Compact self-matched gyrators using edge magnetoplasmons

Non-reciprocal microwave components are indispensable in quantum information processing and cryogenic measurement. Conventional implementations, however, are bulky and incompatible with on-chip scalable integration. Recent efforts to develop compact on-chip alternatives often rely on active modulation or complex circuit architectures, which introduce additional losses and degrade performance. We demonstrate the realization of compact, self-impedance-matched gyrators based on edge magnetoplasmons in a two-dimensional electron gas. Gyrators can be used as building blocks for other non-reciprocal elements such as isolators and circulators. Our devices achieve gyration from 0.2 to 2 GHz, tunable by moderate out-of plane magnetic fields below 400 mT, and sub-mm footprint, two orders of magnitude smaller than conventional ferrite-based components. Using an electrode geometry predicted to minimize reflections, we achieve insertion losses as low as 2 to 4 dB. The self-matched design framework we utilize is broadly applicable, and can be implemented in a wide variety of non-reciprocal device architectures.

💡 Research Summary

The paper presents a compact, cryogenic gyrator that exploits edge magnetoplasmons (EMPs) in a high‑mobility two‑dimensional electron gas (2DEG) to achieve non‑reciprocal microwave transmission without the need for external impedance‑matching networks. Conventional ferrite‑based circulators and isolators are bulky and unsuitable for large‑scale integration in quantum processors. Recent on‑chip approaches often rely on active modulation or intricate circuit topologies, which introduce extra loss and complexity.

In the reported devices, a circular mesa of GaAs‑based 2DEG is patterned with three ports (P1, P2, P3) placed around the perimeter. Each port is capacitively coupled to the edge via a micron‑scale metallic gate, eliminating ohmic contacts and the associated dissipation. Port P3 is deliberately made twice as long as the other ports and is grounded; this asymmetry provides a self‑impedance‑matching condition, ensuring that the characteristic impedance of the transmission line (≈ 50 Ω) automatically matches the effective impedance of the EMP channel. When a perpendicular magnetic field B⊥ (≤ 400 mT) is applied, EMPs propagate chirally along the edge, breaking reciprocity: forward transmission (P1→P2) follows a longer path than reverse transmission (P2→P1).

The authors measured scattering parameters S21 and S12 at temperatures around 50 mK over 0.2–2 GHz. By subtracting the linear cable delay and defining the non‑reciprocal phase difference Δφ = Arg(e^{i(φ21−φ12)}), they obtain a quantity that is free of extrinsic delays. Gyration points are identified where Δφ ≈ ±π, indicating a π‑phase shift in one direction while the opposite direction experiences no phase shift. These points appear as distinct peaks in the magnitude response; three peaks (labeled 1, 2, 3) are observed, with peaks 1 and 3 corresponding to gyration (Δφ ≈ π) and peak 2 representing symmetric transmission (Δφ ≈ 0).

Insertion loss at the lowest‑frequency gyration peak is remarkably low, 2–4 dB, which the authors attribute primarily to intrinsic EMP dissipation rather than impedance mismatch. The EMP velocity is strongly reduced by the metallic gate screening, following v_g ≈ σ0 c_emp, where σ0 is the Hall conductivity and c_emp the gate‑edge capacitance per unit length. This slowdown shifts the operating frequency into the sub‑GHz regime for millimetre‑scale devices.

A theoretical model combines the self‑matching scheme of Ref.