Superconducting Circuits Enable On-Chip Gyration and Non-Reciprocal Microwave Control.

The manipulation of microwave signals within superconducting circuits represents a significant area of development for advanced quantum technologies and signal processing. Achieving non-reciprocal signal transmission, where a signal travels differently depending on its direction, is particularly challenging, often requiring external magnetic fields which introduce unwanted noise and complexity. Researchers at the University of Maryland and the University of Illinois at Urbana–Champaign now report a method for creating ‘pure gyration’ – a specific type of non-reciprocal coupling – within a single chip using only superconducting components and time-varying modulation. Zhiyin Tu, Violet Workman, Gaurav Bahl, and Alicia J. Kollár detail their findings in a new article, “Realization of pure gyration in an on-chip superconducting microwave device”, demonstrating a system capable of substantial signal isolation and offering a pathway towards scalable non-reciprocal metamaterials. Their approach utilises spatially and temporally modulated superconducting resonators, effectively creating a synthetic magnetic field without the need for external magnets.

Researchers report the successful demonstration of pure gyration, a form of non-reciprocal coupling, between degenerate states through the application of spatio-temporal modulation. This means electromagnetic waves propagate differently depending on the direction of travel, a property not typically observed in conventional materials. The experiments utilise microwave superconducting resonators, circuits exhibiting zero electrical resistance at low temperatures, modulated with arrays of dc-SQUIDs, or Superconducting Quantum Interference Devices, extremely sensitive magnetometers. This methodology establishes a technique independent of specific materials or operating frequencies, potentially enabling the creation of large-scale non-reciprocal metamaterials, artificially engineered materials with properties not found in nature.

The study meticulously maps continuous exceptional surfaces within the modulation parameter space. Exceptional surfaces represent conditions where the system’s behaviour changes dramatically, allowing for arbitrarily large contrasts in signal magnitude. Crucially, robust volumes of phase contrast, differences in the timing of the wave, also exist within this space. The intersection of these surfaces reliably generates pure gyration, providing precise control over the non-reciprocal behaviour and offering a pathway to tailor device characteristics for specific applications. This detailed mapping provides valuable insights into the underlying physics and allows for optimisation of device performance.

Rigorous calibration procedures address systematic errors inherent in the experimental setup, ensuring the accuracy and reliability of the results. Researchers account for frequency-dependent phase delays and signal loss in transmission lines, subtracting offsets from magnitude and phase contrast measurements to isolate the true non-reciprocal response. Furthermore, they correct for differential phase offsets arising from the modulation signal pathways, utilising the anti-symmetry of the response – the predictable opposite behaviour when the input and output are reversed – to extract and compensate for these effects.

Experimental results demonstrate substantial isolation, quantified in decibels, a logarithmic unit measuring signal strength, and the first on-chip gyrator constructed solely from superconducting circuit elements. A gyrator is a circuit that mimics the behaviour of an ideal inductor, and in this case, exhibits non-reciprocal behaviour. This achievement validates the effectiveness of the spatio-temporal modulation technique and the accuracy of the implemented calibration procedures, confirming the potential for practical applications. The demonstrated performance establishes a benchmark for future research and development in this area, and opens up new avenues for exploring the fundamental physics of non-reciprocal devices.

Future work will focus on scaling this approach to create more complex non-reciprocal metamaterials, expanding the capabilities and functionalities of these devices. Expanding these capabilities requires a thorough investigation of the impact of increased system size and exploration of alternative modulation schemes. Researchers plan to investigate novel modulation techniques and optimise device parameters to enhance performance and functionality. Further research will also explore the potential for integrating these devices with other quantum circuits, potentially enabling novel functionalities and applications in quantum information processing.