Some of the most important quantum effects powering today\u2019s technology happen on scales that are almost impossible to see. One such phenomenon, called the Josephson effect, lies at the heart of quantum computers, ultra-precise voltage standards, and sensitive medical tools used to measure brain activity.<\/p>\n
However, despite its importance, the microscopic processes inside a Josephson junction<\/a> remain largely hidden. Now, researchers in Germany have found a way to uncover them.\u00a0<\/p>\n By recreating this solid-state quantum effect using clouds of ultracold atoms, they have directly observed Shapiro steps<\/a>\u2014a key quantum phenomenon once thought to belong only to superconductors.<\/p>\n \u201cIn our experiment, we were able to visualize the resulting excitations for the first time. The fact that this effect now appears in a completely different physical system, an ensemble of ultracold atoms, confirms that Shapiro steps are a universal phenomenon,\u201d Herwig Ott, lead researcher and a physicist at Rhineland-Palatinate Technical University (RPTU), said<\/a>.<\/p>\n According to the researchers, this achievement is an important step towards opening new ways to study, visualize, and ultimately control quantum behavior.<\/p>\n Recreating an atomic Josephson junction<\/p>\n A traditional Josephson junction consists of two superconductors<\/a> separated by an extremely thin insulating barrier. Quantum mechanics allows electric current to flow across this barrier without resistance.\u00a0<\/p>\n When the current becomes strong enough, dissipation appears. If the junction is also exposed to microwave radiation, the current\u2013voltage curve develops flat plateaus known as Shapiro steps. These steps are so reliable that they form the basis of the global standard for the volt.<\/p>\n The problem is that the microscopic processes behind these steps, how energy is lost and how excitations form, are almost impossible to observe directly inside a solid superconductor. To overcome this challenge, the RPTU team turned to a technique called quantum simulation.<\/p>\n Instead of electrons in a solid, the researchers used Bose\u2013Einstein condensates<\/a> (BECs), ultracold gases in which atoms behave collectively like a single quantum wave. They created two such condensates and separated them using an extremely thin optical barrier, formed by a focused laser beam. This setup acts as an atomic Josephson junction.<\/p>\n To mimic the effect of microwave radiation, the team moved the laser barrier back and forth with a periodically modulated speed. This motion played the same role as an alternating electromagnetic field in a superconducting junction<\/a>.\u00a0<\/p>\n As the barrier moved, atoms flowed between the two condensates, and the researchers measured the resulting difference in chemical potential, the atomic equivalent of voltage. The result was surprising. Shapiro steps appeared in the atomic system.\u00a0<\/p>\n \u201cA quantum mechanical effect from solid-state physics is transferred to a completely different system\u2014and yet its essence remains the same. This builds bridges between the quantum worlds of electrons and atoms,\u201d Ott said.<\/p>\n Is it possible to link multiple Josephson Junctions?<\/p>\n The experiment is clear proof that Shapiro steps are truly universal, appearing not only in electronic superconductors but also in gases of ultracold atoms<\/a>. This also confirms that the underlying physics depends only on fundamental constants and driving frequency, not on the specific particles involved.<\/p>\n Moreover, the research suggests that atomic systems make quantum behavior visible in ways solid materials cannot, offering a powerful new platform to study dissipation, coherence, and non-equilibrium quantum dynamics.\u00a0<\/p>\n However, the current setup is still a simplified model and cannot yet reproduce the full complexity of real electronic circuits<\/a>.<\/p>\n Next, the researchers plan to connect multiple atomic Josephson junctions, creating full-fledged atomic circuits\u2014a growing field known as atomtronics. Such circuits could serve as testbeds for future quantum technologies and help scientists understand electronic components at a much deeper, microscopic level.<\/p>\n