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Automated Magnetic Field Stabilization for Trapped-Ion Quantum Networking Experiments

Saketha Male

Published Jun 5, 2026• 0 citations
Publisher

Abstract

Magnetic field fluctuations are a dominant source of dephasing in Zeeman-encoded trapped-ion qubits, limiting coherence time and the fidelity of quantum operations, particularly in modular quantum networks where phase stability must be maintained across spatially separated nodes. In this work, we design and characterize an active magnetic field stabilization system based on real-time feedback using a high-resolution magnetometer, a digitally controlled Red Pitaya platform, and a coil actuator with a measured gain of ∼ −5.37 G/V. Spectral measurements of the laboratory environment reveal dominant 60 Hz magnetic noise with amplitudes on the order of 400 µG and significant harmonic content. The system operates at a sampling rate of ∼ 605 Hz with an actuator bandwidth of ∼ 85 Hz, enabling targeted suppression of these low-frequency components. Given a Zeeman sensitivity of ∼ 2.8 MHz/G, such fluctuations correspond to kilohertz-scale frequency noise, which rapidly dephases qubit superpositions. Suppression of these fluctuations is therefore expected to reduce phase noise and extend coherence times, with first-order estimates suggesting improvement from the submillisecond regime into the few-millisecond regime for Zeeman-sensitive qubits. These results demonstrate that active magnetic-field stabilization is a practical approach to reducing environmental dephasing and improving the performance of trapped-ion quantum network experiments.

DOI
10.17615/pybd-4704

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Magnetic field fluctuations are a dominant source of dephasing in Zeeman-encoded trapped-ion qubits, limiting coherence time and the fidelity of quantum operations, particularly in modular quantum networks where phase stability must be maintained across spatially separated nodes. In this work, we design and characterize an active magnetic field stabilization system based on real-time feedback using a high-resolution magnetometer, a digitally controlled Red Pitaya platform, and a coil actuator with a measured gain of ∼ −5.37 G/V. Spectral measurements of the laboratory environment reveal dominant 60 Hz magnetic noise with amplitudes on the order of 400 µG and significant harmonic content. The system operates at a sampling rate of ∼ 605 Hz with an actuator bandwidth of ∼ 85 Hz, enabling targeted suppression of these low-frequency components. Given a Zeeman sensitivity of ∼ 2.8 MHz/G, such fluctuations correspond to kilohertz-scale frequency noise, which rapidly dephases qubit superpositions. Suppression of these fluctuations is therefore expected to reduce phase noise and extend coherence times, with first-order estimates suggesting improvement from the submillisecond regime into the few-millisecond regime for Zeeman-sensitive qubits. These results demonstrate that active magnetic-field stabilization is a practical approach to reducing environmental dephasing and improving the performance of trapped-ion quantum network experiments.
Abstract Superconducting qubits are a promising platform for building a larger-scale quantum processor capable of solving otherwise intractable problems. In order for the processor to reach practical viability, the gate errors need to be further suppressed and remain stable for extended periods of time. With recent advances in qubit control, both single- and two-qubit gate fidelities are now in many cases limited by the coherence times of the qubits. Here we experimentally employ closed-loop feedback to stabilize the frequency fluctuations of a superconducting transmon qubit, thereby increasing its coherence time by 26% and reducing the single-qubit error rate from (8.5 ± 2.1) × 10−4 to (5.9 ± 0.7) × 10−4. Importantly, the resulting high-fidelity operation remains effective even away from the qubit flux-noise insensitive point, significantly increasing the frequency bandwidth over which the qubit can be operated with high fidelity. This approach is helpful in large qubit grids, where frequency crowding and parasitic interactions between the qubits limit their performance.
We present the experimental realization of a continuous dynamical decoupling scheme which suppresses leading frequency shifts in a multi-ion frequency reference based on 40Caþ. By near-resonant magnetic coupling of the 2S1=2 and 2D5=2 Zeeman sublevels using radio-frequency dressing fields, engineered transitions with reduced sensitivity to magnetic-field fluctuations are obtained. A second stage detuned dressing field reduces the influence of amplitude noise in the first stage driving fields and decreases 2nd-rank tensor shifts, such as the electric quadrupole shift. Suppression of the quadratic dependence of the quadrupole shift to 3ð2Þ mHz=μm2 and coherence times of 290(20) ms on the optical transition are demonstrated even within a laboratory environment with significant magnetic field noise. Besides removing inhomogeneous line shifts in multi-ion clocks, the demonstrated dynamical decoupling technique may find applications in quantum computing and simulation with trapped ions by a tailored design of decoherence-free subspaces.
We demonstrate a trapped-ion protocol in which a nearby, dedicated "monitor" qubit tracks magnetic-field drifts in real time without interrupting data-qubit operations. Using two 40Ca+^{40}\mathrm{Ca}^+40Ca+ ions and the optical--metastable--ground architecture, we encode the data qubit in the ground-state manifold and the monitor qubit in a metastable-state manifold to achieve spectral separation. The monitor qubit senses common magnetic fluctuations during data-qubit experiments, enabling feedforward corrections to the qubit-control drives. Under applied magnetic noise with a realistic spectrum (1/f21/f^{2}1/f2), the protocol maintains coherence and, when compared with interleaved calibration, it extends usable data-qubit probe times by up to a factor of ∼2{\sim}\sqrt{2}∼2​ and doubles the experimental duty cycle. These results establish monitor qubits as a scalable tool for real-time recalibration in quantum information processors.
We demonstrate a trapped-ion protocol in which a nearby, dedicated monitor qubit tracks magnetic field drifts in real time without interrupting data-qubit operations. Using two 40Ca+^{40}\mathrm{Ca}^{+}40Ca+ ions and the optical-metastable-ground architecture, we encode the data qubit in the ground-state manifold and the monitor qubit in a metastable-state manifold to achieve spectral separation. The monitor qubit senses common magnetic fluctuations during data-qubit experiments, enabling feedforward corrections to the qubit-control drives. Under applied magnetic noise with a realistic spectrum (1/f21/{f}^{2}1/f2), the protocol maintains coherence and, when compared with interleaved calibration, it extends usable data-qubit probe times by up to a factor of approximately 2\sqrt{2}2​ and doubles the experimental duty cycle. These results establish monitor qubits as a scalable tool for real-time recalibration in quantum information processors.
Abstract The low-frequency magnetic field noise resulting from the lab[1]oratory power supply is one of the main decoherence sources of trapped-ion qubits. Synchronizing the experimental sequence to the phase of the power line is widely used to mitigate this problem, but it will greatly reduce the ex[1]perimental efficiency. In this paper, we experimentally demonstrate a simple active compensation method to reduce the observed 50 Hz and 150 Hz strong magnetic noise in an ion trap induced by the power line. In our method, a single 40Ca+ ion is used as the noise probe and an reverse compensation sig[1]nal is generated by a programmable arbitrary waveform generator (AWG).After compensation, an 86% reduction of the periodic magnetic field fluc[1]tuation and over 35-fold extension of the coherence time from 70 µs to 2500 µs were observed. This method can also be applied to compensate other spectral components of the magnetic field noise related to the power line, and it is also useful for other atomic systems such as neutral atoms.
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.