Fundamental Physics with Cold Radioactive Heavy Elements

The electric dipole moment (EDM) for an elementary particle can be used to study physics beyond the Standard Model. A non-zero EDM leads to CP violation, which is important for understanding the mater-antimatter asymmetry in the Universe. In heavy paramagnetic atoms, due to relativistic effects, the electron EDM results in an atomic EDM that is enhanced by a factor equal to the 3rd power of the nuclear charge. The heaviest alkali element francium (Fr) has the largest enhancementof about 895. At present, the construction of a high-intensity laser-cooled Fr beam line at RIKEN is almost ready. The development to measure the EDM by atomic interferometry with an accuracy of 10^-29 ecm is in progress. In this experiment, we will use quantum optics techniques such as laser cooling and trapping in an optical lattice to achieve longer interaction times and better use of radioactive atoms. Low-energy Fr ions will be produced by nuclear fusion evaporation reactions, and they will be neutralized by depositing them on a suitable neutralizing element. Then Fr atoms are rapidly decelerated and trapped by laser cooling in a magneto-optical trap (MOT). They will then be transferred to an optical lattice to measure the spin precession of the Fr with the Ramsey resonance method.

Furthermore, we are developing the new quantum sensing technique using ultracold entangled Fr states in an optical lattice. The dependence of the uncertainty in the EDM on a single-photon Rabi frequency inside the optical cavity is investigated. The quantum sensing using the spin squeezed state of a Fr atom for the measurement of its EDM is predicted to offer an uncertainty below the standard quantum limit, which means the measurement accuracy of the EDM at a level below 10⁻³⁰ ecm. The Fr trapped in the optical lattice can be also used to detect the parity non-conservation in the atomic system to study the weak interaction in the quantum many-body system. The status of the fundamental physics using cold Fr source is presented in this paper.