Laser Cooling of Antihydrogen Atoms

The antihydrogen atom, composed of an antiproton and a positron, presents a unique opportunity to test fundamental symmetries and principles of physics by comparing its properties to those of hydrogen. Precision measurements require antihydrogen at the lowest possible kinetic energies. Laser cooling, a well-established technique for matter, has never been applied to antimatter due to the challenges in generating and manipulating vacuum ultraviolet light, the limited optical access to trapped antihydrogen, and the extremely low density of antihydrogen samples. This paper addresses this challenge by describing a successful laser cooling experiment using the Antihydrogen Laser Physics Apparatus (ALPHA). The work builds on previous studies that demonstrated the feasibility of laser cooling antihydrogen through simulations, suggesting that long confinement times and higher laser repetition rates could overcome the limitations imposed by the low excitation rate. These simulations also predicted the possibility of three-dimensional cooling with one-dimensional laser access via anharmonic coupling of antihydrogen's motional degrees of freedom in the trap, enhanced by tailoring the magnetic trapping field. The successful application of laser cooling to antihydrogen opens avenues for significantly improving the precision of future experiments testing fundamental physics.

Significant advancements have been made in antihydrogen studies, including techniques for production, confinement, and interrogation using microwaves and lasers. Experiments are underway to measure the gravitational properties of antimatter. However, the high kinetic energies of anti-atoms have limited the precision of these measurements. Prior work focused on demonstrating the feasibility of laser cooling antihydrogen through simulations, highlighting the potential for overcoming the challenges posed by the low density and limited optical access using long cooling durations and higher laser repetition rates. These studies also indicated the possibility of achieving three-dimensional cooling with one-dimensional laser access by leveraging anharmonic coupling of the atom's motional degrees of freedom in the magnetic trap. This forms the theoretical basis for the current experimental approach.

The experiment was conducted using the ALPHA-2 apparatus. Antihydrogen atoms, initially trapped in a magnetic minimum trap with a depth of about 50 µeV, were subjected to a pulsed 121.6 nm Lyman-α laser. The laser frequency was detuned to achieve Doppler cooling (red detuning) or heating (blue detuning). The experiment involved four main series: no laser, heating, cooling, and ‘stack and cool’. The ‘stack and cool’ series involved continuous laser cooling during the antihydrogen accumulation phase. The 1s–2p transition was probed at various frequencies to measure the energy distribution of the antihydrogen. The time-of-flight (TOF) method was used to reconstruct the transverse kinetic energy of the anti-atoms. In addition, a spectroscopy experiment using 243.1 nm light to drive the two-photon 1s–2s transition was performed on both laser-cooled and uncooled samples to compare spectral linewidths. Numerical simulations were used to model the antihydrogen's motion and interaction with the laser light. The simulations tracked the motion of trapped antihydrogen atoms, including their interactions with cooling and probing laser radiation. A parameter, W_a (the total energy injected into the atom by the laser), was adjusted empirically to match the experimental results. This provided a qualitative understanding of the cooling process, despite incomplete knowledge of some experimental parameters. The TOF distributions were used to reconstruct transverse kinetic energies, and these were analyzed to reveal correlations between longitudinal and transverse energies. This experimental setup and procedure enabled the detailed study of laser cooling's effectiveness on antihydrogen samples.

The experimental results demonstrate successful laser cooling of antihydrogen. The spectral lineshape of the 1s–2p transition narrowed significantly when a red-detuned laser was applied, indicating longitudinal cooling. The TOF distributions showed a substantial reduction in the transverse kinetic energy of the anti-atoms, particularly in the ‘stack and cool’ series, where the median transverse energy was reduced by an order of magnitude, with a substantial fraction of atoms reaching submicroelectronvolt energies. The comparison with simulations showed qualitative agreement between experimental and simulated lineshapes and TOF distributions, confirming the observed energy reduction. Analysis of the correlation between longitudinal and transverse energies revealed that in the cooling series, transversely cold atoms were also longitudinally colder. In the spectroscopy experiment, the 1s–2s transition linewidth was significantly narrower (by a factor of four) for the laser-cooled sample compared to the uncooled sample, demonstrating that the reduced velocity spread of the atoms due to laser cooling leads to an improvement of the transition spectral resolution. The experimental data demonstrates a significant reduction in anti-hydrogen's final energy and a noticeable increase in cooling rates. The achieved results in energy reduction and spectral line narrowing highlight the effectiveness of the laser cooling technique and its potential for future applications.

The successful laser cooling of antihydrogen addresses a significant challenge in the field of antimatter research. The observed reduction in energy and the narrowing of the spectral lines demonstrate the feasibility of this technique and its potential to improve the precision of future experiments. The ability to manipulate the motion of antimatter atoms with laser light has significant implications for improving the precision of spectroscopy and gravitational measurements on antihydrogen, thereby enhancing tests of fundamental physics principles. The quantitative analysis of energy reductions and spectral line narrowing validates the theoretical predictions and confirms that laser cooling is an effective tool for manipulating antimatter atoms. The observed correlations between longitudinal and transverse energies provide further insights into the cooling dynamics of dilute antihydrogen samples in magnetic traps. Future studies need to concentrate on a detailed understanding of these dynamics to further optimize cooling techniques. The developed 'stack and cool' method transforms the current antihydrogen accumulation phase into an efficient cooling period, increasing the overall efficiency of the experiment.

This paper reports the first successful demonstration of laser cooling of antihydrogen atoms, significantly reducing their kinetic energy and leading to narrower spectral lines. This breakthrough has immediate implications for precision spectroscopy experiments aimed at testing fundamental symmetries and opens exciting new possibilities for future research involving antimatter, including anti-atomic fountains and anti-atom interferometry. Further optimizations of laser parameters and exploration of combined cooling schemes are expected to lead to even colder and denser antihydrogen samples. The demonstrated 'stack and cool' procedure improves the experimental cycle efficiency.

The current experiment employs one-dimensional laser access, potentially limiting the cooling efficiency due to indirect coupling between motional degrees of freedom. The empirical adjustment of a simulation parameter (W_a) to match experimental results indicates that some aspects of the experimental system are not fully understood and require further investigation. While the achieved level of cooling is significant, further reduction of antihydrogen energy is possible through optimizations and exploring complementary cooling techniques. A more detailed investigation of the system's parameters and dynamics will enhance the interpretation and generalizability of the experimental results.