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AIP conference proceedings

A Laser-cooled Single-atom-on-demand Source For Si Quantum Computing

W. M. Fairbank, S. A. Lee, William Czajkowski, Jonathan Kluck, Timothy C. Ralph, Ping Koy Lam

Published Jan 1, 2011• 1 citation
Publisher

Abstract

A promising proposal by B. Kane for a scalable silicon quantum computer requires the placement of 31P atoms 20 nm apart and 10 nm below the surface in pure 28Si to 1 nm precision. This paper presents progress on a scheme for laser cooling and trapping single 31Si atoms in a magneto‐optical trap (MOT), resonantly ionizing that one atom near threshold, and softly depositing the single 31Si+ ion in Si to nm precision at ∼100 eV. A few hours after deposition, 31Si beta decays to 31P+. The hyperfine structure and isotope shifts of the 221.7 nm cooling transition for the stable isotopes of Si have been measured with precision for the first time. Sufficient power for the MOT at this wavelength has also been demonstrated. New autoionizing states near threshold have been found and ionization cross sections determined. The velocity distributions of laser ablated Si atoms have been measured, and unexpected results have been found. Construction of the MOT and design of the ion optics are underway.

DOI
10.1063/1.3630174

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A promising proposal by B. Kane for a scalable silicon quantum computer requires the placement of 31P atoms 20 nm apart and 10 nm below the surface in pure 28Si to 1 nm precision. This paper presents progress on a scheme for laser cooling and trapping single 31Si atoms in a magneto‐optical trap (MOT), resonantly ionizing that one atom near threshold, and softly depositing the single 31Si+ ion in Si to nm precision at ∼100 eV. A few hours after deposition, 31Si beta decays to 31P+. The hyperfine structure and isotope shifts of the 221.7 nm cooling transition for the stable isotopes of Si have been measured with precision for the first time. Sufficient power for the MOT at this wavelength has also been demonstrated. New autoionizing states near threshold have been found and ionization cross sections determined. The velocity distributions of laser ablated Si atoms have been measured, and unexpected results have been found. Construction of the MOT and design of the ion optics are underway.
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We realize a magneto-optical trap (MOT) of titanium (Ti) atoms, performing laser cooling on the 498 nm transition between the long-lived 3 d 3 ( F 4 ) 4 s a 5 F 5 metastable state and the 3 d 3 ( F 4 ) 4 p y 5 G 6 o excited state. Without the addition of any repumping light, we observe MOTs of the three stable, I = 0 bosonic isotopes, Ti 46 , Ti 48 , and Ti 50 . Up to 8.30 ( 26 ) × 10 5 Ti 48 atoms are trapped at a maximum density of 1.3 ( 4 ) × 10 11 cm − 3 and at a temperature of 90 ( 15 ) µ K . By measuring the decay of the MOT, we constrain the leakage branching ratio of the cooling transition ( ≤ 2.5 × 10 − 6 ) and the two-body loss coefficient ( ≤ 2 × 10 − 10 cm 3 s − 1 ). Our approach to laser cooling Ti can be applied to other transition metals, enabling a significant expansion of the elements that can be laser cooled.
We demonstrate launching of laser-cooled Yb atoms in a cold atomic fountain. Atoms in a collimated thermal beam are first cooled and captured in a magneto-optical trap (MOT) operating on the strongly allowed ${{}^{1}S}_{0}\ensuremath{\rightarrow}{{}^{1}P}_{1}$ transition at 399 nm (blue line). They are then transferred to a MOT on the weakly allowed ${{}^{1}S}_{0}\ensuremath{\rightarrow}{{}^{3}P}_{1}$ transition at 556 nm (green line). Cold atoms from the green MOT are launched against gravity at a velocity of around 2.5 m/s using a pair of green beams. We trap more than 107{10}^{7}107 atoms in the blue MOT and transfer up to 7070%70 into the green MOT. The temperature for the odd isotope 171Yb^{171}\mathrm{Yb}171Yb is  1mK~1 \mathrm{mK} 1mK in the blue MOT, and reduces by a factor of 40 in the green MOT.
Neutral atom quantum computing is a promising avenue toward the realization of a physical quantum computer. The diffraction pattern formed by laser light immediately behind a circular aperture can be used as optical atomic dipole traps, and has the potential to be scaled up to create a two dimensional array of individually addressable qubit sites. In working towards experimental demonstration of the dipole traps, we are constructing a MOT. The function of the MOT is to cool and trap 87Rb in a localized cloud in our vacuum chamber, which will be used to load the dipole traps. One critical aspect to the functioning of the MOT is the precise tuning of the trapping laser to the red of the F=2→F’=3 hyperfine transition and the pump laser to the F=1→F’=2 hyperfine transition. We have successfully constructed a saturated absorption apparatus which works to distinguish the hyperfine structure of Rb within the Doppler broadened transition peaks. The saturated absorption scans serve to correlate the voltage applied to the piezo with the laser light frequencies causing hyperfine transitions in Rb, allowing for the precise tuning of the lasers needed to trap atoms.
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.