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Enhancement of Superconducting Qubit Coherence via Structured Low-Frequency Toroidal Magnetic Fields

Eduardo Parra

Published Apr 12, 2026• 0 citations
Publisher

Abstract

This work demonstrates a novel approach to enhancing superconducting qubit coherence through controlled low-frequency electromagnetic environments. A toroidal coil geometry is used to generate spatially coherent magnetic fields at cryogenic temperatures, enabling precise control of the qubit environment without modifying device materials or fabrication processes. A Ta/Si λ/4 CPW resonator is placed at the center of a 3D-printed toroidal coil, and Bayesian optimization is employed to tune the applied magnetic field parameters. The optimization converges to a narrow frequency band centered at 7.831 Hz, resulting in a significant increase in qubit relaxation time from 1.02 ms to 4.7 ms (4.6× improvement), along with a substantial enhancement in internal quality factor. Control experiments confirm that the observed effect is not attributable to thermal, mechanical, or broadband electromagnetic artifacts. The results suggest that structured low-frequency electromagnetic environments can act as an additional control parameter for suppressing decoherence, introducing environmental engineering as a complementary axis alongside materials and circuit design.

DOI
10.5281/zenodo.19554984

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This work demonstrates a novel approach to enhancing superconducting qubit coherence through controlled low-frequency electromagnetic environments. A toroidal coil geometry is used to generate spatially coherent magnetic fields at cryogenic temperatures, enabling precise control of the qubit environment without modifying device materials or fabrication processes. A Ta/Si λ/4 CPW resonator is placed at the center of a 3D-printed toroidal coil, and Bayesian optimization is employed to tune the applied magnetic field parameters. The optimization converges to a narrow frequency band centered at 7.831 Hz, resulting in a significant increase in qubit relaxation time from 1.02 ms to 4.7 ms (4.6× improvement), along with a substantial enhancement in internal quality factor. Control experiments confirm that the observed effect is not attributable to thermal, mechanical, or broadband electromagnetic artifacts. The results suggest that structured low-frequency electromagnetic environments can act as an additional control parameter for suppressing decoherence, introducing environmental engineering as a complementary axis alongside materials and circuit design.
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For quantum computing to become fault tolerant, the underlying quantum bits must be effectively isolated from the noisy environment. It is well known that including an electromagnetic bandgap around the qubit operating frequency improves coherence for superconducting circuits. However, investigations of bandgaps to other environmental coupling mechanisms remain largely unexplored. Here, we present a method to enhance the coherence of superconducting circuits by introducing a phononic bandgap around the device operating frequency. The phononic bandgaps block resonant decay of defect states within the gapped frequency range, removing the electromagnetic coupling to phonons at the gap frequencies. We construct a multiscale model that derives the decrease in the density of states due to the bandgap and the resulting increase in defect state T1 times. We demonstrate that emission rates from in-plane defect states can be suppressed by up to two orders of magnitude. We combine these simulations with theory for resonators operating in the continuous-wave regime and show that improvements in quality factors are expected by up to the enhancement in defect T1 times. Furthermore, we use full master equation simulation to demonstrate the suppression of qubit energy relaxation even when interacting with 200 defect states. We conclude with an exploration of device implementation including tradeoffs between fabrication complexity and qubit performance.
The coherence of superconducting transmon qubits is often disrupted by fluctuations in the energy relaxation time (T1), limiting their performance for quantum computing. While background magnetic fields can be harmful to superconducting devices, we demonstrate that both trapped magnetic flux and externally applied static magnetic fields can suppress temporal fluctuations in T1 without significantly degrading its average value or qubit frequency. Using a three-axis Helmholtz coil system, we applied calibrated magnetic fields perpendicular to the qubit plane during cooldown and operation. Remarkably, transmon qubits based on tantalum-capped niobium (Nb/Ta) capacitive pads and aluminum-based Josephson junctions (JJs) maintained T1 lifetimes near 300 μs even when cooled in fields as high as 600 mG. Both trapped flux up to 600 mG and applied fields up to 400 mG reduced T1 fluctuations by more than a factor of two, while higher field strengths caused rapid coherence degradation. We attribute this stabilization to the polarization of paramagnetic impurities, the role of trapped flux as a sink for non-equilibrium quasiparticles (QPs), and partial saturation of fluctuating two-level systems (TLSs). These findings challenge the conventional view that magnetic fields are inherently detrimental and introduce a strategy for mitigating noise in superconducting qubits, offering a practical path toward more stable and scalable quantum systems.
Abstract Here we consider a tunable superconducting cavity that can be used either as a tunable coupler to a qubit inside the cavity or as a tunable low noise, low temperature, RF filter. Our design consists of an array of radio-frequency superconducting quantum interference devices (rf SQUIDs) inside a superconducting cavity. This forms a tunable metamaterial structure which couples to the cavity through its magnetic plasma frequency. By tuning the resonant frequency of the metamaterial through an applied magnetic flux, one can tune the cavity mode profile. This allows us to detune the cavity initially centered at 5.593 GHz by over 200 MHz. The maximum quality factor approaches that of the empty cavity, which is 4.5 × 106. The metamaterial electromagnetic response is controlled via a low-frequency or dc magnetic flux bias, and we present a control line architecture that is capable of applying sufficient magnetic flux bias with minimal parasitic coupling. Together this design allows for an in-situ tunable cavity which enables low-temperature quantum control applications.
Fault-tolerant superconducting quantum processors are limited by decoherence, limited connectivity, and the wiring burden at cryogenic temperatures. This review surveys superconducting “metamaterials”: circuits built from many repeating, deeply subwavelength building blocks (unit cells) that tailor dispersion, impedance, and bandgaps. By shaping the electromagnetic (and sometimes acoustic) environment of a qubit, these networks can suppress or enhance radiative decay and frequency shifts (Purcell and Lamb effects) and provide compact multimode structures for multiplexed control and photon-mediated coupling. We emphasize the practical trade-offs—added dielectric participation, fabrication disorder, and leakage through spurious chip/package modes—and the measurements needed to quantify them. The review summarizes design and tuning approaches (including Josephson/Superconducting Quantum Interference Device implementations), materials and fabrication limits, and mode-resolved benchmarks, and it highlights emerging directions such as topological lattices and engineered reservoirs while clearly separating demonstrations from proposals.