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Ion-Based Characterization of Laser Beam Profiles for Quantum Information Processing

Ilyoung Jung, Frank G. Schroer, Philip Richerme

TL;DR

This work tackles the challenge of characterizing laser beams at the ion location to optimize trapped-ion quantum gates. It introduces and demonstrates aion-based sensing via the differential four-photon Stark shift in $^{171}$Yb$^+$ driven by mode-locked Raman beams, enabling direct extraction of polarization, spot size, and alignment. The approach reveals that four-photon Stark-shift diagnostics yield higher sensitivity to beam misalignment and polarization imperfections than conventional Rabi-frequency methods, and it leads to faster, more robust Raman-driven gates. Overall, ions can serve as local sensors to calibrate laser beams, with broad applicability to other Zeeman/hyperfine qubits and quantum information platforms.

Abstract

Laser-driven operations are a common approach for engineering one- and two-qubit gates in trapped-ion arrays. Measuring key parameters of these lasers, such as beam sizes, intensities, and polarizations, is central to predicting and optimizing gate speeds and stability. Unfortunately, it is challenging to accurately measure these properties at the ion location within an ultra-high vacuum chamber. Here, we demonstrate how the ions themselves may be used as sensors to directly characterize the laser beams needed for quantum gate operations. Making use of the four-photon Stark Shift effect in $^{171}$Yb$^+$ ions, we measure the profiles, alignments, and polarizations of the lasers driving counter-propagating Raman transitions. We then show that optimizing the parameters of each laser individually leads to higher-speed Raman-driven gates with smaller susceptibility to errors. Our approach demonstrates the capability of trapped ions to probe their local environments and to provide useful feedback for improving system performance.

Ion-Based Characterization of Laser Beam Profiles for Quantum Information Processing

TL;DR

This work tackles the challenge of characterizing laser beams at the ion location to optimize trapped-ion quantum gates. It introduces and demonstrates aion-based sensing via the differential four-photon Stark shift in Yb driven by mode-locked Raman beams, enabling direct extraction of polarization, spot size, and alignment. The approach reveals that four-photon Stark-shift diagnostics yield higher sensitivity to beam misalignment and polarization imperfections than conventional Rabi-frequency methods, and it leads to faster, more robust Raman-driven gates. Overall, ions can serve as local sensors to calibrate laser beams, with broad applicability to other Zeeman/hyperfine qubits and quantum information platforms.

Abstract

Laser-driven operations are a common approach for engineering one- and two-qubit gates in trapped-ion arrays. Measuring key parameters of these lasers, such as beam sizes, intensities, and polarizations, is central to predicting and optimizing gate speeds and stability. Unfortunately, it is challenging to accurately measure these properties at the ion location within an ultra-high vacuum chamber. Here, we demonstrate how the ions themselves may be used as sensors to directly characterize the laser beams needed for quantum gate operations. Making use of the four-photon Stark Shift effect in Yb ions, we measure the profiles, alignments, and polarizations of the lasers driving counter-propagating Raman transitions. We then show that optimizing the parameters of each laser individually leads to higher-speed Raman-driven gates with smaller susceptibility to errors. Our approach demonstrates the capability of trapped ions to probe their local environments and to provide useful feedback for improving system performance.

Paper Structure

This paper contains 8 sections, 17 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Methods
  3. Experimental Setup
  4. Four-photon Stark shift
  5. Results
  6. Beam Power and Polarization
  7. Beam Intensity Profiles and Alignments
  8. Discussion and Conclusions

Figures (4)

  • Figure 1: Experimental Setup. (a) Diagram showing the ion trap within an ultra-high vacuum chamber. Also shown are the Raman beam configurations, ideal beam polarizations (double-headed arrows), and ideal magnetic field direction. QWP = Quarter-Wave Plate; HWP = Half-Wave Plate (b) Relevant energy level structure of $^{171}$Yb$^+$. Frequency comb components from a pair of 355 nm laser beams can drive Raman transitions, and each beam individually generates a differential four-photon Stark shift between qubit states.
  • Figure 2: (a) Experimental sequence used to measure the four-photon Stark shift. (b) The four-photon Stark shift exhibits a quadratic dependence (fitted orange line) on the input Raman beam power. (c) The shift also shows an oscillatory dependence as the HWP angle is scanned. The oscillation amplitude, offset, and shape reveal information about the incoming beam polarization and external magnetic field direction. The solid line is a fit to Equation \ref{['eq:dw4']}, using the polarizations in Equation \ref{['eq:genepsilon']} as inputs.
  • Figure 3: Raman beam laser profiles as measured via the four-photon Stark shift. Panel (a) shows the profiles before alignment optimization; panel (b) is after optimization.
  • Figure 4: (a) Rabi frequency versus ion position, before and after the alignment optimizations shown in Figure \ref{['profile']}. Optimization only slightly increases the measured two-photon Rabi frequency. (b) When one Raman beam is misaligned by a distance $d$, the resulting Rabi frequency curve shifts by $d/2$ with little change to its peak amplitude. (c) The four-photon Stark shift signal is much more sensitive to misalignments than the Rabi frequency signal, with increasing sensitivity for larger deviations.