Spotting the spin of the Majorana fermion under the microscope

Majorana detection
The figure shows a schematic of the experiment. A magnetized scanning tunneling microscope tip was used to probe the spin property of the quantum wave function of the Majorana fermion at the end of a chain of iron atoms on the surface of a superconductor made of lead. Image courtesy of Yazdani Lab, Princeton University.

By Catherine Zandonella, Office of the Dean for Research

Researchers at Princeton University have detected a unique quantum property of an elusive particle notable for behaving simultaneously like matter and antimatter. The particle, known as the Majorana fermion, is prized by researchers for its potential to open the doors to new quantum computing possibilities.

In the study published this week in the journal Science, the research team described how they enhanced an existing imaging technique, called scanning tunneling microscopy, to capture signals from the Majorana particle at both ends of an atomically thin iron wire stretched on the surface of a crystal of lead. Their method involved detecting a distinctive quantum property known as spin, which has been proposed for transmitting quantum information in circuits that contain the Majorana particle.

“The spin property of Majoranas distinguishes them from other types of quasi-particles that emerge in materials,” said Ali Yazdani, Princeton’s Class of 1909 Professor of Physics.  “The experimental detection of this property provides a unique signature of this exotic particle.”

The finding builds on the team’s 2014 discovery, also published in Science, of the Majorana fermion in a single atom-wide chain of iron atoms atop a lead substrate. In that study, the scanning tunneling microscope was used to visualize Majoranas for the first time, but provided no other measurements of their properties.

“Our aim has been to probe some of the specific quantum properties of Majoranas. Such experiments provide not only further confirmation of their existence in our chains, but open up possible ways of using them.” Yazdani said.

First theorized in the late 1930s by the Italian physicist Ettore Majorana, the particle is fascinating because it acts as its own antiparticle. In the last few years, scientists have realized that they can engineer one-dimensional wires, such as the chains of atoms on the superconducting surface in the current study, to make Majorana fermions emerge in solids.  In these wires, Majoranas occur as pairs at either end of the chains, provided the chains are long enough for the Majoranas to stay far enough apart that they do not annihilate each other. In a quantum computing system, information could be simultaneously stored at both ends of the wire, providing a robustness against outside disruptions to the inherently fragile quantum states.

Previous experimental efforts to detect Majoranas have used the fact that it is both a particle and an antiparticle. The telltale signature is called a zero-bias peak in a quantum tunneling measurement. But studies have shown that such signals could also occur due to a pair of ordinary quasiparticles that can emerge in superconductors. Professor of Physics Andrei Bernevig and his team, who with Yazdani’s group proposed the atomic chain platform, developed the theory that showed that spin-polarized measurements made using a scanning tunneling microscope can distinguish between the presence of a pair of ordinary quasi-particles and a Majorana.

Typically, scanning tunneling microscopy (STM) involves dragging a fine-tipped electrode over a structure, in this case the chain of iron atoms, and detecting its electronic properties, from which an image can be constructed.  To perform spin-sensitive measurements, the researchers create electrodes that are magnetized in different orientations. These “spin-polarized” STM measurements revealed signatures that agree with the theoretical calculations by Bernevig and his team.

“It turns out that, unlike in the case of a conventional quasi-particle, the spin of the Majorana cannot be screened out by the background. In this sense it is a litmus test for the presence of the Majorana state,” Bernevig said.

The quantum spin property of Majorana may also make them more useful for applications in quantum information. For example, wires with Majoranas at either end can be used to transfer information between far away quantum bits that rely on the spin of electrons. Entanglement of the spins of electrons and Majoranas may be the next step in harnessing their properties for quantum information transfer.

The STM studies were conducted by three co-first authors in the Yazdani group: scientist Sangjun Jeon, graduate student Yonglong Xie, and former postdoctoral research associate Jian Li (now a professor at Westlake University in Hangzhou, China).  The research also included contributions from postdoctoral research associate Zhijun Wang in Bernevig’s group.

This work has been supported by the Gordon and Betty Moore Foundation as part of the EPiQS initiative (grant GBMF4530), U.S. Office of Naval Research (grants ONR-N00014-14-1-0330, ONR-N00014-11-1-0635, and ONR- N00014-13-1-0661) , the National Science Foundation through the NSF-MRSEC program (grants DMR-142054 and DMR-1608848) and an EAGER Award (grant NOA -AWD1004957), the U.S. Army Research Office MURI program (grant W911NF-12-1-046), the U.S. Department of Energy Office of Basic Energy Sciences, the Simons Foundation, the David and Lucile Packard Foundation, and the Eric and Wendy Schmidt Transformative Technology Fund at Princeton.

The study, “Distinguishing a Majorana zero mode using spin resolved measurements,” was published in the journal Science on Thursday, October 12, 2017.

When scaling the quantum slopes, veer for the straight path (Physical Review A)

Research image
Princeton University researchers found that the “landscape” for quantum control (above) — a representation of quantum mechanics that allows the dynamics of atoms and molecules to be manipulated — can be unexpectedly simple, which could help scientists realize the next generation of technology by harnessing atoms and molecules to create small but incredibly powerful devices. Scientists achieve quantum control by finding the ideal radiation field (top of the graphic) that leads to the desired response from the system. Like a mountain hiker, a scientist can take a difficult, twisting path that requires frequent stops to evaluate the next step (right path). Or, they can opt for a straighter trail that cuts directly to the summit (left path). The researchers provide in their paper an algorithm that scientists can use to identify the starting point of the straight path to their desired quantum field. (Image courtesy of Arun Nanduri)

By Morgan Kelly, Office of Communications

Like any task, there is an easy and a hard way to control atoms and molecules as quantum systems, which are driven by tailored radiation fields. More efficient methods for manipulating quantum systems could help scientists realize the next generation of technology by harnessing atoms and molecules to create small but incredibly powerful devices such as molecular electronics or quantum computers.

Of course, controlling quantum systems is as painstaking as it sounds, and requires scientists to discover the ideal radiation field that leads to the desired response from the system. Scientists know that reaching that state of quantum nirvana can be a long and expensive slog, but Princeton University researchers have found that the process might be more straightforward than previously thought.

The researchers report in the journal Physical Review A that quantum-control “landscapes” — the path of a system’s response from the initial field to the final desired field — appears to be unexpectedly simple. Although still a mountain of a task, finding a good control radiation field turns out to be very much like climbing a mountain, and scientists need only choose the right path. Like a hiker, a scientist can take a difficult, twisting path that requires frequent stops to evaluate which step to take next. Or, as the Princeton researchers show, they can opt for a straighter trail that cuts directly to the summit.

The researchers observe in their paper that these fast tracks toward the desired control field actually exist, and are scattered all over the landscape. They provide an algorithm that scientists can use to identify the starting point of the straight path to their desired quantum field.

The existence of nearly straight paths to reach the best quantum control was surprising because the landscapes were assumed to be serpentine, explained first author Arun Nanduri, who received his bachelor’s degree in physics from Princeton in 2013 and is working in the laboratory of Herschel Rabitz, Princeton’s Charles Phelps Smyth ’16 *17 Professor of Chemistry.

“We found that not only can you always climb to the top, but you can climb along a simple path to the top,” Nanduri said. “If we could consistently identify where these paths are located, a scientist could efficiently climb the landscape. Looking around for the next good step along an unknown path takes great effort. However, starting along a straight path requires you to look around once, and you can keep walking forward with your eyes closed, as it were.”

Following a straighter path could be a far more efficient way of achieving control of atoms and molecules for a host of applications, including manipulating chemical reactions and operating quantum computers, Nanduri said. The source of much scientific excitement, quantum computers would use “qubits” that can be entangled to potentially give them enormous storage and computational capacities far beyond the capabilities of today’s digital computers.

If the Princeton research helps scientists quickly and easily find the control fields they need, it could also allow them to carry out improved measurements of quantum systems and design new ones, Nanduri said.

“We don’t know if our discovery will directly lead to futuristic quantum devices, but this finding should spur renewed research,” Nanduri said. “If straight paths to good quantum control solutions can be routinely found, it would be remarkable.”

Read the abstract.

Nanduri, Arun, Ashley Donovan, Tak-San Ho, Herschel Rabitz. 2013. Exploring quantum control landscape structure. Physical Review A. Article published: Sept. 30, 2013. DOI: 10.1103/PhysRevA.88.033425

The work was funded by the Program in Plasma Science and Technology at Princeton University, the Army Research Office, and the U.S. Department of Energy.

Quantum computing moves forward (Science)

By Catherine Zandonella, Office of the Dean for Research

New technologies that exploit quantum behavior for computing and other applications are closer than ever to being realized due to recent advances, according to a review article published this week in the journal Science.

A silicon chip levitates individual atoms used in quantum information processing. Photo: Curt Suplee and Emily Edwards, Joint Quantum Institute and University of Maryland. Credit: Science.

These advances could enable the creation of immensely powerful computers as well as other applications, such as highly sensitive detectors capable of probing biological systems. “We are really excited about the possibilities of new semiconductor materials and new experimental systems that have become available in the last decade,” said Jason Petta, one of the authors of the report and an associate professor of physics at Princeton University.

Petta co-authored the article with David Awschalom of the University of Chicago, Lee Basset of the University of California-Santa Barbara, Andrew Dzurak of the University of New South Wales and Evelyn Hu of Harvard University.

Two significant breakthroughs are enabling this forward progress, Petta said in an interview. The first is the ability to control quantum units of information, known as quantum bits, at room temperature. Until recently, temperatures near absolute zero were required, but new diamond-based materials allow spin qubits to be operated on a table top, at room temperature. Diamond-based sensors could be used to image single molecules, as demonstrated earlier this year by Awschalom and researchers at Stanford University and IBM Research (Science, 2013).

The second big development is the ability to control these quantum bits, or qubits, for several seconds before they lapse into classical behavior, a feat achieved by Dzurak’s team (Nature, 2010) as well as Princeton researchers led by Stephen Lyon, professor of electrical engineering (Nature Materials, 2012). The development of highly pure forms of silicon, the same material used in today’s classical computers, has enabled researchers to control a quantum mechanical property known as “spin”. At Princeton, Lyon and his team demonstrated the control of spin in billions of electrons, a state known as coherence, for several seconds by using highly pure silicon-28.

Quantum-based technologies exploit the physical rules that govern very small particles — such as atoms and electrons — rather than the classical physics evident in everyday life. New technologies based on “spintronics” rather than electron charge, as is currently used, would be much more powerful than current technologies.

In quantum-based systems, the direction of the spin (either up or down) serves as the basic unit of information, which is analogous to the 0 or 1 bit in a classical computing system. Unlike our classical world, an electron spin can assume both a 0 and 1 at the same time, a feat called entanglement, which greatly enhances the ability to do computations.

A remaining challenge is to find ways to transmit quantum information over long distances. Petta is exploring how to do this with collaborator Andrew Houck, associate professor of electrical engineering at Princeton. Last fall in the journal Nature, the team published a study demonstrating the coupling of a spin qubit to a particle of light, known as a photon, which acts as a shuttle for the quantum information.

Yet another remaining hurdle is to scale up the number of qubits from a handful to hundreds, according to the researchers. Single quantum bits have been made using a variety of materials, including electronic and nuclear spins, as well as superconductors.

Some of the most exciting applications are in new sensing and imaging technologies rather than in computing, said Petta. “Most people agree that building a real quantum computer that can factor large numbers is still a long ways out,” he said. “However, there has been a change in the way we think about quantum mechanics – now we are thinking about quantum-enabled technologies, such as using a spin qubit as a sensitive magnetic field detector to probe biological systems.”

Read the abstract.

Awschalom D.D., Bassett L.C., Dzurak A.S., Hu E.L. & Petta J.R. (2013). Quantum spintronics: engineering and manipulating atom-like spins in semiconductors. Science 339 (6124) 1174-1179. PMID:

The research at Princeton University was supported by the Alfred P. Sloan Foundation, the David and Lucile Packard Foundation, US Army Research Office grant W911NF-08-1-0189, DARPA QuEST award HR0011-09-1-0007 and the US National Science Foundation through the Princeton Center for Complex Materials (DMR-0819860) and CAREER award DMR-0846341.