Cold Lithium Interferometry and the Equivalence Principle

Does gravity truly accelerate all masses at the same rate? Einstein's (weak) Equivalence Principle states that a gravitational field accelerates all objects equally, regardless of mass. A violation of the Εquivalence Principle is intimately related to a violation of Lorentz symmetry. The Standard Model Extension (SME) is a general framework that parameterizes Lorentz violations by describing different gravitational couplings for electrons, protons, and neutrons, and can be used to describe violations of the Equivalence Principle. For a bound system of these particles like the atoms we use in atom interferometers, the total deviation of the gravitational acceleration depends on the kinetic energy of the particles within the nucleus and the number of neutrons. A precise measurement of the difference in gravitational acceleration for lithium-6 and lithium-7 provides one of the most sensitive tests of the SME parameters available. We will use atom interferometry to test the Equivalence Principle by measuring the difference in gravitational acceleration felt by lithium-6 and lithium-7 to a projected sensitivity of Δg/g~10-14.

Lithium is the lightest alkali species, recoiling from photons with a larger velocity than its bloated cousins rubidium and cesium. Lithium's large recoil velocity is an advantage for atom interferometry, enclosing a larger spacetime area for a given number of photon recoils. But lithium's large recoil velocity also makes it difficult to cool. Even still, we have cooled our cloud to 200 μK and built the world's first cold lithium-7 atom interferometer. The atom optics of our interferometer are built using stimulated Raman transitions that change the hyperfine state while kicking the atom with the momentum of two photons. We will first apply our interferometer to measure the fine-structure constant, to perform hyperfine spectroscopy on lithium-7, and to measure its gravitational acceleration.

The Lithium Team

Kayleigh Cassella

Eric Copenhaver

Past team members

Paul Hamilton

Geena Kim

Dennis Schlippert

Biswaroop Mukherjee

Trinity Joshi

Daniel Tiarks


  1. Lithium recoil measurement without a subrecoil sample. Kayleigh Cassella, Eric Copenhaver, Brian Estey, Yanying Feng, Chen Lai, and Holger Müller, to appear in Phys. Rev. Lett. and arXiv 1610.07588

  2. Sisyphus cooling of lithium. Paul Hamilton, Geena Kim, Trinity Joshi, Biswaroop Mukherjee, Daniel Tiarks, and Holger Müller, Phys. Rev. A 89, 023409 (2014). arxiv:1308.1935

  3. Equivalence principle and bound kinetic energy. Michael Hohensee, Holger Müller, and R. B. Wiringa, Phys. Rev. Lett. 111, 151102 (2013). arxiv:1308.2936