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You asked IBM Research scientist Jessie Rosenberg “anything” about silicon photonics

IBM Research

You asked IBM Research scientist Jessie Rosenberg “anything” about silicon photonics  
Here’s what she said about flying qubits, wafer-scale photonic test systems, and more

Silicon photonics uses light, versus electricity, to send signals from a microchip. IBM engineers use these pulses of light to increase the connectivity and bandwidth between datacenters for faster data transfer over longer distances. But silicon photonics isn’t just for datacenters. IBM sees the technology helping improve everything from IoT sensors to quantum photonics.

Jessie Rosenberg PhD and past Forbes 30 Under 30 innovator recently joined The Optical Society (OSA) on Reddit for an Ask Me Anything to discuss all things silicon photonics. And questions ranged from all-optical processing to interstellar travel.

Question 1: Is [silicon photonics] intended to replace the electronic connections between two silicon chips? Would we have motherboards
with fiber optics on them?
Jessie Rosenberg: As data rates get higher, the propagation distance over copper interconnects gets shorter. At some distance, there’s a crossover point where transitioning to optics and back will actually save you energy. As optical transceivers become more efficient and required bandwidths get higher, that crossover point becomes shorter. It started out with telecom distances, over many many kilometers, and now is
down to the hundreds-of-meters level in datacenters. Over time I definitely think optics will reach into the circuit board!

There has been work, at IBM and other places, on an
“optical backplane,” which is essentially what you described. Generally on PCBs the approach is to use an optical routing layer containing polymer waveguides. See, for example, these two papers from IBM on card-to-card and module-to-module data transmission over optical PCBs.

Question 2: Thanks for doing this AMA! What key technical hurdles do you foresee needing to be overcome as you push to higher performance, particularly in terms of bandwidth? Any thoughts on the potential of other technologies such as Lipson’s graphene on SiN modulator?
JR: The next big jump will come from moving to higher-order modulation formats – encoding data with multiple intensity levels, polarizations, or additional wavelengths instead of just zeros and ones. We already have wavelength multiplexing, but we could add more wavelength channels there. We also have demonstrated a monolithic PAM-4 56 Gb/s modulator, which uses four intensity levels to code additional data at the same clock rate.

Past that, there are many new material systems that are very interesting, such as graphene, polymers, or nonlinear materials. The challenge there will be integrating them with high volume manufacturing and achieving high yield in order to develop a reliable commercial process. Integrating new materials with CMOS technology tends to be a slow process, however history shows that it can be done when it proves necessary!

Question 3: Since photons follow the laws of quantum physics, will we be able to make qubits with light and integrate them into a chip within the next decades?
JR: Many groups are already working on this! There are many different ways to make qubits – using optical photons as qubits is difficult, as the nonlinearities at optical frequencies are low and it is challenging to create deterministic single photon sources (though researchers are making progress on this!). At IBM we use superconducting qubits, which store information as microwave photons. The quantum computing group here has demonstrated a 5-qubit quantum computer integrated on a chip, which is presented as the IBM Quantum Experience for anyone to run experiments on. The Quantum Experience site is full of tutorials and videos, it’s a great resource to learn about quantum computing in general as well as our work at IBM.

These are stationary qubits, but we also need to be able to transmit qubits over long distances. For that, we need to convert between stationary and long-range qubits, so-called “flying” qubits. Optical photons work well as flying qubits, since they can propagate with low loss in optical fiber. Coupling between stationary and flying qubits is a significant challenge, since it has to be done with high fidelity and without disturbing the information in the qubit. This is a main research area for our group right now.

Question 4: What would you say is your biggest contribution/advancement thus far? How will the future of photonics science help propel our evolution, and possible interstellar travel?
JR: Personally, I’m proud of the inline waferscale photonic test system I designed and constructed with the optical test team, that allows us to automatically measure all electrical and optical devices across a silicon wafer, even before the wafers are finished processing. The test system sits in the actual cleanroom of the semiconductor foundry, and tests every photonics wafer early on in the process, checking for certain wafer acceptance criteria. If any of the specs are out of line, that wafer can be scrapped before it goes through the long and expensive series of later processing steps and packaging.
As far as evolution – well, silicon photonics enables larger datacenters and supercomputers.

Supercomputers can perform biological simulations such as protein folding, understanding protein folding helps develop better medical treatments, better medical treatments –> evolution???

For interstellar travel – larger datacenters can perform more analytics on bigger astronomical datasets (IBM is already working with the Netherlands Institute for Radio Astronomy to use Big Data to decode the Big Bang), which may help us find more Earth-like planets to colonize out among the stars. Once we solve that pesky problem of getting there…

Question 5: Thanks for doing the chat today Dr. Rosenberg! I’m attending CLEO for the first time next month. I know you have been involved with the technical committees, and I was wondering if you had any advice on what I should make sure to see or if you had any tips for getting the most out of the CLEO experience.
JR: Congratulations on attending your first CLEO! I’d suggest attending as many tutorials as you can, we make sure to pick great speakers who can give an informative overview of their respective fields. It’s an excellent way to broaden your knowledge base and come up with new ideas!

The post deadline sessions are also one of the highlights of the week, there are only a few concurrent sessions and they showcase the most exciting breaking research. I’d also suggest a walk through the exhibit floor, it can be very valuable to network with vendors and understand what’s available on the market. Hope you enjoy your trip!

Read more from the IBM Research blog on this topic and many others. Thank you to Dr. Rosenberg and the IBM Research team allowing participants to “Ask her Anything.”


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