From Pennsylvania State University: “The case of the elusive Majorana: The so-called ‘angel particle’ still a mystery”

From Pennsylvania State University

January 03, 2020
Sam Sholtis

An exotic quantum state known as a “chiral Majorana fermion” is predicted in devices wherein a superconductor is affixed on top of a quantum anomalous Hall (QAH) insulator (left panel). Experiments performed at Penn State and the University of Würzburg in Germany show that the millimeter-size superconductor strip used in the proposed device geometry creates an electrical short, preventing the detection of chiral Majoranas (right panel). Image: Cui-Zu Chang, Penn State

A 2017 report of the discovery of a particular kind of Majorana fermion — the chiral Majorana fermion, referred to as the “angel particle” — is likely a false alarm, according to new research. Majorana fermions are enigmatic particles that act as their own antiparticle and were first hypothesized to exist in 1937. They are of immense interest to physicists because their unique properties could allow them to be used in the construction of a topological quantum computer.

A team of physicists at Penn State and the University of Wurzburg in Germany led by Cui-Zu Chang, an assistant professor of physics at Penn State, studied over three dozen devices similar to the one used to produce the angel particle in the 2017 report. They found that the feature that was claimed to be the manifestation of the angel particle was unlikely to be induced by the existence of the angel particle. A paper describing the research appears on Jan. 3 in the journal Science.

“When the Italian physicist Ettore Majorana predicted the possibility of a new fundamental particle which is its own antiparticle, little could he have envisioned the long-lasting implications of his imaginative idea,” said Nitin Samarth, Downsbrough Department Head and professor of physics at Penn State. “Over 80 years after Majorana’s prediction, physicists continue to actively search for signatures of the still elusive ‘Majorana fermion’ in diverse corners of the universe.”

In one such effort, particle physicists are using underground observatories that seek to prove whether the ghost-like particle known as the neutrino — a subatomic particle that rarely interacts with matter — might be a Majorana fermion.

On a completely different front, condensed matter physicists are seeking to discover manifestations of Majorana physics in solid-state devices that combine exotic quantum materials with superconductors. In such devices, electrons are theorized to dress themselves as Majorana fermions by stitching together a fabric constructed from core aspects of quantum mechanics, relativistic physics, and topology. This analogous version of Majorana fermions has particularly captured the attention of condensed-matter physicists because it may provide a pathway for constructing a “topological quantum computer” whose qubits (quantum versions of binary 0s and 1s) are inherently protected from environmental decoherence — the loss of information that results when a quantum system is not perfectly isolated, and a major hurdle in the development of quantum computers.

“An important first step toward this distant dream of creating a topological quantum computer is to demonstrate definitive experimental evidence for the existence of Majorana fermions in condensed matter,” said Chang. “Over the past seven or so years, several experiments have claimed to show such evidence, but the interpretation of these experiments is still debated.”

The team studied devices fashioned from a quantum material known as a “quantum anomalous Hall insulator,” wherein the electrical current flows only at the edge. A recent study predicted that when the edge current is in clean contact with a superconductor, propagating chiral Majorana fermions are created and the electrical conductance of the device should be “half-quantized” — a value of e2/2h where “e” is the electron charge and “h” is Planck constant — when subject to a precise magnetic field. The Penn State-Wurzburg team studied over three dozen devices with several different materials configurations and found that devices with a clean superconducting contact always show the half-quantized value regardless of magnetic field conditions. This occurs because the superconductor acts like an electrical short and is thus not indicative of the presence of the Majorana fermion, said the researchers.

“The fact that two laboratories — at Penn State and at Wurzburg — found completely consistent results using a wide variety of device configurations casts serious doubt on the validity of the theoretically proposed experimental geometry and questions the 2017 claim of observing the angel particle,” said Moses Chan, Evan Pugh Professor Emeritus of Physics at Penn State.

“I remain optimistic that the combination of quantum anomalous Hall insulators and superconductivity is an attractive scheme for realizing chiral Majoranas,” said Morteza Kayyalha, a postdoctoral research associate at Penn State who carried out the device fabrication and measurements. “But our theorist colleagues need to rethink the device geometry.”

“This is an excellent illustration of how science should work,” said Samarth. “Extraordinary claims of discovery need to be carefully examined and reproduced. All of our postdocs and students worked really hard to make sure they carried out very rigorous tests of the past claims. We are also making sure that all of our data and methods are shared transparently with the community so that our results can be critically evaluated by interested colleagues.”

In addition to Chang, Samarth, Chan and Kayyalha, the research team includes Penn State faculty member Qi Li, and Wurzburg faculty members Laurens Molenkamp and Charles Gould. The project relied on materials synthesis carried out at Penn State’s 2D Crystal Consortium user facility for synthesis of 2D quantum materials and was funded by the U.S. National Science Foundation, the Office of Naval Research, the U.S. Department of Energy, the Army Research Office, and the European Research Council.

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About Penn State

WHAT WE DO BEST

We teach students that the real measure of success is what you do to improve the lives of others, and they learn to be hard-working leaders with a global perspective. We conduct research to improve lives. We add millions to the economy through projects in our state and beyond. We help communities by sharing our faculty expertise and research.

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From Princeton University: “In leap for quantum computing, silicon quantum bits establish a long-distance relationship”

From Princeton University

Dec. 30, 2019
Catherine Zandonella, Office of the Dean for Research

Imagine a world where people could only talk to their next-door neighbor, and messages must be passed house to house to reach far destinations.

Until now, this has been the situation for the bits of hardware that make up a silicon quantum computer, a type of quantum computer with the potential to be cheaper and more versatile than today’s versions.

Now a team based at Princeton University has overcome this limitation and demonstrated that two quantum-computing components, known as silicon “spin” qubits, can interact even when spaced relatively far apart on a computer chip. The study was published in the journal Nature.

Researchers at Princeton University have made an important step forward in the quest to build a quantum computer using silicon components, which are prized for their low cost and versatility compared to the hardware in today’s quantum computers. The team showed that a silicon-spin quantum bit (shown in the box) can communicate with another quantum bit located a significant distance away on a computer chip. The feat could enable connections between multiple quantum bits to perform complex calculations. Image by Felix Borjans

“The ability to transmit messages across this distance on a silicon chip unlocks new capabilities for our quantum hardware,” said Jason Petta, the Eugene Higgins Professor of Physics at Princeton and leader of the study. “The eventual goal is to have multiple quantum bits arranged in a two-dimensional grid that can perform even more complex calculations. The study should help in the long term to improve communication of qubits on a chip as well as from one chip to another.”

Quantum computers have the potential to tackle challenges beyond the capabilities of everyday computers, such as factoring large numbers. A quantum bit, or qubit, can process far more information than an everyday computer bit because, whereas each classical computer bit can have a value of 0 or 1, a quantum bit can represent a range of values between 0 and 1 simultaneously.

To realize quantum computing’s promise, these futuristic computers will require tens of thousands of qubits that can communicate with each other. Today’s prototype quantum computers from Google, IBM and other companies contain tens of qubits made from a technology involving superconducting circuits, but many technologists view silicon-based qubits as more promising in the long run.

Silicon spin qubits have several advantages over superconducting qubits. The silicon spin qubits retain their quantum state longer than competing qubit technologies. The widespread use of silicon for everyday computers means that silicon-based qubits could be manufactured at low cost.

The challenge stems in part from the fact that silicon spin qubits are made from single electrons and are extremely small.

“The wiring or ‘interconnects’ between multiple qubits is the biggest challenge towards a large scale quantum computer,” said James Clarke, director of quantum hardware at Intel, whose team is building silicon qubits using using Intel’s advanced manufacturing line, and who was not involved in the study. “Jason Petta’s team has done great work toward proving that spin qubits can be coupled at long distances.”

To accomplish this, the Princeton team connected the qubits via a “wire” that carries light in a manner analogous to the fiber optic wires that deliver internet signals to homes. In this case, however, the wire is actually a narrow cavity containing a single particle of light, or photon, that picks up the message from one qubit and transmits it to the next qubit.

The two qubits were located about half a centimeter, or about the length of a grain of rice, apart. To put that in perspective, if each qubit were the size of a house, the qubit would be able to send a message to another qubit located 750 miles away.

The key step forward was finding a way to get the qubits and the photon to speak the same language by tuning all three to vibrate at the same frequency. The team succeeded in tuning both qubits independently of each other while still coupling them to the photon. Previously the device’s architecture permitted coupling of only one qubit to the photon at a time.

“You have to balance the qubit energies on both sides of the chip with the photon energy to make all three elements talk to each other,” said Felix Borjans, a graduate student and first author on the study. “This was the really challenging part of the work.”

Each qubit is composed of a single electron trapped in a tiny chamber called a double quantum dot. Electrons possess a property known as spin, which can point up or down in a manner analogous to a compass needle that points north or south. By zapping the electron with a microwave field, the researchers can flip the spin up or down to assign the qubit a quantum state of 1 or 0.

“This is the first demonstration of entangling electron spins in silicon separated by distances much larger than the devices housing those spins,” said Thaddeus Ladd, senior scientist at HRL Laboratories and a collaborator on the project. “Not too long ago, there was doubt as to whether this was possible, due to the conflicting requirements of coupling spins to microwaves and avoiding the effects of noisy charges moving in silicon-based devices. This is an important proof-of-possibility for silicon qubits because it adds substantial flexibility in how to wire those qubits and how to lay them out geometrically in future silicon-based ‘quantum microchips.’”

The communication between two distant silicon-based qubits devices builds on previous work by the Petta research team. In a 2010 paper in the journal Science, the team showed it is possible to trap single electrons in quantum wells. In the journal Nature in 2012, the team reported the transfer of quantum information from electron spins in nanowires to microwave-frequency photons, and in 2016 in Science they demonstrated the ability to transmit information from a silicon-based charge qubit to a photon. They demonstrated nearest-neighbor trading of information in qubits in 2017 in Science. And the team showed in 2018 in Nature that a silicon spin qubit could exchange information with a photon.

Jelena Vuckovic, professor of electrical engineering and the Jensen Huang Professor in Global Leadership at Stanford University, who was not involved in the study, commented: “Demonstration of long-range interactions between qubits is crucial for further development of quantum technologies such as modular quantum computers and quantum networks. This exciting result from Jason Petta’s team is an important milestone towards this goal, as it demonstrates non-local interaction between two electron spins separated by more than 4 millimeters, mediated by a microwave photon. Moreover, to build this quantum circuit, the team employed silicon and germanium – materials heavily used in the semiconductor industry.”

In addition to Borjans and Petta, the following contributed to the study: Xanthe Croot, a Dicke postdoctoral fellow; associate research scholar Michael Gullans; and Xiao Mi, who earned his Ph.D. at Princeton in Petta’s group and is now a research scientist at Google.

The study was funded by Army Research Office (grant W911NF-15-1-0149) and the Gordon and Betty Moore Foundation’s EPiQS Initiative (grant GBMF4535).

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About Princeton: Overview

Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

From Scientific American: “‘Qutrit’ Experiments Are a First in Quantum Teleportation”

From Scientific American

August 6, 2019
Daniel Garisto

The proof-of-concept demonstrations herald a major step forward in quantum communications.

(aleksandarnakovski/iStock)

Credit: Getty Images

For the first time, researchers have teleported a qutrit, a tripartite unit of quantum information. The independent results from two teams are an important advance for the field of quantum teleportation, which has long been limited to qubits—units of quantum information akin to the binary “bits” used in classical computing.

These proof-of-concept experiments demonstrate that qutrits, which can carry more information and have greater resistance to noise than qubits, may be used in future quantum networks.

Chinese physicist Guang-Can Guo and his colleagues at the University of Science and Technology of China (USTC) reported their results in a preprint paper on April 28, although that work remains to be published in a peer-reviewed journal. On June 24 the other team, an international collaboration headed by Anton Zeilinger of the Austrian Academy of Sciences and Jian-Wei Pan of USTC, reported its results in a paper in Physical Review Letters. That close timing—as well as the significance of the result—has each team vying for credit and making critiques of the other’s work.

“Each of these [experiments] is an important advance in the technology of teleportation,” says William Wootters, a physicist at Williams College, who was not involved with either study.

The name quantum teleportation brings to mind a technology out of Star Trek, where “transporters” can “beam” macroscale objects—even living humans—between far-distant points in space. Reality is less glamorous. In quantum teleportation, the states of two entangled particles are what is transported—for instance, the spin of an electron. Even when far apart, entangled particles share a mysterious connection; in the case of two entangled electrons, whatever happens to one’s spin influences that of the other, instantaneously.

“Teleportation” also conjures visions of faster-than-light communication, but that picture is wrong, too. If Alice wants to send Bob a message via quantum teleportation, she has to accompany it with classical information transported via photons—at the speed of light but no faster. So what good is it?

Oddly enough, quantum teleportation may also have important utility for secure communications in the future, and much of the research is funded with cybersecurity applications in mind. In 2017 Pan, Zeilinger and their colleagues used China’s Micius satellite to perform the world’s longest communication experiment, across 7,600 kilometers.

Illustration of the three cooperating ground stations (Graz, Nanshan, and Xinglong). Listed are all paths

Two photons—each acting as a qubit—were beamed to Vienna and China. By taking information about the state of the photons, the researchers in each location were able to effectively construct an unhackable password, which they used to conduct a secure video call. The technique acts like a wax seal on a letter: any eavesdropping would interfere and leave a detectable mark.

Researchers have attempted to teleport more complicated states of particles with some success. In a study published in 2015 [Physics World] Pan and his colleagues managed to teleport two states of a photon: its spin and orbital angular momentum. Still, each of these states was binary—the system was still using qubits. Until now, scientists had never teleported any more complicated state.

A classical bit can be a 0 or 1. Its quantum counterpart, a qubit, is often said to be 0 and 1—the superposition of both states. Consider, for instance, a photon, which can exhibit either horizontal or vertical polarization. Such qubits are breezily easy for researchers to construct.

A classical trit can be a 0, 1 or 2—meaning a qutrit must embody the superposition of all three states. This makes qutrits considerably more difficult to make than qubits.

To create their qutrits, both teams used the triple-branching path of a photon, expressed in carefully orchestrated optical systems of lasers, beam splitters and barium borate crystals. One way to think about this arcane arrangement is the famous double-slit experiment, says physicist Chao-Yang Lu, a co-author of the new paper by Pan and Zeilinger’s team. In that classic experiment, a photon goes through two slits at the same time, creating a wavelike interference pattern. Each slit is a state of 0 and 1, because a photon goes through both. Add a third slit for a photon to traverse, and the result is a qutrit—a quantum system defined by the superposition of three states in which a photon’s path effectively encodes information.

Creating a qutrit from a photon was only the opening skirmish in a greater battle. Both teams also had to entangle two qutrits together—no mean feat, because light rarely interacts with itself.

Crucially, they had to confirm the qutrits’ entanglement, also known as the Bell state. Bell states, named after John Stewart Bell, a pioneer of quantum information theory, are the conditions in which particles are maximally entangled. Determining which Bell state qutrits are in is necessary to extract information from them and to prove that they conveyed that information with high fidelity.

What constitutes “fidelity” in this case? Imagine a pair of weighted dice, Wootters says: If Alice has a die that always lands on 3, but after she sends it to Bob, it only lands on 3 half of the time, the fidelity of the system is low—the odds are high it will corrupt the information it transmits. Accurately transmitting a message is important, whether the communication is quantum or not. Here, the teams are in dispute about the fidelity. Guo and his colleagues believe that their Bell state measurement, taken over 10 states, is sufficient for a proof-of-concept experiment. But Zeilinger and Pan’s group contends that Guo’s team failed to measure a sufficient number of Bell states to definitively prove that it has high enough fidelity.

Despite mild sniping, the rivalry between the groups remains relatively friendly, even though provenance for the first quantum teleportation of a qutrit hangs in the balance. Both teams agree that each has teleported a qutrit, and they both have plans to go beyond qutrits: to four level systems—ququarts—or even higher.

Some researchers are less convinced, though. Akira Furusawa, a physicist at the University of Tokyo, says that the method used by the two teams is ill-suited for practical applications because it is slow and inefficient. The researchers acknowledge the criticism but defend their results as a work in progress.

“Science is step by step. First, you make the impossible thing possible,” Lu says. “Then you work to make it more perfect.”

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Scientific American, the oldest continuously published magazine in the U.S., has been bringing its readers unique insights about developments in science and technology for more than 160 years.

From Joint Quantum Institute: “Remote Quantum Systems Produce Interfering Photons”

From Joint Quantum Institute

December 17, 2019

Research Contact
Steve Rolston
rolston@umd.edu

Story by Jillian Kunze

A schematic showing the paths taken by photons from two different sources in neighboring buildings. (Credit: S. Kelley/NIST)

Scientists at the Joint Quantum Institute (JQI) have observed, for the first time, interference between particles of light created using a trapped ion and a collection of neutral atoms. Their results could be an essential step toward the realization of a distributed network of quantum computers capable of processing information in novel ways.

In the new experiment, atoms in neighboring buildings produced photons—the quantum particles of light—in two distinct ways. Several hundred feet of optical cables then brought the photons together, and the research team, which included scientists from JQI as well as the Army Research Lab, measured a telltale interference pattern. It was the first time that photons from these two particular quantum systems were manipulated into having the same wavelength, energy and polarization—a feat that made the particles indistinguishable. The result, which may prove vital for communicating over quantum networks of the future, was published recently in the journal Physical Review Letters.

“If we want to build a quantum internet, we need to be able to connect nodes of different types and functions,” says JQI Fellow Steve Rolston, a co-author of the paper and a professor of physics at the University of Maryland. “Quantum interference between photons generated by the different systems is necessary to eventually entangle the nodes, making the network truly quantum.”

The first source of photons was a single trapped ion—an atom that is missing an electron—held in place by electric fields. Collections of these ions, trapped in a chain, are leading candidates for the construction of quantum computers due to their long lifetimes and ease of control. The second source of photons was a collection of very cold atoms, still in possession of all their electrons. These uncharged, or neutral, atomic ensembles are excellent interfaces between light and matter, as they easily convert photons into atomic excitations and vice versa. The photons produced by each of these two systems are typically different, limiting their ability to work together.

In one building, researchers used a laser to excite a trapped barium ion to a higher energy. When it transitioned back to a lower energy, it emitted a photon at a known wavelength but in a random direction. When scientists captured a photon, they stretched its wavelength to match photons from the other source.

In an adjacent building, a cloud of tens of thousands of neutral rubidium atoms generated the photons. Lasers were again used to pump up the energy of these atoms, and that procedure imprinted a single excitation across the whole cloud through a phenomenon called the Rydberg blockade. When the excitation shed its energy as photons, they traveled in a well-defined direction, making it easy for researchers to collect them.

The team used an interferometer to measure the degree to which two photons were identical. A single photon entering the interferometer is equally likely to take either of two possible exits. And two distinguishable photons entering the interferometer at the same time don’t notice each other, acting like two independent single photons.

But when researchers brought together the photons from their two sources, they almost always took the same exit—a result of quantum interference and an indication that they were nearly identical. This was precisely what the research team had hoped for: the first demonstration of interference between photons from these two very different quantum systems.

In this experiment, photons traveled from the first building to the second via hundreds of feet of optical fiber. Due to this distance, sending photons from both systems to meet at the interferometer simultaneously was a feat of precise timing. Detectors were placed at the exits of the interferometer to detect where the photons came out, but the team often had to wait—gathering all the data took 24 hours over a period of 3 days.

Further experimental upgrades could be used to generate a special quantum connection called entanglement between the ion and the neutral atoms. In entanglement, two quantum objects become so closely linked that the results from measuring one are correlated with the results from measuring the other, even if the objects are separated by a huge distance. Entanglement is necessary for the speedy algorithms that scientists hope to run on quantum computers in the future.

Generating entanglement between different quantum systems usually requires identical photons, which the researchers were able to create. Unfortunately, trapped ions emit photons in a random direction, making the probability of catching them low. This meant that only about eight photons from the trapped ion made it to the interferometer each second. If the researchers attempted to perform more intricate experiments with that rate, the data could take months to collect. However, future work may increase how frequently the ion emits photons and allow for a useful rate of entanglement production.

“This is a stepping-stone on the way to being able to entangle these two systems,” says Alexander Craddock, a graduate student at JQI and the lead author of this study. “And that would be fantastic, because you can then take advantage of all the different weird and wonderful properties of both of them.”

In addition to Rolston and Craddock, co-authors of the paper include JQI graduate students John Hannegan, Dalia Ornelas-Huerta, and Andrew Hachtel, JQI postdoctoral researcher James Siverns, Army Research Laboratory scientists and JQI Affiliates Elizabeth Goldschmidt (now an Assistant Professor of Physics at the University of Illinois) and Qudsia Quraishi, and JQI Fellow Trey Porto.

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JQI supported by Gordon and Betty Moore Foundation

We are on the verge of a new technological revolution as the strange and unique properties of quantum physics become relevant and exploitable in the context of information science and technology.

The Joint Quantum Institute (JQI) is pursuing that goal through the work of leading quantum scientists from the Department of Physics of the University of Maryland (UMD), the National Institute of Standards and Technology (NIST) and the Laboratory for Physical Sciences (LPS). Each institution brings to JQI major experimental and theoretical research programs that are dedicated to the goals of controlling and exploiting quantum systems.

From University of Chicago: Women in STEM -“Physicist taps quantum mechanics to crack molecular secrets” Giulia Galli

From University of Chicago

Dec 16, 2019
Louise Lerner

Prof. Giulia Galli’s work predicts how to harness molecular behavior to improve technology, such as purifying water.
Photo by Jean Lachat

There are few scientists who would describe condensed matter physics—a branch that studies the behavior of solid matter—as “simple.” But to Prof. Giulia Galli, it’s less complex than the problems she works on at the University of Chicago.

“Problems like water and energy are much more complicated than what I was trained for in condensed matter physics,” she said. “All of my work is driven by problems.”

It’s complex problems like these that the Pritzker School for Molecular Engineering—the first of its kind to focus on this emerging field—was set up to solve. And it’s the kind of innovative research that Galli, a theorist who uses computational models to figure out the behavior of molecules and materials, is helping tackle through her pioneering work.

The focus of Galli’s studies is to understand and predict how to harness molecular behavior to improve technology, particularly in the areas of purifying water, speeding up computation and sensing with quantum technology, and perfecting renewable energy technology.

“Essentially, we predict how atoms arrange themselves,” explained Galli, the Liew Family Professor of Molecular Engineering at UChicago. “We do this by developing theoretical algorithms and powerful codes and simulations in order to understand the quantum mechanics at play in a given material.”

For example, her group can use theory to predict which material will make a cheaper solar cell, or suggest a new configuration for a quantum bit made from electron spins. “Energy and water are incredibly important problems—even a small improvement from your science can have a huge impact,” she said. “This is really important to me.”

One of Galli’s favorite parts of her day is working with her group, including postdoctoral researcher Elizabeth Lee (left) and graduate student Hien Vo (right). Photo by Jean Lachat

Galli, who also heads the Midwest Integrated Center for Computational Materials, has garnered international recognition for her work in helping shape the field. She recently received the Feynman Theory Prize, an annual honor highlighting extraordinary work in harnessing quantum mechanics for the public interest. It was her fourth such major award in her field this year.

“It is not difficult to understand why Giulia has been recognized as a scientific leader by a diverse set of scientific organizations,” said Matthew Tirrell, the founding Pritzker director and dean of the Pritzker School of Molecular Engineering. “She wields powerful and versatile computational tools that she has deployed to learn about many important scientific matters, ranging from how water behaves to materials being explored for quantum device engineering.”

Deciphering atomic rules

Quantum mechanics describes the rules of atomic behavior at incredibly tiny scales—a world full of the unexpected, which Galli seeks to explain using computer codes. But the challenge of modeling the interactions between hundreds of thousands of atoms in a material is a Herculean task. Often she uses the Research Computing Center at the University, but for more complex simulations, her team uses the extremely powerful supercomputers at UChicago-affiliated Argonne National Laboratory, where Galli has a joint appointment.

The simulations may take months, depending on the problem; in fact, that Galli’s group is constantly running simulations on as many machines as they can get ahold of: “We’re running simulations every day, many at the same time. We probably have 15 projects running right now,” she said.

_______________________________________
“The job of a good scientist is to constantly doubt your answers.”
—Prof. Giulia Galli
_______________________________________

At the same time, she’s usually writing four or five papers at any given time; in between, she’s traveling to conferences, teaching, or working with students and postdoctoral researchers in her group.

Her field has changed a great deal over the years, as computers and data capacity have improved, but to Galli, it keeps her energized. “The problems are always changing. Nothing is ever boring.”

Since she moved to the PME from the University of California-Davis, she’s been able to work much more closely with scientists on the experimental side, creating a loop where their experiments validate and explore her theoretical predictions, and her insights suggest new avenues for experiments.

One such collaborator is David Awschalom, the Liew Family professor in molecular engineering and director of the Chicago Quantum Exchange, who has worked with Galli for years at PME.

“Giulia’s innovative work on exploring materials for quantum information science and technology is guiding research programs at the University of Chicago and around the world,” said Awschalom. “Her innovative research is based on identifying important problems in materials science, developing a unique theoretical approach that is informed by experimental measurement, and ultimately resolving outstanding questions about the dynamics of complex systems with predictive models.”

Addressing a ‘data crisis’

More recently, she’s become interested in addressing a problem in the field of science known as the data reproducibility crisis. All good experiments and calculations have to be able to produce the same results, no matter who’s doing the experiment or carrying out the simulations; but as simulations grow more complex and the amount of data skyrockets, it becomes harder for other scientists to be able to check someone’s work.

A recent Galli study examined inorganic links between nanoparticles for applications in solar panels and optical devices.
Illustration by Peter Allen.

Galli began providing links for interested parties to download the data (and codes) from her work, but that was only a local solution. To address the problem on a larger scale, Galli created a publicly available tool called Qresp that provides a framework for researchers to share their data and workflows, so that others can see how the results were reached—and try to poke holes in it.

She sees this as essential for science—and for scientists.

“The job of a good scientist is to constantly doubt your answers,” Galli said. “The minute you get results, you have to think about how to validate them. How to find a different way to evaluate them. To push and challenge yourself. To do what you don’t yet know how to do. That’s what I tell my graduate students.

“The real job of a scientist is to come up with a way to solve a problem that nobody else knows how to solve. And then to challenge yourself, over and over again, to make sure your solution is correct and robust.”

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An intellectual destination

One of the world’s premier academic and research institutions, the University of Chicago has driven new ways of thinking since our 1890 founding. Today, UChicago is an intellectual destination that draws inspired scholars to our Hyde Park and international campuses, keeping UChicago at the nexus of ideas that challenge and change the world.

The University of Chicago is an urban research university that has driven new ways of thinking since 1890. Our commitment to free and open inquiry draws inspired scholars to our global campuses, where ideas are born that challenge and change the world.

We empower individuals to challenge conventional thinking in pursuit of original ideas. Students in the College develop critical, analytic, and writing skills in our rigorous, interdisciplinary core curriculum. Through graduate programs, students test their ideas with UChicago scholars, and become the next generation of leaders in academia, industry, nonprofits, and government.

UChicago research has led to such breakthroughs as discovering the link between cancer and genetics, establishing revolutionary theories of economics, and developing tools to produce reliably excellent urban schooling. We generate new insights for the benefit of present and future generations with our national and affiliated laboratories: Argonne National Laboratory, Fermi National Accelerator Laboratory, and the Marine Biological Laboratory in Woods Hole, Massachusetts.

The University of Chicago is enriched by the city we call home. In partnership with our neighbors, we invest in Chicago’s mid-South Side across such areas as health, education, economic growth, and the arts. Together with our medical center, we are the largest private employer on the South Side.

In all we do, we are driven to dig deeper, push further, and ask bigger questions—and to leverage our knowledge to enrich all human life. Our diverse and creative students and alumni drive innovation, lead international conversations, and make masterpieces. Alumni and faculty, lecturers and postdocs go on to become Nobel laureates, CEOs, university presidents, attorneys general, literary giants, and astronauts.

From Caltech: “How Electrons Break the Speed Limit”

From Caltech

December 09, 2019
Robert Perkins
(626) 395‑1862
rperkins@caltech.edu

New understanding of charge transport reveals an exotic quantum mechanical regime.

In work that may have broad implications for the development of new materials for electronics, Caltech scientists for the first time have developed a way to predict how electrons interacting strongly with atomic motions will flow through a complex material. To do so, they relied only on principles from quantum mechanics and developed an accurate new computational method.

Studying a material called strontium titanate, postdoctoral researcher Jin-Jian Zhou and Marco Bernardi, assistant professor of applied physics and materials science, showed that charge transport near room temperature cannot be explained by standard models. In fact, it violates the Planckian limit, a quantum speed limit for how fast electrons can dissipate energy while they flow through a material at a given temperature.

Their work was published in the journal Physical Review Research on December 2.

The standard picture of charge transport is simple: electrons flowing through a solid material do not move unimpeded but instead can be knocked off course by the thermal vibrations of atoms that make up the material’s crystalline lattice. As the temperature of a material changes, so too does the amount of vibration and the resulting effect of this vibration on charge transport.

Individual vibrations can be thought of as quasiparticles called phonons, which are excitations in materials that behave like individual particles, moving and bouncing around like an object. Phonons behave like the waves in the ocean, while electrons are like a boat sailing across that ocean, jostled by the waves. In some materials, the strong interaction between electrons and phonons in turn creates a new quasiparticle known as a polaron.

“The so-called polaron regime, in which electrons interact strongly with atomic motions, has been out of reach for first-principles calculations of charge transport because it requires going beyond simple perturbative approaches to treat the strong electron-phonon interaction,” says Bernardi. “Using a new method, we have been able to predict both the formation and the dynamics of polarons in strontium titanate. This advance is crucial since many semiconductors and oxides of interest for future electronics and energy applications exhibit polaron effects.”

Strontium titanate is known as a complex material because at different temperatures its atomic structure changes dramatically, with the crystal lattice shifting from one shape to another, which in turn shifts the phonons that electrons have to navigate. Last year, Zhou and Bernardi showed in a Physical Review Letters paper that they can describe the phonons associated with these structural phase transitions and include them in their computational workflow to accurately predict the temperature dependence of the electron mobility in strontium titanate.

Now, they have developed a new method that can describe the strong interactions between the electrons and phonons in strontium titanate. This allows them to explain the formation of polarons and accurately predict both the absolute value and the temperature dependence of the electron mobility, a key charge-transport property in materials.

In doing so, they uncovered an exotic feature of strontium titanate: charge transport near room temperature cannot be explained with the simple standard picture of electrons scattering with atomic vibrations in the material. Rather, transport occurs in a subtle quantum mechanical regime in which the electrons carry electricity collectively rather than individually, allowing them to violate the theoretical limit for charge transport.

“In strontium titanate, the usual mechanism of charge transport due to electrons scattering with phonons has been widely accepted for the last half century. However, the picture that emerges from our study is far more complicated,” says Zhou. “At room temperature, it’s as if roughly half of each electron contributes to charge transport through the usual phonon scattering mechanism, while the other half of the electron contributes to a collective form of transport that is not yet fully understood.”

In addition to representing a fundamental advance in the understanding of charge transport, the new method by Zhou and Bernardi can be applied to many semiconductors as well as to materials such as oxides and perovskites, and to new quantum materials exhibiting polaron effects. Besides charge transport, Zhou and Bernardi plan to investigate materials with unconventional thermoelectricity (the generation of electricity from heat) and superconductivity (electric current without resistance). In these materials, existing calculations have not yet been able to take into account polaron effects.

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The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”

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From University of Chicago: “In surprise breakthrough, scientists create quantum states in everyday electronics”

From University of Chicago

Dec 6, 2019
Louise Lerner

(From left) graduate students Kevin Miao, Chris Anderson, and Alexandre Bourassa monitor quantum experiments at the Pritzker School of Molecular Engineering.

UChicago team discovers innovative way to tune quantum signals

After decades of miniaturization, the electronic components we’ve relied on for computers and modern technologies are now starting to reach fundamental limits. Faced with this challenge, engineers and scientists around the world are turning toward a radically new paradigm: quantum information technologies.

Quantum technology, which harnesses the strange rules that govern particles at the atomic level, is normally thought of as much too delicate to coexist with the electronics we use every day in phones, laptops and cars. However, scientists with the University of Chicago’s Pritzker School of Molecular Engineering announced a significant breakthrough: Quantum states can be integrated and controlled in commonly used electronic devices made from silicon carbide.

“The ability to create and control high-performance quantum bits in commercial electronics was a surprise,” said lead investigator David Awschalom, the Liew Family Professor in Molecular Engineering at UChicago and a pioneer in quantum technology. “These discoveries have changed the way we think about developing quantum technologies—perhaps we can find a way to use today’s electronics to build quantum devices.”

In two papers published in Science and Science Advances, Awschalom’s group demonstrated they could electrically control quantum states embedded in silicon carbide. The breakthrough could offer a means to more easily design and build quantum electronics—in contrast to using exotic materials scientists usually need to use for quantum experiments, such as superconducting metals, levitated atoms or diamonds.

These quantum states in silicon carbide have the added benefit of emitting single particles of light with a wavelength near the telecommunications band. “This makes them well suited to long-distance transmission through the same fiber-optic network that already transports 90% of all international data worldwide,” said Awschalom, senior scientist at Argonne National Laboratory and director of the Chicago Quantum Exchange.

Moreover, these light particles can gain exciting new properties when combined with existing electronics. For example, in the Science Advances paper, the team was able to create what Awschalom called a “quantum FM radio;” in the same way music is transmitted to your car radio, quantum information can be sent over extremely long distances.

“All the theory suggests that in order to achieve good quantum control in a material, it should be pure and free of fluctuating fields,” said graduate student Kevin Miao, first author on the paper. “Our results suggest that with proper design, a device can not only mitigate those impurities, but also create additional forms of control that previously were not possible.”

In the Science paper, they describe a second breakthrough that addresses a very common problem in quantum technology: noise.

“Impurities are common in all semiconductor devices, and at the quantum level, these impurities can scramble the quantum information by creating a noisy electrical environment,” said graduate student Chris Anderson, a co-first author on the paper. “This is a near-universal problem for quantum technologies.”

But, by using one of the basic elements of electronics—the diode, a one-way switch for electrons—the team discovered another unexpected result: The quantum signal suddenly became free of noise and was almost perfectly stable.

“In our experiments we need to use lasers, which unfortunately jostle the electrons around. It’s like a game of musical chairs with electrons; when the light goes out everything stops, but in a different configuration,” said graduate student Alexandre Bourassa, the other co-first author on the paper. “The problem is that this random configuration of electrons affects our quantum state. But we found that applying electric fields removes the electrons from the system and makes it much more stable.”

By integrating the strange physics of quantum mechanics with well-developed classical semiconductor technology, Awschalom and his group are paving the way for the coming quantum technology revolution.

“This work brings us one step closer to the realization of systems capable of storing and distributing quantum information across the world’s fiber-optic networks,” Awschalom said. “Such quantum networks would bring about a novel class of technologies allowing for the creation of unhackable communication channels, the teleportation of single electron states and the realization of a quantum internet.”

For its research, the team used the Chicago Materials Research Center and the Pritzker Nanofabrication Facility. Awschalom is also working with the Polsky Center for Entrepreneurship and Innovation at the University of Chicago to advance these discoveries.

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One of the world’s premier academic and research institutions, the University of Chicago has driven new ways of thinking since our 1890 founding. Today, UChicago is an intellectual destination that draws inspired scholars to our Hyde Park and international campuses, keeping UChicago at the nexus of ideas that challenge and change the world.

The University of Chicago is an urban research university that has driven new ways of thinking since 1890. Our commitment to free and open inquiry draws inspired scholars to our global campuses, where ideas are born that challenge and change the world.

We empower individuals to challenge conventional thinking in pursuit of original ideas. Students in the College develop critical, analytic, and writing skills in our rigorous, interdisciplinary core curriculum. Through graduate programs, students test their ideas with UChicago scholars, and become the next generation of leaders in academia, industry, nonprofits, and government.

UChicago research has led to such breakthroughs as discovering the link between cancer and genetics, establishing revolutionary theories of economics, and developing tools to produce reliably excellent urban schooling. We generate new insights for the benefit of present and future generations with our national and affiliated laboratories: Argonne National Laboratory, Fermi National Accelerator Laboratory, and the Marine Biological Laboratory in Woods Hole, Massachusetts.

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In all we do, we are driven to dig deeper, push further, and ask bigger questions—and to leverage our knowledge to enrich all human life. Our diverse and creative students and alumni drive innovation, lead international conversations, and make masterpieces. Alumni and faculty, lecturers and postdocs go on to become Nobel laureates, CEOs, university presidents, attorneys general, literary giants, and astronauts.

From Science News: “Trapping atoms in a laser beam offers a new way to measure gravity”

From Science News

November 7, 2019
Maria Temming

The technique can measure slight gravitational variations, which could help in mapping terrain.

A new type of experiment to measure the strength of gravity uses atoms suspended in laser light (with the machinery pictured above), rather than free-falling atoms. V. Xu.

By watching how atoms behave when they’re suspended in midair, rather than in free fall, physicists have come up with a new way to measure Earth’s gravity.

Traditionally, scientists have measured gravity’s influence on atoms by tracking how fast atoms tumble down tall chutes. Such experiments can help test Einstein’s theory of gravity and precisely measure fundamental constants (SN: 4/12/18). But the meters-long tubes used in free-fall experiments can be unwieldy and difficult to shield from environmental interference such as stray magnetic fields. With a new tabletop setup, physicists can gauge the strength of Earth’s gravity by monitoring atoms suspended a couple millimeters in the air by laser light.

This redesign, described in the Nov. 8 Science, could better probe the gravitational forces exerted by small objects. The technique also could be used to measure slight gravitational variations at different places in the world, which may help in mapping the seafloor or finding oil and minerals underground (SN: 2/12/08).

Physicist Victoria Xu and colleagues at the University of California, Berkeley began by launching a cloud of cesium atoms into the air and using flashes of light to split each atom into a superposition state. In this weird quantum limbo, each atom exists in two places at once: one version of the atom hovering a few micrometers higher than the other. Xu’s team then trapped these split cesium atoms in midair with light from a laser.

Got you, atom. To measure gravity, physicists split atoms into a weird quantum state called superposition — where one version of the atom is slightly higher than the other (blue dots connected by vertical yellow bands in this illustration). The researchers trap these atoms in midair using laser light (horizontal blue bands). While held in the light, each version of a single atom behaves slightly differently, due to their different positions in Earth’s gravitational field. Measuring those differences allows physicists to determine the strength of Earth’s gravity at that location.

Measuring the strength of gravity with atoms that are held in place, rather than being tugged downward by a gravitational field, requires tapping into the atoms’ wave-particle duality (SN: 11/5/10). That quantum effect means that, much as light waves can act like particles called photons, atoms can act like waves. And for each cesium atom caught in superposition, the higher version of the atom wave undulates a little faster than its lower counterpart, due to the atoms’ slightly different positions in Earth’s gravitational field. By tracking how fast the waviness of the two versions of an atom gets out of sync, physicists can calculate the strength of Earth’s gravity at that spot.

“Very impressive,” says physicist Alan Jamison of MIT. To him, one big promise of the new technique is more controlled measurements. “It’s quite a challenge to work on these drop experiments, where you have a 10-meter-long tower,” he says. “Magnetic fields are hard to shield, and the environment produces them all over the place — all the electrical systems in your building, and so forth. Working in a smaller volume makes it easier to avoid those environmental noises.”

More compact equipment can also measure shorter-range gravity effects, says study coauthor Holger Müller. “Let’s say you don’t want to measure the gravity of the entire Earth, but you want to measure the gravity of a small thing, such as a marble,” he says. “We just need to put the marble close to our atoms [and hold it there]. In a traditional free-fall setup, the atoms would spend a very short time close to our marble — milliseconds — and we would get much less signal.”

Physicist Kai Bongs of the University of Birmingham in England imagines using the new kind of atomic gravimeter to investigate the nature of dark matter or test a fundamental facet of Einstein’s theory of gravity called the equivalence principle (SN: 4/28/17). Many unified theories of physics proposed to reconcile quantum mechanics and Einstein’s theory of gravity — which are incompatible — violate the equivalence principle in some way. “So looking for violations might guide us to the grand unified theory,” he says. “That’s one of the Holy Grails in physics.”

See the full article here .

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From MIT News: “Chemists observe “spooky” quantum tunneling”

From MIT News

November 4, 2019
Anne Trafton

MIT chemists have observed, for the first time, inversion of the umbrella-like ammonia molecule by quantum tunneling. Image: Chelsea Turner, MIT.

Extremely large electric fields can prevent umbrella-shaped ammonia molecules from inverting.

A molecule of ammonia, NH3, typically exists as an umbrella shape, with three hydrogen atoms fanned out in a nonplanar arrangement around a central nitrogen atom. This umbrella structure is very stable and would normally be expected to require a large amount of energy to be inverted.

However, a quantum mechanical phenomenon called tunneling allows ammonia and other molecules to simultaneously inhabit geometric structures that are separated by a prohibitively high energy barrier. A team of chemists that includes Robert Field, the Robert T. Haslam and Bradley Dewey Professor of Chemistry at MIT, has examined this phenomenon by using a very large electric field to suppress the simultaneous occupation of ammonia molecules in the normal and inverted states.

“It’s a beautiful example of the tunneling phenomenon, and it reveals a wonderful strangeness of quantum mechanics,” says Field, who is one of the senior authors of the study.

Heon Kang, a professor of chemistry at Seoul National University, is also a senior author of the study, which appears this week in the Proceedings of the National Academy of Sciences. Youngwook Park and Hani Kang of Seoul National University are also authors of the paper.

Suppressing inversion

The experiments, performed at Seoul National University, were enabled by the researchers’ new method for applying a very large electric field (up to 200,000,000 volts per meter) to a sample sandwiched between two electrodes.

This assembly is only a few hundred nanometers thick, and the electric field applied to it generates forces nearly as strong as the interactions between adjacent molecules.

“We can apply these huge fields, which are almost the same magnitude as the fields that two molecules experience when they approach each other,” Field says. “That means we’re using an external means to operate on an equal playing field with what the molecules can do themselves.”

This allowed the researchers to explore quantum tunneling, a phenomenon often used in undergraduate chemistry courses to demonstrate one of the “spookinesses” of quantum mechanics, Field says.

As an analogy, imagine you are hiking in a valley. To reach the next valley, you need to climb a large mountain, which requires a lot of work. Now, imagine that you could tunnel through the mountain to get to the next valley, with no real effort required. This is what quantum mechanics allows, under certain conditions. In fact, if the two valleys have exactly the same shape, you would be simultaneously located in both valleys.

In the case of ammonia, the first valley is the low-energy, stable umbrella state. For the molecule to reach the other valley — the inverted state, which has exactly the same low-energy — classically it would need to ascend into a very high-energy state. However, quantum mechanically, the isolated molecule exists with equal probability in both valleys.

Under quantum mechanics, the possible states of a molecule, such as ammonia, are described in terms of a characteristic energy level pattern. The molecule initially exists in either the normal or inverted structure, but it can tunnel spontaneously to the other structure. The amount of time required for that tunneling to occur is encoded in the energy level pattern. If the barrier between the two structures is high, the tunneling time is long. Under certain circumstances, such as application of a strong electric field, tunneling between the regular and inverted structures can be suppressed.

For ammonia, exposure to a strong electric field lowers the energy of one structure and raises the energy of the other (inverted) structure. As a result, all of the ammonia molecules can be found in the lower energy state. The researchers demonstrated this by creating a layered argon-ammonia-argon structure at 10 kelvins. Argon is an inert gas which is solid at 10 K, but the ammonia molecules can rotate freely in the argon solid. As the electric field is increased, the energy states of the ammonia molecules change in such a way that the probabilities of finding the molecules in the normal and inverted states become increasingly far apart, and tunneling can no longer occur.

This effect is completely reversible and nondestructive: As the electric field is decreased, the ammonia molecules return to their normal state of being simultaneously in both wells.

“This manuscript describes a burgeoning frontier in our ability to tame molecules and control their underlying dynamics,” says Patrick Vaccaro, a professor of chemistry at Yale University who was not involved in the study. “The experimental approach set forth in this paper is unique, and it has enormous ramifications for future efforts to interrogate molecular structure and dynamics, with the present application affording fundamental insights into the nature of tunneling-mediated phenomena.”

Lowering the barriers

For many molecules, the barrier to tunneling is so high that tunneling would never happen during the lifespan of the universe, Field says. However, there are molecules other than ammonia that can be induced to tunnel by careful tuning of the applied electric field. His colleagues are now working on exploiting this approach with some of those molecules.

“Ammonia is special because of its high symmetry and the fact that it’s probably the first example anybody would ever discuss from a chemical point of view of tunneling,” Field says. “However, there are many examples where this could be exploited. The electric field, because it’s so large, is capable of acting on the same scale as the actual chemical interactions,” offering a powerful way of externally manipulating molecular dynamics.

The research was funded by the Samsung Science and Technology Foundation and the National Science Foundation.

See the full article here .

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From Caltech: “Untangling Quantum Entanglement”

From Caltech

Caltech Magazine Fall 2019
Whitney Clavin

In Erwin Schrödinger’s famous thought experiment, a cat is trapped in a box with a bit of poison the release of which is controlled by a quantum process. The cat therefore exists in a quantum state of being both dead and alive until somebody opens the box and finds the cat either dead or alive.

The perplexing phenomenon of quantum entanglement is central to quantum computing, quantum networking, and the fabric of space and time.

The famous “Jim twins,” separated soon after birth in the 1940s, seemed to live parallel lives even though they grew up miles apart in completely different families. When they were reunited at the age of 39, they discovered many similarities between their life stories, including the names of their sons, wives, and childhood pets, as well as their preferences for Chevrolet cars, carpentry, and more.

A similar kind of parallelism happens at a quantum level, too. The electrons, photons, and other particles that make up our universe can become inextricably linked, such that the state observed in one particle will be identical for the other. That connection, known as entanglement, remains strong even across vast distances.

“When particles are entangled, it’s as if they are born that way, like twins,” says Xie Chen, associate professor of theoretical physics at Caltech. “Even though they might be separated right after birth, [they’ll] still look the same. And they grow up having a lot of personality traits that are similar to each other.”

The phenomenon of entanglement was first proposed by Albert Einstein and colleagues in the 1930s. At that time, many questioned the validity of entanglement, including Einstein himself. Over the years and in various experiments, however, researchers have generated entangled particles that have supported the theory. In these experiments, researchers first entangle two particles and then send them to different locations miles apart. The researchers then measure the state of one particle: for instance, the polarization (or direction of vibration) of a photon. If that entangled photon displays a horizontal polarization, then so too will its faithful partner.

“It may be tempting to think that the particles are somehow communicating with each other across these great distances, but that is not the case,” says Thomas Vidick, a professor of computing and mathematical sciences at Caltech. “There can be correlation without communication.” Instead, he explains, entangled particles are so closely connected that there is no need for communication; they “can be thought of as one object.”

As baffling as the concept of two entangled particles may be, the situation becomes even more complex when more particles are involved. In natural settings such as the human body, for example, not two but hundreds of molecules or even more become entangled, as they also do in various metals and magnets, making up an interwoven community. In these many-body entangled systems, the whole is greater than the sum of its parts.

“The particles act together like a single object whose identity lies not with the individual components but in a higher plane. It becomes something larger than itself,” says Spyridon (Spiros) Michalakis, outreach manager of Caltech’s Institute for Quantum Information and Matter (IQIM) and a staff researcher. “Entanglement is like a thread that goes through every single one of the individual particles, telling them how to be connected to one another.”

Associate Professor of Theoretical Physics Xie Chen specializes in the fields of condensed matter physics and quantum information.

At Caltech, researchers are focusing their studies on many-body entangled systems, which they believe are critical to the development of future technologies and perhaps to cracking fundamental physics mysteries. Scientists around the world have made significant progress applying the principles of many-body entanglement to fields such as quantum computing, quantum cryptography, and quantum networks (collectively known as quantum information); condensed-matter physics; chemistry; and fundamental physics. Although the most practical applications, such as quantum computers, may still be decades off, according to John Preskill, the Richard P. Feynman Professor of Theoretical Physics at Caltech and the Allen V.C. Davis and Lenabelle Davis Leadership Chair of the Institute of Quantum Science and Technology (IQST), “entanglement is a very important part of Caltech’s future.”

Entanglement Passes Tests with Flying Colors

In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen published a paper on the theoretical concept of quantum entanglement, which Einstein called “spooky action at a distance.” The physicists described the idea, then argued that it posed a problem for quantum mechanics, rendering the theory incomplete. Einstein did not believe two particles could remain connected to each other over great distances; doing so, he said, would require them to communicate faster than the speed of light, something he had previously shown to be impossible.

Today, experimental work leaves no doubt that entanglement is real. Physicists have demonstrated its peculiar effects across hundreds of kilometers; in fact, in 2017, a Chinese satellite named Micius sent entangled photons to three different ground stations, each separated by more than 1,200 kilometers, and broke the distance record for entangled particles.

Entanglement goes hand in hand with another quantum phenomenon known as superposition, in which particles exist in two different states simultaneously. Photons, for example, can display simultaneously both horizontal and vertical states of polarization.

Or, to simplify, consider two “entangled” quarters, each hidden under a cup. If two people, Bob and Alice, were each to take one of those quarters to a different room, the quarters would remain both heads and tails until one person lifted the cup and observed his or her quarter; at that point, it would randomly become either heads or tails. If Alice were to lift her cup first and her quarter was tails, then when Bob observed his quarter, it would also be tails. If you repeated the experiment and the coins are covered once more, they would go back to being in a state of superposition. Alice would lift her cup again and might find her quarter as heads this time. Bob would then also find his quarter as heads. Whether the first quarter is found to be heads or tails is entirely random.

Similarly, when a researcher entangles two photons and then sends each one in different directions under carefully controlled conditions, they will continue to be in a state of superposition, both horizontally and vertically polarized. Only when one of the photons is measured do both randomly adopt just one of the two possible polarization states.

“Quantum correlations are deeply different than ordinary correlations,” says Preskill. “And randomness is the key. This spooky intrinsic randomness is actually what bothered Einstein. But it is essential to how the quantum world works.”

“Scientists often use the word correlation to explain what is happening between these particles,” adds Oskar Painter, the John G Braun Professor of Applied Physics and Physics at Caltech. “But, actually, entanglement is the perfect word.”

Entanglement to the Nth Degree

Untangling the relationship between two entangled particles may be difficult, but the real challenge is to understand how hundreds of particles, if not more, can be similarly interconnected.

According to Manuel Endres, an assistant professor of physics at Caltech, one of the first steps toward understanding many-body entanglement is to create and control it in the lab. To do this, Endres and his team use a brute force approach: they design and build laboratory experiments with the goal of creating a system of 100 entangled atoms.

“This is fundamentally extremely difficult to do,” says Endres. In fact, he notes, it would be difficult even at a much smaller scale. “If I create a system where I generate, for instance, 20 entangled particles, and I send 10 one way and 10 another way, then I have to measure whether each one of those first 10 particles is entangled with each of the other set of 10. There are many different ways of looking at the correlations.”

While the task of describing those correlations is difficult, describing a system of 100 entangled atoms with classical computer bits would be unimaginably hard. For instance, a complete classical description of all the quantum correlations among as many as 300 entangled particles would require more bits than the number of atoms in the visible universe. “But that’s the whole point and the reason we are doing this,” Endres says. “Things get so entangled that you need a huge amount of space to describe the information. It’s a complicated beast, but it’s useful.”

“Generally, the number of parameters you need to describe the system is going to scale up exponentially,” says Vidick, who is working on mathematical and computational tools to describe entanglement. “It blows up very quickly, which, in general, is why it’s hard to make predictions or simulations, because you can’t even represent these systems in your laptop’s memory.”

To solve that problem, Vidick and his group are working on coming up with computational representations of entangled materials that are simpler and more succinct than models that currently exist.

“Quantum mechanics and the ideas behind quantum computing are forcing us to think outside the box,” he says.

A Fragile Ecosystem

Another factor in creating and controlling quantum systems has to do with their delicate nature. Like Mimosa pudica ,a member of the pea family also known as the “sensitive plant,” which droops when its leaves are touched, entangled states can easily disappear, or collapse, when the environment changes even slightly. For example, the act of observing a quantum state destroys it. “You don’t want to even look at your experiment, or breathe on it,” jokes Painter. Adds Preskill, “Don’t turn on the light, and don’t even dare walk into the room.”

The problem is that entangled particles become entangled with the environment around them quickly, in a matter of microseconds or faster. This then destroys the original entangled state a researcher might attempt to study or use. Even one stray photon flying through an experiment can render the whole thing useless.

“You need to be able to create a system that is entangled only with itself, not with your apparatus,” says Endres. “We want the particles to talk to one another in a controlled fashion. But we don’t want them to talk to anything in the outside world.”

In the field of quantum computing, this fragility is problematic because it can lead to computational errors. Quantum computers hold the promise of solving problems that classical computers cannot, including those in cryptography, chemistry, financial modeling, and more. Where classical computers use binary bits (either a “1” or a “0”) to carry information, quantum computers use “qubits,” which exist in states of “1” and “0” at the same time. As Preskill explains, the qubits in this mixed state, or superposition, would be both dead and alive, a reference to the famous thought experiment proposed by Erwin Schrödinger in 1935, in which a cat in a box is both dead and alive until the box is opened, and the cat is observed to be one or the other. What’s more, those qubits are all entangled. If the qubits somehow become disentangled from one another, the quantum computer would be unable to execute its computations.

To address these issues, Preskill and Alexei Kitaev (Caltech’s Ronald and Maxine Linde Professor of Theoretical Physics and Mathematics and recipient of a 2012 Breakthrough Prize in Fundamental Physics), along with other theorists at Caltech, have devised a concept to hide the quantum information within a global entangled state, such that none of the individual bits have the answer. This approach is akin to distributing a code among hundreds of people living in different cities. No one person would have the whole code, so the code would be much less vulnerable to discovery.

Manuel Endres, assistant professor of physics, here pictured with Adam Shaw (left) and Ivaylo Madjarov (right), uses laser-based techniques in his lab to create many-body entanglement.

“The key to correcting errors in entangled systems is, in fact, entanglement,” says Preskill. “If you want to protect information from damage due to the extreme instability of superpositions, you have to hide the information in a form that’s very hard to get at,” he says. “And the way you do that is by encoding it in a highly entangled state.”

Spreading the Entanglement

At Caltech, this work on the development of quantum-computing systems is conducted alongside with research into quantum networks in which each quantum computer acts as a separate node, or connection point, for the whole system. Painter refers to this as “breaking a quantum computer into little chunks” and then connecting them together to create a distributed network. In this approach, the chunks would behave as if they were not separated. “The network would be an example of many-body entanglement, in which the bodies are the different nodes in the network,” says Painter.

Quantum networks would enhance the power of quantum computers, notes Preskill.

“We’d like to build bigger and bigger quantum computers to solve harder and harder problems. And it’s hard to build one piece of hardware that can handle a million qubits,” he says. “It’s easier to make modular components with 100 qubits each or something like that. But then, if you want to solve harder problems, you’ve got to get these different little quantum computers to communicate with one another. And that would be done through a quantum network.”

Quantum networks could also be used for cryptography purposes, to make it safer to send sensitive information; they would also be a means by which to distribute and share quantum information in the same way that the World Wide Web works for conventional computers. Another future use might be in astronomy. Today’s telescopes are limited. They cannot yet see any detail on, for instance, the surface of distant exoplanets, where astronomers might want to look for signs of life or civilization. If scientists could combine telescopes into a quantum network, it “would allow us to use the whole Earth as one big telescope with a much-improved resolution,” says Preskill.

“Up until about 20 years ago, the best way to explore entanglement was to look at what nature gave us and try to study the exotic states that emerged,” notes Painter. “Now our goal is to try to synthesize these systems and go beyond what nature has given us.”

At the Root of Everything

While entanglement is the key to advances in quantum-information sciences, it is also a concept of interest to theoretical physicists, some of whom believe that space and time itself are the result of an underlying network of quantum connections.

“It is quite incredible that any two points in space-time, no matter how far apart, are actually entangled. Points in space-time that we consider closer to each other are just more entangled than those further apart,” says Michalkis.

The link between entanglement and space-time may even help solve one of the biggest challenges in physics: establishing a unifying theory to connect the macroscopic laws of general relativity (which describe gravity) with the microscopic laws of quantum physics (which describe how subatomic particles behave).

The quantum error-correcting schemes that Preskill and others study may play a role in this quest. With quantum computers, error correction ensures that the computers are sufficiently robust and stable. Something similar may occur with space-time. “The robustness of space may come from a geometry where you can perturb the system, but it isn’t affected much by the noise, which is the same thing that happens in stable quantum-computing schemes,” says Preskill.

“Essentially, entanglement holds space together. It’s the glue that makes the different pieces of space hook up with one another,” he adds.

At Caltech, the concept of entanglement connects various labs and buildings across campus. Theorists and experimentalists in computer science, quantum-information science, condensed-matter physics, and other fields regularly work across disciplines and weave together their ideas.

“We bring our ideas from condensed-matter physics to quantum-information folks, and we say, ‘Hey, I have a material you can use for quantum computation,’” says Chen. “Sometimes we borrow ideas from them. Many of us from different fields have realized that we have to deal with entanglement head-on.”

Preskill echoes this sentiment and is convinced entanglement is an essential part of Caltech’s future: “We are making investments and betting on entanglement as being one of the most important themes of 21st-century science.”

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The California Institute of Technology (commonly referred to as Caltech) is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering. Its 124-acre (50 ha) primary campus is located approximately 11 mi (18 km) northeast of downtown Los Angeles. “The mission of the California Institute of Technology is to expand human knowledge and benefit society through research integrated with education. We investigate the most challenging, fundamental problems in science and technology in a singularly collegial, interdisciplinary atmosphere, while educating outstanding students to become creative members of society.”

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