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arXiv:0811.3971 · Physical Review A

Prospects for application of ultracold Sr2 molecules in precision measurements

Svetlana Kotochigova, Tanya Zelevinsky, Jun Ye

Published Jan 8, 2009• 48 citations
arXivHTMLPDF

Abstract

Precision measurements with ultracold molecules require development of robust and sensitive techniques to produce and interrogate the molecules. With this goal, we theoretically analyze factors that affect frequency measurements between rovibrational levels of the ${\mathrm{Sr}}_{2}$ molecule in the electronic ground state. This measurement can be used to constrain the possible time variation of the proton-electron mass ratio. ${\mathrm{Sr}}_{2}$ is expected to be a strong candidate for achieving high precision due to the spinless nature and ease of cooling and perturbation-free trapping of Sr [T. Zelevinsky et al., Phys. Rev. Lett. 100, 043201 (2008)]. The analysis includes calculations of two-photon transition dipole moments between deeply and weakly bound vibrational levels, lifetimes of intermediate excited states, and Stark shifts of the vibrational levels by the optical lattice field, including possibilities of Stark-cancellation trapping.

DOI
10.1103/physreva.79.012504

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Precision measurements with ultracold molecules require development of robust and sensitive techniques to produce and interrogate the molecules. With this goal, we theoretically analyze factors that affect frequency measurements between rovibrational levels of the Sr2{\mathrm{Sr}}_{2}Sr2​ molecule in the electronic ground state. This measurement can be used to constrain the possible time variation of the proton-electron mass ratio. Sr2{\mathrm{Sr}}_{2}Sr2​ is expected to be a strong candidate for achieving high precision due to the spinless nature and ease of cooling and perturbation-free trapping of Sr [T. Zelevinsky et al., Phys. Rev. Lett. 100, 043201 (2008)]. The analysis includes calculations of two-photon transition dipole moments between deeply and weakly bound vibrational levels, lifetimes of intermediate excited states, and Stark shifts of the vibrational levels by the optical lattice field, including possibilities of Stark-cancellation trapping.
Abstract : Ultracold molecules provide an exciting testing ground for studies of fundamental interactions, new states of matter, and metrology. Diatomic molecules based on two-electron atoms are especially suitable for precise tests of interatomic interactions, molecular quantum electrodynamics, electron-proton mass ratio variations, and other measurements in molecular and fundamental physics. This thesis describes the construction of a new strontium apparatus, from initial vacuum system setup through characterization of ultracold atom samples, followed by a new method of efficient, all-optical production of ultracold 88Sr2 molecules in an optical lattice, with detection via optical fragmentation. High-Q spectra of the weakly bound molecules in magnetic fields are studied, yielding precise binding energies, anomalously large molecular g factors resulting from large nonadiabatic effects, and strongly enhanced magnetic susceptibility. The thesis then concludes with an outlook on future experiments in our lab, including studies of forbidden molecular transitions, and longer term studies of fundamental physics from deeply bound Sr2.
Ultracold atoms and molecules provide ideal stages for precision tests of fundamental physics. With microkelvin neutral strontium atoms confined in an optical lattice, we have achieved a fractional resolution of 4 × 10-15 on the 1S0–3P0 doubly forbidden 87 Sr clock transition at 698 nm. Measurements of the clock line shifts as a function of experimental parameters indicate systematic errors below the 10-15 level. The ultrahigh spectral resolution permits resolving the nuclear spin states of the clock transition at small magnetic fields, leading to measurements of the 3P0 magnetic moment and metastable lifetime. In addition, photoassociation spectroscopy performed on the narrow 1S0–3P1 transition of 88 Sr shows promise for efficient optical tuning of the ground state scattering length and production of ultracold ground state molecules. Lattice-confined Sr 2 molecules are suitable for constraining the time variation of the proton–electron mass ratio. In a separate experiment, cold, stable, ground state polar molecules are produced from Stark decelerators. These cold samples have enabled an order-of-magnitude improvement in the measurement precision of ground state, Λ doublet microwave transitions in the OH molecule. Comparing the laboratory results to those from OH megamasers in interstellar space will allow a sensitivity of 10-6 for measuring the potential time variation of the fundamental fine structure constant Δα/α over 1010 years. These results have also led to improved understandings of the molecular structure. The study of the low magnetic field behavior of OH in its 2Π3/2 ro-vibronic ground state precisely determines a differential Landé g factor between opposite parity components of the Λ doublet.
in a lattice. Sr2 has a range of electronically excited bound states which are readily accessible with optical wavelengths using the narrow 1S0->3P1 intercombination line. As in Nat. Phys. 11, 32, we measure the lifetimes of the narrow, deeply-bound subradiant states in the 1g (1S0+3P1 dissociative limit) potential, allowing for coherent control of molecules and a comparison with theoretical predictions of the lifetimes and transition strengths of these states. Next, we study ultracold photodissociation of Sr2 molecules through abortion of one and two photons near the atomic intercombination line. This allows us to observe the vector character of transition elements through the angular dissociation pattern and to directly measure barrier heights in the excited state potentials. Finally, as shown in PRL 114, 023001, we demonstrate that in a non-magic lattice, a narrow transition can be used to measure the trapped gas temperature through the linewidth of the spectral feature corresponding to the carrier transitions. We use this technique to measure the temperature of Sr2 molecules to 10x higher precision than with standard techniques. We discuss future prospects with this molecular lattice clock. 1 Funding from NIST, ARO, and NSF IGERT
Over the past few decades, rapid development of laser cooling techniques and narrow-linewidth lasers have allowed atom-based quantum clocks to achieve unprecedented precision. Techniques originally developed for atomic clocks can be extended to ultracold molecules, with applications ranging from quantum-state-controlled ultracold chemistry to searches for new physics. Because of the richness of molecular structure, quantum metrology based on molecules provides possibilities for testing physics that is beyond the scope of traditional atomic clocks. This thesis presents the work performed to establish a state-of-the-art quantum clock based on ultracold molecules. The molecular clock is based on a frequency difference between two vibrational levels in the electronic ground state of 88Sr2 diatomic molecules. Such a clock allows us test molecular QED, improve constraints on nanometer-scale gravity, and potentially provide a model-independent test of temporal variations of the proton-electron mass ratio. Trap-insensitive spectroscopy is crucial for extending coherent molecule-light interactions and achieving a high quality factor Q. We have demonstrated a magic wavelength technique for molecules by manipulating the optical lattice frequency near narrow polarizability resonances. This general technique allows us to increase the coherence time to tens of ms, an improvement of a factor of several thousand, and to narrow the linewidth of a 25 THz vibrational transition initially to 30 Hz. This width corresponds to the quality factor Q=8x10^11. Besides the molecular quantum metrology, investigations of novel phenomena in state-selected photodissociation are also described in this thesis, including magnetic-field control of photodissociation and observation of the crossover from ultracold to quasiclassical chemistry.
Ultracold atoms and molecules provide ideal stages for precision tests of fundamental physics. With microkelvin neutral strontium atoms confined in an optical lattice, we have achieved a fractional resolution of 4 × 10−15 on the 1S0 − 3P0 doubly‐forbidden 87Sr clock transition at 698 nm. The overall systematic uncertainty of the clock is evaluated below the 10−15 level. The ultrahigh spectral resolution permits resolving the nuclear spin states of the clock transition at small magnetic fields, leading to measurements of the 3P0 magnetic moment and metastable lifetime. In addition, photoassociation spectroscopy performed on the narrow 1S0 − 3P1 transition of 88Sr shows promise for efficient optical tuning of the ground state scattering length and production of ultracold ground‐state molecules. Lattice‐confined Sr2 molecules are suitable for constraining the time‐variation of electron‐proton mass ratio. In a separate experiment, cold, ground state polar molecules produced from Stark decelerators have enabled an order of magnitude improvement in measurement precision of ground‐state, Λ‐doublet microwave transitions in the OH molecule. Comparing the laboratory results to those from OH megamasers in interstellar space will allow a sensitivity of 10−6 for measuring the potential time variation of the fundamental fine structure constant Δα/α over 1010 years. These results have also led to improved understandings in the molecular structure. The study of the low magnetic field behavior of OH in its 2Π3/2 ro‐vibronic ground state precisely determines a differential Landé g‐factor between opposite parity components of the Λ‐doublet.
We have produced large samples of stable ultracold (88)Sr(2) molecules in the electronic ground state in an optical lattice. The fast, all-optical method of molecule creation involves a near-intercombination-line photoassociation pulse followed by spontaneous emission with a near-unity Franck-Condon factor. The detection uses excitation to a weakly bound electronically excited vibrational level corresponding to a very large dimer and yields a high-Q molecular vibronic resonance. This is the first of two steps needed to create deeply bound (88)Sr(2) for frequency metrology and ultracold chemistry.