Cr-doped quantum anomalous Hall (QAH) insulators are emerging as key components for directing one-way flow of signals in future circuits. This design uses the unique properties of QAH materials, which maintain stable edge channels even without an applied magnetic field, and opens opportunities for stronger plasmon-photon interactions due to a high-impedance plasmon mode.

QAH Insulator Properties Enable Resonator Coupling

A high-impedance plasmon mode, exhibiting a Chern number, within quantum anomalous Hall (QAH) materials markedly amplifies electric-dipole coupling to nearby quantum emitters; this characteristic makes these materials ideal for strong asymmetric plasmon-photon coupling and potential applications in nonreciprocal electronics. The impedance of this mode reaches, a value that directly influences the scale of zero-point voltage fluctuations and, consequently, the strength of the interaction with quantum emitters. This amplified coupling is central to a newly demonstrated three-port circulator design, where a QAH plasmon resonator is asymmetrically linked to a pair of LC resonators.

These inductors, combined with stray capacitance, form LC resonators tuned to closely match the EMP resonance frequency; this frequency is determined by the wave vector, Hall conductivity, effective dielectric constant, drift velocity of the 1D edge state, and the physical width of the EMP mode. Equivalent circuit diagrams reveal parasitic capacitance, stray capacitance and edge capacitance as key components influencing the device’s performance, specifically the anticlockwise transport of EMPs between resonators. The design draws inspiration from Hatano-Nelson chains, non-Hermitian systems where asymmetric couplings induce nonreciprocal transport.

The six quintuple layers of the magnetic topological insulator thin films used in this work were grown using ultrahigh-vacuum molecular beam epitaxy, a precise method for creating the necessary material structure. Unlike traditional Quantum Hall systems requiring external magnetic fields, these QAH materials maintain a stable number of edge channels even at zero magnetic field, simplifying device fabrication and operation. This stability ensures reliable nonreciprocal signal routing by maintaining consistent performance regardless of external magnetic interference.

The observed enhancement in transmission is a direct consequence of the unique properties of the QAH material and its ability to facilitate strong plasmon-photon interactions. The configuration relies on interference between the EMP and parasitic capacitive pathways, leading to asymmetric coupling and ultimately enabling the circulator’s function.

EMP Resonance and Non-Hermitian Coupling Mechanisms

Cr-doped quantum anomalous Hall (QAH) materials exhibit asymmetric coupling strengths critical to the operation of a newly designed circulator, as revealed by a minimal coupled-mode matrix analysis of the device’s response. The analysis, detailed in a recent publication, describes a configuration where differences in propagation time and loss for opposing modes arise from the chiral nature of the EMP resonator, despite identical resonator characteristics.

These differences, dependent on path length and input microwave power, are fundamental to achieving non-reciprocal signal routing. The circulator’s architecture relies on a four-mode model described by a non-Hermitian matrix, where two LC resonators and two directional EMP channels interact. The model assumes idealized nonreciprocity propagation.

Specifically, the equation demonstrates how the coupling strength between the LC resonators and EMP modes dictates the overall performance, with the off-diagonal structure of the matrix defining the directional sequence of signal flow. The configuration produces exceptionally high isolation, exceeding at 543.8 MHz within an approximately bandwidth, due to broadband suppression of transmission from port 1 to port 3 and a narrow resonance feature.

“Given the almost identical resonance frequencies of the input and output LC resonators,” the researchers write, “we set and.” The observed asymmetric coupling is a direct result of the QAH material’s chiral nature, rather than simply differing resonator properties. This chirality manifests as inequivalent chiral propagation paths around the QAH mesa, leading to variations in phase shifts and dissipation between the two EMP channels.

By eliminating intermediate EMP modes through coupled-mode theory, the team derived a transmission spectrum expression that highlights the interplay between LC and EMP resonator characteristics. This detailed analysis provides a pathway for optimizing the circulator’s performance and tailoring its response for specific applications requiring precise, one-way signal transmission. The minimal device-level model, based on the non-Hermitian four-mode matrix, offers a powerful tool for understanding and designing future non-reciprocal devices based on QAH materials.

Hatano-Nelson Model Describes Asymmetric Plasmonic System

An impedance of characterizes the high-impedance plasmon mode central to a new understanding of asymmetric signal transmission, with a corresponding Chern number defining its topological properties. While previous work by Mahoney et al. explored similar architectures using quantum Hall systems, that research did not connect the observations to Hatano-Nelson-type couplings, a key link established in this latest study.

Researchers interpret the observed behavior as consistent with an effective two-site Hatano-Nelson-type plasmonic system, where asymmetric coupling drives the system toward a strongly nonreciprocal, non-Hermitian transport regime. This model arises from a frequency-domain coupled-mode treatment, initially described by the equation and subsequently reduced by eliminating intermediate edge magnetoplasmonic (EMP) modes.

The resulting effective model, expressed as, captures unequal direction-dependent couplings between neighboring unit cells, a key feature of the asymmetric system. The differing lengths and dissipation inherent in the two EMP propagation paths, inequivalent chiral paths around the QAH mesa, lead to asymmetric effective coupling, a phenomenon captured by the terms and describing the frequency-domain complex response of each path.

This effective model accurately predicts the phase behavior observed experimentally, particularly in the regime of strongest asymmetric transport. To refine the comparison between simulation and experimental data, a high-resolution scan of the phase response was conducted, subtracting the phase of from, effectively removing line delay and enabling a more precise analysis.

Circulator Configuration with LC Resonators and EMPs

This enhancement stems from the configuration of two LC resonators and directional electromagnetic propagation (EMP) channels described by a minimal non-Hermitian coupled-mode matrix, allowing for idealized nonreciprocity. Analysis of the circulator’s performance indicates a stable transmission magnitude, with the magnitude of remaining nearly constant over an input power range exceeding. This robustness is particularly notable when comparing the LC-resonator-coupled configuration to EMP circulators without LC coupling, demonstrating the benefit of this design choice for maintaining signal integrity across varying power levels.

Microwave Transmission Reveals Nonreciprocal Behavior

Experimental characterization of the circulator reveals a direct method for determining a key value without requiring an impedance-matching circuit, a simplification that eases fabrication and testing. Microwave transmission coefficients were measured using a detailed scheme outlined in supplementary materials, with all cryogenic measurements performed at approximately.. Device and Hatano-Nelson model. The nonreciprocity arises from constructive and destructive interference between the EMP mode and signals transmitted through parasitic capacitance formed between the gold pad and EMP resonator.

The magnitude of the transmitted signal remains nearly constant across an input power range exceeding 10 dBm, indicating robust performance. 5 GHz, as shown in accompanying figures. 5 GHz, a performance level exceeding that of comparable electromagnetic propagation (EMP) circulators.

Detailed measurements of transmission magnitude and phase at an input power of 0 dBm, where asymmetric transport is strongest, confirm strongly asymmetric coupling between the two LC resonators. The researchers report, highlighting the stability of the design.

These results suggest a new mechanism for harnessing the intrinsic unidirectionality of QAH edge states to engineer nonreciprocal devices, with promising implications for scaling superconducting quantum processors and improving axion haloscope sensitivity.

👉 More information
🗞 Circulators based on coupled quantum anomalous Hall insulators and resonators
✍️ Luis A. Martinez et al.
🧠 DOI: http://link.aps.org/doi/10.1103/c5hf-mn4s

More like this