Second-Scale $^9\text{Be}^+$ Spin Coherence in a Compact Penning Trap

TL;DR

This paper demonstrates long-lived nuclear spin coherence in a compact Penning trap using $^9$Be$^+$ ions, with innovative magnetic field stability and spin readout techniques, advancing quantum information and precision measurement technologies.

Contribution

It introduces a reconfigurable compact Penning trap with near-insensitive hyperfine transition, achieving over 1 second spin coherence and novel optical detection without a shelving laser.

Findings

Spin coherence time exceeds 1 second.

Magnetic field stability below 20 ppb at 43 seconds.

Effective sympathetic cooling reduces reactive loss.

Abstract

We report microwave spectroscopy of co-trapped $^9\text{Be}^+$ and $^{40}\text{Ca}^+$ within a compact permanent-magnet-based Penning ion trap. The trap is constructed with a reconfigurable array of NdFeB rings providing a 0.654 T magnetic field that is near the 0.6774-T magnetic-field-insensitive hyperfine transition in $^9\text{Be}^+$. Performing Ramsey spectroscopy on this hyperfine transition, we demonstrate nuclear spin coherence with a contrast decay time of >1 s. The $^9\text{Be}^+$ is sympathetically cooled by a Coulomb crystal of $^{40}\text{Ca}^+$, which minimizes $^9\text{Be}^+$ illumination and thus mitigates reactive loss. Introducing a unique high-magnetic-field optical detection scheme for $^{40}\text{Ca}^+$, we perform spin state readout without a 729~nm shelving laser. We record a fractional trap magnetic field instability below 20 ppb (<13 nT) at 43 s of averaging time with no magnetic shielding and only passive thermal isolation. We discuss potential applications of this compact, reconfigurable Penning trap.