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arXiv:1611.08414 · New Journal of Physics

Cooling atomic ions with visible and infra-red light

Frieder Lindenfelser, Matteo Marinelli, Vlad Negnevitsky, Simon Ragg, Jonathan Home

Published May 5, 2017• 20 citations
arXivHTMLPDF

Abstract

We demonstrate the ability to load, cool and detect singly charged calcium ions in a surface electrode trap using only visible and infrared lasers for the trapped-ion control. As opposed to the standard methods of cooling using dipole-allowed transitions, we combine power broadening of a quadrupole transition at 729 nm with quenching of the upper level using a dipole allowed transition at 854 nm. By observing the resulting 393 nm fluorescence we are able to perform background-free detection of the ion. We show that this system can be used to smoothly transition between the Doppler cooling and sideband cooling regimes, and verify theoretical predictions throughout this range. We achieve scattering rates which reliably allow recooling after collision events and allow ions to be loaded from a thermal atomic beam. This work is compatible with recent advances in optical waveguides, and thus opens a path in current technologies for large-scale quantum information processing. In situations where dielectric materials are placed close to trapped ions, it carries the additional advantage of using wavelengths which do not lead to significant charging, which should facilitate high rate optical interfaces between remotely held ions.

DOI
10.1088/1367-2630/aa7150

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•Jul 23, 2014•0 citations
We demonstrate the ability to load, cool and detect singly charged calcium ions in a surface electrode trap using only visible and infrared lasers for the trapped-ion control. As opposed to the standard methods of cooling using dipole-allowed transitions, we combine power broadening of a quadrupole transition at 729 nm with quenching of the upper level using a dipole allowed transition at 854 nm. By observing the resulting 393 nm fluorescence we are able to perform background-free detection of the ion. We show that this system can be used to smoothly transition between the Doppler cooling and sideband cooling regimes, and verify theoretical predictions throughout this range. We achieve scattering rates which reliably allow recooling after collision events and allow ions to be loaded from a thermal atomic beam. This work is compatible with recent advances in optical waveguides, and thus opens a path in current technologies for large-scale quantum information processing. In situations where dielectric materials are placed close to trapped ions, it carries the additional advantage of using wavelengths which do not lead to significant charging, which should facilitate high rate optical interfaces between remotely held ions.
Doppler cooling of calcium ions has been experimentally demonstrated using the ${S}_{1∕2}\ensuremath{\rightarrow}{D}_{5∕2}$ dipole-forbidden transition. Scattering forces and fluorescence levels a factor of 5 smaller than for the usual Doppler cooling on the dipole-allowed ${S}_{1∕2}\ensuremath{\rightarrow}{P}_{1∕2}$ transition have been achieved. Since the light scattered from the ions can be monitored at (violet) wavelengths that are very different from the excitation wavelengths, single ions can be detected with an essentially zero background level. This, as well as other features of the cooling scheme, can be extremely valuable for ion-trap-based quantum-information processing.
This thesis reports on the setup of a new ion trap apparatus designed for experiments with single ⁴⁰Ca⁺ ions to perform molecular spectroscopy. The calcium ion is laser cooled, allowing for sympathetic cooling of the nonfluorescing molecular ion. The aim of these experiments is to explore loading and identifying molecular ions in RF-Paul traps, as well as developing new spectroscopic tools to measure transitions of molecular ions via the fluorescence of co-trapped ⁴⁰Ca⁺ ions. Ground state cooling of a mixed ion pair is implemented as a first step towards increasing the sensitivity of our technique to the level necessary to measure transitions with low scattering rates (like those present in molecular ions). Doppler cooling on the S(1/2)->P(1/2) transition of the calcium ion results in the formation of a Coulomb crystal, the behavior of which may be used to infer properties of the molecular ion. Following cooling, sideband spectroscopy on the narrow S(1/2)->D(5/2) quadrupole transition of calcium may be used to identify the mass of single molecular ions. This method is verified via a non-destructive measurement on ⁴⁰CaH⁺ and ⁴⁰Ca¹⁶O⁺. The normal modes of the Coulomb crystal can also be used to extract information from the target ion to the control ion. By driving the blue side of a transition, laser induced heating can be put into the two ion system, which leads to changes in fluorescence of the ⁴⁰Ca⁺ ion, first demonstrated with two Ca⁺ isotopes. Increasing the sensitivity of this technique requires ground state cooling of both the ⁴⁰Ca⁺ ion and the ion of interest, enabling the transfer of the ion's motional state into the ground state with high probability. This thesis demonstrates ground state cooling of the atomic ion and sympathetic cooling of a second ion (⁴⁴Ca⁺). Once in the ground state, heating of the Coulomb crystal by scattering photons off of the spectroscopy ion can be measured by monitoring the resolved motional sidebands of the S(1/2)->D(5/2) transition of ⁴⁰Ca⁺, allowing for spectral lines to be inferred. Future experiments will investigate this technique with molecular ions.
We demonstrate loading of ions into a surface-electrode trap (SET) from a remote, laser-cooled source of neutral atoms. We first cool and load $\ensuremath{\sim}$106{10}^{6}106 neutral 88{}^{88}88Sr atoms into a magneto-optical trap from an oven that has no line of sight with the SET. The cold atoms are then pushed with a resonant laser into the trap region where they are subsequently photoionized and trapped in an SET operated at a cryogenic temperature of 4.6 K. We present studies of the loading process and show that our technique achieves ion loading into a shallow (15 meV depth) trap at rates as high as 125 ions/s while drastically reducing the amount of metal deposition on the trap surface as compared with direct loading from a hot vapor. Furthermore, we note that due to multiple stages of isotopic filtering in our loading process, this technique has the potential for enhanced isotopic selectivity over other loading methods. Rapid loading from a clean, isotopically pure, and precooled source may enable scalable quantum-information processing with trapped ions in large, low-depth surface-trap arrays that are not amenable to loading from a hot atomic beam.
Laser cooling is fundamental to quantum computing and metrology using atomic systems. Precise control often requires cooling atoms' motional degrees of freedom to the quantum ground state, imposing operation time and architectural limitations particularly in large-scale systems. Here we demonstrate how the integrated optical control of interest for scaling trapped-ion systems additionally enables laser cooling that bypasses limitations of conventional schemes. Leveraging multi-channel integrated delivery of ultraviolet to infrared wavelengths for calcium ion control including in passively phase-stable ultraviolet standing waves (SWs), we experimentally verify a long-standing prediction by Cirac et al., realizing Doppler cooling to below the conventional Doppler limit at a SW node. We also present the first realization of ground-state cooling via electromagnetically induced transparency (EIT) using a ``probe" beam delivered as a SW with atoms positioned at a node, predicted to enable multi-mode sub-recoil-limit laser cooling. We demonstrate cooling of motional modes spanning an approximately 5 MHz bandwidth from the Doppler temperature to near the ground state within 150~\textmu s, reaching nˉ≈0.05\bar n \approx 0.05nˉ≈0.05 phonon number occupancies for the target mode. Direct evaluation against the comparable running-wave (RW) scheme shows the SW implementation's simultaneous advantage in cooling rate, motional mode bandwidth, and final phonon number, as previously theoretically predicted. Our work leverages capabilities enabled by integrated optical delivery to demonstrate fast cooling of multiple modes to the quantum ground state, and more broadly how scalable approaches to optical control can enable enhancements in fundamental atomic functionalities.
We report the laser cooling of a single 40Ca+^{40}\text{Ca}^+40Ca+ ion in a Penning trap to the motional ground state in one dimension. Cooling is performed in the strong binding limit on the 729-nm electric quadrupole S1/2↔D5/2S_{1/2}\leftrightarrow D_{5/2}S1/2​↔D5/2​ transition, broadened by a quench laser coupling the D5/2D_{5/2}D5/2​ and P3/2P_{3/2}P3/2​ levels. We find the final phonon number to be nˉ=0.014±0.009\bar{n}=0.014\pm0.009nˉ=0.014±0.009. We measure the heating rate of the trap to be very low with nˉ˙=2.5±0.3s−1\dot{\bar{n}}=2.5\pm 0.3\textrm{s}^{-1}nˉ˙=2.5±0.3s−1 and a scaled spectral noise density of ωSE(ω)∼1.6−8V2m−2Hz−1s−1\omega S_{E}(\omega)\sim1.6^{-8}\textrm{V}^2\textrm{m}^{-2}\textrm{Hz}^{-1}\textrm{s}^{-1}ωSE​(ω)∼1.6−8V2m−2Hz−1s−1, which is consistent with the large ion-electrode distance. We perform Rabi oscillations on the sideband-cooled ion and observe a coherence time of 0.7±0.1ms0.7\pm 0.1\textrm{ms}0.7±0.1ms, noting that the practical performance is currently limited by the intensity noise of the probe laser.