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Optical Characterization Of A Phase Fresnel Lens For Trapped Ion Quantum Computing

B. G. Norton, Erik W. Streed, Justin James Chapman, D. Kielpinski

Published Jan 1, 2009• 0 citations
OpenAlex

Abstract

To efficiently collect light from ions in a trapped ion quantum computer, a lens with a high numerical aperture is advantageous. Using phase Fresnel lenses with large working distances and high numerical apertures, we can couple a large fraction of the fluorescence from an ion into a single optical propagation mode (TEM00). We have optically profiled a phase Fresnel lens with a numerical aperture of 0.64 and a focal length of 3 mm. A 350ᱵ nm beam waist and a beam quality M2 of 1.08ᰮ05 was measured near the diffraction limit.

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Mar 8, 2014•0 citations
To efficiently collect light from ions in a trapped ion quantum computer, a lens with a high numerical aperture is advantageous. Using phase Fresnel lenses with large working distances and high numerical apertures, we can couple a large fraction of the fluorescence from an ion into a single optical propagation mode (TEM00). We have optically profiled a phase Fresnel lens with a numerical aperture of 0.64 and a focal length of 3 mm. A 350ᱵ nm beam waist and a beam quality M2 of 1.08ᰮ05 was measured near the diffraction limit.
Efficient ion-photon coupling is an important component for large-scale ion-trap quantum computing. We propose that arrays of phase Fresnel lenses (PFLs) are a favorable optical coupling technology to match with multi-zone ion traps. Both are scalable technologies based on conventional micro-fabrication techniques. The large numerical apertures (NAs) possible with PFLs can reduce the readout time for ion qubits. PFLs also provide good coherent ion-photon coupling by matching a large fraction of an ion's emission pattern to a single optical propagation mode (TEM00_{00}00​). To this end we have optically characterized a large numerical aperture phase Fresnel lens (NA=0.64) designed for use at 369.5 nm, the principal fluorescence detection transition for Yb+^++ ions. A diffraction-limited spot w0=350±15w_0=350\pm15w0​=350±15 nm (1/e21/e^21/e2 waist) with mode quality M2=1.08±0.05M^2= 1.08\pm0.05M2=1.08±0.05 was measured with this PFL. From this we estimate the minimum expected free space coherent ion-photon coupling to be 0.64\%, which is twice the best previous experimental measurement using a conventional multi-element lens. We also evaluate two techniques for improving the entanglement fidelity between the ion state and photon polarization with large numerical aperture lenses.
Efficient ion-photon coupling is an important component for large-scale ion-trap quantum computing. We propose that phase Fresnel lenses (PFLs) are a favorable optical coupling technology to match with multi-zone ion traps. Both are scalable technologies based on conventional micro-fabrication techniques. Large numerical aperture (NA) PFLs can reduce the readout time for ion qubits and provide good coherent coupling to a single optical mode. From optical characterization of a suitable PFL we predict a coherent coupling efficiency of >0.64%, twice the best experimental efficiency achieved to date. In addition, high NA entanglement fidelity limits can be removed by optical filtering.
Designing and integrating micro-optical components into atom and ion traps are enabling steps toward miniaturizing trap dimensions in quantum computation applications.The micro-optic must have a high numerical aperture for precise illumination of the ion and should not introduce scatter.Due to the extreme optical efficiency requirements in trapped ion and atom-based quantum information processing, even slight losses from integrated micro-optics are detrimental.We have designed and fabricated aspheric micro-lenses through grayscale transfer into a fused silica in an effort to realize increased transmissive efficiency and decreased scatter compared to an equivalent diffractive optical element.The fabricated grayscale lens profile matched the desired lens profile well, and the measured and predicted optical performances were in good agreement.The pattern was transferred via coupled plasma reactive-ion etching smoothly into the fused silica with a RMS roughness ~35 nm.The micro-lens had a diameter of 88 um and 14.2 um sag, with an as-designed focal length of 149 um and spot diameter of 2.6 um.The maximum measured efficiency was ~80% (86% of theoretical, possibly due to rms roughness).This realized efficiency is superior to the equivalent diffractive lens efficiency, designed to the same use parameters.The grayscale approach demonstrated an increase in collection efficiency, at the desired optical focal length, providing the potential for further refinement.
The integration of optics for efficient light delivery and the collection of fluorescence from trapped ions in surface electrode ion traps is a key component to achieving scalability for quantum information processing.Diffractive optical elements (DOEs) present a promising approach as compared to bulk optics because of their small physical profile and their flexibility in tailoring the optical wavefront.The precise alignment of the optics for coupling fluorescence to and from the ions, however, poses a particular challenge.Excitation and manipulation of the ions requires a high degree of optical access, significantly restricting the area available for mounting components.The ion traps, DOEs, and other components are compact, constraining the manipulation of various elements.For efficient fluorescence collection from the ions the DOE must be have a large numerical aperture (NA), which results in greater sensitivity to misalignment.The ion traps are sensitive devices, a mechanical approach to alignment such as contacting the trap and using precision motors to back-off a set distance not only cannot achieve the desired alignment precision, but risks damage to the ion trap.We have developed a non-contact precision optical alignment technique.We use line foci produced by off-axis linear Fresnel zone plates (FZPs) projected on alignment targets etched in the top metal layer of the ion trap and demonstrate micron-level alignment accuracy.
The integration of optics for efficient light delivery and the collection of fluorescence from trapped ions in surface electrode ion traps is a key component to achieving scalability for quantum information processing. Diffractive optical elements (DOEs) present a promising approach as compared to bulk optics because of their small physical profile and their flexibility in tailoring the optical wavefront. The precise alignment of the optics for coupling fluorescence to and from the ions, however, poses a particular challenge. Excitation and manipulation of the ions requires a high degree of optical access, significantly restricting the area available for mounting components. The ion traps, DOEs, and other components are compact, constraining the manipulation of various elements. For efficient fluorescence collection from the ions the DOE must be have a large numerical aperture (NA), which results in greater sensitivity to misalignment. The ion traps are sensitive devices, a mechanical approach to alignment such as contacting the trap and using precision motors to back-off a set distance not only cannot achieve the desired alignment precision, but risks damage to the ion trap. We have developed a non-contact precision optical alignment technique. We use line foci produced by off-axis linear Fresnel zone plates (FZPs) projected on alignment targets etched in the top metal layer of the ion trap and demonstrate micron-level alignment accuracy.