Superfluid Currents Switch Off in Two Steps with Added Dissipation

Scientists at the Institute of Science Tokyo, in collaboration with Osaka Metropolitan University, have revealed a new phase transition in superfluids by modelling energy loss, offering fundamental insight into quantum dynamics. Soma Takemori and colleagues modelled a system of interconnected superfluids to demonstrate how dissipation sharply alters current flow within them. Their research details a two-step process. Increasing dissipation initially enables current to flow through specific junctions, before ultimately extinguishing it altogether, a phenomenon known as a nonequilibrium dynamical phase transition. This understanding of how superfluids respond to energy loss provides key insight into the dynamics of these quantum systems and could inform the development of more robust quantum technologies.

Enhanced dissipation rates enable observation of complete current suppression in coupled superfluids

For strong tunneling amplitudes, V₃₁, a nonequilibrium dynamical phase transition (NDPT) occurs, characterised by the simultaneous vanishing of all dc Josephson currents, at a dissipation rate nearly twice as high as previously achievable with single junction studies. This represents a significant advancement in understanding the limits of superconductivity in open quantum systems. Previously, stepwise current decay was documented in similar systems, hindering observation of this simultaneous vanishing. This new observation is enabled by modelling a triad of interconnected fermionic superfluids, allowing for a more holistic understanding of dissipation’s impact. The use of a triad, rather than isolated junctions, introduces complex interactions and allows for the observation of collective behaviour not present in simpler models. A clear threshold exists where the system transitions from maintaining current flow through multiple junctions to complete current suppression, offering insights into the durability of quantum coherence in complex systems. This threshold is particularly important for applications requiring sustained quantum states.

These superfluids, systems where electrons pair up and flow without resistance due to the formation of Cooper pairs, were modelled using a sophisticated dissipative BCS theory. The BCS (Bardeen-Cooper-Schrieffer) theory is a cornerstone of superconductivity, describing how electrons form Cooper pairs and condense into a superfluid state. This work extends the BCS framework to incorporate dissipation through two-body loss, the removal of particles from the system, which represents a fundamental source of decoherence. The researchers employed a ladder representation to accurately describe the complex interactions between the superfluids and the dissipation mechanism. This method allows for a systematic approximation of the many-body problem, crucial for modelling interacting fermionic systems. Analytical calculations, using a simplified model based on rate equations, corroborated these numerical findings, strengthening the evidence for this enhanced NDPT. The agreement between analytical and numerical results validates the theoretical framework and increases confidence in the observed phase transition. The implications of this finding extend to the design of sensitive quantum sensors and information processing systems, as understanding the limits of current flow is vital for maintaining quantum coherence and preventing errors in quantum computations. Specifically, the observed NDPT sets a limit on the rate at which information can be processed without significant loss of signal.

Dissipation strength governs current behaviour in linked superfluid systems

Understanding how superfluids, materials capable of flowing without resistance, respond to energy loss is vital for building next-generation quantum technologies. Superfluidity arises from the macroscopic quantum coherence of Cooper pairs, and any process that disrupts this coherence, such as two-body loss, introduces dissipation. This work details how introducing dissipation into interconnected superfluids alters the flow of current, revealing a surprising dependence on the strength of the links between them. The strength of these links is quantified by the tunneling amplitude, V₃₁, which determines the probability of Cooper pairs tunneling between the superfluids. A larger tunneling amplitude facilitates stronger coupling and more pronounced collective effects. Although the current modelling relies on a simplified approach, neglecting potentially important factors like variations in magnetic fields or the impact of different types of particle loss, it establishes a key baseline understanding of how energy loss impacts these systems. Future research could incorporate these additional complexities to provide a more complete picture.

Real-world superfluids experience magnetic field fluctuations and varied particle loss mechanisms not fully accounted for in this study, but this research provides valuable insight for designing sensitive quantum sensors and information processing systems. Energy loss induces rotation in the superfluid order parameter upon examination of interconnected superfluids, generating direct current Josephson currents between the materials; a Josephson junction is a weak link allowing supercurrent to flow. The behaviour of these currents is critically dependent on the tunneling amplitude, which governs the strength of connections between superfluids, and it dictates the manner in which these currents vanish during a nonequilibrium dynamical phase transition, or NDPT. The observed phase transition is not merely a gradual decrease in current, but a sharp, collective suppression, indicating a fundamental change in the system’s quantum state. This sudden change in the system’s behaviour highlights the sensitivity of interconnected superfluids to external influences and provides a foundation for further investigation into the control of quantum currents. The ability to precisely control and manipulate these currents is essential for developing advanced quantum devices. Furthermore, the study’s findings contribute to a broader understanding of open quantum systems, which are constantly interacting with their environment, and the challenges of maintaining quantum coherence in such systems. The dissipation rate at which the NDPT occurs is a crucial parameter for characterising the robustness of the superfluid state against environmental noise.

The research demonstrated that energy loss in interconnected superfluids causes rotation in the superfluid order parameter and generates direct current Josephson currents. This is significant because the behaviour of these currents depends on the strength of connections between the superfluids, leading to a sharp, collective suppression of current during a nonequilibrium dynamical phase transition. Researchers found that the way currents vanish differs depending on the tunneling amplitude between superfluids. The authors suggest future work will incorporate additional complexities, such as magnetic field fluctuations, to refine this understanding of open quantum systems.