Demonstration of one-dimensional chirp cooling of positronium

Positronium (Ps), a bound state of an electron and its antiparticle, the positron, provides a unique system for probing fundamental physics. Owing to its composition of light leptons, precise calculations of positronium energy levels and decay rates can be performed using Quantum Electrodynamics theory, a cornerstone of physics. Precision spectroscopy of positronium offer significant opportunities for testing particle physics theories, which are currently under scrutiny due to recent cosmological observations.

However, the experimental accuracy and precision have been compromised by the challenges associated with cooling Ps. Traditional experiments typically deal with Ps atom clouds at several hundreds of Kelvin, introducing significant measurement uncertainties. Due to the short lifetime of approximately 142 ns for the spin-triplet ground state of Ps, rapid cooling is essential.

In this presentation, we demonstrate one-dimensional laser cooling of Ps down to approximately 1 K within 100 ns. This is accomplished using a chirp cooling scheme, which employs a laser pulse train with the center frequency shifting upwards at a rate of 0.5 GHz/ns to maintain resonance with the decelerating Ps atoms. Approximately 10% of the Ps atoms created was cooled successfully from the initial temperature of 600 K.

We will also present a comparison of the resultant Ps velocity distribution with simulations based on the Lindblad equation formalism. Furthermore, we will discuss the implications of this work for future precision spectroscopy studies.