# Learnings from the trip to UC Berkeley
### Feb 19th, 2023, Youngik

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* Overview
* I had a chance to visit UC Berekley for 5 days.
* There were numerous meetings including lab tours, and I had a chance to give a talk to UC Berkeley faculties and researchers.
* Here's my impression US vs Korea: if I make an analogy of R&D using a war, it is like that they fight with guns while we use spears. We need to learn and prepare our own guns.
* Of course, we cannot copy rich groups and universities exactly the same due to the budget. However, it is more of lack of information rather than money.

* Meeting with Dr. Park and Dr. Son
* During our conversation, I realized that rapid adiabatic coupler is the way to go for having broadband, robust 50:50 power splitting.
* Honewell has a publication on multi-layer, low-loss fiber-to-chip coupler work. We want to benchmark this in the future.

* Jelena Vuckovic's lab
* free space Montana is 100k
* large space Montana is 300k with fiber feedthrough, but their quality is somehow very bad
* Amir Safavi-Naieni's lab
* Fiber-to-chip gluing was done in this setup
* Each side has manual 3-axis of freedom
* There are fiber holders on each side that can be rotated around its longitudinal axis
* There is a hovering microscope so that you can see the alignment live by yourself
* there was xy translation plus rotation for the middle sample holder
* there were some additional translation stages
* LN etching
* They use ion beam milling machine (https://snsf.stanford.edu/facilities/fab/fcr/ionmill) to etch thin film lithium niobate
* Surprisingly! They can etch 700nm now with this machine, after some tweaking of etch parameters: this will help dispersion engineering
* poling
* poling setup was very simple, and looks similar to a regular grating coupler setup
* instead of grating coupler, they have probing needs which are used to apply an electric pulse
* The whole setup was prepared on a movable cart
* Under the chip setup, there was a machine that could apply high voltage electric pulse
* It took 6 months for them to get the first successful result
* For poling patterns, large one can be made even with photolithograph (Heidelberg MLA150 Mask Writer, https://docs.google.com/document/d/1l7oRbOvrYVHcSjxSGK8_1Yyc0gQ3jg0O6vdkbvoPNmQ/edit). But you need e-beam lithography for visible applications
* Alp Sipahigil's lab
* Alp can reliably make G centers, and just figured out how to make T centers
* Alp and Ming's groups worked together to achieve 5dB per facet cryogenic fiber-to-chip coupling for 16 fiber arrays. Outermost fibers were used to optimized the signal and the rest of 14 arrays were automatically aligned. Its performance was even slightly better than room temperature. They also studied Amir's paper and used cryogenic-friendly glue to do this.
* Alp's arXiv paper (https://arxiv.org/abs/2211.09305) showed self-HOM measurement from G-center.
* UC Berkeley has a lot of contractros who can do fabs for customers with some fees.
* UC Berekley fab is unlikely to take materials like lithium niobate even though they have an ion milling machine. This is due to the cleanroom's CMOS-oriented character.
* Ming Wu's lab
* Ming Wu is working on MEMS optical switches
* He mentioned that the feed-forward was previously pursued in optical communication system with the name called optical packet switching, but it did not go very far
* He thought our GMZI idea is interesting and it was news to him
* He introduced a paper from Journal of Lightwave Technologies published around 2020, that shows lots of architectures
* His lab already achieved flip-chip bonding. His e-dies are much smaller than photonic dies because he produced the cheap, identical e-dies.
* His lab has the collests setup ever! It can be used for both packaging and measurement. 6DOF with 100nm closed loop control stage is being used, even with automatic injection of epoxies. The whole system is worth $300k. It has two cameras above, and one camera on front and back side each.
* Prof. Myung-Ki Kim from Korea University had a sabbatical year at UC Berkeley and developed the micro-lenses for very low-insertion loss coupling from on-chip waveguides to SMF-28 fiber around 0.7 dB. They plan to publish it soon.
* His lab pursues very low crossing loss, and experimentally reaches 10 mdB per device. Still this is not enough for their very large network.
* I mentioned the idea of monitoring light signals from above using sensitivie camera, which is completely non-destructive. Prof. Wu thinks that may be difficult because of stochastic nature of these waveguides.
* Prof. Wu kindly invited me for a visit during his sabbatical year.

* Vladimir Stojanovic group graduate student
* They are taping out from Global foundries with the goal of integration of various devices
* Davd (the graduate student) was piggybacking on Ayar Lab's tapeout chance, and Alp was piggybacking on Stojanovic group's tapeout
* GF has a nice system that integrated photonics and electronics are combined on a single chip. Interestingly, on the same surface, they have both phtonics chip and electronics interface. And they are internally linked through GF / Ayar Labs fab technologies.
* GF has a roadmap of putting BTO in the far future.
* GF turnaround time is 6 months.
* They also do AIM photonics.
* Others
* Even in UC Berekley, no one really knew about LOQC field and they appreciated for my presentation since it helped their understanding of it.
* After talking to researcher's there, I am convinced in these two things.
* Nobody really knows the importance of our nonlinear quantum photonics simulator.
* Prof. Ming Wu approved that studying on-chip multiplexing (or called switching in his field) is a very good direction.
* Read M. Fejer group graduate Dr. Jason Pelc's thesis to get an idea of the efficts of poling pattern disorders.
* Prof. Eli Yablonovitch thinks there's a resemblence between MBQC and quantum repeater approach. I had an honor to have a one-to-one chat with him.