Novel Design May Boost Efficiency of On-Chip Frequency Combs

On the cover of the Pink Floyd album Dark Side of the Moon, a prism splits a ray of light into all the colors of the rainbow. This multicolored medley, which owes its emergence to the fact that light travels as a wave, is almost always hiding in plain sight; a prism simply reveals that it was there. For instance, sunlight is a mixture of many different colors of light, each bobbing up and down with their own characteristic frequency. But taken together the colors merge into a uniform yellowish glow.

A prism, or something like it, can also undo this splitting, mixing a rainbow back into a single beam. Back in the late 1970s, scientists figured out how to generate many colors of light, evenly spaced in frequency, and mix them together—a creation that became known as a frequency comb because of the spiky way the frequencies lined up like the teeth on a comb. They also overlapped the crests of the different frequencies in one spot, making the colors come together to form short pulses of light rather than one continuous beam.

As frequency comb technology developed, scientists realized that they could enable new laboratory developments(link is external), such as ultra-precise optical atomic clocks, and by 2005 frequency combs had earned two scientists a share of the Nobel Prize(link is external) in physics. These days, frequency combs are finding uses in modern technology, by helping self-driving cars to “see” and allowing optical fibers to transmit many channels worth of information at once, among others.

Now, a collaboration of researchers at the University of Maryland (UMD) has proposed a way to make chip-sized frequency combs ten times more efficient by harnessing the power of topology—a field of abstract math that underlies some of the most peculiar behaviors of modern materials. The team, led by Mohammad Hafezi and Kartik Srinivasan, as well as Yanne Chembo, an associate professor of electrical and computer engineering at UMD and a member of the Institute for Research in Electronics and Applied Physics, published their result(link is external) recently in the journal Nature Physics.

“Topology has emerged as a new design principle in optics in the past decade,” says Hafezi, “and it has led to many intriguing new phenomena, some with no electronic counterpart. It would be fascinating if one also finds an application of these ideas.”

Small chips that can generate a frequency comb have been around for almost fifteen years. They are produced with the help of micro-ring resonators—circles of material that sit atop a chip and guide light around in a loop. These circles are usually made of a silicon compound that is 10 to 100 microns in diameter and printed directly on a circuit board.

Light can be sent into the micro-ring from an adjacent piece of silicon compound, deposited in a straight line nearby. If the frequency of light matches one of the natural frequencies of the resonator, the light will go around and around thousands of times—or resonate—building up the light intensity in the ring before leaking back out into the straight-line trace.

Circling around thousands of times gives the light many chances to interact with the silicon (or other compound) it’s traveling through. This interaction causes other colors of light to pop up, distinct from the color sent into the resonator. Some of those colors will also resonate, going around and around the circle and building up power. These resonant colors are at evenly spaced frequencies—they correspond to wavelengths of light that are an integer fraction of the ring circumference, folding neatly into the circle and forcing the frequencies to form the teeth of a comb. At precisely the right input power and color, the crests of all the colors overlap automatically, making a stable comb. The evenly spaced colors that make up the comb come together to form a single, narrow pulse of light circulating around the ring.

“If you tune the power and the frequency of the light going into the resonator to be just right, magically at the output you get these pulses of light,” says Sunil Mittal, a postdoctoral researcher at the Joint Quantum Institute (JQI) and the lead author of the paper.

On-chip frequency combs allow for compact appRendering of a light-guiding lattice of micro-rings that researchers predict will create a highly efficient frequency comb. (Credit: S. Mittal/JQI)Rendering of a light-guiding lattice of micro-rings that researchers predict will create a highly efficient frequency comb. (Credit: S. Mittal/JQI)lications. For example, light detection and ranging (LIDAR) allows self-driving cars to detect what’s around them by bouncing short pulses of light produced by a frequency comb off its surroundings. When the pulse comes back to the car, it’s compared against another frequency comb to get an accurate map of the surroundings. In telecommunications, combs can be used to transmit more information in one optical fiber by writing different data onto each of the comb teeth using a technique called wavelength-division multiplexing (WDM).

But chip-scale frequency combs also have their limitations. In one micro-ring, the fraction of power that can be converted from the input into a comb at the output—the mode efficiency—is fundamentally limited to only 5%.

Mittal, Hafezi, and their collaborators have previously pioneered a micro-ring array with built-in topological protection, and used it to supply single photons on demand and generate made-to-order entangled photons. They wondered if a similar setup—a square lattice of micro-ring resonators with extra “link” rings—could also be adapted to improve frequency comb technology.

In this setting, the micro-rings along the outer edge of the lattice become distinct from all the rings in the middle. Light sent into the lattice spends most of its time along this outer edge and, due to the nature of the topological constraints, it doesn’t scatter into the center. The researchers call this outer circle of micro-rings a super-ring.

The team hoped to find magic conditions that would form a frequency comb in the pulses circulating around the super-ring. But this is tricky: Each of the rings in the lattice can have its own pulse of light circling round and round. To get one big pulse of light going around the super-ring, the pulses within each micro-ring would have to work together, syncing up to form an overall pulse going around the entire boundary.

Mittal and his collaborators didn’t know at what frequency or power this would happen, or if it would work at all. To figure it out, Mittal wrote computer code to simulate how light would traverse the 12 by 12 ring lattice. To the team’s surprise, not only did they find parameters that made the micro-ring pulses sync up into a super-ring pulse, but they also found that the efficiency was a factor of ten higher than possible for a single ring comb.

With “magic” input color and power, a lattice of micro-rings produces a single pulse of light circulating around the super-ring outer edge. This pulse is made up of equally spaced frequencies forming a highly efficient comb. (Credit: S. Mittal/JQI)

This improvement owes everything to the cooperation between micro-rings. The simulation showed that the comb’s teeth were spaced in accordance with the size of individual micro-rings, or wavelengths that fold neatly around the small circle. But if you zoomed in on any of the individual teeth, you’d see that they were really subdivided into smaller, more finely spaced sub-teeth, corresponding to the size of the super-ring.  Simply put, the incoming light was coupled with a few percent efficiency into each of these extra sub-teeth, allowing the aggregate efficiency to top 50%.

The team is working on an experimental demonstration of this topological frequency comb. Using simulations, they were able to single out silicon nitride as a promising material for the micro-rings, as well as figure out what frequency and power of light to send in. They believe constructing their superefficient frequency comb should be within reach of current state-of-the art experimental techniques.

If such a comb is built, it may become important to the future development of several key technologies. The higher efficiency could benefit applications like LIDAR in self-driving cars or compact optical clocks. Additionally, the presence of finely spaced sub-teeth around each individual tooth could, for example, also help add more information channels in a WDM transmitter.

And the team hopes this is just the beginning.  “There could be many applications which we don't even know yet,” says Mittal. “We hope that there'll be much more applications and more people will be interested in this approach.”

Original story by Dina Genkina:

 In addition to Mittal, Chembo, Hafezi (who is also a professor of physics and of electrical and computer engineering at UMD, as well as a member of the Quantum Technology Center and the The Institute for Research in Electronics and Applied Physics), and Srinivasan (who is also a Fellow of the National Institute of Standards and Technology), the team included Assistant Research Scientist Gregory Moille.

Foundational Step Shows Quantum Computers Can Be Better Than the Sum of Their Parts

Pobody’s nerfect—not even the indifferent, calculating bits that are the foundation of computers. But College Park Professor Christopher Monroe’s group, together with colleagues from Duke University, have made progress toward ensuring we can trust the results of quantum computers(link is external) even when they are built from pieces that sometimes fail. They have shown in an experiment, for the first time, that an assembly of quantum computing pieces can be better than the worst parts used to make it. In a paper published in the journal Nature(link is external) on Oct. 4, 2021, the team shared how they took this landmark step toward reliable, practical quantum computers.

In their experiment, the researchers combined several qubits—the quantum version of bits—so that they functioned together as a single unit called a logical qubit. They created the logical qubit based on a quantum error correction code so that, unlike for the individual physical qubits, errors can be easily detected and corrected, and they made it to be fault-tolerant—capable of containing errors to minimize their negative effects.

“Qubits composed of identical atomic ions are natively very clean by themselves,” says Monroe, who is also a Fellow of the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science. “However, at some point, when many qubits and operations are required, errors must be reduced further, and it is simpler to add more qubits and encode information differently. The beauty of error correction codes for atomic ions is they can be very efficient and can be flexibly switched on through software controls.”

This is the first time that a logical qubit has been shown to be more reliable than the most error-prone step required to make it. The team was able to successfully put the logical qubit into its starting state and measure it 99.4% of the time, despite relying on six quantum operations that are individually expected to work only about 98.9% of the time.A chip containing an ion trap that researchers use to capture and control atomic ion qubits (quantum bits). (Credit: Kai Hudek/JQI)A chip containing an ion trap that researchers use to capture and control atomic ion qubits (quantum bits). (Credit: Kai Hudek/JQI)

That might not sound like a big difference, but it’s a crucial step in the quest to build much larger quantum computers. If the six quantum operations were assembly line workers, each focused on one task, the assembly line would only produce the correct initial state 93.6% of the time (98.9% multiplied by itself six times)—roughly ten times worse than the error measured in the experiment. That improvement is because in the experiment the imperfect pieces work together to minimize the chance of quantum errors compounding and ruining the result, similar to watchful workers catching each other's mistakes.

The results were achieved using Monroe’s ion-trap system at UMD, which uses up to 32 individual charged atoms—ions—that are cooled with lasers and suspended over electrodes on a chip. They then use each ion as a qubit by manipulating it with lasers.

“We have 32 laser beams,” says Monroe. “And the atoms are like ducks in a row; each with its own fully controllable laser beam. I think of it like the atoms form a linear string and we're plucking it like a guitar string. We're plucking it with lasers that we turn on and off in a programmable way. And that's the computer; that's our central processing unit.”

By successfully creating a fault-tolerant logical qubit with this system, the researchers have shown that careful, creative designs have the potential to unshackle quantum computing from the constraint of the inevitable errors of the current state of the art. Fault-tolerant logical qubits are a way to circumvent the errors in modern qubits and could be the foundation of quantum computers that are both reliable and large enough for practical uses.

Correcting Errors and Tolerating Faults

Developing fault-tolerant qubits capable of error correction is important because Murphy’s law is relentless: No matter how well you build a machine, something eventually goes wrong. In a computer, any bit or qubit has some chance of occasionally failing at its job. And the many qubits involved in a practical quantum computer mean there are many opportunities for errors to creep in.

Fortunately, engineers can design a computer so that its pieces work together to catch errors—like keeping important information backed up to an extra hard drive or having a second person read your important email to catch typos before you send it. Both the people or the drives have to mess up for a mistake to survive. While it takes more work to finish the task, the redundancy helps ensure the final quality.

Some prevalent technologies, like cell phones and high-speed modems, currently use error correction to help ensure the quality of transmissions and avoid other inconveniences. Error correction using simple redundancy can decrease the chance of an uncaught error as long as your procedure isn’t wrong more often than it’s right—for example, sending or storing data in triplicate and trusting the majority vote can drop the chance of an error from one in a hundred to less than one in a thousand.

So while perfection may never be in reach, error correction can make a computer’s performance as good as required, as long as you can afford the price of using extra resources. Researchers plan to use quantum error correction to similarly complement their efforts to make better qubits and allow them to build quantum computers without having to conquer all the errors that quantum devices suffer from.

“What's amazing about fault tolerance, is it's a recipe for how to take small unreliable parts and turn them into a very reliable device,” says Kenneth Brown, a professor of electrical and computer engineering at Duke and a coauthor on the paper. “And fault-tolerant quantum error correction will enable us to make very reliable quantum computers from faulty quantum parts.”

But quantum error correction has unique challenges—qubits are more complex than traditional bits and can go wrong in more ways. You can’t just copy a qubit, or even simply check its value in the middle of a calculation. The whole reason qubits are advantageous is that they can exist in a quantum superposition of multiple states and can become quantum mechanically entangled with each other. To copy a qubit you have to know exactly what information it’s currently storing—in physical terms you have to measure it. And a measurement puts it into a single well-defined quantum state, destroying any superposition or entanglement that the quantum calculation is built on.

So for quantum error correction, you must correct mistakes in bits that you aren’t allowed to copy or even look at too closely. It’s like proofreading while blindfolded. In the mid-1990s, researchers started proposing ways to do this using the subtleties of quantum mechanics, but quantum computers are just reaching the point where they can put the theories to the test.

The key idea is to make a logical qubit out of redundant physical qubits in a way that can check if the qubits agree on certain quantum mechanical facts without ever knowing the state of any of them individually.

Can’t Improve on the Atom

There are many proposed quantum error correction codes to choose from, and some are more natural fits for a particular approach to creating a quantum computer. Each way of making a quantum computer has its own types of errors as well as unique strengths. So building a practical quantum computer requires understanding and working with the particular errors and advantages that your approach brings to the table.

The ion trap-based quantum computer that Monroe and colleagues work with has the advantage that their individual qubits are identical and very stable. Since the qubits are electrically charged ions, each qubit can communicate with all the others in the line through electrical nudges, giving freedom compared to systems that need a solid connection to immediate neighbors.

“They’re atoms of a particular element and isotope so they're perfectly replicable,” says Monroe. “And when you store coherence in the qubits and you leave them alone, it exists essentially forever. So the qubit when left alone is perfect. To make use of that qubit, we have to poke it with lasers, we have to do things to it, we have to hold on to the atom with electrodes in a vacuum chamber, all of those technical things have noise on them, and they can affect the qubit.”

For Monroe’s system, the biggest source of errors is entangling operations—the creation of quantum links between two qubits with laser pulses. Entangling operations are necessary parts of operating a quantum computer and of combining qubits into logical qubits. So while the team can’t hope to make their logical qubits store information more stably than the individual ion qubits, correcting the errors that occur when entangling qubits is a vital improvement.

The researchers selected the Bacon-Shor code as a good match for the advantages and weaknesses of their system. For this project, they only needed 15 of the 32 ions that their system can support, and two of the ions were not used as qubits but were only needed to get an even spacing between the other ions. For the code, they used nine qubits to redundantly encode a single logical qubit and four additional qubits to pick out locations where potential errors occurred. With that information, the detected faulty qubits can, in theory, be corrected without the “quantum-ness” of the qubits being compromised by measuring the state of any individual qubit.

“The key part of quantum error correction is redundancy, which is why we needed nine qubits in order to get one logical qubit,” says Laird Egan (PhD, '21), who is the first author of the paper. “But that redundancy helps us look for errors and correct them, because an error on a single qubit can be protected by the other eight.”

The team successfully used the Bacon-Shor code with the ion-trap system. The resulting logical qubit required six entangling operations—each with an expected error rate between 0.7% and 1.5%. But thanks to the careful design of the code, these errors don't combine into an even higher error rate when the entanglement operations were used to prepare the logical qubit in its initial state.

The team only observed an error in the qubit's preparation and measurement 0.6% of the time—less than the lowest error expected for any of the individual entangling operations. The team was then able to move the logical qubit to a second state with an error of just 0.3%. The team also intentionally introduced errors and demonstrated that they could detect them.

“This is really a demonstration of quantum error correction improving performance of the underlying components for the first time,” says Egan. “And there's no reason that other platforms can't do the same thing as they scale up. It's really a proof of concept that quantum error correction works.”

As the team continues this line of work, they say they hope to achieve similar success in building even more challenging quantum logical gates out of their qubits, performing complete cycles of error correction where the detected errors are actively corrected, and entangling multiple logical qubits together.

“Up until this paper, everyone's been focused on making one logical qubit,” says Egan. “And now that we’ve made one, we're like, ‘Single logical qubits work, so what can you do with two?’”

Original story by Bailey Bedford:

In addition to Monroe, Brown and Egan, the coauthors of the paper are Marko Cetina, Andrew Risinger, Daiwei Zhu, Debopriyo Biswas, Dripto M. Debroy, Crystal Noel, Michael Newman and  Muyuan Li.

IonQ Joins the New York Stock Exchange

IonQ debuts on the NYSE, 10/1/21.IonQ debuts on the NYSE, 10/1/21.

On October 1, 2021, IonQ, a company founded on research based at the University of Maryland Department of Physics, joined the New York Stock Exchange. College Park Professor Chris Monroe is IonQ’s Co-Founder and Chief Scientist, and many Terp alumni hold positions in the company.

“It is exciting to see the fruits of the efforts at UMD Physics and the JQI lead to this significant step toward a quantum future,” said physics chair Steve Rolston. “Much of the underlying science and technology were developed here, and many of IonQ’s technical staff are former UMD graduate students and postdocs.”

Monroe joined UMD Physics in 2007, and he and his students, postdocs and colleagues registered a terrific run of achievements. They created the first quantum logic gate and demonstrated the first entanglement of multiple qubits. Monroe’s group also produced the first quantum entanglement between two widely separated atoms, and made headlines worldwide by reporting the first teleportation of quantum information between individual atoms a meter apart.

Not long after, Monroe’s Trapped Ion Quantum Information lab took the lead role in devising a comprehensive plan for a complete, modular, scalable, fault-tolerant quantum-computer architecture in which information would be stored in assemblies of elementary logic units consisting of registers of trapped, entangled ion qubits.    

These and other developments led to the creation of IonQ in 2015. The company headquarters is just off campus, near the College Park Metro Station.

UMD President Darryl Pines traveled to New York for the NYSE premiere. Pines touted the development in an op-ed for the Baltimore Sun: Quantum physics will revolutionize the DMV region.

For more on the NYSE opening:

UMD Researchers Included in New NSF Quantum Initiatives

The National Science Foundation (NSF) announced a $5 million, two-year award to a University of Maryland-led multi-institutional team to develop quantum interconnects—crucial technology to connect quantum computers and pave the way for a quantum internet.

The team, QuaNeCQT (Quantum Networks to Connect Quantum Technology), has been developing the quantum versions of a modem and a router—familiar equipment in the world of standard, or “classical” computing, but a challenge to build for use with devices that operate based on the principles of quantum.

The devices allow “ion trap” quantum computers—a leading approach to quantum information processing developed in part at the University of Maryland—to exchange quantum information over distances measured in kilometers, eventually leading to the development of networks that could revolutionize numerous industries and help solve vexing societal problems.

Quantum networks are at an inflection point with the potential for significant expansion, said Edo Waks, a professor of electrical and computer engineering and of physics and the associate director of UMD’s Quantum Technology Center (QTC). But the scale-up can’t happen without standardized modular hardware between the new computers that are emerging and the vast infrastructure of the current internet.

“The hardware we are developing will address the critical gap, opening up the door to the future quantum internet that can connect quantum computers over continental distances,” said Waks.

Other UMD team members include physics Assistant Professor and QTC Fellow Norbert Linke, and Mid-Atlantic Crossroads (MAX) Executive Director Tripti Sinha, assistant vice president and chief technology officer for UMD’s Division of Information Technology. The team also includes Dirk Englund of the Massachusetts Institute of Technology and Saikat Guha of the University of Arizona.

The researchers plan to deploy this new technology in the Mid-Atlantic Region Quantum Internet (MARQI), UMD's regional quantum network footprint. The MARQI network will interconnect quantum computers at UMD, the Army Research Laboratory, MAX and IonQ—a leading quantum computing company focused on ion-trap computers that operates in UMD’s Discovery District—with a potential for significant expansion.

During the first phase of research, the team developed working prototypes of the quantum router and modem. Using a process called quantum frequency conversion, the modem converts signals from a quantum computer to infrared photons that can propagate through optical fibers over long distances. The router is powered by a silicon photonic chip that manipulates quantum signals in the network using quantum teleportation—an effect demonstrated in 2009 by researchers at UMD’s Joint Quantum Institute that allows quantum states to be transferred between particles that are physically separate. The team has deployed these prototypes in the MARQI network and established direct links with the various nodes of the network.

A quantum network could revolutionize numerous industries that take advantage of quantum computing including computing, banking, medicine and data analytics It would also enable connection of many multiple small quantum computers into powerful distributed quantum computers that could potentially solve problems with significant societal impact, from curing diseases to new approaches to fighting climate change.

As quantum technology converges with the Internet, a new technology sector would emerge, the researchers say, bringing with it the potential for major economic growth by producing rapid technological innovation and creating a large number of new jobs for the future “quantum workforce,” just as the emergence of the Internet did toward the late 20th century.

In addition, The National Science Foundation has announced a five-year, $25 million grant to fund a multi-institutional center including the University of Maryland that will conduct research to bring atomic-level precision to the devices and technologies that underpin much of modern life.

The Center for Integration of Modern Optoelectronic Materials on Demand (IMOD) is a collaboration of scientists and engineers at 11 universities led by the University of Washington. UMD faculty involved with IMOD include three professors of electrical and computer engineering and physics: WaksRonald Walsworth, founding director of the QTC; and Mohammad Hafezi, a fellow of QTC.

IMOD research will center on new semiconductor materials and scalable manufacturing processes for new devices based on optoelectronics, the study and application of electronic devices that produce, detect and control light. Applications would range from displays and sensors to a technological revolution, under development today, that’s based on harnessing the principles of quantum mechanics.

“Our work will develop new classes of colloidal materials that can generate quantum light with unprecedented efficiency, and enable strong photon-photon interactions,” said Waks, who serves as UMD’s lead investigator. “These are the key building blocks for photonics-based quantum information processing.”

The goal of the center is to realize high-impact platforms for quantum networking and sensing, Walsworth said.

“As a key part of IMOD, QTC researchers will lead efforts to establish a new class of quantum materials that combine pristine optical properties and long qubit coherence times,” he said.

The other academic institutions in IMOD are the University of Pennsylvania; Lehigh University; Columbia University; Georgia Institute of Technology; Northwestern University; City College of New York; the University of Chicago; the University of Colorado at Boulder and the University of Maryland, Baltimore County.

External partners include Amazon, Corning, Microsoft and Nanosys, as well as government organizations like the National Renewable Energy Laboratory and the Pacific Northwest National Laboratory.

Adapted from stories by Kara Stamets: