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arXiv:1907.03413 · Physical Review Letters

Second-Scale Coherence Measured at the Quantum Projection Noise Limit with Hundreds of Molecular Ions

Yan Zhou, Yuval Shagam, William B. Cairncross, Kia Boon Ng, Tanya Roussy, Tanner Grogan, Kevin Robert Boyce, Antonio Vigil, Madeline Pettine, Tanya Zelevinsky, Jun Ye, Eric Cornell

Published Feb 4, 2020• 30 citations
arXivHTMLPDF

Abstract

Cold molecules provide an excellent platform for quantum information, cold chemistry, and precision measurement. Certain molecules have enhanced sensitivity to beyond standard model physics, such as the electron's electric dipole moment (eEDM). Molecular ions are easily trappable and are therefore particularly attractive for precision measurements where sensitivity scales with interrogation time. Here, we demonstrate a spin precession measurement with second-scale coherence at the quantum projection noise (QPN) limit with hundreds of trapped molecular ions, chosen for their sensitivity to the eEDM rather than their amenability to state control and readout. Orientation-resolved resonant photodissociation allows us to simultaneously measure two quantum states with opposite eEDM sensitivity, reaching the QPN limit and fully exploiting the high count rate and long coherence.

DOI
10.1103/physrevlett.124.053201

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Cold molecules provide an excellent platform for quantum information, cold chemistry, and precision measurement. Certain molecules have enhanced sensitivity to beyond standard model physics, such as the electron's electric dipole moment (eEDM). Molecular ions are easily trappable and are therefore particularly attractive for precision measurements where sensitivity scales with interrogation time. Here, we demonstrate a spin precession measurement with second-scale coherence at the quantum projection noise (QPN) limit with hundreds of trapped molecular ions, chosen for their sensitivity to the eEDM rather than their amenability to state control and readout. Orientation-resolved resonant photodissociation allows us to simultaneously measure two quantum states with opposite eEDM sensitivity, reaching the QPN limit and fully exploiting the high count rate and long coherence.
Cold molecules provide a platform for a variety of scientific applications such as quantum computation, simulation, cold chemistry, and searches for physics beyond the Standard Model. Mastering quantum state control and measurement of diverse molecular species is critical for enabling these applications. However, state control and readout are difficult for the majority of molecular species due to the lack of optical cycling transitions. Here we demonstrate internal state cooling and orientation-selective photofragment imaging in a spin precession measurement with multi-second coherence, allowing us to achieve the quantum projection noise (QPN) limit in a large ensemble of trapped molecular ions. We realize this scheme for both HfF+ and ThF+ --- molecules chosen for their sensitivity to beyond Standard Model physics rather than their amenability to state control and readout.
This paper presents an experimental platform designed to facilitate quantum logic control of polar molecular ions in a segmented ring ion trap, paving the way for precision measurements. This approach focuses on achieving near-unity state preparation and detection, as well as long spin-precession coherence. A distinctive aspect lies in separating state preparation and detection conducted in a static frame from parity-selective spin precession in a rotating frame. Moreover, the method is designed to support spatially and temporally coincident measurements on multiple ions prepared in states with different sensitivity to the new physics of interest. This provides powerful techniques to probe and minimize potential sources of systematic error. While the primary focus of this paper is on detecting the electron's electric dipole moment (eEDM) using 232ThF+^{232}\mathrm{ThF}^{+}232ThF+ ions, the proposed methodology holds promise for broader applications, particularly with ion species that exhibit enhanced sensitivity to the nuclear magnetic quadruple moment (nMQM).
We present the conceptual design of an experiment to measure the electron's electric dipole moment (eEDM) using 138^{138}138BaOH molecules in an optical lattice. The BaOH molecule is laser-coolable and highly sensitive to the eEDM, making it an attractive candidate for such a precision measurement, and capturing it in an optical lattice offers potentially very long coherence times. We study possibilities and limitations of this approach, identify the most crucial limiting factors and ways to overcome them. The proposed apparatus can reach a statistical error of 10−30 e 10^{-30}\,e\,10−30ecm by measuring spin precession on a total number of 5×1095 \times 10^95×109 molecules over a span of 120 days.
Heavy diatomic molecules have been identified as good candidates for use in electron electric dipole moment (eEDM) searches. Suitable molecular species can be produced in pulsed beams, but with a total flux and/or temporal evolution that varies significantly from pulse to pulse. These variations can degrade the experimental sensitivity to changes in the spin precession phase of an electrically polarized state, which is the observable of interest for an eEDM measurement. We present two methods for measurement of the phase that provide immunity to beam temporal variations, and make it possible to reach shot-noise-limited sensitivity. Each method employs rapid projection of the spin state onto both components of an orthonormal basis. We demonstrate both methods using the eEDM-sensitive $H{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Delta}}_{1}$ state of thorium monoxide, and use one of them to measure the magnetic moment of this state with increased accuracy relative to previous determinations.
The search for a permanent electron electric dipole moment (eEDM) serves as a test of fundamental symmetry violations and of physics beyond the Standard Model. Trapped molecular ions in the 3Δ1 metastable electronic state are suitable candidates for an eEDM search due to their large effective electric fields and long electron spin coherence times. This thesis presents the quantum state manipulation and coherent spectroscopy of trapped HfF+ molecular ions in rotating bias fields for an eEDM search. The quantum state manipulation, which involves preparation of a large fraction of molecular ions in a single desired quantum state as well as rotational-state-resolved detection, is complicated by the lack of HfF+ spectroscopic information prior to the start of this thesis. We performed state preparation by first state-selectively autoionizing neutral HfF such that 35% of the HfF+ are formed in a single rovibrational level of the electronic ground state 1Σ+, and then transferring those ions into the desired Stark levels of a single hyperfine-rovibrational manifold of the 3Δ1 state. Rotational-state-resolved detection is accomplished by both laser-induced fluorescence and resonance-enhanced multi-photon photodissociation, where the latter is preferred as the state detection method of choice because its efficiency is two orders of magnitude higher compared to fluorescence. With the quantum state manipulation techniques developed, we performed Ramsey spectroscopy of the trapped HfF+ ions in the presence of rotating bias electric and magnetic fields, demonstrating electron spin coherence times as long as 150 ms. Finally, we present a preliminary measurement of the eEDM at the |de| < 10-25 e cm level.
Heavy polar molecules can be used to measure the electric dipole moment of the electron, which is a sensitive probe of physics beyond the Standard Model. The value is determined by measuring the precession of the molecule’s spin in a plane perpendicular to an applied electric field. The longer this precession evolves coherently, the higher the precision of the measurement. For molecules in a trap, this coherence time could be very long indeed. We evaluate the sensitivity of an experiment where neutral molecules are trapped electrically, and compare this to an equivalent measurement in a molecular beam. We consider the use of a Stark decelerator to load the trap from a supersonic source, and calculate the deceleration efficiency for YbF molecules in both strong-field seeking and weak-field seeking states. With a 1 s holding time in the trap, the statistical sensitivity could be ten times higher than it is in the beam experiment, and this could improve by a further factor of five if the trap can be loaded from a source of larger emittance. We study some effects due to field inhomogeneity in the trap and find that rotation of the electric field direction, leading to an inhomogeneous geometric phase shift, is the primary obstacle to a sensitive trap-based measurement. 1