This article is based on a conversation between Irfan Siddiqi and Daniel Rodán Legrain as part of the Quantum Builders series, sponsored by Qblox. Watch the full webinar for more on quantum trajectories, the Advanced Quantum Testbed, and California's quantum policy push.
Irfan Siddiqi is a professor of physics and electrical engineering at UC Berkeley, a faculty scientist at Lawrence Berkeley National Laboratory, and director of the Advanced Quantum Testbed. He leads the Quantum Nanoelectronics Lab, where his work on quantum-limited microwave amplifiers, quantum jumps, and quantum trajectories has shaped how the field measures superconducting qubits.
He began his work before superconducting qubits existed as a measurable object. "I started grad school before superconducting qubits were first measured," Siddiqi said. His path ran through cryogenics and AMO physics as an undergraduate at Harvard, a summer internship at Hypres working on flux trapping, and a PhD at Yale built around detectors for astronomy.
The turn toward qubit measurement came through his postdoc with Michel Devoret at Yale. "Michel is the one that always asks the hard questions. What's the limit? How does it work, et cetera," Siddiqi said. "And that's how we got started in this measurement business."
Working with graduate student Rajamani Vijayaraghavan, the lab began studying bifurcation amplifiers, mapping a qubit's two states onto a larger anharmonic oscillator to get a single-shot digital readout. That work raised a harder question about back action, and what happens to a qubit when a measurement is made.
Siddiqi frames the entire measurement research program around a single idea, one he described sharing at a Pontifical Academy of Science meeting. "Quantum mechanics to me is your theory," he said. "For me, in the development of theories of physics, it's one of the first times, if not the first time, human beings have said there is a reality that's more complicated than we can observe. And it says something even more profound, that you may never be able to observe it in its entirety."
He returned to this later using a different image. "What is the structure of this entanglement? How much do you have? How much does it flow?" he asked. "I think we will learn something very profound about how quantum mechanics operates in many-body systems, and in particular in open many-body systems."
Siddiqi traced this shift back to a basic principle of measurement. "For every bit of information that you try to extract from the system, you must do some damage to your cubit," he said. The lab's early goal was reducing the number of unmeasured modes in their circuits, since those extra modes carry away information as uncontrolled decoherence.
That discipline led to one of the hardest experiments his group has run: testing whether measurement truly collapses a quantum state to a single point. "It turned out to be a very beautiful result that show that basically you don't collapse the wave function to a point," Siddiqi said. "You collapse it to on the blocks of your ring when you measure two things somewhat simultaneously."
Standing-wave amplifiers run into limits on gain and bandwidth, which pushed Siddiqi's group toward nonlinear transmission lines. The hard part was phase matching, keeping a pump and a signal in sync long enough to transfer energy between them.
The breakthrough came from an unexpected pairing. Natalie Antler, a student in Siddiqi's lab working on diamond defect centers, was housemates with Kevin O'Brien, a mechanical engineering student working on photonics. "Kevin is the one that came up with this architecture to match with resonators and to have this resonant phase matching," Siddiqi said. Working with Will Oliver's group at MIT Lincoln Laboratory, they built the first two working devices.
The Advanced Quantum Testbed grew out of the push behind the National Quantum Initiative Act, where Siddiqi testified before the Senate on its first authorization. The model was to open Berkeley Lab's hardware to outside teams with an idea worth testing. "We published, I don't know, 40 manuscripts, right, in the first five years in top journals," Siddiqi said.
Openness was the operating principle. "We had promised everyone and continue to promise that we would keep your IP separate," Siddiqi said. "The output for us were joint papers, which I think everyone has been on board with."
That model is winding down as DOE priorities shift, and Siddiqi's team is now thinking about a horizontal ecosystem model instead, one where different partners build and improve standard components the way a semiconductor supply chain does. "We don't actually make any of our own amplifiers, we buy them at the moment," he said. "I'm happy to buy from others in the field."
Governor Newsom signed AB 940 after visiting Siddiqi's Berkeley lab. Siddiqi sees the state's interest as a long-horizon bet rather than a short-term return calculation. Asked by investors about ROI timelines, he said his own answer runs against the grain. "I said, 50 years for me," Siddiqi said. "Because we go ahead and we're trying to come up with some idea that has long-term impact. It's not a fad."
He points to California's existing technical density as the reason the state can move quickly. "It is already distributed. There are many people in Silicon Valley and other places in Southern California, Northern California, that think about very technical problems," he said, citing conversations with Nvidia and Keysight as examples of that base engaging with quantum.
As Berkeley's physics department chair and a Distinguished Teaching Award winner, Siddiqi has watched his introductory quantum mechanics course grow from roughly 20 students to 350. His approach to the license a physics doctorate confers is direct. "We give you a license to think," he said. "The worst thing you can do is not think."
His advice to students who hear the field is settled runs the same direction. "Don't let anybody ever tell you the problem is solved," Siddiqi said. "Most likely the problem is not solved. And quantum mechanics by any means is not solved."
Siddiqi is focused on using machine learning to characterize noise in multi-qubit systems, particularly systems that do not follow simple Markovian behavior. "I'm quite excited about this idea of using tools, different mathematical tools, not for this kind of intelligence business," he said. "But use AI to learn a structure, and then try to do something with that structure."
Measurement, testbeds, and policy all point at the same open question for Siddiqi. The field still needs to learn how entanglement behaves at scale. Qblox works alongside research teams building that measurement infrastructure, from control stacks to readout. If you're exploring how to move from single-qubit measurement to system-level characterization, we'd be glad to connect.
Contact us to learn how Qblox supports quantum measurement infrastructure.
