The formation of strongly correlated fermion pairs is fundamental for the emergence of fermionic superfluidity and superconductivity¹. For instance, Cooper pairs made of two electrons of opposite spin and momentum at the Fermi surface of the system are a key ingredient of Bardeen–Cooper–Schrieffer (BCS) theory—the microscopic explanation of the emergence of conventional superconductivity². Understanding the mechanism behind pair formation is an ongoing challenge in the study of many strongly correlated fermionic systems³. Controllable many-body systems that host Cooper pairs would thus be desirable. Here we directly observe Cooper pairs in a mesoscopic two-dimensional Fermi gas. We apply an imaging scheme that enables us to extract the full in situ momentum distribution of a strongly interacting Fermi gas with single-particle and spin resolution⁴. Our ultracold gas enables us to freely tune between a completely non-interacting, unpaired system and weak attractions, where we find Cooper pair correlations at the Fermi surface. When increasing the attractive interactions even further, the pairs gradually turn into deeply bound molecules that break up the Fermi surface. Our mesoscopic system is closely related to the physics of nuclei, superconducting grains or quantum dots^5,6,7. With the precise control over the interactions, particle number and potential landscape in our experiment, the observables we establish in this work provide an approach for answering longstanding questions concerning not only such mesoscopic systems but also their connection to the macroscopic world.

强相关费米子对的形成是费米子超流和超导¹出现的基础。例如，在系统费米表面由两个自旋和动量相反的电子组成的库珀对是 Bardeen-Cooper-Schrieffer (BCS) 理论的关键成分——这是对传统超导性出现的微观解释²。了解配对形成背后的机制是研究许多强相关费米子系统的一个持续挑战³. 因此，托管库珀对的可控多体系统将是可取的。在这里，我们直接观察介观二维费米气体中的库珀对。^{我们应用了一种成像方案，使我们能够提取具有单粒子和自旋分辨率4}的强相互作用的费米气体的完整原位动量分布。我们的超冷气体使我们能够在完全不相互作用、不成对的系统和弱吸引力之间自由调整，我们在费米表面发现库珀对相关性。当进一步增加有吸引力的相互作用时，这些对逐渐变成破坏费米表面的深度结合的分子。我们的介观系统与原子核、超导晶粒或量子点的物理学密切相关^5,6,7. 通过在我们的实验中对相互作用、粒子数和潜在景观进行精确控制，我们在这项工作中建立的可观测物提供了一种方法，可以回答长期存在的问题，这些问题不仅涉及此类介观系统，还涉及它们与宏观世界的联系。