Stevens Just Switched on the Nation's First Campus Hybrid Quantum-Communications Network

Late at night, a driver veers off a highway to avoid a collision — safely, but just barely. That same instant, the driver's twin sibling, thousands of miles away, suddenly wakes in a cold sweat with a terrible premonition about a car accident and rushes to call the driver.

That's more or less how the spooky, yet all-too-real, world of quantum communication works.

Now Stevens Institute of Technology's Center for Quantum Science and Engineering (CQSE), led by physics professor Yuping Huang, has built and unveiled a small-scale demonstration that utilizes the quantum concept to address grand challenges in cybersecurity.

Last spring, the CQSE team successfully completed a line-of-sight quantum link across a campus street to the high-tech Hanlon 2 financial data visualization laboratory in the university's Babbio Center.

Then, this February, a third node was completed, tested and verified: a quantum link, via underground fiber-optic cables, to a kiosk on the first floor of the S.C. Williams Library on the upper Stevens campus.

"There has been tremendous effort and progress made by worldwide researchers toward bringing quantum communications to real life," explains Huang. "This hybrid-quantum network, which could be the first of its kind in a campus setting, will serve as a testbed for engineering innovations and, more importantly, as an open platform to encourage and engage students and scholars from broad backgrounds."

Lithium crystals, splitting elusive photons into twins

The idea of quantum communication, first conceived in the 1980s, has yet to be implemented on an industrial scale.

"The physics have long been known, and the concept has been proven in experiments, including using a satellite," notes Huang. "But the engineering of this, in a cost-effective manner that could scale widely, has proven to take more time."

A laser in Stevens’ QuEST Lab fires photons at a lithium niobite crystal.

The Stevens team hit upon the idea of building both line-of-sight and optic links (which are interconnected and fast-switched) into a secure system on campus for demonstration, verification and educational purposes.

It works like this: A source laser in the Burchard building creates twin photons in a lithium niobate waveguide, making 10 million attempts per second. The photons are routed simultaneously both to Babbio — via line of sight, but using unconventional wavelengths — and to the library via fiber-optic cables. Three detectors, one in each of the three locations, then flag individual photons to create private keys for data encryption.

It is neither as easy nor as straightforward to do in practice as it sounds in theory.

"This has been very challenging to engineer," notes Stevens postdoctoral researcher Yong Meng Sua who is leading much of the technical work on the project. "One challenge is that the vast majority of the photons do not make it to the other end, as they are easily lost by scattering, deflection and absorption during transmission. Another challenge is that we need to tell the true signal-carrying photons from background photons that may come from a variety of sources."

As one of the twin photons is directed to the financial lab in Babbio and the other is routed underground to the library, measurements of the pair are instantaneously made and statistical probabilities are applied to verify that the photon pair is appearing in both locations at the same moment.

That's where it gets really interesting.

Applications include cybersecurity, trading platforms

The central phenomenon of quantum mechanics is known as entanglement: the idea that, whenever any manipulation is made to one photon, its distant sibling also responds and changes as a result — which seems impossible, but in fact actually occurs.

"In plain terms, if you create a pair of twin photons, and then measure what happens at the library, you don't even have to go over to Babbio to find out what measurement outcome the photon in Babbio would have, because it will be predictable," says Sua. "It's remarkable."

This unusual, inexplicable property has created great excitement in industry and academic for years. Potential applications of quantum communications, if the technology can be harvested and scaled up, include ultra-secure financial transactions as well as defense, medical and other sensitive data storage and computation.

"Quantum communications provide an absolutely secure way of transferring data," notes doctoral candidate Lac Nguyen, another member of Huang's team.

That's because current security technology depends upon algorithms which, though highly complex, can never be completely random nor foolproof. Very powerful computers could, in theory, be able to crack them.

Quantum communications methods, if realized, offer a potentially powerful solution.

"Quantum communications provide ultimate security," explains Huang, "because it is instantly known when information on the photons has been touched or altered, thanks to the principles of quantum mechanics."

To increase the practical utility of the technique, the CQSE team also leverages so-called "high-dimensional" entanglement, meaning each photon spans many quantum states. Doing so enables Stevens' researchers to gather much more information from each pair, which helps mitigate the loss issue.

"We can send photons at very high confidence, but there are always factors that lead to loss," notes Huang. "We will be attempting to use the fine features of the entangled photons to greatly increase the information capacity of the communications sent across this network, which will also enhance their robustness in practical settings."

Meanwhile, the team will work to harvest the tremendous potential of integrated photonics also being developed at CQSE under two National Science Foundation (NSF)-supported projects. Those and other approaches will be tested and perfected using Stevens’ new quantum network. New ideas may emerge, as well.

Increasing collaborations, demonstrations, outreach

For now, when a visitor types in a message in the Stevens library kiosk, it can be made to appear simultaneously and securely at a terminal in the CQSE lab or the Hanlon financial lab. (A monitor in the library also signals and displays the random arrival of single photons.)

"We wanted to make this fun," says Nguyen, who designed and maintains the kiosk as part of her contribution to the project. "We want people to touch this technology and begin to get interested in it and perhaps join us to participate in future iterations of the research."

In that vein, as research continues to develop further ways of scaling up the pilot network and keeping it cost-effective, Huang says Stevens' effort will draw in increasingly interdisciplinary collaborators — both from across campus and from external partners — as time goes on.

"When we established this project, we wanted to greatly involve students from other departments beyond just physics, including computer science, electrical engineering, mechanical engineering and financial engineering, and we have begun to do so. This work opens another door to these students and helps guide them to futures in technology; when they graduate they are a step ahead of their peers from other institutions."

That collaboration will continue to grow and evolve, taking in additional faculty as well.

"What we are building," concludes Huang, "is really a platform that we can invite students and researchers from many different backgrounds to collectively look into the advantages and challenges of this technology, which is going to become huge. We know we cannot solve all the quantum problems ourselves, so we need to bring lots of talented people on board — and who knows what great ideas they will have to solve those grand challenges?"