Shapiro steps, which are characterized by sudden changes in the voltage-current relationship of a Josephson junction exposed to microwave radiation, have now been observed for the very first time in ultracold gases by research teams from Germany and Italy. This groundbreaking work on atomic Josephson junctions sheds new light on this fascinating phenomenon and could pave the way for establishing a standard for chemical potential.
The concept originated in 1962 when Brian Josephson from the University of Cambridge theorized that if two superconductors were separated by a thin layer of insulation, the difference in phase between the wavefunctions on either side would enable quantum tunneling, resulting in a current even when there was no voltage applied.
A year later, Sidney Shapiro and his team at Arthur D. Little demonstrated that by using a microwave field to induce an alternating electric current, the phase of the wavefunctions across a Josephson junction would evolve at different rates. This process leads to quantized increments in the potential difference across the junction itself. The magnitude of these so-called "Shapiro steps" is solely dependent on the frequency of the applied field and the electrical charge, making it a crucial reference point for the volt standard.
In their subsequent research, scientists have created analogs of Josephson junctions in various systems, including liquid helium and ultracold atomic gases. In this latest study, two independent teams successfully detected Shapiro steps within ultracold quantum gases. Instead of employing a fixed insulator, the researchers utilized focused laser beams to generate barriers that segmented the traps into two distinct sections. They then adjusted the positions of these barriers to modify the potential experienced by the atoms on either side.
Herwig Ott from RPTU University Kaiserslautern-Landau in Germany, who led one of the research groups, explains, "When we move the atoms at a constant speed, it results in a continuous flow of atoms through the barrier. This mimics a direct current. To implement the Shapiro protocol, we must apply alternating current, which can be achieved by varying the barrier over time."
Collaborating with teams from Hamburg and the United Arab Emirates, Ott and his colleagues employed a Bose–Einstein condensate (BEC) composed of rubidium-87 atoms. Meanwhile, Giulia Del Pace from the European Laboratory for Nonlinear Spectroscopy at the University of Florence and her team, which included the same UAE collaborators, studied ultracold lithium-6 atoms, which are classified as fermions.
Both research groups confirmed the existence of the theoretically predicted Shapiro steps. However, Ott and Del Pace emphasize that their findings go beyond mere validation of existing theories. "What we found is that regardless of the underlying mechanisms at play, the occurrence of Shapiro steps is a universal phenomenon," Ott notes. While in superconductors, these steps result from the breaking of Cooper pairs, in ultracold atomic gases, they arise from the formation of vortex rings. Despite the differing origins, the mathematics governing both scenarios remains consistent, which Ott finds particularly noteworthy.
Del Pace expressed initial skepticism about whether Shapiro steps could manifest in strongly interacting fermions, which exhibit significantly more interactions than electrons in superconductors. She posed a critical question: "Is the strong interaction a hindrance or does it actually facilitate the dynamics? It turns out it enhances the dynamics."
To investigate this, Del Pace’s group manipulated a variable magnetic field to transition their system between a BEC of molecules, characterized by Cooper pair dominance, and a unitary Fermi gas, where particles interacted at the maximum allowable level dictated by quantum mechanics. The size of the observed Shapiro steps varied based on the intensity of interparticle interaction.
Both researchers suggest that their findings could eventually lead to a reference standard for chemical potential—a measure of atomic interaction strength or the equation of state within a system. "While the equation of state is well-defined for a BEC or a strongly interacting Fermi gas, there exists a range of interaction strengths where the equation of state remains entirely unexplored. We can draw inspiration from the application of Josephson junctions in superconductors to investigate the equation of state in regions where it is currently unknown," Del Pace elaborates.
The two studies are featured together in the journal Science: Del Pace's article and Ott's article.
RocĂo Jáuregui Renaud from the Autonomous University of Mexico has expressed admiration for the research, particularly for demonstrating these effects in both bosonic and fermionic systems. "These studies are significant, and while their findings align, the experimental setups differ," she states. "Currently, the goal isn't to enhance our understanding of superconductivity directly but rather to gain insights into phenomena that may be elusive in electronic systems but observable in neutral atoms."
But here's where it gets controversial—could the implications of these findings reshape our understanding of quantum interactions? What do you think about the universality of Shapiro steps across different systems? Share your thoughts in the comments!
