Speaker
Description
Ion-trap-based quantum computers have attracted significant attention as a promising platform because ions used as qubits exhibit long coherence times and low error rates. For scaling up these systems, it is important to maintain a uniform qubit environment. Considering the dependence of energy levels on external magnetic fields, both the uniformity and stability of the magnetic field are key factors. In fact, magnetic field gradients have been reported to reduce the coherence time of qubits [1]. Therefore, it is necessary to locally measure and evaluate the magnetic field at each ion position. In this study, we develop a method to evaluate the magnetic field at individual ion positions by analyzing the dark resonance spectra of trapped ions. Furthermore, to investigate the effect of magnetic materials in the packaged trap used in our system, we constructed an experimental setup that allows easy exchange of traps, aiming to visualize the magnetic field distribution.
We use a planar trap to confine $^{40}\mathrm{Ca}^+$ ions and perform Doppler cooling using 397 nm and 866 nm lasers, addressing the $^{2}S_{1/2}-{}^{2}P_{1/2}$ and $^{2}D_{3/2}-{}^{2}P_{1/2}$ transitions, respectively. First, using simulations with QuTiP (Quantum Toolbox in Python) [2], we examined the dark resonance spectra under applied magnetic fields. The results showed that the spacing of dark resonances increases with the magnetic field strength. We also confirmed that the measurement resolution depends on the laser linewidth, frequency stability, and intensity. These results indicate that it is possible to quantitatively evaluate the magnetic field through the observation of dark resonance spectra. The expected magnetic field resolution is on the order of 1 mG, estimated from the laser linewidth (100–200 kHz) and the frequency scanning step (10 kHz). In the experiment, by scanning the wavelength of the 866 nm laser under an applied magnetic field, we observed dips corresponding to dark resonances. However, we found that the current laser linewidth limits the spectral contrast, making quantitative evaluation of the magnetic field challenging, and we plan to improve the laser system to achieve higher-resolution measurements.
Furthermore, this study focuses on the influence of trap materials on the magnetic field at the ion positions. The land grid array (LGA), which supports the trap chip, contains magnetic materials that may affect the local magnetic field. To investigate this effect, we constructed an experimental setup in which the trap can be easily exchanged and performed ion trapping experiments using this setup. In addition, to reduce external magnetic noise, we incorporated a single-layer magnetic shield and magnetic-field compensation coils. Based on electromagnetic simulations of our setup, a magnetic field gradient of approximately 13 mG/mm is expected. Assuming a magnetic field resolution of 1 mG, the spatial resolution is estimated to be about 77 µm. Therefore, the magnetic field distribution at this scale can be evaluated using an ion string.
[1] Shannon X. Wang et al., Appl. Phys. Lett. 94, 094103 (2009)
[2] J. R. Johansson et al., Comp. Phys. Comm.183, 8, pp.1760-1772 (2012)
| Academic level | Master's Student |
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