In a groundbreaking development, scientists at the European Organization for Nuclear Research (CERN) have successfully demonstrated laser cooling of positronium – an exotic atom consisting of an electron and its anti-particle, a positron. This achievement opens up new possibilities for precision spectroscopy of positronium to test the foundations of physics.
Background
- Positronium (Ps) is an exotic hydrogen-like atom made up of an electron and a positron (antimatter counterpart of electron) orbiting each other. First predicted in 1934 by Austrian-Swiss physicist Erwin Schrödinger, positronium was experimentally observed in 1951.
- As a purely leptonic system with reduced complexity compared to regular atoms and molecules, positronium enables highly sensitive tests of bound state Quantum Electrodynamics (QED). Precise spectroscopy of Ps can be used to measure fundamental physical constants and search for physics beyond the Standard Model.
- However, precision spectroscopy requires preparation of Ps in the ground state, which is hampered by the annihilation of electron-positron pair. The lifetime of para-Positronium (p-Ps) in the ground state is only 125 picoseconds.
- Techniques like Zeeman deceleration and buffer gas cooling have led to improvements in Ps beam characteristics. However, laser cooling – a widely used technique for atomic, ionic and molecular systems – had proven elusive for exotic Ps atoms.
The Technological Breakthrough
- In a technological breakthrough, the AEgIS collaboration at CERN has demonstrated Doppler laser cooling of positronium for the first time.
- They have successfully confined Ps atoms in a magnetic trap and implemented laser cooling in one dimension using a technique called Zeeman relaxation.
- This led to an increase in the fraction of positronium in the ground state from 10% to ~40% – the highest ever achieved for trapped positronium.
Details of the Experiment
Magnetic Trapping Setup
- A 22Na radioisotope source generates positrons which are moderated using a cryogenic Neon moderator.
- The Ps atoms formed in vacuum chamber are captured in a magnetic trap consisting of 5 coils in pentagonal geometry operated at 1 Tesla magnetic field.
- The elongated magnetic trap cools the transverse degrees of motion via cyclotron radiation while longitudinal confinement is through reflections at the end coils.
Laser System
- A 205 nm UV laser system drives the bound-bound Lyman-α transition between n=2 and n=1 states for Doppler cooling:
n=2 -> n=1 + 205 nm photon
- The laser bandwidth is adjusted to be larger than the Doppler width associated with the energy distribution of trapped Ps.
Doppler Cooling Dynamics
- When Ps atoms move towards laser source, the Doppler effect brings the absorption line closer to the laser frequency causing photon absorption and spontaneous decay.
- Each photon absorption-emission cycle applies a small momentum kick opposite to Ps velocity causing gradual velocity reduction and cooling.
Diagnostics
- Lyman-α fluorescence spot at the trap center provided real-time evidence of successful laser cooling.
- Annihilation detectors mapped 3D distribution of cold Ps for the first time.
Key Results
- 40% of trapped Ps cooled to ground state compared to ~10% before laser cooling. This is the highest ground state fraction reported for trapped positronium.
- Narrowing of annihilation spatial distribution showing evidence of cooling in all 3 dimensions.
- Measurement of Ps temperature and establishment of cooling cycles.
Significance of This Achievement
Fundamental Physics Studies
- Precision spectroscopy of cold Ps can provide most stringent tests of bound state QED calculations.
- Also enables search for new physics like dark matter candidates and compact dimensions.
Towards Bose-Einstein Condensation
- Further laser and evaporative cooling can produce degenerate Bose-Einstein condensate (BEC) of positronium as an electron-positron macroscopic quantum state.
Antimatter Gravity Experiments
- AEgIS collaboration aims to measure gravitational acceleration of antihydrogen and cold Ps is the first step towards that goal.
Enabling Precision Measurements
- Narrow velocity distribution and increased ground state fraction will substantially reduce systematic uncertainties in proposed measurements like:
- Gravitational acceleration of Ps
- Ps hyperfine splitting
- Ps decay rate
- Lamb shift
- Determination of Rydberg constant
- Test of CPT symmetry
Outstanding Remarks
The demonstrated laser cooling of positronium to milliKelvin temperatures is the first implementation of a technique that has profoundly impacted atomic physics, for a system with antimatter constituents.
- The exciting prospects of studying cold Ps for fundamental physics tests combined with applications like Ps BEC warrants deeper investigations into laser and evaporative cooling to nanokelvin scale.
- Given rapid advances in antimatter capabilities at facilities like CERN-ADA and GBAR experiment, laser-cooled Ps promises to provide great scientific dividends in the future.