A breakthrough in antimatter physics has been achieved by an international collaboration of researchers, who have successfully demonstrated the laser cooling of positronium, a short-lived atom that provides an ideal testing ground for bound-state quantum electrodynamics. The experiment, performed at CERN, could pave the way for new studies on the interactions between light and charged matter, as well as the possibility of creating an antimatter Bose-Einstein condensate.
What is positronium and why is it important?
Positronium is a fundamental atom that consists of an electron and a positron, which are both leptons and interact through electromagnetic and weak forces. Unlike ordinary atoms, which are made of a mixture of baryons and leptons, positronium is purely leptonic and has no nuclear matter. It is also unstable and annihilates in vacuum with a lifetime of only 142 nanoseconds.
Positronium is important for quantum studies because it is a simple system that can be used to test the theory of quantum electrodynamics, which describes how light and matter interact at the quantum level. Quantum electrodynamics is one of the most successful theories in physics, but it still faces some challenges and uncertainties, especially when applied to bound states of particles. By measuring the energy levels and transitions of positronium with high precision, physicists can test the validity and accuracy of quantum electrodynamics and look for possible deviations or new phenomena.
Positronium is also important for antimatter research because it can be used to produce antihydrogen atoms, which are the simplest antimatter atoms and consist of an antiproton and a positron. Antihydrogen atoms can be used to study the properties and behaviour of antimatter, which is one of the biggest mysteries in physics. Antimatter is the opposite of matter, with opposite charge and spin, and it annihilates when it meets matter. Antimatter is predicted to have the same mass and energy as matter, but this has not been confirmed experimentally. Moreover, antimatter is extremely rare in nature, and physicists do not know why there is more matter than antimatter in the universe, which violates the symmetry between matter and antimatter. By comparing antihydrogen with hydrogen, physicists can test whether antimatter obeys the same laws of physics as matter, such as the weak equivalence principle, which states that all bodies fall at the same rate in a gravitational field regardless of their mass or composition.
How was positronium laser cooled?
Laser cooling of positronium has been a long-standing challenge in physics, as it requires high-intensity, large-bandwidth and long-duration laser pulses to reduce the temperature of the atoms during their short lifespan. The AEgIS collaboration, which includes physicists from 19 European groups and one Indian group, has developed a special alexandrite-based laser system that meets these requirements. The team has reported in Physical Review Letters that they have cooled positronium atoms from about 380 Kelvin to 170 Kelvin, corresponding to a decrease of the transversal component of their velocity from 54 km/s to 37 km/s.
The experiment was performed at CERN, the European Organization for Nuclear Research, where a beam of positrons was generated by smashing protons into a metal target. The positrons were then slowed down and injected into a porous silicon target, where they formed positronium atoms by capturing electrons from the silicon atoms. The positronium atoms exited from the target with a high temperature and velocity due to their thermal motion and their recoil from the formation process. To cool them down, the researchers used a pulsed laser that emitted light at a wavelength of 243 nanometers, which matched the transition between two energy levels of positronium. By tuning the frequency of the laser slightly below the resonance frequency of the transition, the researchers exploited the Doppler effect, which causes a shift in the frequency of light depending on the relative motion of the source and the observer. In this way, only the positronium atoms that were moving towards the laser could absorb photons from it and lose kinetic energy. The photons were then re-emitted in random directions, resulting in a net decrease of velocity along the direction of the laser beam. By applying this process repeatedly with multiple laser pulses from different directions, the researchers achieved a three-dimensional cooling of positronium.
The cooling process was monitored by detecting the annihilation gamma rays emitted by positronium when it decayed. By measuring the time interval between successive gamma rays from each positronium atom, the researchers could infer its velocity distribution and temperature. The results showed that after applying 12 laser pulses within 60 nanoseconds, about 10% of the positronium atoms were cooled from 380 Kelvin to 170 Kelvin, while the rest remained at the initial temperature or were lost due to collisions or absorption by the target.
What are the implications and future prospects of positronium laser cooling?
The achievement of positronium laser cooling has significant implications for antimatter research, as it could enable the production of more antihydrogen atoms, which are formed by the reaction between excited positronium and trapped antiprotons. The lower the positronium velocity, the higher the probability of antihydrogen formation, which is crucial for increasing the number and quality of antihydrogen atoms available for experiments. The AEgIS collaboration aims to measure the gravitational acceleration of antihydrogen as a test of the weak equivalence principle for antimatter, which could reveal new physics if a difference is found between matter and antimatter.
Moreover, laser cooling of positronium could open up new possibilities for basic science, such as precision spectroscopy of its energy levels, which could test quantum electrodynamics with unprecedented accuracy. Quantum electrodynamics is the theory that describes how light and matter interact at the quantum level. It is one of the most successful theories in physics, but it still faces some challenges and uncertainties, especially when applied to bound states of particles. By measuring the energy levels and transitions of positronium with high precision, physicists can test the validity and accuracy of quantum electrodynamics and look for possible deviations or new phenomena.
Another potential application is the creation of an antimatter Bose-Einstein condensate, which is a state of matter where quantum phenomena become macroscopic. A Bose-Einstein condensate is formed when a large number of atoms are cooled to very low temperatures and occupy the same quantum state. A Bose-Einstein condensate of positronium has never been observed before and could reveal new properties of this exotic system. For example, it could exhibit superfluidity, which is a phenomenon where a fluid flows without friction or viscosity. It could also show quantum interference effects, such as matter-wave diffraction and interference patterns. Furthermore, it could allow the study of quantum entanglement between matter and antimatter, which is a phenomenon where two particles share a quantum state and influence each other even when separated by large distances.
The AEgIS experiment is one of the many efforts to explore the mysteries of antimatter at CERN, the European Organization for Nuclear Research. By using advanced technologies and innovative methods, the researchers hope to shed light on some of the fundamental questions in physics, such as why there is more matter than antimatter in the universe, and whether antimatter behaves differently from matter under gravity.
Recent Blog : NVIDIA Partners with Indian Govt for Sovereign AI