Tyler O'Neal, Staff Editor SYSTEMS

Michigan Tech physicists catch electrons in action in an antiferromagnetic nanowire

The electron is one of the fundamental particles in nature we read about in school. Its behavior holds clues to new ways to store digital data.

In a study published in Nano Letters, physicists from Michigan Technological University explore alternative materials to improve capacity and shrink the size of digital data storage technologies. Ranjit Pati, professor of physics at Michigan Tech, led the study and explains the physics behind his team's new nanowire design.

"Thanks to a property called spin, electrons behave like tiny magnets," Pati said. "Similar to how a bar magnet's magnetization is dipolar, pointing from south to north, the electrons in a material have magnetic dipole moment vectors that describe the material's magnetization."

When these vectors are in random orientation, the material is nonmagnetic. When they are parallel to each other, it's called ferromagnetism and antiparallel alignments are antiferromagnetism. Current data storage technology is based on ferromagnetic materials, where the data are stored in small ferromagnetic domains. This is why a strong enough magnet can mess up a mobile phone or other electronic storage.

Depending on the direction of magnetization (whether pointing up or down), data are recorded as bits (either a 1 or 0) in ferromagnetic domains. However, there are two bottlenecks, and both hinge on proximity. First, bring an external magnet too close, and its magnetic field could alter the direction of magnetic moments in the domain and damage the storage device. And, second, the domains each have a magnetic field of their own, so they can't be too close to each other either. The challenge with smaller, more flexible, more versatile electronics is that they demand devices that make it harder to keep ferromagnetic domains safely apart. A diagram of a nanowire cross-section shows the interaction between atoms in the nanowire core. Yellow atoms: germanium; red atoms: chromium; purple atoms around the outside of the yellow-red connections: silicon shell. CREDIT Ranjit Pati/Michigan Tech

"Ultrahigh-density data packing would be a daunting task with ferromagnetic memory domains," Pati said. "Antiferromagnetic materials, on the other hand, are free from these issues."

On their own antiferromagnetic materials aren't great for electronic devices, but they're not influenced by outside magnetic fields. This ability to resist magnetic manipulation started getting more attention from the research community and Pati's team used a predictive quantum many-body theory that considers electron-electron interactions. The team found that chromium-doped nanowires with a germanium core and silicon shell can be antiferromagnetic semiconductors.

Several research groups have recently demonstrated the manipulation of individual magnetic states in antiferromagnetic materials using electrical current and lasers. They observed spin dynamics in the terahertz frequency -- much faster than the frequency used in our current data storage devices. This observation has opened up a plethora of research interests in antiferromagnetism and could lead to faster, higher-capacity data storage.

"In our recent work, we have successfully harnessed the intriguing features of an antiferromagnet into a low-dimensional, complementary metal-oxide compatible semiconductor (CMOS) nanowire without destroying the semiconducting property of the nanowire," Pati said. "This opens up possibilities for smaller and smarter electronics with higher capacity data storage and manipulation."

Pati adds that the most exciting part of the research for his team was uncovering the mechanism that dictates antiferromagnetism. The mechanism is called superexchange and it controls the spin of electrons and the antiparallel alignment that makes them antiferromagnetic. In the team's nanowire, germanium electrons act as a go-between, an exchanger, between unconnected chromium atoms.

"The interaction between the magnetic states of the chromium atoms is mediated by the intermediate atoms they are bonded to. It is a cooperative magnetic phenomenon," Pati said. "In a simple way, let us say there are two people A and B: They are far apart and cannot communicate directly. But A has a friend C and B has a friend D. C and D are close friends. So, A and B can interact indirectly through C and D."

Better understanding how electrons communicate between atomic friends enables more experiments to test the potential of materials like chromium-doped nanowires. A better understanding of the germanium-silicon nanowire material's antiferromagnetic nature is what boosts the potential for smaller, smarter, higher-capacity electronics.

Purdue prof leads research into deep fakes

Tyler O'Neal, Staff Editor SYSTEMS

Falsified images, research documents among the latest areas of focus

Deep fake videos of actor Tom Cruise on Tik Tok can create some confusing fans. A deep fake video of a drug company CEO announcing COVID-19 vaccine failures, however, could cause panic.

Ferreting out deep fake videos is the work of Edward Delp, the Charles William Harrison Distinguished Professor of Electrical and Computer Engineering at Purdue University, who is leading one of the teams in the Semantic Forensics program created by the Defense Advanced Research Projects Agency for the U.S. Department of Defense.

The program, which has been ongoing since late last year, is a continuation of Delp’s innovative previous work into tools to detect deep fakes and manipulated media. Only now the research is extending much further to include other video styles and media such as images and documents.

“Not only do we want to be able to detect when a piece of media has been manipulated, but we want to be able to attribute it: Who did it, why did they do it and what was their intent?” said Delp, director of the Video and Imaging Processing Laboratory at Purdue. “It’s called detection, attribution, and characterization. We’re trying to solve the whole problem.” Edward Delp, the Charles William Harrison Distinguished Professor of Electrical and Computer Engineering and director of the Video and Imaging Processing Laboratory at Purdue University, is leading a team of experts for the U.S. Department of Defense researching ways to detect several forms of manipulated media. (Purdue University photo/John Underwood)

Delp has worked on video tampering as part of a larger research into media forensics, focusing on sophisticated techniques based on artificial intelligence and machine learning to create an algorithm that detects deep fakes.

For this program, Delp is overseeing a team of technical experts from the University of Notre Dame, the University of Campinas in Brazil, and three Italian universities: Politecnico di Milano, the University of Siena, and the University Federico II of Naples.

The team has complementary skills and backgrounds in computer vision and biometrics, machine learning, digital forensics, as well as signal processing and information theory.

The team is moving away from studying deep fakes that concentrate only on faces, instead of looking at fake street scenes and fake biological imagery like microscope images and X-rays.

“Think about the insurance fraud if you have a doctor who could generate fake X-rays of patients and start turning those into the insurance companies,” Delp said.

Something as seemingly harmless as a driver putting a camera in a car to record heading down the road can have other applications. Delp said the video can be manipulated to make it look like it’s a different road or neighborhood and used to try and fool an autonomous vehicle system.

Delp said the team is doing some unique things with document validity, particularly with regard to research papers.

“One of the things we’re looking at is, with the COVID pandemic there has been a lot of scientific papers put forward that have falsified data,” he said. “We are building a tool that will look at a scientific paper; in particular look at the images and figures and determine whether they’ve been manipulated.”

Delp said the problem, called research integrity, has been an ongoing issue for almost 15 years, particularly in the biomedical community.

Also being examined are articles trying to be passed off as originating from specific newspapers. Delp said the work is called style detection and examines the writing style guides of a number of nationwide newspapers.

Delp said the premise is if someone writes an article and then tries to fake it, saying it comes from the New York Times; they probably did not read the style guide and will not have the appropriate word usage as a result.

Style detection is one of the team’s long-term projects.

China demos universal quantum error-correcting code with superconducting qubits

Tyler O'Neal, Staff Editor SYSTEMS

Universal fault-tolerant quantum supercomputing relies on the implementation of quantum error correction. An essential milestone is the achievement of error-corrected logical qubits that genuinely benefit from error correction, outperforming simple physical qubits. Although tremendous efforts have been devoted to demonstrating quantum error-correcting codes with different quantum hardware, previous realizations are limited to be against certain types of errors or to prepare special logical states. It remains one of the greatest and also notoriously difficult challenges to realize a universal quantum error-correcting code for more than a decade.

In a new research article published in the Beijing-based National Science Review, scientists at the University of Science and Technology of China, the Tsinghua University, and at the University of Oxford, present their latest work on the experimental exploration of five-qubit quantum error-correcting code with superconducting qubits. The authors realized the [[5,1,3]] code on a superconducting quantum processor, verified the viability of experimental realization of quantum error-correcting codes with superconducting qubits.

These scientists completed the important step towards the implementation of quantum error correction. This is achieved first by dedicated experimental optimization of superconducting quantum qubits, enabling the realization of more than a hundred quantum gates. Focusing on the five-qubit quantum error-correcting code, the so-called 'perfect code' that corrects single generic qubit errors, they theoretically compiled and optimized its encoding process to have the minimal possible number (eight) of nearest-neighbor controlled-phase gates. These experimental and theoretical advances finally enabled the realization of the basic ingredients of a fully functional five-qubit error-correcting code, involving the encoding of a general logical qubit into an error-correcting code, with the subsequent verification of all key features including the identification of an arbitrary physical error, the power for transversal manipulation of the logical state, and state decoding.

"The device for the implementation of the five-qubit error-correcting code is a 12-qubit superconducting quantum processor. Among these 12 qubits, we chose five adjacent qubits to perform the experiment. The qubits are capacitively coupled to their nearest neighbors. The capacitively coupled XY control lines enable the application of single-qubit rotation gates by applying microwave pulses, and the inductively coupled Z control lines enable the double-qubit controlled-phase gates by adiabatically tune the two-qubit state |11> close to the avoid level crossing of |11> and |02>. After careful calibrations and gate optimizations, we have the average gate fidelities as high as 0.9993 for single-qubit gates and 0.986 for two-qubit gates. With the implementation of only single-qubit rotation gates and double-qubit controlled-phase gates, we realized the circuit for encoding and decoding of the logical state." they state in an article titled "Experimental exploration of five-qubit quantum error-correcting code with superconducting qubits."

"On a superconducting quantum processor, we experimentally realized the logical states |0>_L, |1>_L, |±>_L, and |±i>_L that are eigenstates of the logical Pauli operators X_L, Y_L, and Z_L, and the magic state |T>_L= (|0>_L+e^{i\pi/4}|1>_L)/\sqrt{2} that cannot be realized by applying Clifford operations on any eigenstate of the logical Pauli operators," they add. "Finally, the state fidelity of |T>_L reaches 54.5(4)%."

"The quality of the prepared logical states can also be divided into its overlap with the logical code space and its agreement with the target logical state after projecting it into the code space," they stated. After projecting to the code space, the average value is as high as 98.6(1)%. "Since projecting to the code space is equivalent to post-selecting all +1 stabilizer measurements, our result also indicates the possibility of high fidelity logical state preparation with future non-destructive stabilizer measurements."

After the realization of the logical state, the scientists proceed with the verification of error correction/detection ability of the five qubit code. "As shown in Fig.2(a) we do indeed find, for each case, the corresponding syndrome pattern that identifies the location of the single-qubit error," they added.

Then, the scientists implemented and verified the transversal logical operations, and performed the quantum process tomography within the code space to characterize these logical operations. "We determine gate fidelities of the logical X_L, Y_L, and Z_L operations to be 97.2(2)%, 97.8(2)%, and 97.3(2)%, respectively," they stated.

"Finally, after encoding the single-qubit input state into the logical state, we apply the decoding circuit, see Fig. 4(a), to map it back to the input state," they added. "After quantum process tomography from the four output states, the process fidelity is determined as 74.5(6)% as shown in Fig. 4(b)."

"An essential milestone on the road to fault-tolerant quantum computing is the achievement of error-corrected logical qubits that genuinely benefit from error correction, outperforming simple physical qubits," they add. "Direction for future works include the realization of non-destructive error detection and error correction, and the implementation of logical operations on multiple logical qubits for the five-qubit code. Our work also has applications in error mitigation for near-term quantum supercomputing."