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arXiv:1412.0782 · New Journal of Physics

Sympathetic cooling of molecular ion motion to the ground state

René Rugango, J E Goeders, T H Dixon, J M Gray, Ncamiso B. Khanyile, G Shu, R J Clark, Kenneth R. Brown

Published Mar 12, 2015• 42 citations
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Abstract

We demonstrate sympathetic sideband cooling of a 40 CaH + molecular ion co-trapped with a 40 Ca + atomic ion in a linear Paul trap. Both axial modes of the two-ion chain are simultaneously cooled to near the ground state of motion. The center of mass mode is cooled to an average quanta of harmonic motion , corresponding to a temperature of . The breathing mode is cooled to , corresponding to a temperature of .

DOI
10.1088/1367-2630/17/3/035009

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We demonstrate sympathetic sideband cooling of a 40 CaH + molecular ion co-trapped with a 40 Ca + atomic ion in a linear Paul trap. Both axial modes of the two-ion chain are simultaneously cooled to near the ground state of motion. The center of mass mode is cooled to an average quanta of harmonic motion , corresponding to a temperature of . The breathing mode is cooled to , corresponding to a temperature of .
We demonstrate sympathetic Doppler cooling and our progress towards sideband cooling of a CaH+^{+}+ ion co-trapped with a Ca+^{+}+ atomic ion in a linear Paul trap. Molecular ions are generally difficult to laser cool due to a lack of closed electronic transitions as a result of vibrational and rotational states. Despite this challenge, they can be cooled indirectly through their Coulombic interaction with a fluorescent atomic ion that is being directly laser cooled. Ions are firstly Doppler cooled to get to the Lamb-Dicke regime, where the ion motion is small relative to the excitation wavelength and then sideband cooled reaching temperatures below 1 μ\muμK. All the ions' axial modes (center of mass and breathing mode) and radial modes ( two center of mass and two tilt modes) are addressed, and the temperature is determined by examining the ratio of sidebands.
Abstract We achieve the sympathetic sideband cooling of a 40 Ca + – 27 Al + pair. Both axial modes of the two-ion chain are cooled simultaneously to near the ground state of the motion. The center of mass mode is cooled to an average phonon number of 0.052(9), and the breathing mode is cooled to 0.035(6). The heating rates of both a single 40 Ca + and the 40 Ca + – 27 Al + pair are measured and compared. This work is a fundamental step toward the implementation of a 40 Ca + – 27 Al + quantum logic clock.
Control of the external degree of freedom of trapped molecular ions is essential for their promising applications to spectroscopy, precision measurements of fundamental constants, and quantum information technology. Here, we demonstrate near ground-state cooling of the axial motional modes of a calcium mono-oxide ion via sympathetic sideband cooling with a cotrapped calcium ion. We also show that the phonon state of the axial out-of-phase mode of the ion chain is maintained while the mode frequency is adiabatically ramped up and/or down. The adiabatic ramping of the motional mode frequency is a prerequisite for searching for the proposed molecular dipole-phonon interaction [W. C. Campbell and E. R. Hudson, Phys. Rev. Lett. 125, 120501 (2020); M. Mills et al., Phys. Chem. Chem. Phys. 22, 24964 (2020)].
We have recently demonstrated the laser cooling of a single 40Ca+ ion to the motional ground state in a Penning trap using the resolved-sideband cooling technique on the electric quadrupole transition S12↔ D52. Here we report on the extension of this technique to small ion Coulomb crystals made of two or three 40Ca+ ions. Efficient cooling of the axial motion is achieved outside the Lamb-Dicke regime on a two-ion string along the magnetic field axis as well as on two- and three-ion planar crystals. Complex sideband cooling sequences are required in order to cool both axial degrees of freedom simultaneously. We measure a mean excitation after cooling of n¯COM=0.30(4) for the centre of mass (COM) mode and n¯B=0.07(3) for the breathing mode of the two-ion string with corresponding heating rates of 11(2) s-1 and 1(1)s-1 at a trap frequency of 162 kHz. The occupation of the ground state of the axial modes (ntilt=nCOM=0) is above 75% for the two-ion planar crystal and the associated heating rates 0.8(5) s-1 at a trap frequency of 355 kHz.
We have cooled a two-ion crystal to the ground-state of its collective modes of motion. Laser cooling, more specifically resolved sideband cooling, is performed sympathetically by illuminating only one of the two 40 Ca + ions in the crystal. The heating rates of the motional modes of the crystal in our linear trap have been measured, and we found them considerably smaller than those previously reported by Turchette et al (2000 Phys.Rev. A 61 063418) in the case of trapped 9 Be + ions. After the ground state is prepared, coherent quantum state manipulation of the atomic population can be performed. Up to 12 Rabi oscillations are observed, showing that many coherent manipulations can be achieved. Coherent excitation of each ion individually and ground state cooling are important tools for the realization of quantum information processing in ion traps.
We use electromagnetically-induced-transparency laser cooling to cool motional modes of a linear ion chain. As a demonstration, we apply electromagnetically-induced-transparency cooling on 24Mg+^{24}\mathrm{Mg}^{+}24Mg+ ions to cool the axial modes of a $^{9}\mathrm{Be}^{+}\mathrm{\text{\ensuremath{-}}}^{24}\mathrm{Mg}^{+}$ ion pair and a $^{9}\mathrm{Be}^{+}\mathrm{\text{\ensuremath{-}}}^{24}\mathrm{Mg}^{+}\mathrm{\text{\ensuremath{-}}}^{24}\mathrm{Mg}^{+}\mathrm{\text{\ensuremath{-}}}^{9}\mathrm{Be}^{+}$ ion chain, thereby sympathetically cooling the 9Be+^{9}\mathrm{Be}^{+}9Be+ ions. Compared to previous implementations of conventional Raman sideband cooling, we achieve approximately an order-of-magnitude reduction in the duration required to cool the modes to near the ground state and significant reduction in required laser intensity.