Quantum technology harnesses the radical power of quantum systems — such as isolated atoms, photons and electrons — to transform how we process and communicate information. But that potential can be realized only if our nation’s resources are focused in a way that helps bring quantum research from the laboratory to the marketplace.

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An international team of scientists, with key contributions from researchers at the University of Maryland, has found the first evidence of a source of high-energy cosmic neutrinos—ghostly subatomic particles that travel to Earth unhindered for billions of light years from the most extreme environments in the universe.

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Transistors are tiny switches that form the bedrock of modern computing—billions of them route electrical signals around inside a smartphone, for instance.

Quantum computers will need analogous hardware to manipulate quantum information. But the design constraints for this new technology are stringent, and today’s most advanced processors can’t be repurposed as quantum devices. That’s because quantum information carriers, dubbed qubits, have to follow different rules laid out by quantum physics.

Scientists can use many kinds of quantum particles as qubits, even the photons that make up light. Photons have added appeal because they can swiftly shuttle information over long distances and they are compatible with fabricated chips. However, making a quantum transistor triggered by light has been challenging because it requires that the photons interact with each other, something that doesn’t ordinarily happen on its own.

Now, researchers at the Joint Quantum Institute (JQI), led by JQI Fellow Edo Waks have cleared this hurdle and demonstrated the first single-photon transistor using a semiconductor chip. The device, described in the July 6 issue of Science, is compact: Roughly one million of these new transistors could fit inside a single grain of salt. It is also fast, able to process 10 billion photonic qubits every second.

“Using our transistor, we should be able to perfResearchers demonstrate the first single-photon transistor using a semiconductor chip. They used a single photon, stored in a quantum memory, to toggle the state of other photons. (Image Credit: E.Edwards/JQI)orm quantum gates between photons,” says Waks. “Software running on a quantum computer would use a series of such operations to attain exponential speedup for certain computational problems.

The photonic chip is made from a semiconductor with numerous holes in it, making it appear much like a honeycomb. Light entering the chip bounces around and gets trapped by the hole pattern; a small crystal called a quantum dot sits inside the area where the light intensity is strongest. Analogous to conventional computer memory, the dot stores information about photons as they enter the device. The dot can effectively tap into that memory to mediate photon interactions—meaning that the actions of one photon affect others that later arrive at the chip.

“In a single-photon transistor the quantum dot memory must persist long enough to interact with each photonic qubit,” says Shuo Sun, the lead author of the new work who is a Postdoctoral Research Fellow at Stanford University*. “This allows a single photon to switch a bigger stream of photons, which is essential for our device to be considered a transistor.”

To test that the chip operated like a transistor, the researchers examined how the device responded to weak light pulses that usually contained only one photon. In a normal environment, such dim light might barely register. However, in this device, a single photon gets trapped for a long time, registering its presence in the nearby dot.

The team observed that a single photon could, by interacting with the dot, control the transmission of a second light pulse through the device. The first light pulse acts like a key, opening the door for the second photon to enter the chip. If the first pulse didn’t contain any photons, the dot blocked subsequent photons from getting through. This behavior is similar to a conventional transistor where a small voltage controls the passage of current through it’s terminals. Here, the researchers successfully replaced the voltage with a single photon and demonstrated that their quantum transistor could switch a light pulse containing around 30 photons before the quantum dot’s memory ran out.

Waks, who is also a professor in the University of Maryland Department of Electrical and Computer Engineering, said that his team had to test different aspects of the device’s performance prior to getting the transistor to work. “Until now, we had the individual components necessary to make a single photon transistor, but here we combined all of the steps into a single chip,” Waks says.

Sun says that with realistic engineering improvements their approach could allow many quantum light transistors to be linked together. The team hopes that such speedy, highly connected devices will eventually lead to compact quantum computers that process large numbers of photonic qubits.

*Other contributors and affiliations

• Edo Waks has affiliations with the University of Maryland Department of Electrical and Computer Engineering (ECE), Department of Physics, Joint Quantum Institute, and the Institute for Research in Electronics and Applied Physics (IREAP).
• Shuo Sun was a UMD graduate student at the time of this research. He is now a postdoctoral research fellow at Stanford University.
• JQI Fellow Glenn Solomon, a physicist at the National Institute of Standards and Technology, grew the sample used in this research.
• Hyochul Kim was a postdoctoral research at UMD at the time of the research. He is now at Samsung Advanced Institute of Technology.
• Zhouchen Luo is currently a UMD ECE graduate student.

Paper reference

"A single-photon switch and transistor enabled by a solid-state quantum memory,” Shuo Sun, Hyochul Kim, Zhouchen Luo, Glenn S. Solomon, and Edo Waks, Science, 361, 57 (2018)

Research contact:

Edo Waks: waks [AT] umd [DOT] edu

Michelle Girvan, who was promoted to the rank of Professor, works in the emerging area of network science, which focuses on complex connectivity patterns among interacting units and joins physics with the domains of mathematics, biology, environmental studies, economics, sociology, and psychology, among others. Her analysis of networks helps explain developments in settings as diverse as gene encoding and the nation’s electric grid. Girvan received her Ph.D. in 2004 from Cornell University, and has held appointments at the Santa Fe Institute and the Institute for Advanced Study. She holds a joint appointment in the Institute for Physical Sciences and Technology. In 2017 she received the Richard A. Ferrell Distinguished Faculty Fellowship and was elected a Fellow of the American Physical Society.

Min Ouyang, who was promoted to the rank of Professor, is a member of the Center for Nanophysics and Advanced Materials. His experiments at the juncture of physics and chemistry involve creating novel and complex nanomaterials via the bottom-up synthetic strategy and understanding nanoscale physics by using ultrafast and single photon optics, with potential applications ranging from quantum information processing to thermal management fabrics. He received his Ph.D. in 2001 from Harvard University and did postdoctoral work at the University of California in Santa Barbara before joining UMD. Among his honors are an Alfred P. Sloan Fellowship, an NSF Career Award, an Office of Naval Research Young Investigator Award, a Beckman Young Investigator Award and a Scialog Fellowship from the Research Corporation.

Ayush Gupta, who was promoted to the rank of Associate Research Professor, works in physics education research, developing new materials and teaching practices to help students gain greater competence with disciplinary content and practice. He has contributed to the articulation and modeling of the contextual dynamics of core disciplinary practices in STEM such as mathematical sense-making and tinkering. In another thread of work, he has contributed to modeling how cultural practices influence the creation of more/less inclusive experiences for STEM students. His work has also introduced novel models for how engineering students think about ethics and social responsibility, connecting cognitive theories with social theory and ideas from Science and Technology Studies. He received his Ph.D. in electrical engineering from this campus, and is also a Keystone Instructor in the Clark School of Engineering.

Ivan Burenkov has been promoted to Assistant Research Scientist. He received his Ph.D. in 2012 from Moscow State University, and has been a postdoctoral researcher with Adjunct Professor Alan Migdall since 2015. His interests include quantum enhanced measurements for advanced optical communication, bio-medical applications and photon frequency conversion

Nicholas Butch, who was a Rolfe Glover Postdoctoral Fellow in CNAM from 2008-11, was promoted to Adjunct Associate Professor. In addition, three other NIST scientists now have appointments in the department: Thomas Purdy and Michael Zwolak as Adjunct Assistant Professors, and Sergey Polyakov as Adjunct Associate Professor.