Orchestrating the nanoscale: exploring light and matter for quantum science

Light. Matter. Interactions. 

The pithy trio of terms belies the wide range of science explored by Randall Goldsmith, a professor of chemistry at the University of Wisconsin–Madison. There, he directs nature’s smallest constituents to follow his cues on an atom-scale stage.  

On one side are photons — particles of light. On the other are molecules — particles of matter. Goldsmith explores and orchestrates their interactions, uncovering phenomena that could be the bases for devices that can detect a single diseased cell in human tissue or carry information over a hackerproof network. 

“It’s so cool how the modern QIS toolkit can control, seemingly at will, the fate of the electronic states of molecules and atoms … that, for me, is what’s really amazing.” — Randall Goldsmith, University of Wisconsin–Madison

While such next-generation technologies are in their very early stages, they are expected to have widespread impact in the coming decades thanks to accelerating advances in quantum information science, or QIS, which harnesses the power of atoms and molecules for practical use.  

As part of Q-NEXT — a U.S. Department of Energy (DOE) National QIS Research Center led by DOE’s Argonne National Laboratory — Goldsmith is advancing QIS by choreographing light-and-matter interplay. 

“All of these partners kind of dance together in ways that can really give you a powerful new perspective on what the molecules are doing,” Goldsmith said. ​“We could potentially build black boxes that can be deployed in biotechnology, in pharma, in environmental sensing. New opportunities emerge when you use nanodevices or nanostructures.” 

Goldsmith is building photonic interfaces — minuscule mirrors and lenses — that measure and manipulate light to illuminate and influence molecules in specific ways. 

Take the microcavity technique developed by Goldsmith and his team. This photonic interface is a tiny space that traps light for a few nanoseconds. The trapped light passes through the molecule as the molecule passes through the cavity, revealing detailed information about the molecule’s shape and motion. 

Typically, researchers add fluorescing compounds — compounds that emit light — to chemical reactions to track and visualize them. Goldsmith’s microcavity technique enables scientists to get a vivid picture of molecular behavior without the traditional use of fluorescent labels, which can distort how molecules act in nature. 

“These kinds of photonic devices give us a whole new fully stocked sandbox of new knobs to play with,” Goldsmith said. ​“You have to get all the molecules’ various states right to fully capture the physics of the system.” 

And one has to capture the physics of the system to design molecules as custom qubits — the fundamental units of quantum information.  

Molecular qubits are only one class of qubit, the variety of which could fill a book. Each class has its advantages. Goldsmith is drawn to molecular qubits’ versatility, which makes for a well-equipped quantum playground. 

“The advantage of molecules is that there’s a hundred years of experience of learning how to build them,” he said. ​“With molecules, you could essentially dial in whatever you want because you have control over the items you put in.” 

By tuning the molecule’s various photonic features, scientists can control the qubit lifetime and the features of the light it emits as a signal. That fine-tunability enables researchers to design the perfect qubit for taking the temperature of a living cell or blasting data through a quantum communication network. 

“Say your photonic interfaces increase the rate at which qubits couple to each other. If you want to get a meaningful data transmission rate, you need that photonic interface so you’re not hostage to the sloth-like behavior of molecules that will emit whenever they damn please,” he said. ​“If you put them in a photonic interface, you can really tell them to hurry up. And this applies to any of the diversity of different types of materials that are being looked at in Q-NEXT.” 

Goldsmith is one of several collaborators within Q-NEXT working on molecular qubits. Along with Q-NEXT Director David Awschalom at the University of Chicago and Danna Freedman at MIT, Goldsmith is developing customizable qubits that can be used in multiple applications — an increasingly powerful area of exploration at the quantum research center. 

Goldsmith’s enthusiasm for molecular exploration began as an undergraduate at Cornell University, when he learned how pliable molecules could be. 

“I got more and more excited about what molecules could do and using light to learn about what the molecules were doing,” he said. ​“I was reading pop science papers about this idea of using molecules as the key elements in very small electronics, as transistors, as wires to move around charge.” 

Following graduate work at Northwestern University and a postdoctoral appointment at Stanford University, Goldsmith joined the University of Wisconsin as a faculty member. There he began the ​“wacky project,” as he called it — it was a new research area for him — of building photonic devices and deploying them to look at molecules. 

“I’d been moving into an area that I really had no experience in, so it was sort of a high-risk, high-return project,” he said. ​“Thankfully, I had some adventuresome and very, very capable, creative, brilliant and tenacious early students who helped us all learn together to get into photonics.” 

And tenacity is a must, given the challenge of maneuvering information at nature’s smallest scales. 

“As a community, we’ve become very dependent on these photonic devices and nano devices. Making them is not easy. Making them in a way that’s scalable or reproducible is not easy,” Goldsmith said. ​“We burn through a lot of them, so we have to make a whole bunch of them. Developing ways of smoothing that process is not glamorous work, but it’s important. 

And it takes all kinds, he says — physicists, chemists, materials scientists, biologists, engineers, technicians — for QIS to live up to its potential. 

“It’s so cool how the modern QIS toolkit can control, seemingly at will, the fate of the electronic states of molecules and atoms,” Goldsmith said. ​“And that, for me, is what’s really amazing.” 

This work was supported by the U.S. Department of Energy Office of Science National Quantum Information Science Research Centers as part of the Q-NEXT center. 

About Q-NEXT

Q-NEXT is a U.S. Department of Energy National Quantum Information Science Research Center led by Argonne National Laboratory. Q-NEXT brings together world-class researchers from national laboratories, universities and U.S. technology companies with the goal of developing the science and technology to control and distribute quantum information. Q-NEXT collaborators and institutions have established two national foundries for quantum materials and devices, develop networks of sensors and secure communications systems, establish simulation and network test beds, and train the next-generation quantum-ready workforce to ensure continued U.S. scientific and economic leadership in this rapidly advancing field. For more information, visit https://​q​-next​.org/.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology by conducting leading-edge basic and applied research in virtually every scientific discipline. Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.

The U.S. Department of Energy’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://​ener​gy​.gov/​s​c​ience.