After finishing my PhD in physics at UC Berkeley, I considered a few choices for the next stage in my career. I could apply for industry jobs with a STEM focus, or find a position in a national lab, or continue academic research at a university. That last option typically means a postdoctoral appointment, the next step in the academic career path. Postdocs are members of research groups at universities and national labs who already have a PhD, and who spend most of their time doing research, either on their own or alongside students. Some postdocs also have teaching appointments.
For me, a postdoctoral position was an opportunity to learn a whole new set of experimental skills. During my PhD in Prof. Dan Stamper-Kurn’s Ultracold Atomic Physics group, I built and tuned lasers, cooled and trapped atoms, and measured quantum noise and forces. (All of this could fill up another entire blog post!) After six years, I was still enamored of the idea of testing quantum mechanics in the lab, but ready to try my hand at some new techniques. One building over, in LeConte Hall, I found a group also studying quantum mechanics, but with a different set of tools.
I am now a postdoc in Prof. Irfan Siddiqi’s Quantum Nanoelectronics Lab. We study fundamentals of quantum mechanics, asking questions like: How delicate are effects like quantum superposition and quantum entanglement? What is the impact of an observer (a snooping postdoc or an errant photon) on a quantum system? Can we build quantum devices to sufficient scale that they can be applied to real-world problems?
My experimental tools in QNL look quite different from my ultracold lab, but the underlying physics is surprisingly similar. Instead of atoms, we use home-built quantum particles called superconducting qubits. These are nanofabricated circuits made out of superconducting material like aluminum. The quantum nature comes from the Josephson junction, a circuit element that controls the tunneling of Cooper pairs, or pairs of correlated electrons in superconductors. Superconducting qubits behave much like atoms. They have quantized energy levels that can be addressed by applying microwave excitations, and they can be prepared in quantum superpositions of those energy levels, the nanoscale equivalent of Schrödinger’s cat being at once alive and dead.
I enjoy quantum nanoelectronics research for the same reason I enjoyed ultracold atoms research. I get to have a hand in every level of the experiment, from design of the chip, to nanofabrication, to packaging and installing samples in our dilution refrigerator (superconductivity requires chilly conditions, 10-30 millikelvin, or less than -459 degrees Fahrenheit), to wiring up our microwave lines, designing and applying pulse sequences, acquiring measurements, and analyzing time-series data. We work in teams of students and postdocs, meaning that I always have colleagues on hand to help with everything from interpreting results to hoisting heavy magnetic shields. The whole experimental process is often cyclical, with the data analysis pointing us to improvements needed in chip design, so that we start back at the beginning, better informed. In the end, a lot of satisfaction comes from building up the ingredients for observing quantum phenomena by hand!
One of my current experiments explores a new method to entangle many qubits that are far apart from each other on a chip. Entanglement is one of the building blocks of quantum information processing, giving quantum computing its power. Many entanglement schemes for superconducting qubits are limited to nearest-neighbor interactions – they only work when the qubits are next to each other. I’m working to design circuits that overcome this limitation. Along the way, we’ll try to understand more about the delicacy of entanglement, how measurements can sometimes help and sometimes hinder, and what new simulations this tool might enable.
It’s a very exciting time to be doing research in quantum information, and if you’d like to learn more about QNL, please visit us at qnl.berkeley.edu. Thanks to L’Oreal USA For Women in Science, you can also see a video of me that features the inside of a dilution refrigerator and proper cleanroom garb (facebook link is https://www.facebook.com/LOrealUSA.FWIS/videos/10159637972075367/).