Lasing threshold in the conventional lasers is the minimum input power required to initiate laser oscillation. It has been widely accepted that the conventional laser threshold occurring around a unity intracavity photon number can be eliminated in the input-output curve by making the so-called β parameter approach unity. The recent experiments, however, have revealed that even in this case the photon statistics still undergo a transition from coherent to thermal statistics when the intracavity mean photon number is decreased below unity. Since the coherent output is only available above the diminished threshold, the long-sought promise of thresholdless lasers to produce always coherent light has become questionable. Here, we present an always-coherent thresholdless laser based on superradiance by two-level atoms in a quantum superposition state with the same phase traversing a high-Q cavity. Superradiant lasing was observed without the conventional lasing threshold around the unity photon number and the photon statistics remained near coherent even below it. The coherence was improved by reducing the coupling constant as well as the excited-state amplitude in the superposition state. Our results pave a way toward always-coherent thresholdless lasers with more practical media such as quantum dots, nitrogen-vacancy centers and doped ions in crystals.

传统激光器中的激光阈值是启动激光振荡所需的最小输入功率。人们普遍认为，通过使所谓的β参数接近1，可以在输入-输出曲线中消除在单位腔内光子数周围出现的传统激光阈值。然而，最近的实验表明，即使在这种情况下，当腔内平均光子数降低到单位以下时，光子统计仍然经历从相干统计到热统计的转变。由于相干输出仅在降低的阈值以上才可用，因此长期以来寻求的无阈值激光器产生始终相干光的承诺已变得值得怀疑。在这里，我们提出了一种始终相干的无阈值激光器，基于量子叠加态的两级原子的超辐射，具有相同的相位穿过高 Q 腔。在没有围绕统一光子数的传统激光阈值的情况下观察到超辐射激光，并且即使低于该阈值，光子统计数据仍然接近相干。通过降低耦合常数以及叠加态的激发态振幅来提高相干性。我们的研究结果为使用更实用的介质（如量子点、氮空位中心和晶体中的掺杂离子）实现始终相干的无阈值激光器铺平了道路。