Laser Cooling of Antihydrogen Atoms

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 collision. Slowing the translational motion of atoms and ions by application of such a force²³, known as laser cooling, was first demonstrated 40 years ago⁴⁵. 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 antihydrogen²⁹, the antimatter counterpart of hydrogen with an aproton and a positron. By exciting the 1s–2p transition in antihydrogen with pulsed, narrow-linewidth, Lyman-α laser radiation²⁴, we Doppler-cool samples of magnetically trapped antihydrogen. Although we sharply resonant laser cooling on individual anti-atoms, leading to energy reduction in the mean transverse energy by more than an order of magnitude—while a substantial fraction of the anti-atoms attaining submicrovolt energies traverse kinetic energies. We also report the observation of the laser-driven 1s–2s transition in samples of laser-cooled antihydrogen atoms. The observed spectral lines are 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 spectroscopy¹³ and gravitational³⁴ 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.

C. J. Baker, W. Bertsche, A. Capra, C. Carruth, C. L. César, M. Charlton, A. Christensen, R. Collier, A. Cridland Mathad, S. Eriksson, A. Evans, N. Evetts, J. Fajans, T. Friesen, M. C. Fujiyama, R. D. Gilt, P. Grandemange, P. Grann, J. S. Hangs, V. N. Hardy, M. E. Hayden, D. Hodgjkinson, E. Hunter, C. A. Isaac, M. A. Johnson, J. M. Jones, S. A. Jones, S. Jonsell, A. Kharamov, R. Knapp, L. Kurchanov, N. Madsen, D. Maxwell, J. T. K. McKenna, S. Menary, J. M. Michal, T. Momose, P. S. Mullard, J. J. Munich, K. Olchanski, A. Olin, J. Pszka, A. Powell, P. Pusa, C. O. Rasmussen, F. Robicheaux, R. L. Sacramonte, M. Sameed, E. Sarid, D. M. Silveira, D. M. Starko, C. S. O. Sutter, A. Thibeault, R. I. Thompson, D. P. van der Werf, J. S. Wurtzele