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Train delays can cascade into stalled commutes, economic losses, and vacation snags. Scheduling trains is computationally complex, though: It can take hours or days to solve large transportation networks on traditional computers, when disruptions like train breakdowns or traffic accidents demand much quicker solutions. A new study led by UMBC researchers—and focused on Baltimore’s Light RailLink, a hybrid tram-rail network sharing roads with cars inside Baltimore City—harnesses quantum computing to address this challenge, using an approach that blends physics, computer science, and mathematics.

In their new paper, Sebastian Deffner, associate professor of physics; postdoctoral fellow Emery Doucet; doctoral candidate Reece Robertson; and collaborators Krzysztof Domino and Bartłomiej Gardas at the Institute of Theoretical and Applied Informatics in the Polish Academy of Sciences tested whether quantum devices could manage train schedules under real-world conditions. The team leveraged the “noise” inherent in quantum computers—random, unwanted disturbances that cause an effect similar to radio static—to model unpredictable train travel times. 

Their results suggest that quantum computers can solve transportation scheduling problems, but more advanced hardware is needed to make using quantum devices practical, especially for larger networks. 

Randomness on the rails

Doucet and Robertson discussed the work at Baltimore’s Camden Yards LightRail Link Station, while Orioles baseball fans unloaded from trains at regular intervals and delivery trucks rumbled by. Their enthusiasm cut through the urban din, their voices rising over the clatter and clank of passing Baltimore Light RailLink cars.

“How long it takes you to get between two stations where you have a lot of shared infrastructure in between—you can’t really predict that precisely,” Doucet says, gesturing toward the tracks and their intersection with a nearby stoplight. This randomness complicates scheduling, but the team’s diverse expertise—spanning theoretical physics, algorithm design, and quantum hardware—facilitated creative solutions.

Noisy doesn’t have to be bad

Current quantum computers are classified as “NISQ,” or “noisy intermediate-scale quantum,”  pronounced “nisk.” That means they’re error-prone with only moderate power. Rather than fighting the noise, though, the researchers used it to mimic everyday randomness, like traffic delays. 

“The ‘N’ in NISQ stands for ‘noisy,’ but that doesn’t mean that the noise has to always be deleterious,” Doucet explains. “We wondered if maybe we could use the noise that the device is subject to as a tool to model the chaos and randomness.”

The team tested their approach on two different quantum computers, one made by IonQ, which is headquartered in Maryland, and the other by D-Wave. Each company’s quantum devices use quantum bits, or qubits, slightly differently to process information. The research team was able to solve scheduling problems with up to 12 trains on D-Wave’s system, which contains thousands of qubits, and only two trains on IonQ’s 25-qubit system. 

This proof-of-principle work demonstrates that quantum computers can tackle concrete problems, though they’re not yet faster or cheaper than classical supercomputers for large networks—the experiments cost about $65,000. 

“What we’ve shown is that with the currently available hardware, you can already solve practical problems,” Deffner, senior author on the new paper, says. The study highlights the need for larger, less noisy quantum systems to handle bigger networks.

Merging expertise, expanding possibilities

The potential impact is significant; rapid rescheduling could prevent network-wide disruptions. 

“If you have an issue on a train network, everything has to stop until you reschedule, at least in that region—and the longer it takes you to come up with a new schedule, the more disruptive the original problem becomes,” Doucet noted, as a train coasted noisily into the platform. 

Robertson, a Ph.D. candidate in computer science, added, “Within the next few generations of quantum technology, the problems we could address will get larger, approaching problems that are intractable on current hardware.” 

Robertson’s computer science background complements the team’s physics expertise.

“Someone else might be able to help me with physics intuition, and then I can help them by suggesting an algorithm we could use to test their idea, or by applying some computational intuition that we could use in designing our quantum solution.”

“Quantum information science is truly interdisciplinary,” adds Deffner, who is also affiliated with the UMBC computer science and electrical engineering department and has master’s level math training.

Beyond trains 

This interdisciplinary, quantum-based approach could eventually optimize logistics, financial portfolios, or drug discovery—fields with complex, random variables. The study, funded through Deffner’s fellowship at the National Quantum Laboratory, involved coding, theoretical modeling, and experiments on real quantum devices—a departure for Deffner’s typically theory-focused research group. 

Working on the Baltimore system was a fun challenge, Deffner says, because the LightRail Link transitions from operating as a train unaffected by traffic outside the city to a tram navigating city streets and stopping at traffic lights inside Baltimore. “Because of its unusual characteristics, it was just a unique problem. And of course, it’s cool to work on a local system,” Deffner says.

By uniting diverse expertise, UMBC’s team is turning quantum noise into a strength, paving the way for efficient solutions to real-world problems.

For over 500 days, the Hyper-Angular Rainbow Polarimeter 2 (HARP2), a high-tech instrument built by UMBC researchers and students, has been orbiting Earth on NASA’s Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite, capturing stunning data about our planet’s atmosphere, surface, and oceans. UMBC celebrated this milestone with lab tours, a poster session showcasing student research, and talks from the PACE instrument and science teams and university leadership, highlighting the instrument’s success and the hands-on role students play in this NASA mission.

“There is palpable pride on our campus in UMBC’s contributions to the PACE mission,” shares Karl V. Steiner, vice president for research and creative achievement. “Because much of the HARP2 instrument was created with strong involvement from both undergraduate and graduate students, this mission truly combines the best of our research and education missions. HARP2 is clearly a milestone in our 30-plus year partnership with NASA Goddard.’’   

HARP2’s story builds on UMBC’s earlier work with the original HARP, a pint-sized CubeSat launched in 2019 that earned the American Institute of Aeronautics and Astronautics’ SmallSat Mission of the Year award in 2021. That tiny satellite showed the world how capturing data from many angles could reveal a more complete picture of the atmospheric composition, including the roles of clouds, dust, and smoke. 

HARP2, launched in February 2024, takes HARP’s work further, collecting detailed data from a survey of the entire globe every two days. HARP2 collects in half a day the same amount of data collected by HARP in its entire two-year flight. Researchers have also enhanced data analysis methods, allowing scientists to glean even more accurate and detailed information from the raw data transmitted from space. Alongside PACE’s Ocean Color Instrument (OCI), HARP2 is helping scientists study everything from air quality to climate patterns, and painting a fuller picture of Earth’s systems.

What sets HARP2 apart is its ability to see the atmosphere in 3D-like detail, thanks to its unique “hyper-angular” views. “The instrument is working, it’s producing great data, and the community is starting to use it,” shares Vanderlei Martins, director of the Earth and Space Institute (ESI) at UMBC and the HARP team lead. Scientists across the U.S. and beyond are tapping into HARP2’s data to track pollution, measure cloud droplet properties, and more. Unlike other satellites, HARP2 can distinguish whether particles in the air are smoke, dust, or pollution, offering clues that help us understand air quality and climate impacts.

To ground-truth the data coming from HARP2, some members of the team traveled to Bolivia. They flew drones over high-altitude Lake Titicaca and the Salar de Uyuni salt flat to collect data that complements HARP2’s space-based views. With less atmosphere above them, these sites offer a clearer match to satellite observations, helping refine the science.

An “explosion” of science

Students at UMBC are deeply involved, from building and calibrating HARP2 to digging into its data. Graduate students in the atmospheric physics program work closely with faculty and NASA engineers, even on the most sensitive elements of the project. 

During testing for HARP2, “I was given the opportunity not only to see the initial calibration process, but then to see the team respond to a catastrophic failure, and then come back from it,” recalls Rachel Smith, an atmospheric physics Ph.D. student. “To watch them come together in support of the project and not miss a beat putting it back together was really incredible to see.” Early in her time with the group, Smith got to hold the instrument. “It’s a cool feeling, that I picked up and worked on this thing that’s now in space,” she says.  

Nirandi Jayasinghe, another atmospheric physics Ph.D. student, recalls the earliest data coming in. “We were all here—graduate students, scientists, and engineers—in this very room, doing stuff piece by piece to visualize ‘first light’ from HARP2,” she says. “I don’t think I’ve seen this much synergy between people anywhere else.”

Over time, the team has grown to meet the rising demands of the project. “I’ve seen how the group has gone from just me, Dominik [Cieslak], Roberto [Borda], and a couple of other engineers to now three, four, five grad students at once tackling different and interesting science questions,” shares Brent McBride ’14, physics, Ph.D. ’22, atmospheric physics, who today is an instrument scientist with the ESI. “It’s been an explosion of not only the capacity of the group, but the science that we’re capable of doing.” 

“There’s huge power there”

“Because of its hyper-angular capability, there are things we can see with HARP2 that we have never seen before,” Martins explains. This includes new insights into cloud properties and aerosol types, which are critical for understanding climate and pollution. HARP2 also helps OCI see past atmospheric haze, boosting studies over land and water. 

HARP2 has been rock-steady since launch, with no major issues. It even uses the moon for monthly calibration checks to keep its data sharp. Designed for at least three years but with fuel for potentially a decade, it’s poised to keep delivering. The data is freely available, sparking discoveries worldwide. 

“HARP2 is helping us monitor and understand Earth’s systems and come up with ways to improve life,” Martins says.“We can study everything from fires to red tides and even floods, all the way from natural disasters to the effect of pollution on nature in general. There’s huge power there.” 

As the team celebrates HARP2’s more than 500 days in orbit, its steady stream of data continues to fuel discoveries that deepen our understanding of Earth’s atmosphere and oceans. From its roots as a small CubeSat to its role in NASA’s PACE mission, UMBC’s HARP2 showcases the power of collaboration among students, scientists, and NASA, driving science that informs everything from air quality forecasts to climate solutions. With years of potential ahead, HARP2’s impact is only beginning to unfold, inspiring new questions and innovations from UMBC’s campus to the global scientific community.