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as in the fiber arrays, but alignment would materials and geometric properties or recommendations expressed in this
not be required, though at the cost of a more impacting power handling at input facets, material are those of the author(s) and
complex fabrication process, and potential loss in tapered waveguide couplers, and do not necessarily reflect the views
incompatibilities with other integrated scatter of uncoupled light into unwanted of the Under Secretary of Defense for
technologies such as complementary metal directions. (Near-resonant scattered light Research and Engineering, Defense
oxide semiconductor (CMOS) electronics, can lead to decoherence of qubits housed Advanced Research Projects Agency, and
unless a complete back-end-of-line process in atomic systems.) Some atomic species Department of Defense.
is developed. Pulled and tapered fibers of interest, including those in which the
laid along input waveguides can allow for highest-fidelity two-qubit operations References
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though with slightly reduced modularity for further development of lower-loss “Patterned photostimulation via
because fiber preparation is required. materials, and alignment methods with visible-wavelength photonic probes
Another approach is the fusing of individual tighter tolerances, in photonics packaging. for deep brain optogenetics,”
fibers to etched prominences in the facet Perhaps counterintuitively, reducing the Neurophotonics, vol. 4, pp. 011002–
at each waveguide location by means of need for fiber-to-waveguide coupling may 011002, Jan. 2017.
pulsed-laser heating and welding. Inherently be a necessary step; in this regard, on- 2. G. Moody, V. Sorger, P. Juodawlkis,
a serial technique, this method may be chip active components for amplification, W. Loh, C. Sorace-Agaskar, M.
challenging for large numbers of channels, for instance, would reduce the amount Davanco, et al.,“Roadmap on
and it is still to be determined if the optical of power that must be coupled from off- integrated quantum photonics,” arXiv
quality of the facet can be maintained during chip. Altogether, the best approach will preprint arXiv:2102.03323, 2021.
welding, but the inherent strength of the likely involve holistic consideration of 3. C. D. Bruzewicz, J. Chiaverini, R.
connection without the need for adhesives system requirements, weighing the ability McConnell, J. M. Sage, “Trapped-ion
is an attractive attribute for small numbers to design improved materials and prove- quantum computing: Progress and
of inputs. Each of these novel techniques, out new fabrication processes against the challenges,” Appl. Phys. Rev., vol. 6,
though promising, requires further challenges associated with achieving and no. 2, p. 021314, 2019.
development before it will be beneficial for maintaining tight alignment tolerances 4. R. Niffenegger, J. Stuart, C. Sorace-
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The variety of unproven technologies diameter and structure. Likely, combining C. Bruzewicz, et al., “Integrated
for on-chip coupling described above all of these advances will be required for multi-wavelength control of an ion
accentuates the fact that, in general, we are the most challenging, and therefore the qubit,” Nature, vol. 586, no. 7830, pp.
still in the early stages of current research most ultimately rewarding, applications. 538–542, 2020.
in multi-wavelength, multi-channel on- 5. K . K . Me ht a , C. Z h a n g , M .
chip coupling of visible light. While the Acknowledgments Malinowski, T.-L. Nguyen, M.
methodology demonstrated here allows We acknowledge contributions to Stadler, J. P. Home, “Integrated optical
for robust delivery of multiple visible this work from C. Sorace-Agaskar, P. multi-ion quantum logic,” Nature, vol.
wavelengths to on-chip waveguides, Callahan, D. Kharas, R. McConnell, and 586, no. 7830, pp. 533–537, 2020.
enabling the development of proof-of- J.M. Sage. We thank Chris Thoummaraj 6. S. Bramhavar, C. Sorace-Agaskar,
principle devices utilizing trapped ions as and Peter Murphy for substrate bonding D. Kharas, W. Loh, R. Maxson,
quantum sensors or computing elements, and chip packaging assistance. This G. N. West, et al., “A visible-light
significant research in this area is required to material is based upon work supported integrated photonic platform for
determine if these or related approaches are by the Under Secretary of Defense for atomic systems,” in Integrated Optics:
extensible to the number of sites and power Research and Engineering, Defense Devices, Materials, and Technologies
levels needed for systems with performance Advanced Research Projects Agency, XXIII (S. M. García-Blanco and
that exceeds classical capabilities. and Department of Defense under Air P. Cheben, eds.), vol. 10921, pp.
Additional specific challenges include Force Contract No. FA8702-15-D-0001. 172–176, Inter. Soc. for Optics and
obtaining a deeper understanding of Any opinions, findings, conclusions Photonics, SPIE, 2019.

Biographies
Robert Niffenegger recently completed his post-doc work at Lincoln Laboratory, Massachusetts Institute of
Technology, Lexington, MA USA, and received his PhD in Physics from Purdue U. in 2015. He spent four years
at Intel as an Integration and Yield Engineer developing front-end process technology for the 7nm node and
patented a new metal gate process. After that, he joined the Quantum Information and Integrated Nanosystems
Group, MIT Lincoln Laboratory, working on trapped ions and integrated photonics; email bobnifty@gmail.com.

David Reens is a Postdoctoral Research Associate in the Quantum Information and Integrated Nanosystems
group at MIT Lincoln Laboratory, Lexington, MA USA. He received his Physics Masters and PhD from Colorado U. at Boulder
in 2019, where he studied low temperature gases of neutral hydroxyl radicals with Professor Jun Ye. He focuses primarily on the
application of chip-scale integrated trapped ion technologies to the development of portable atomic clocks.

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