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Physical Review A

Tests of time independence of the electron and nuclear masses with ultracold molecules

Stephan Schiller, V. I. Korobov

Published Mar 17, 2005• 182 citations
Publisher

Abstract

We propose to use laser spectroscopy of ultracold molecules to establish improved limits on the time independence of electron-to-nuclear and nuclear-nuclear mass ratios by comparing, via an optical frequency comb, the frequencies of suitable sets of transitions in the ground electronic state. Hydrogen molecular ions trapped in a radiofrequency trap, sympathetically cooled by atomic ions, are identified as an accessible system. We show that the dipole-allowed rovibrational transition frequencies of $\mathrm{H}{\mathrm{D}}^{+}$ are suitable probes for a time dependence of ${m}_{e}∕{m}_{p}$ or ${m}_{p}∕{m}_{d}$. Separate bounds on the time independence of these constants can be obtained from a comparison of $\mathrm{H}{\mathrm{D}}^{+}$ and $\mathrm{H}_{2}{}^{+}$ transitions frequencies. Spectroscopy of single molecular ions via a quantum jump method is proposed as an approach toward ultrahigh precision.

DOI
10.1103/physreva.71.032505

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We propose to use laser spectroscopy of ultracold molecules to establish improved limits on the time independence of electron-to-nuclear and nuclear-nuclear mass ratios by comparing, via an optical frequency comb, the frequencies of suitable sets of transitions in the ground electronic state. Hydrogen molecular ions trapped in a radiofrequency trap, sympathetically cooled by atomic ions, are identified as an accessible system. We show that the dipole-allowed rovibrational transition frequencies of HD+\mathrm{H}{\mathrm{D}}^{+}HD+ are suitable probes for a time dependence of me∕mp{m}_{e}∕{m}_{p}me​∕mp​ or mp∕md{m}_{p}∕{m}_{d}mp​∕md​. Separate bounds on the time independence of these constants can be obtained from a comparison of HD+\mathrm{H}{\mathrm{D}}^{+}HD+ and H2+\mathrm{H}_{2}{}^{+}H2​+ transitions frequencies. Spectroscopy of single molecular ions via a quantum jump method is proposed as an approach toward ultrahigh precision.
Narrow ro-vibrational transitions in ultracold molecules are excellent candidates for frequency references in the near-IR to visible spectral domain and interesting systems for fundamental tests of physics, in particular for a satellite test of the gravitational redshift of clocks. We have performed laser spectroscopy of several ro-vibrational overtone transitions υ = 0 → υ = 4 in HD+ ions at around 1.4 μm. 1+1 REMPD was used as a detection method, followed by measurement of the number of remaining molecules. The molecular ions were stored in a linear radiofrequency trap and cooled to millikelvin temperatures, by sympathetic cooling using laser-cooled Be+ ions simultaneously stored in the same trap.
Narrow ro-vibrational transitions in ultracold molecules are excellent candidates for frequency references in the near-IR to visible spectral domain and interesting systems for fundamental tests of physics, in particular for a satellite test of the gravitational redshift of clocks. We have performed laser spectroscopy of several ro-vibrational overtone transitions υ = 0 → υ = 4 in HD + ions at around 1.4 μm. 1+1 REMPD was used as a detection method, followed by measurement of the number of remaining molecules. The molecular ions were stored in a linear radiofrequency trap and cooled to millikelvin temperatures, by sympathetic cooling using laser-cooled Be + ions simultaneously stored in the same trap.
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.
We have measured the ro-vibrational infrared transition (v = 0, J = 2) → (v = 4, J = 1) in ultracold HD+ molecular ions embedded inside Be+ Coulomb crystals, by 1+1 REMPI spectroscopy. The molecular ions, trapped in a linear radiofrequency trap, were cooled to temperatures of few tens of mK, via sympathetic cooling using laser-cooled Be+ ions. For detection, we have used secular excitation of the HD+ ions to monitor their loss rate. The transition frequency agrees with a theoretical prediction.
A single, trapped and ultracold molecular hydrogen ion is an attractive quantum system for exploring various aspects of fundamental physics, such as the determination of fundamental constants and testing their time-independence. Here we demonstrate, for the first time, controlled loading, sympathetic cooling, mass spectrometric identification, and vibrational excitation of ultracold single HD+ ions trapped in a tightly confining radiofrequency trap using single laser-cooled Be+ ions for sympathetic cooling. The apparatus can be used also for preparing other single ions, both lighter and heavier than the coolant ion.
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.