1. Beryllium ions in segmented Paul trap
Cade Perkins

2. Spectroscopy of 𝐻𝑒 +
Ultimate objective is spectroscopy of 1s-2s transition in 𝐻𝑒 +
4 𝐻𝑒 + is a hydrogen-like one-electron system, suitable for further testing of fundamental theories, e.g. QED.
There is no suitable cooling transition in 𝐻𝑒 + ,
But 𝐻𝑒 + may be cooled sympathetically via Coulomb interaction with co-stored, laser-cooled ions
Some candidate ions with cooling transitions reachable by current laser systems: 𝐵𝑒 + , 𝑀𝑔 + , 𝐶𝑎 +
Magnesium ions have been explorer more by this group primarily due to accessible laser systems

3. Beryllium ions as coolant
Similar charge-to-mass ratio of the coolant and cooled ions is necessary for reaching lower temperatures.
Graph shows result of molecular dynamics simulation of 20 𝑀 + ions of given mass, sympathetically cooled by 60 coolant ions in linear Paul trap.
9 𝐵𝑒 + is only of the listed laser-cooled candidate ions to cool mass-4 ions (i.e 𝐻𝑒 + ) to milli-Kelvin temperatures.
Okada, et. al. Phys. Rev. A, vol. 81, p , 2010

4. Laser-cooling level scheme for 9 𝐵 𝑒 +
Primary cooling and fluorescence transition: 2s 𝑆 ↔2𝑝 𝑃 (Fraunhofer D2 transition) at nm
Hyperfine splitting must be “bridged” to keep the excited state from decaying to a hyperfine level outside the optical pumping transition

5. Laser cooling system
626 nm generated by sum frequency generation (SFG) of two infrared OrangeOne fiber lasers from Menlo Systems:
1051 nm from ytterbium-doped fiber laser
1550 nm from erbium-doped fiber laser
313 nm generated by second harmonic generation (SHG) in BBO crystal in Hänsch-Couillaud doubling cavity.
Split 313 nm beam and frequency-shift each side to “bridge” hyperfine splitting in 2s ground state by using double-pass AOM setup.

6. Sum Frequency Generation
626 𝑛𝑚≈ 𝑐 𝑐 1051 𝑛𝑚 + 𝑐 1550 𝑛𝑚 = 𝑐 𝑓 1 + 𝑓 2
Single-pass through bulk Periodically Poled Lithium Niobate (PPLN) crystal
Both fundamental beams are mode matched for optimum SFG
Up to 5 W output from each fiber lasers, with linewidth about 70 MHz
Have obtained up to 2.16 W red light at 626 nm, far beyond requirements
Batteiger, V., Ph.D. Dissertation, LMU, 2011

7. Sum Frequency Generation
Both fiber laser beams were characterized for proper mode matching into PPLN crystal

8. Sum Frequency Generation
Mode matching using basic Gaussian beam ray tracing.
Used GaussianBeam v0.4 software for convenient modeling:
Common focusing lens
F = 60 mm
1051 nm beam
1550 nm beam

9. Sum Frequency Generation
Both fiber lasers are tunable by temperature control of oscillator.
They also allow feedback/finer control via piezoelectric actuator - currently not used.
Overall tunable range spans both the D1 and D2 transition, allowing for various cooling and re-pumping schemes.
SFG Tunable range
+253 GHz
-223 GHz

10. Second Harmonic Generation
Monitor and tune 626nm wavelength in absence of wavelength meter for UV light.
8mm BBO Crystal
Demonstrated 580 mW of 313nm light before the double-pass AOM
Point out electron trap rods, segments and rings.
Point out ovens and electron guns.
Named after Wolfgang Paul (who shared 1989 Nobel Prize with Hans Dehmelt for his work)

11. Robin, J., Masters thesis, LMU, 2012
Double-pass AOM Setup
Acousto-optical Modulator (AOM) generates an acoustic wave moving inside a transparent medium (e.g. quartz crystal).
An incident optical beam undergoes a Doppler shift. The frequency shift is equal to the acoustic frequency and can be negative or positive – selected by incident angle. (Much of the original beam passes through the AOM unchanged.)
Further, the acoustic wave establishes a periodic density “grating”, thereby diffracting the shifted beam by some angle.
AOM
Beam splitter
See Fig 12 and Fig 13 in Feng’s disseration.
Robin, J., Masters thesis, LMU, 2012

12. Robin, J., Masters thesis, LMU, 2012
Double-pass AOM Setup
In the double-pass setup, the shifted, 1st diffracted order is reflected back along the same path.
When the shifted beam passes back through the AOM, it produces a beam shifted twice the acoustic frequency from the original.
Passing in the other direction, the shifted beam is again diffracted, but the angle is in the opposite direction so that it now overlaps with the original beam. This is a critical feature of the double-pass setup, because it allows minor frequency adjustment without causing the output beam to wander.
AOM
Beam splitter
See Fig 12 and Fig 13 in Feng’s disseration.
Robin, J., Masters thesis, LMU, 2012

13. Robin, J., Masters thesis, LMU, 2012
Double-pass AOM Setup
The final result: Two overlapping beams which center wavelengths bridge the hyperfine splitting of Beryllium ground state:
nm nm
AOM
Beam splitter
See Fig 12 and Fig 13 in Feng’s disseration.
Robin, J., Masters thesis, LMU, 2012

14. Segmented Ion Trap RF applied to large solid electrodes
12 independent segmented electrode pairs provide tight axial confinement and field shaping for ion shuttling
What are wavelenghts for ionization and laser cooling laser?
Verdi: 532 nm
560nm to 280 nm Cooling
570nm to 285 nm ionization
Mg: Z=12, standard weight =
285nm for resonant enhanced two-photon ionization: (1s2 2s2 2p6 3s2) to 3s3p to ionized atom
24^Mg: 79; 25^Mg: 10; 26^Mg:11

15. Tank Circuit and Helical Resonator
Upon initial trap construction, resonator was measured to have Q factor of > 400, very good for RF tank circuit.
After trap completion and connection of various feedthrough components (e.g. ovens and DC power supplies), Q factor is only about 100, comparable to other tank circuit designs.
What are wavelenghts for ionization and laser cooling laser?
Verdi: 532 nm
560nm to 280 nm Cooling
570nm to 285 nm ionization
Mg: Z=12, standard weight =
285nm for resonant enhanced two-photon ionization: (1s2 2s2 2p6 3s2) to 3s3p to ionized atom
24^Mg: 79; 25^Mg: 10; 26^Mg:11
Design reference: Siverns, J. D., et al. (2012). "On the application of radio frequency voltages to ion traps via helical resonators." Applied Physics B-Lasers and Optics 107(4):

16. Imaging System
Single 𝑓=80𝑚𝑚 condenser: asphericon A50-80FPX-X-S, fused silica asphere
Electron Multiplying Charge Coupled Device (EMCCD) camera: Andor iXon X3 model DU-885K, claimed to be able to detect single-photon events
Magnification currently x7.2. Planned addition of microscope objective will give about x90 magnification