Journal club 10 Feb. 2020

Integrated optical control and enhanced coherence of ion qubits via multi-wavelength photonics

Introduction

$^{88}$Sr$^+$ ion level system

Loading by photoionization:

• 461 nm, 405 nm

Cooling and detection:

• 422 nm
• 1092 nm (repumping)

Qubit operations, state preparation, sideband cooling:
(electric quadrupole transitions)

• 674 nm

Groundstate cooling / state preparation:

• 1033 nm (dark state quench)
  • repumping in sideband cooling

Proxy for measuring the 405 nm:

• 408 nm

Surface-trap chip and integrated photonics

§1: Deliver light from: external source -> ion

§2: Description of the chip waveguides

• materials:
  • guiding layer: SiN
  • cladding: SiO$_2$
• losses:
  • 0.5 dB/cm above 633 nm
  • 10 dB/cm at 405 nm

• adiabatic tapering at end to expand the spatial mode before grating coupler

§3: Description of the grating

• periodic pattern etching of widened waveguide
• curved teeth to focus beam
• 10% efficiencies

§4: Description of the trap, electrodes and optics, ion position and grating positions

• ion @ 55 $\mu$m
• 20 $\times$ 20 $\mu$m$^2$ apertures
  • 55 $\mu$m to center
• Thin ITO (indium tin oxide) film prevents charging of the exposed waveguides

§5: Chip fabrication

• size: 1 cm × 1 cm
  • via wafer dicing
• polished chip edge
  • (avoid light scatter)
• epoxyed fiber array

§6: Six wavelengths

• See $^{88}$Sr$^+$ level scheme
• Shared waveguides

§7: Different angles grating emission

• Lateral focus only --> both address the ion

Photonic ion trap characterization and operation

§1: Characterization with microsope

• high NA microscope $\to$ CMOS detector
  • 3D profiles: precise beam position

• Intersection 10 μm above RF min (55 μm)
  • n discrepancy: design vs. fab (SiN)

• displace horizontally (no vertical possible)
  • up to 3 beams at a time

§2, §3, §4: Demonstrations with ion (1 beam at a time):

• Loading via photoionization:
  • 405 nm + 461 nm free space
  • 461 nm + 405 nm free space
• Dark state quenching:
  • 1033 nm
• Repumping:
  • 1092 nm
  • (for Doppler cooling and detection)
• Qubit transition spectroscopy:
  • 674 nm beam path
  • State preparation:
    • optical pumping
    • sideband cooling $\overline{n} < 1$ in axial mode
  • Rabi oscillations
  • Ramsey interferometry
• Detection:
  • 422 nm
  • high NA lens $\to$ Photomultiplier
  • spatial filtering of scattered grating photons
  • 99.6% state detection fidelity

§5: Characterization with ion

• strength of laser-ion interaction
• shuttle ion using the DCs
  • ion position on x-axis

• 674 nm:
  • Rabi freq $\to$ beam intensity
• 422 nm and 1092 nm:
  • ion fluorescence (photon counts)
• 1033 nm:
  • quench / bright state prob. ~ beam intensity

§6: Agreement ion $\leftrightarrow$ high-NA microscope

§7: Beam combination

• 422 nm and 1092 nm:
  • Doppler cooling, state detection
  • lifetime of hours
  • 99.5% state detection fidelity

• 674 nm, 1033 nm, 1092 nm:
  • qubit state preparation (optical pumping)
  • carrier transitions
  • sideband transitions

Vibration resilience

§1: (Hypothesis) Inherent stability of integrated optics

• ion and optics vibrate with common mode
• reduced decoherence from:
  • optical phase variations
  • amplitude modulation

§2, §3, §4: Test with cryocooler

• partial / total cryocooler-trap coupling

• Ramsey experiment: contrast decay time vs. acceleration

Conclusion

• vibration resilient quantum control
• robust, portable, ion traps

Methods

• Profiles at different positions

Questions

Is this applicable to UV light?
Not in the form that is described here

Why are you telling us all of this then?
There are being efforts of making this happen for UV light (ask Amado or Tania). I presented this paper because the benefits reported could be also of use to us; so take it as a motivation to pursue this integrated optics approach.