本期封面所示为一个量子化的超流体“原子线路”(atomtronic circuit)中的滞后现象，是根据一个处于束缚状态的环形“玻色-爱因斯坦凝聚态” (BEC)的图像做出的图示。滞后现象是指一个系统的物理性质强烈依赖于所施加扰动的历史，在包括硬盘驱动器和磁通门磁力计等在内的电子线路中被广泛利用，并且是射频SQUIDs（“超导量子干涉装置”的英文首字母缩写）发挥功能所必需的。滞后现象也是“超流体性”的基础，曾被预测会出现在超流体原子气体如BEC中。现在，Gretchen Campbell及同事报告了对由被一个转动的弱连结(weak link)阻隔的一个超流体BEC环形成的一个线路中的量子化循环状态之间的滞后现象所做的首次直接探测。这一系统中滞后现象的存在对新兴的“原子技术”(atomtronics)领域有重要意义（在该领域中，超冷原子所扮演的角色与电子在电子技术中所扮演的角色相似）。原子线路中的受控滞后现象对于实用设备的研发来说可能会被证明是一个极为重要的特性。
Atomtronics is an emerging interdisciplinary field that seeks to develop new functional methods by creating devices and circuits where ultracold atoms, often superfluids, have a role analogous to that of electrons in electronics. Hysteresis is widely used in electronic circuits—it is routinely observed in superconducting circuits and is essential in radio-frequency superconducting quantum interference devices. Furthermore, it is as fundamental to superfluidity5 (and superconductivity) as quantized persistent currents critical velocity and Josephson effects. Nevertheless, despite multiple theoretical predictions, hysteresis has not been previously observed in any superfluid, atomic-gas Bose–Einstein condensate. Here we directly detect hysteresis between quantized circulation states in an atomtronic circuit formed from a ring of superfluid Bose–Einstein condensate obstructed by a rotating weak link (a region of low atomic density). This contrasts with previous experiments on superfluid liquid helium where hysteresis was observed directly in systems in which the quantization of flow could not be observed, and indirectly in systems that showed quantized flow. Our techniques allow us to tune the size of the hysteresis loop and to consider the fundamental excitations that accompany hysteresis. The results suggest that the relevant excitations involved in hysteresis are vortices, and indicate that dissipation has an important role in the dynamics. Controlled hysteresis in atomtronic circuits may prove to be a crucial feature for the development of practical devices, just as it has in electronic circuits such as memories, digital noise filters (for example Schmitt triggers) and magnetometers (for example superconducting quantum interference devices).