William Witczak-Krempa was a postdoctoral researcher at Perimeter Institute for Theoretical Physics. Now he is a professor at the University of Montreal, untangling the secrets of entanglement.
Perimeter Institute alumni have gone on to a wide variety of roles after leaving the Institute. The universal principles of Nature have always fascinated William Witczak-Krempa, and that fascination has led him to a particular knotty issue: understanding the very structure of entanglement in quantum matter. We reached out to William to learn more about his journey.
This interview has been lightly edited for clarity and length.
What is your current role, and how are you trying to push boundaries in your field?
I am an associate professor in the department of physics at the University of Montreal, and an adjunct faculty at McGill University. I hold a Canada Research Chair on “Quantum Phase Transitions.” I am a founding member of the Courtois Institute, of which I hold a Courtois Chair. My group’s research lies at the intersection of condensed matter physics, quantum field theory, and quantum information. One key question that we strive to answer is: What is the structure of entanglement in quantum matter and architectures (including quantum computers)? Entanglement is an extremely rich and challenging phenomenon as it exists in collective forms that transcend any two-party characterization. We have been charting the structure of quantum entanglement in quantum matter both in and out of equilibrium. Our focus has been on the collective forms, called multi-party entanglement, for which much less is known in many-body systems. One of our key findings has been to establish the fate of entanglement under general types of evolutions. As one application, it yields a striking result: all forms of entanglement are short-ranged in matter in equilibrium.
What brought you to where you are now?
I’ve always been driven by a desire to delve deeper into the universal principles of Nature. Are there simple ingredients that allow for the emergence of the rich phenomena that we observe? My fellowship at Perimeter Institute led to the biggest transformational change in my career: during one of the stimulating lunchtime discussions, Professor Roger Melko brought to my attention a puzzle regarding entanglement near an important quantum phase transition. This led to a collaboration with Professors Roger Melko and Robert Myers that ignited my quest to map entanglement, or as I call it “quantum DNA.” I was also able to interact with other key thinkers, including Professors Daniel Gottesman, Sung-Sik Lee, Subir Sachdev, Guifre Vidal and Xiao-Gang Wen.
What are you passionate about?
I’m a husband, and father of three wonderful kids. Besides family, physics, and golden retrievers, I’m passionate about natural habitats (forests, oceans, and mountains), and protecting them. As a physicist, I see it as a responsibility to protect the environment. I believe in the importance of questioning why and how we do science. Our discoveries have consequences, and it is wise to reflect on them.
How has your work impacted your industry and community?
My group has contributed to the understanding of quantum entanglement in many-body systems, both in and out of equilibrium. First, we have established the short-range nature of multi-party entanglement in quantum matter in equilibrium, which extends our finding regarding the bipartite negativity for conformal field theories (CFTs) in any dimension. Second, we argued for the fate of entanglement in generic time-evolution protocols, with a focus on quantum quenches. Third, we have shown how the interplay between measurements and unitary evolution can lead to long-range entanglement in quantum circuits. These fixed points described by non-unitary CFTs possess new “entanglement critical exponents” not possible in equilibrium. Fourth, we have begun to chart multiparty entanglement via an approach called “entanglement microscopy.” The idea is to perform full tomography for a microscopic subregion (using quantum Monte Carlo, say), and then analyze its full entanglement structure. This allowed us to show that multipartite entanglement arises solely in loops in quantum spin liquids, shedding new light on the phase diagram of important Hamiltonians (Kagome Heisenberg model, Kitaev model in a magnetic field). We further argued that genuine entanglement loops constitute a general property of quantum gauge theories.
How do you give back to your community?
I have successfully supervised numerous students and postdocs, five of which are now physics faculty in Canada, China, France, and the United States. My enthusiasm for teaching has been recognized by the award for best physics teaching at University of Montreal. I have shared my love of physics beyond academia by giving public lectures, as well as recording an online interview about quantum physics at the popular “Sans Filtre Podcast.”
Finally, I’m a founding member of the Institut Courtois, which was made possible by one of the largest donations for fundamental research in Canada. This Institute will not only contribute to advancing science beyond what can be done in conventional universities, but also to the dissemination of science.