ions. Let us note that for low-temperature samples
(e.g. in crystallized regime) the ion density increases
by increasing Vrf , provided that the rf-heating remains
low enough to prevent the melting of the crystal.
We also observed that, when working in the regime of
relatively high Vrf , that is optimal for EB (Vrf = 500V ),
1100±5% ions can be trapped by EB to be compared to
1200 ± 5% ions obtained by photoionisation. This small
discrepancy might be explained by space charge effects
due to electrons that can perturb the trap electrostatic
potential. Let us also finally remark that the selectivity
of the TPPI technique allowed us to load pure Sr + clouds
in the trap at very low oven temperatures (on the order
of 120 ◦ C), contrary to the case of EB (on the order
of 165 ◦ C). As mentioned above, this lower temperature
implies a vapour pressure reduction of several orders of
magnitude.
4 Conclusion
We demonstrated the loading of Sr + in a linear Paul trap
using two-photon absorption of ultrafast pulses centered
at 431 nm. We compared this technique to the electron
bombardment loading and observed several advantages,
already mentioned in previous experiments concerning
other species or other photo-excitation paths. In particular
this technique allowed us to selectively load pure
Sr + clouds, to explore trapping regimes with low RF
voltages, to obtain large cooled-ion clouds, and to improve
the vacuum quality by lowering the power in the
Sr oven. Concerning this last point, it has been possible
to reduce the expected Sr partial pressure near the
oven filament by roughly 4 orders of magnitude. Our results
have been obtained with a home-made femtosecond
source that delivers relatively low energy pulses.
We expect an improvement of two orders of magnitudes
in the photoionization rate in the case of commercially
available Ti:Sa oscillators delivering routinely 10 nJ per
pulse. Let us finally mention that the trapping of up to
4 × 10 4 cooled Sr + ions is an important step towards the
realization of an ion-based quantum memory[31].
5 Acknowledgments
We thank P. Lepert for technical support. The authors
would also like to thank M. Joffre for the loan of the femtosecond
oscillator. This work was supported by ANR
“jeunes chercheuses et jeunes chercheurs” research contract
JC05 61454.
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