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arXiv:physics/0412053 · Physical Review Letters

Sympathetic Cooling of He+4 Ions in a Radio-Frequency Trap

Bernhard Roth, U. Fröhlich, Stephan Schiller

Published Feb 7, 2005• 46 citations
arXivHTMLPDF

Abstract

We have generated Coulomb crystals of ultracold $^{4}\mathrm{H}\mathrm{e}^{+}$ ions in a linear radio-frequency trap, by sympathetic cooling via laser-cooled $^{9}\mathrm{B}\mathrm{e}^{+}$. Stable crystals containing up to $150$ localized ${\mathrm{H}\mathrm{e}}^{+}$ ions at $\ensuremath{\sim}20\text{ }\text{ }\mathrm{m}\mathrm{K}$ were obtained. Ensembles or single ultracold ${\mathrm{H}\mathrm{e}}^{+}$ ions open up interesting perspectives for performing precision tests of QED and measurements of nuclear radii. This Letter also indicates the feasibility of cooling and crystallizing highly charged atomic ions using $^{9}\mathrm{B}\mathrm{e}^{+}$ as coolant.

DOI
10.1103/physrevlett.94.053001

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In this work, the setup of an experiment aimed at high-precision laser spectroscopy of ultracold molecular hydrogen ions, started at the University of Konstanz in 1999 [1,2], was continued and first results were obtained. In particular, a new linear rf trap and a new trap supply electronics were built, the latter being capable of driving the linear rf trap with a maximum rf amplitude of > 1000V at a frequency of 14.2MHz. This made possible the simultaneous trapping of a large mass range (2u For laser cooling of Be+ ions, doubly resonant sum-frequency generation (DR-SFG) of 313nm laser light was performed between a resonantly frequency-doubled monolithic Nd:Yag laser and a Ti:sapphire laser, emitting at 532 and 760nm wavelength, respectively. An output power of P_UV > 100mW was achieved at 313nm wavelength, with a high spectral purity (rms short-term frequency fluctuations With this experimental setup, sympathetic cooling of N+, BeD+, BeH+, 4He+, HD+, and D+ ions by the laser-cooled Be+ ions was demonstrated, using laser-cooled fluorescence mass spectrometry (LCF-MS) to detect the sympathetically cooled species. Accordingly, sympathetic cooling at a minimum mass ratio of m(H2+)/m(Be+) = 0.22 was shown to be possible with both the sympathetically and laser-cooled ions in a cloud-like state. This points out the incorrectness of the theoretical lower limit m_SC/m_LC = 0.53 which was predicted in Ref.[3]. Here, the subscripts SC and LC denote the sympathetically and the laser-cooled species, respectively. In a further experiment, almost pure Be+ ion plasmas were laser-cooled down to temperatures in the milli-Kelvin range, leading to the crystallization of the Be+ ion plasmas. The outer shapes of different-sized Be+ ion crystals in a fully anisotropic trap potential were compared with the charged fluid predictions, showing good agreement at sufficiently large numbers of Be+ ions. By fitting Voigt profiles to the measured fluorescence signal, a typical temperature T(Be+) of 30mK was estimated for the Be+ ions. Finally, an ultracold sample of 4He+ ions [T(He+) approximately 40mK] was produced for the first time by sympathetic crystallization of the 4He+ ions. This may open up perspectives for the high-precision laser spectroscopy of 4He+ (and 3He+). [1] T.K.W. Boley, Diplomarbeit, Universitat Konstanz (1999) [2] H. Schnitzler, Dissertation, Universitat Konstanz (2001) [3] T. Baba and I. Waki, Appl.Phys.B, 74, 375-382 (2002)

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We have generated Coulomb crystals of ultracold 4He+^{4}\mathrm{H}\mathrm{e}^{+}4He+ ions in a linear radio-frequency trap, by sympathetic cooling via laser-cooled 9Be+^{9}\mathrm{B}\mathrm{e}^{+}9Be+. Stable crystals containing up to 150150150 localized He+{\mathrm{H}\mathrm{e}}^{+}He+ ions at \ensuremath∼20  mK\ensuremath{\sim}20\text{ }\text{ }\mathrm{m}\mathrm{K}\ensuremath∼20  mK were obtained. Ensembles or single ultracold He+{\mathrm{H}\mathrm{e}}^{+}He+ ions open up interesting perspectives for performing precision tests of QED and measurements of nuclear radii. This Letter also indicates the feasibility of cooling and crystallizing highly charged atomic ions using 9Be+^{9}\mathrm{B}\mathrm{e}^{+}9Be+ as coolant.
In this work, the setup of an experiment aimed at high-precision laser spectroscopy of ultracold molecular hydrogen ions, started at the University of Konstanz in 1999 [1,2], was continued and first results were obtained. In particular, a new linear rf trap and a new trap supply electronics were built, the latter being capable of driving the linear rf trap with a maximum rf amplitude of > 1000V at a frequency of 14.2MHz. This made possible the simultaneous trapping of a large mass range (2u For laser cooling of Be+ ions, doubly resonant sum-frequency generation (DR-SFG) of 313nm laser light was performed between a resonantly frequency-doubled monolithic Nd:Yag laser and a Ti:sapphire laser, emitting at 532 and 760nm wavelength, respectively. An output power of P_UV > 100mW was achieved at 313nm wavelength, with a high spectral purity (rms short-term frequency fluctuations With this experimental setup, sympathetic cooling of N+, BeD+, BeH+, 4He+, HD+, and D+ ions by the laser-cooled Be+ ions was demonstrated, using laser-cooled fluorescence mass spectrometry (LCF-MS) to detect the sympathetically cooled species. Accordingly, sympathetic cooling at a minimum mass ratio of m(H2+)/m(Be+) = 0.22 was shown to be possible with both the sympathetically and laser-cooled ions in a cloud-like state. This points out the incorrectness of the theoretical lower limit m_SC/m_LC = 0.53 which was predicted in Ref.[3]. Here, the subscripts SC and LC denote the sympathetically and the laser-cooled species, respectively. In a further experiment, almost pure Be+ ion plasmas were laser-cooled down to temperatures in the milli-Kelvin range, leading to the crystallization of the Be+ ion plasmas. The outer shapes of different-sized Be+ ion crystals in a fully anisotropic trap potential were compared with the charged fluid predictions, showing good agreement at sufficiently large numbers of Be+ ions. By fitting Voigt profiles to the measured fluorescence signal, a typical temperature T(Be+) of 30mK was estimated for the Be+ ions. Finally, an ultracold sample of 4He+ ions [T(He+) approximately 40mK] was produced for the first time by sympathetic crystallization of the 4He+ ions. This may open up perspectives for the high-precision laser spectroscopy of 4He+ (and 3He+). [1] T.K.W. Boley, Diplomarbeit, Universitat Konstanz (1999) [2] H. Schnitzler, Dissertation, Universitat Konstanz (2001) [3] T. Baba and I. Waki, Appl.Phys.B, 74, 375-382 (2002)
Coulomb crystals, containing up to a few hundred ions of which more than 50% were cooled sympathetically by the Coulomb interaction with laser cooled Mg+ ions, have been produced in a linear Paul trap. By controlling the balance of the radiation pressure from the two cooling lasers, the Coulomb crystals could be segregated according to ion species. Previous studies of ion crystals and molecular dynamics simulations suggest that the temperature may be around 10 mK or lower. The obtained results indicate that a wide range of atomic and molecular ions, which due to their internal structures are not amenable to direct laser cooling, can be effectively cooled and localized (crystallized) in linear Paul traps. For high resolution spectroscopy of such ions this may turn out to be very useful.
We have cooled ensembles of the molecular hydrogen ions H2+, H3+, and all their deuterated variants to temperatures of a few mK in a radio frequency trap, by sympathetic cooling with laser-cooled beryllium ions. The molecular ions are embedded in the central regions of Coulomb crystals. Mass spectroscopy and molecular dynamics simulations were used to accurately characterize the properties of the ultracold multispecies crystals. We demonstrate species-selective purification of multispecies ensembles. These molecules are of fundamental importance as the simplest of all molecules, and have the potential to be used for precision tests of molecular structure theory, tests of Lorentz invariance, and measurements of electron to nuclear mass ratios and their time variation.
Control over the motional degrees of freedom of atoms, ions, and molecules in a field-free environment enables unrivalled measurement accuracies but has yet to be applied to highly charged ions (HCIs), which are of particular interest to future atomic clock designs and searches for physics beyond the Standard Model. Here, we report on the Coulomb crystallization of HCIs (specifically (40)Ar(13+)) produced in an electron beam ion trap and retrapped in a cryogenic linear radiofrequency trap by means of sympathetic motional cooling through Coulomb interaction with a directly laser-cooled ensemble of Be(+) ions. We also demonstrate cooling of a single Ar(13+) ion by a single Be(+) ion-the prerequisite for quantum logic spectroscopy with a potential 10(-19) accuracy level. Achieving a seven-orders-of-magnitude decrease in HCI temperature starting at megakelvin down to the millikelvin range removes the major obstacle for HCI investigation with high-precision laser spectroscopy.
It is shown that the translational degrees of freedom of a large variety of molecules, from light diatomic to heavy organic ones, can be cooled sympathetically and brought to rest (crystallized) in a linear Paul trap. The method relies on endowing the molecules with an appropriate positive charge, storage in a linear radio frequency trap, and sympathetic cooling. Two well-known atomic coolant species 9Be+{}^{9}{\mathrm{Be}}^{+}9Be+ and 137Ba+{}^{137}{\mathrm{Ba}}^{+}137Ba+ are sufficient for cooling the molecular mass range from 2 to 20 000 amu. The large molecular charge required for simultaneous trapping of heavy molecules and of the coolant ions can easily be produced using electrospray ionization. Crystallized molecular ions offer vast opportunities for novel studies.
We sympathetically cool highly charged ions (HCI) in Coulomb crystals of Doppler-cooled Be+ ions confined in a cryogenic linear Paul trap that is integrated into a fully enclosing radio-frequency resonator manufactured from superconducting niobium. By preparing a single Be+ cooling ion and a single HCI, quantum logic spectroscopy toward frequency metrology and qubit operations with a great variety of species are enabled. While cooling down the assembly through its transition temperature into the superconducting state, an applied quantization magnetic field becomes persistent, and the trap becomes shielded from subsequent external electromagnetic fluctuations. Using a magnetically sensitive hyperfine transition of Be+ as a qubit, we measure the fractional decay rate of the stored magnetic field to be at the 10-10 s-1 level. Ramsey interferometry and spin-echo measurements yield coherence times of >400 ms, demonstrating excellent passive magnetic shielding at frequencies down to DC.