Researchers from Seoul National University, Korea Research Institute of Standards and Science, Ulsan National Institute of Science and Technology, and Samsung Electronics have developed a compact broadband Purcell filter for fast multiplexed superconducting qubit readout. The filter suppresses Purcell loss, a spontaneous emission loss that occurs when measurement speed is increased. The team used 200 nm-thick niobium films to fabricate the filter, which showed a bandwidth of over 790 MHz within a compact size of 0.29 mm2. The filter design is expected to be easily integrated into existing superconducting quantum circuits.
Broadband Purcell Filters for Superconducting Qubit Readout
A team of researchers from Seoul National University, Korea Research Institute of Standards and Science, Ulsan National Institute of Science and Technology, and Samsung Electronics have developed a broadband Purcell filter with a compact footprint for fast multiplexed superconducting qubit readout. The team includes Seong Hyeon Park, Gahyun Choi, Gyunghun Kim, Jaehyeong Jo, Bumsung Lee, Geonyoung Kim, Kibog Park, YongHo Lee, and Seungyong Hahn.
Engineering the Admittance of External Environments
The researchers have found that engineering the admittance of external environments connected to superconducting qubits is essential. Increasing the measurement speed introduces spontaneous emission loss to superconducting qubits, known as Purcell loss. They have developed a broadband Purcell filter within a small footprint, effectively suppressing Purcell loss without losing the fast measurement speed.
Characterization of the Filter’s Frequency Response
The team characterized the filter’s frequency response at 4.3 K and estimated Purcell loss suppression by finite-element-method simulations of superconducting planar circuit layouts with the proposed filter design. The filter is fabricated with 200 nm-thick niobium films and shows the measured bandwidth over 790 MHz within 0.29 mm2 of compact size owing to densely packed spiral resonators.
Protection of the Qubit from Purcell Loss
The estimated lifetime enhancement indicates the adequate protection of the qubit from Purcell loss. The presented filter design is expected to be easily integrated into existing superconducting quantum circuits for fast and multiplexed readout without occupying a large footprint.
Circuit Quantum Electrodynamics
In circuit quantum electrodynamics (cQED), Josephson junction-based superconducting qubits are utilized as artificially engineered atoms. Quantum nondemolition dispersive readout of qubit state is performed by probing the coupled readout resonator’s frequency shift. Recent progresses in scaling up the number of qubits in superconducting quantum circuits, various quantum algorithms, and error correction protocols imply that the fast and high-fidelity multiplexed readout of qubits with distributed readout resonator frequencies is favorable to avoid complex signal wiring and tight cryogenic cooling budgets.
Engineering the Purcell Loss
To protect qubits from Purcell loss, Purcell filters have been developed to impede photon emission from qubits to the connected external circuits at qubit frequency while allowing it at the readout frequency. The researchers have introduced a 4-pole Purcell filter that has been designed following coupled resonator filter synthesis techniques from microwave engineering. The filter comprises symmetric, densely packed superconducting spiral coplanar-waveguide (CPW) resonators coupled to the neighboring spiral CPW resonators.
Significant Features of the Spiral CPW Purcell Filter Design
The significant features of the spiral CPW Purcell filter design include its compact footprint of 0.29 mm2, comparable to a typical transmon qubit footprint size, wide bandwidth of 790 MHz centered at 6.93 GHz in the passband for multiplexed readout application, and large attenuation at frequency out of the passband for suppressed Purcell loss and the fast readout application.
An article titled “Characterization of broadband Purcell filters with compact footprint for fast multiplexed superconducting qubit readout” was published in the Applied Physics Letters on January 22, 2024. The authors of the study are Seong Hyeon Park, Gahyun Choi, G. T. Kim, Jaehyeong Jo, Bumsung Lee, Geonyoung Kim, Kibog Park, Yong-Ho Lee, and Seungyong Hahn. The article can be accessed through its DOI reference https://doi.org/10.1063/5.0182642.
