INQA Conference 2025: Timothy Duty - Qilimanjaro Quantum Tech

Title: The Josephson-junction chain: from super-inductor to superinsulator

Abstract: In the design space for flux qubits, a very large inductance is required to reach the fluxonium regime. This is typically achieved using a chain of Josephson junctions that provide linear inductance orders of magnitude greater than simple geometric inductance. On the other hand, the internal degrees of freedom of such chains exhibit additional dynamics ranging from linear transmission line modes to quantum phase slips. For fluxonium qubits, such modes can adversely affect coherence.
In junction chains where charging energies begin to dominate, quantum
phase slips proliferate, eventually leading to a quantum phase transition
known as the superconductor-insulator transition. Below the transition the
chain becomes a ‘super-insulator’, and it has been imagined that such a chain could function as a QPS circuit element, or in other words, implementation of the so-called ‘dual’ Josephson relation. The existence of such an element would lead to a range of interesting possibilities for quantum circuits and novel devices. Some examples are novel quantum sensors, qubit couplers, or the socalled ‘duomon’ qubit. However, nature is at times not so nice and has made this intriguing possibility quite difficult to achieve. I will show how a Josephson-junction chain can be described as a TomonagaLuttinger system. Using this approach, the insulating state can be understood as a single T-L mode pinned by disorder. The theory indeed predicts a critical voltage, experimentally observable scaling laws in terms of chain parameters, but unfortunately also shows a strongly suppressed dual Josephson effect. Can one circumvent this using disordered nanowires or perhaps some other device? Do we really understand the underlying physics of quantum phase slips? Can we use more complex chains as quantum simulators? Inquiring minds want to know!