Realization of pure gyration coupling in an on-chip superconducting microwave device

Synthetic materials that emulate tight-binding Hamiltonians have enabled a wide range of advances in topological and non-Hermitian physics. A crucial requirement in such systems is the engineering of nonreciprocal couplings and synthetic magnetic fields. More broadly, the development of these capabilities in a manner compatible with quantum-coherent degrees of freedom remains an outstanding challenge, particularly for superconducting circuits, which are highly sensitive to magnetic fields. Here, we demonstrate that —a nonreciprocal coupling with exactly matched magnitude but nonreciprocal <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>π</a:mi> </a:math> phase contrast—can be realized between degenerate states using only spatiotemporal modulation. We first show the existence of continuous exceptional surfaces in modulation parameter space where coupling with arbitrarily large magnitude contrast can be achieved, with robust volumes of <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mi>π</b:mi> </b:math> phase contrast contained within. We then demonstrate that intersection of these volumes necessarily gives rise to continuous surfaces in parameter space where pure gyration is achieved. Our experiments are performed using microwave superconducting resonators that are modulated using dc superconducting quantum interference device arrays, and achieve isolation and pure gyration coupling with only superconducting circuit elements. However, our method is fully agnostic to physical implementation (classical or quantum) or frequency range and paves the way to realizing large-scale nonreciprocal metamaterials in any platform.