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arXiv:2110.03053 · Physical Review Applied

Second-Scale 9 Be + Spin Coherence in a Compact Penning Trap

Brian J. McMahon, Brian C. Sawyer

Published Jan 5, 2022• 12 citations
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Abstract

We report microwave spectroscopy of co-trapped ${}^{9}$$\mathrm{Be}$${}^{+}$ and ${}^{40}$$\mathrm{Ca}$${}^{+}$ ions within a compact permanent-magnet-based Penning ion trap. The trap is constructed with a reconfigurable array of $\mathrm{Nd}$-$\mathrm{Fe}$-$\mathrm{B}$ rings providing a 0.654-T magnetic field that is near the 0.6774-T magnetic-field-insensitive hyperfine transition in ${}^{9}$$\mathrm{Be}$${}^{+}$. Performing Ramsey spectroscopy on this hyperfine transition, we demonstrate nuclear spin coherence with a contrast decay time of >1 s. The ${}^{9}$$\mathrm{Be}$${}^{+}$ is sympathetically cooled by a Coulomb crystal of ${}^{40}$$\mathrm{Ca}$${}^{+}$, which minimizes ${}^{9}$$\mathrm{Be}$${}^{+}$ illumination and thus mitigates reactive loss. Introducing a unique high-magnetic-field optical-detection scheme for ${}^{40}$$\mathrm{Ca}$${}^{+}$, 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.

DOI
10.1103/physrevapplied.17.014005

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We report microwave spectroscopy of co-trapped {}^{9}$$\mathrm{Be}$${}^{+} and {}^{40}$$\mathrm{Ca}$${}^{+} ions within a compact permanent-magnet-based Penning ion trap. The trap is constructed with a reconfigurable array of Nd\mathrm{Nd}Nd-Fe\mathrm{Fe}Fe-B\mathrm{B}B rings providing a 0.654-T magnetic field that is near the 0.6774-T magnetic-field-insensitive hyperfine transition in {}^{9}$$\mathrm{Be}$${}^{+}. Performing Ramsey spectroscopy on this hyperfine transition, we demonstrate nuclear spin coherence with a contrast decay time of >1 s. The {}^{9}$$\mathrm{Be}$${}^{+} is sympathetically cooled by a Coulomb crystal of {}^{40}$$\mathrm{Ca}$${}^{+}, which minimizes {}^{9}$$\mathrm{Be}$${}^{+} illumination and thus mitigates reactive loss. Introducing a unique high-magnetic-field optical-detection scheme for {}^{40}$$\mathrm{Ca}$${}^{+}, 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.
We report the design and experimental demonstration of a compact, reconfigurable Penning ion trap constructed with rare-earth permanent magnets placed outside of a trap vacuum enclosure. We describe the observation of Doppler laser cooling of ions in a permanent magnet Penning trap. We detail a method for quantifying and optimizing the trap magnetic-field uniformity in situ using a thermal beam of neutral 40Ca^{40}\mathrm{Ca}40Ca precursor atoms. Doppler laser cooling of 40Ca+^{40}\mathrm{Ca}^{+}40Ca+ is carried out at 0.65 T, and side-view images of trapped ion fluorescence show crystalline order for both two- and three-dimensional arrays. Measured 40Ca+^{40}\mathrm{Ca}^{+}40Ca+ trap frequencies confirm the magnetic-field characterization with neutral 40Ca^{40}\mathrm{Ca}40Ca. The compact trap described here enables a variety of cold ion experiments with low size, weight, power, and cost requirements relative to traditional electromagnet-based Penning traps.
We sympathetically cool highly charged ions (HCI) in Coulomb crystals of Doppler-cooled Be+ ions confined in a cryogenic linear Paul trap that is integrated into a fully enclosing radio-frequency resonator manufactured from superconducting niobium. By preparing a single Be+ cooling ion and a single HCI, quantum logic spectroscopy toward frequency metrology and qubit operations with a great variety of species are enabled. While cooling down the assembly through its transition temperature into the superconducting state, an applied quantization magnetic field becomes persistent, and the trap becomes shielded from subsequent external electromagnetic fluctuations. Using a magnetically sensitive hyperfine transition of Be+ as a qubit, we measure the fractional decay rate of the stored magnetic field to be at the 10-10 s-1 level. Ramsey interferometry and spin-echo measurements yield coherence times of >400 ms, demonstrating excellent passive magnetic shielding at frequencies down to DC.
Precision measurement of the ground-state hyperfine constant for 9Be+^9Be^+9Be+ in a linear Paul trap via magnetically insensitive hyperfine transitions
Direct measurements of the ground-state magnetically insensitive hyperfine transition |F=2,mF=0>->|F=1,mF=0> of 9Be+^9Be^+9Be+ ions have been performed using microwave-driven state transfer. The 9Be+^9Be^+9Be+ ions are confined and laser-cooled in a linear Paul trap, forming a Coulomb crystal. The transition frequencies have been measured over a magnetic field range of $ \pm 0.5 mT $ centered at zero magnetic field, and the acquired data were fitted accounting for the high-order Zeeman effect. The hyperfine constant A is determined to be -625.008840(35) MHz, achieving a relative precision of $ 5.6 \times 10^{-8}$.
Laser cooled ions in a Penning trap can be isolated from the environment by placing them in vacuum and only interacting with them through optical and RF fields. The number of trapped particles can be varied from a single ion up to thousands. Confinement is provided by a static homogeneous magnetic field and a quadrupole electric potential. In the natural frame of the ions, this appears as a 3D simple harmonic potential. Therefore three dimensional structures can be formed in the absence of any additional RF field which may lead to heating as is the case with RF traps. There are 3N different motional modes for N particles. I present an analysis of the motion of a single particle showing that the energy levels for all three modes are equally spaced. I also describe the interaction between a trapped two level atom and an optical field. During my time in the lab the laser and computer control of the experiment has been significantly improved. In addition, an existing trap was modified to provide greater optical access and fluorescence collection. This allowed the vibrational levels superimposed on the internal states of a single 40Ca+ ion to be resolved via a narrow linewidth, electric quadrupole transition. This is the first observation of magnetron and modified cyclotron sidebands on an optical transition. When more than one calcium ion is laser cooled, and their temperature reduced below 5mK, they form a Coulomb crystal. The locations of the ions minimise the total potential energy which is comprised of the Coulomb repulsion and trap potential. The fluorescence collection optics have been arranged to resolve individual ions in these crystals. Information about the motion of the ions is deduced by comparing photos from the experiment to numerical simulations. Previously, only two ions have ever been aligned along the magnetic field in a Penning trap. I present strings of up to 29 particles and suggest the only limitation, apart from the electrode structure, is the overlap of the laser beams with the ions.
We implement sympathetic cooling of highly charged ions (HCI) by fully enclosing a linear Paul trap within a superconducting radio-frequency resonator. A quantization magnetic field applied while cooling down into the superconducting state remains present in the trap for centuries and external electromagnetic fluctuations are greatly suppressed. A magnetic field decay rate at the 10−10^{-10}−10 s−1^{-1}−1 level is found using trapped Doppler-cooled Be+^++ ions as hyperfine-structure (hfs) qubits. Ramsey interferometry and spin-echo measurements on magnetically-sensitive hfs transitions yield coherence times of >400 ms, showing excellent passive shielding at frequencies down to DC. For sympathetic cooling of HCI, we extract them from an electron beam ion trap (EBIT) and co-crystallize one together with Doppler-cooled Be+^++ ions. By subsequently ejecting all but one Be+^++ ions, we prepare single HCI for quantum logic spectroscopy towards frequency metrology and qubit operations with a great variety of HCI species.
Direct measurements of the ground-state magnetically insensitive hyperfine transition | F = 2 , m F = 0 〉 → | F = 1 , m F = 0 〉 of Be + 9 ions have been performed using microwave-driven state transfer. The Be + 9 ions are confined and laser cooled in a linear Paul trap, forming a Coulomb crystal. The transition frequencies have been measured over a magnetic-field range of ± 0.5 mT centered at zero magnetic field and the acquired data were fitted accounting for the high-order Zeeman effect. The hyperfine constant A is determined to be − 625.008 840 ( 35 ) MHz, achieving a relative precision of 5.6 × 10 − 8 .