When laser radiation is skilfully applied, atoms and molecules can be cooled^1,2,3, allowing the precise measurements and control of quantum systems. This is essential for the fundamental studies of physics as well as practical applications such as precision spectroscopy^4,5,6,7, ultracold gases with quantum statistical properties^8,9,10 and quantum computing. In laser cooling, atoms are slowed to otherwise unattainable velocities through repeated cycles of laser photon absorption and spontaneous emission in random directions. Simple systems can serve as rigorous testing grounds for fundamental physics—one such case is the purely leptonic positronium^11,12, an exotic atom comprising an electron and its antiparticle, the positron. Laser cooling of positronium, however, has hitherto remained unrealized. Here we demonstrate the one-dimensional laser cooling of positronium. An innovative laser system emitting a train of broadband pulses with successively increasing central frequencies was used to overcome major challenges posed by the short positronium lifetime and the effects of Doppler broadening and recoil. One-dimensional chirp cooling was used to cool a portion of the dilute positronium gas to a velocity distribution of approximately 1 K in 100 ns. A major advancement in the field of low-temperature fundamental physics of antimatter, this study on a purely leptonic system complements work on antihydrogen¹³, a hadron-containing exotic atom. The successful application of laser cooling to positronium affords unique opportunities to rigorously test bound-state quantum electrodynamics and to potentially realize Bose–Einstein condensation^{14,15,16,17,18} in this matter–antimatter system.

当巧妙地应用激光辐射时，原子和分子可以被冷却^1,2,3 ，从而可以精确测量和控制量子系统。这对于物理学的基础研究以及精密光谱学^4,5,6,7 、具有量子统计特性的超冷气体^8,9,10和量子计算等实际应用至关重要。在激光冷却中，通过激光光子吸收和随机方向自发发射的重复循环，原子被减慢到其他方式无法达到的速度。简单的系统可以作为基础物理学的严格测试场——一个这样的例子是纯轻子正电子素^11,12 ，这是一种由电子及其反粒子正电子组成的奇异原子。然而，正电子素的激光冷却迄今为止尚未实现。在这里，我们演示了正电子素的一维激光冷却。创新的激光系统发射一系列中心频率逐渐增加的宽带脉冲，用于克服正电子寿命短以及多普勒展宽和反冲效应带来的主要挑战。使用一维线性调频冷却将一部分稀正电子素气体冷却到 100 ns 内大约 1 K 的速度分布。这项关于纯轻子系统的研究是反物质低温基础物理领域的一项重大进展，补充了反氢¹³ （一种含有强子的奇异原子）的工作。 激光冷却在正电子素上的成功应用为严格测试束缚态量子电动力学提供了独特的机会，并有可能在这个物质-反物质系统中实现玻色-爱因斯坦凝聚^{14,15,16,17,18} 。