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arXiv:1105.4974 · Science

Frequency Metrology in Quantum Degenerate Helium: Direct Measurement of the 2 3 S 1 → 2 1 S 0 Transition

Rob van Rooij, Joseph Steven Borbely, Juliette Simonet, Maarten D. Hoogerland, K. S. E. Eikema, R. A. Rozendaal, W. Vassen

Published Jul 7, 2011• 136 citations
arXivHTMLPDF

Abstract

Precision spectroscopy of simple atomic systems has refined our understanding of the fundamental laws of quantum physics. In particular, helium spectroscopy has played a crucial role in describing two-electron interactions, determining the fine-structure constant and extracting the size of the helium nucleus. Here we present a measurement of the doubly forbidden 1557-nanometer transition connecting the two metastable states of helium (the lowest energy triplet state 2 (3)S(1) and first excited singlet state 2 (1)S(0)), for which quantum electrodynamic and nuclear size effects are very strong. This transition is weaker by 14 orders of magnitude than the most predominantly measured transition in helium. Ultracold, submicrokelvin, fermionic (3)He and bosonic (4)He atoms are used to obtain a precision of 8 × 10(-12), providing a stringent test of two-electron quantum electrodynamic theory and of nuclear few-body theory.

DOI
10.1126/science.1205163

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Precision spectroscopy of simple atomic systems has refined our understanding of the fundamental laws of quantum physics. In particular, helium spectroscopy has played a crucial role in describing two-electron interactions, determining the fine-structure constant and extracting the size of the helium nucleus. Here we present a measurement of the doubly forbidden 1557-nanometer transition connecting the two metastable states of helium (the lowest energy triplet state 2 (3)S(1) and first excited singlet state 2 (1)S(0)), for which quantum electrodynamic and nuclear size effects are very strong. This transition is weaker by 14 orders of magnitude than the most predominantly measured transition in helium. Ultracold, submicrokelvin, fermionic (3)He and bosonic (4)He atoms are used to obtain a precision of 8 × 10(-12), providing a stringent test of two-electron quantum electrodynamic theory and of nuclear few-body theory.
We have measured the forbidden $2\text{ }^{3}{S}_{1}\ensuremath{\rightarrow}2\text{ }^{1}{P}_{1}$ transition at 887 nm in a quantum degenerate gas of metastable 4He^{4}\mathrm{He}4He atoms confined in an optical dipole trap. The determined transition frequency is 338 133 594.4 (0.5) MHz, from which we obtain an ionization energy of the 2 1P12\text{ }^{1}{P}_{1}2 1P1​ state of 814 709 148.6 (0.5) MHz. This ionization energy is in disagreement by $>3\ensuremath{\sigma}$ with the most accurate quantum electrodynamics calculations available. Our measurements also provide a new determination of the lifetime of the 2 1P12\text{ }^{1}{P}_{1}2 1P1​ state of $0.551\text{ }(0.004{)}_{\text{stat}}\text{ }{(}_{\ensuremath{-}0.000}^{+0.013}{)}_{\text{syst}}\text{ }\text{ }\mathrm{ns}$, which is the most accurate determination to date and in excellent agreement with theory.
The helium atom, being a calculable system with narrow transitions that can be measured precisely, is an excellent candidate for testing state-of-the-art Quantum Electrodynamics calculations. This thesis presents a series of precise spectroscopic measurements on a narrow transition in ultracold trapped quantum gases of helium, using both the 3-He and 4-He isotopes, which exhibit distinctly different quantum-statistical behaviour in this ultracold regime. From the shift of this transition between the two isotopes, the difference in their nuclear charge radii is extracted, which is used as a benchmark quantity for comparison with other measurements and tests of Quantum Electrodynamics theory.
Abstract We report on interference studies in the internal and external degrees of freedom of metastable triplet helium atoms trapped near quantum degeneracy in a $$ 1.5\kern0.166667em \upmu \mathrm{m} $$ 1.5 μ m optical dipole trap. Applying a single π/2 \pi /2 π/2 π / 2 rf pulse we demonstrate that 50% of the atoms initially in the m=+1 m=+1 m=+1 m = + 1 state can be transferred to the magnetic field insensitive m=0 m=0 m=0 m = 0 state. Two π/2 \pi /2 π/2 π / 2 pulses with varying time delay allow a Ramsey-type measurement of the Zeeman shift for a high precision measurement of the 23S1 {2}^3{S}_1 23S1​ 2 3 S 1 – 21S0 {2}^1{S}_0 21S0​ 2 1 S 0 transition frequency. We show that this method also allows strong suppression of mean-field effects on the measurement of the Zeeman shift, which is necessary to reach the accuracy goal of 0.1 kHz on the absolute transition frequencies. Theoretically the feasibility of using metastable triplet helium atoms in the m=0 m=0 m=0 m = 0 state for atom interferometry is studied demonstrating favorable conditions, compared to the alkali atoms that are used traditionally, for a non-QED determination of the fine structure constant.
We report on interference studies in the internal and external degrees of freedom of metastable triplet helium atoms trapped near quantum degeneracy in a $$1.5 \,\upmu {\mathrm{m}}$$ optical dipole trap. Applying a single π/2\pi /2π/2 rf pulse we demonstrate that 50% of the atoms initially in the m=+1m=+1m=+1 state can be transferred to the magnetic field insensitive m=0m=0m=0 state. Two π/2\pi /2π/2 pulses with varying time delay allow a Ramsey-type measurement of the Zeeman shift for a high precision measurement of the 2 3S12\,^3S_123S1​ – 2 1S02\,^1S_021S0​ transition frequency. We show that this method also allows strong suppression of mean-field effects on the measurement of the Zeeman shift, which is necessary to reach the accuracy goal of 0.1 kHz on the absolute transition frequencies. Theoretically the feasibility of using metastable triplet helium atoms in the m=0m=0m=0 state for atom interferometry is studied demonstrating favorable conditions, compared to the alkali atoms that are used traditionally, for a non-QED determination of the fine structure constant.
A bright beam of metastable helium atoms is useful in various studies. Laser spectroscopy of helium in an atomic beam can provide a determination of the fine-structure constant and the nuclear charge radius, towards a test of quantum electrodynamics (QED). In these measurements, it is necessary to control not only the internal quantum state but also the translation motion of the helium atom. Here we present a setup producing an intense and speed-adjustable continuous beam of helium atoms. By using a combination of laser cooling, focusing, and deflection, helium atoms are prepared at the single quantum state of 23S1{2}^{\phantom{\rule{0.16em}{0ex}}3}{S}_{1}23S1​ (m=0m=0m=0), with a very narrow distribution of the longitudinal speed ($\ensuremath{\delta}{v}_{z}<\ifmmode\pm\else\textpm\fi{}4.5$ m/s). At the same time, we obtain a flux of $1.8\ifmmode\times\else\texttimes\fi{}{10}^{13}$ (atoms/s)/sr with a fractional fluctuation below 0.02%. The atomic-beam quality was investigated by laser spectroscopy, indicating a considerable improvement over those used in previous helium precision-spectroscopy measurements.
Exotic helium atoms act as unique atomic traps for heavy, negatively charged particles, protecting them from nuclear annihilation and nuclear capture on timescales long enough to enable high-precision laser spectroscopy. Such measurements serve as stringent tests of three-body quantum electrodynamics and offer a direct route to determining fundamental particle masses. Motivated by upcoming spectroscopic efforts targeting pionic (π− 4He+π^{-\,4}\mathrm{He}^+π−4He+) and kaonic (K− 4He+K^{-\,4}\mathrm{He}^+K−4He+) helium, we present a rigorous theoretical evaluation of the collisional and density effects governing these systems. Using an ab initio potential energy surface and coupled-channel quantum scattering calculations, we study the collisional stability of the candidate metastable states against inelastic quenching in a cryogenic helium buffer gas. Furthermore, we provide theoretical reference values for the pressure broadening and pressure shift coefficients of the targeted transitions. These results establish an essential benchmark for future experiments, paving the way for refined determinations of the pion and kaon masses.