Laser cooling of antihydrogen atoms

Abstract

The photon—the quantum excitation of the electromagnetic field—is massless but carries momentum. A photon can therefore exert a force on an object upon collision1. Slowing the translational motion of atoms and ions by application of such a force2,3, known as laser cooling, was first demonstrated 40 years ago4,5. It revolutionized atomic physics over the following decades6–8, and it is now a workhorse in many fields, including studies on quantum degenerate gases, quantum information, atomic clocks and tests of fundamental physics. However, this technique has not yet been applied to antimatter. Here we demonstrate laser cooling of antihydrogen9, the antimatter atom consisting of an antiproton and a positron. By exciting the 1S–2P transition in antihydrogen with pulsed, narrow-linewidth, Lyman-α laser radiation10,11, we Doppler-cool a sample of magnetically trapped antihydrogen. Although we apply laser cooling in only one dimension, the trap couples the longitudinal and transverse motions of the anti-atoms, leading to cooling in all three dimensions. We observe a reduction in the median transverse energy by more than an order of magnitude—with a substantial fraction of the anti-atoms attaining submicroelectronvolt transverse kinetic energies. We also report the observation of the laser-driven 1S–2S transition in samples of laser-cooled antihydrogen atoms. The observed spectral line is approximately four times narrower than that obtained without laser cooling. The demonstration of laser cooling and its immediate application has far-reaching implications for antimatter studies. A more localized, denser and colder sample of antihydrogen will drastically improve spectroscopic11–13 and gravitational14 studies of antihydrogen in ongoing experiments. Furthermore, the demonstrated ability to manipulate the motion of antimatter atoms by laser light will potentially provide ground-breaking opportunities for future experiments, such as anti-atomic fountains, anti-atom interferometry and the creation of antimatter molecules.

Primordial Black Holes from Collapsing Antimatter

“…The ALPHA experiment roughly goes as follows (ALPHA Collaboration, 2011, Figure 1) and (Baker et al, 2021, Figure 1). Using CERN's antiproton and positron accelerators and decelerators, soft antiproton and positron beams are injected into a tube of 280 mm axial length and of 44.35 mm diameter.…”

“…However after taking an overview of these efforts we can select for our purposes the most relevant one namely the ALPHA experiment at CERN, which is a very exciting ongoing experiment exhibiting lot of new (but on theoretical grounds expected) facts about antihydrogen atoms, to see whether or not the communicated results can be used to support or reject the Proposal. Latest results have been reported in Baker et al (2021) however from our point of view, i.e. regarding some technical background details, we shall revisit an older paper ALPHA Collaboration (2011) from 2011 too.…”

“…Thus of course this environment consists of ordinary matter having standard thermodynamical properties. What from our viewpoint relevant is the values of the following three parameters in every individual attempt or run of the experiment: the number N of trapped antihydrogen atoms, their temperature T and their confinement time t. Although the cumulative value of N was reported to be about 1000 in ALPHA Collaboration (2011); Baker et al (2021), its average value in individual attempts (i.e. the situation when antiatoms are under sharp observational control), as summarized in (ALPHA Collaboration, 2011, Table 1 and Figure 2) (but not available in Baker et al, 2021), was N ≈ 1 .…”

Low Energy Antimatter Physics

“…As a part of this development, one has to note the remarkable improvement constituted by the first extension of the laser cooling technique to antimatter. This was obtained by the ALPHA collaboration by making use of the 1S-2P dipole-allowed transition and the Lyman-α laser system developed on purpose [48]. Meanwhile, the laser system performs cooling along a specific direction (symmetry axis of the apparatus, see Figure 4a) and the confining effect of the traps couples the spatial degrees of freedom so that overall cooling is demonstrated.…”

An ion trap source of cold atomic hydrogen via photodissociation of the BaH+ molecular ion

“…By comparing it to antihydrogen, hydrogen can be used to test matter-antimatter symmetries with high precision [8]. In the last 10 years, antihydrogen physics has advanced to the point where more than 1000 atoms can be simultaneously trapped [9], cooled [10], and probed with lasers [11,12] and microwaves [13]. The field is rapidly approaching hydrogen-like precision, and hydrogen-antihydrogen comparisons may soon encounter the same class of hard-to-characterise systematic effects that lead to discrepancies when different groups using different techniques try to measure the same parameters, as demonstrated by the aforementioned proton radius puzzle.…”