Written by Leah Hesla
There’s more than one way to wrangle a photon.
Every day, people send photons — particles of light — back and forth to communicate. They transmit information encoded as patterns of light, whether turning a radio dial, entering an address into a GPS unit, or sending an email.
But there’s another way to harness light for communication: not by manipulating patterns, but rather by exploiting an individual photon’s innate features, its quantum nature.
By taking advantage of traits that are accessible only at the level of the individual particles, scientists are developing fundamentally new, powerful ways of sending and receiving messages: as quantum communication. The payoff is expected to be next-level: networks that are invulnerable to attack and vastly stronger connections between high-performance computers — this could enable solutions to the world’s most intractable problems.
Researchers such as Elizabeth Goldschmidt, an assistant professor at the University of Illinois Urbana-Champaign, are developing all manner of maneuvers for managing photons as quantum carriers of information.
“It’s an interesting hard problem, but one that’s really important. The possibilities are massive for quantum communication — potentially world-changing,” said Goldschmidt, who is also a member of the Illinois Quantum Information Science and Technology Center, Q-NEXT, and NSF Hybrid Quantum Architectures and Networks quantum research centers. “But it’s super difficult, a total mess, which is why people are investigating every possible way to do this. We just don’t know what’s going to work best.”
For her part, Goldschmidt is corralling photons into quantum memories.
The fragility of the qubit
A quantum memory is like the memory on your desktop computer. It’s a temporary holding cell that preserves transmitted information until the information is needed later, when the data can be passed down the line.
The information is stored in fundamental units called qubits, analogous to computer bits. Qubits can be encoded in any number of ways, including as atoms, electrons, special circuits — and photons.
A viable quantum memory should be able to hold on to qubits for a relatively long time, a tall order given that quantum information is famously fussy. A qubit’s faintest interactions with its surroundings can extinguish the encoded information.
Add to that the challenges of wrangling photons, the fastest-moving objects in the universe. How do you hold onto them for long enough to be useful? They also get lost easily. How do you compensate for their waywardness? And how do you do all of this in a way that they keep to themselves, without interacting with their environment?
Goldschmidt began confronting the puzzle in her undergraduate days at Harvard University and continues to tackle it as a full-time physicist and professor.
“I really like this problem of getting quantum information back and forth between stuff and light, the quantum light-matter interface,” Goldschmidt said. “We want to do this in a way that can be built and engineered into systems. To do that, I need to somehow find a way to get this photon to some site without losing it, then read it into some system without measuring it, and do that in a way that I can read it back out again at a later time.”
Preserving memories in rare earths
As a part of Q-NEXT, a DOE National Quantum Information Science Research Center led by the U.S. Department of Energy’s Argonne National Laboratory, Goldschmidt investigates rare-earth atoms as quantum memories, vaults of quantum information.
Rare-earth atoms have electron arrangements that make them great for information storage. (See atomic numbers 58 through 71 on the periodic table.) They’re particularly useful as qubit components when embedded inside crystals.
The encoded photons pass into the crystal structure and port their information (thanks to quantum physics’ peculiar features) to the rare-earth atoms. The atoms in turn preserve the information as a particular configuration of electron energy levels. Both the photons and the atoms avoid the kinds of interactions that destroy quantum information, allowing it to be stored and retrieved.