Tyler O'Neal, Staff Editor SYSTEMS

Japanese researchers develop a quantum supercomputer algorithm that surpasses the QPE norm

Osaka City University refines a quantum supercomputer-ready algorithm to measure the vertical ionization energies of atoms and molecules within 0.1 eV of precision.

Quantum supercomputers have seen a lot of attention recently as they are expected to solve certain problems that are outside the capabilities of normal computers. Primary to these problems is determining the electronic states of atoms and molecules so they can be used more effectively in a variety of industries - from lithium-ion battery designs to in silico technologies in drug development. A common way scientists have approached this problem is by calculating the total energies of the individual states of a molecule or atom and then determine the difference in energy between these states. In nature, many molecules grow in size and complexity, and the cost to calculate this constant flux is beyond the capability of any traditional computer or currently establish quantum algorithms. Therefore, theoretical predictions of the total energies have only been possible if molecules are not sizable and isolated from their natural environment. Kenji Sugisaki, Takeji Takui, Kazunobu Sato

"For quantum computers to be a reality, its algorithms must be robust enough to accurately predict the electronic states of atoms and molecules, as they exist in nature, " state Kenji Sugisaki and Takeji Takui from the Graduate School of Science, Osaka City University.

In December 2020, Sugisaki and Takui, together with their colleagues, led a team of researchers to develop a quantum algorithm they call Bayesian eXchange coupling parameter calculator with Broken-symmetry wave functions (BxB), that predicts the electronic states of atoms and molecules by directly calculating the energy differences. They noted that energy differences in atoms and molecules remain constant, regardless to how complex and large they get despite their total energies grow as the system size. "With BxB, we avoided the common practice of calculating the total energies and targeted the energy differences directly, keeping computing costs within polynomial time", they state. "Since then, our goal has been to improve the efficiency of our BxB software so it can predict the electronic states of atoms and molecules with chemical precision."

Using the computing costs of a well-known algorithm called Quantum Phase Estimation (QPE) as a benchmark, "we calculated the vertical ionization energies of small molecules such as CO, O2, CN, F2, H2O, NH3 within 0.1 electron volts (eV) of precision," states the team, using half the number of qubits, bringing the calculation cost on par with QPE.

Their findings will be published online in the March edition of The Journal of Physical Chemistry Letters.

Ionization energy is one of the most fundamental physical properties of atoms and molecules and an important indicator for understanding the strength and properties of chemical bonds and reactions. In short, accurately predicting the ionization energy allows us to use chemicals beyond the current norm. In the past, it was necessary to calculate the energies of the neutral and ionized states, but with the BxB quantum algorithm, the ionization energy can be obtained in a single calculation without inspecting the individual total energies of the neutral and ionized states. "From numerical simulations of the quantum logic circuit in BxB, we found that the computational cost for reading out the ionization energy is constant regardless of the atomic number or the size of the molecule," the team states, "and that the ionization energy can be obtained with a high accuracy of 0.1 eV after modifying the length of the quantum logic circuit to be less than one-tenth of QPE." (See image for modification details)

With the development of quantum supercomputer hardware, Sugisaki and Takui, along with their team, are expecting the BxB quantum algorithm to perform high-precision energy calculations for large molecules that cannot be treated in real-time with conventional computers.

UK researchers identify new material that paves the way to faster supercomputing

Tyler O'Neal, Staff Editor SYSTEMS

Research led by the Cavendish Laboratory at the University of Cambridge has identified a material that could help tackle speed and energy, the two biggest challenges for computers of the future.

Research in the field of light-based supercomputing - using light instead of electricity for computation to go beyond the limits of today's computers - is moving fast, but barriers remain in developing optical switching, the process by which light would be easily turned 'on' and 'off', reflecting or transmitting light on-demand.

The study, published in an academic journal, shows that a material known as Ta2NiSe5 could switch between a window and a mirror in a quadrillionth of a second when struck by a short laser pulse, paving the way for the development of ultra-fast switching in computers of the future.

The material looks like a chunk of pencil lead and acts as an insulator at room temperature, which means that when infrared light strikes the material in this insulating state, it passes straight through like a window. However, when heated, the material becomes a metal that acts as a mirror and reflects light.

"We knew that Ta2NiSe5 could switch between a window and a mirror when it was heated up, but heating an object is a very slow process," said Dr. Akshay Rao, Harding University Lecturer at the Cavendish Laboratory, who led the research. "What our experiments have shown is that a short laser pulse can also trigger this 'flip' in only 10-15 seconds. This is a million times faster than switches in our current computers."

The researchers were looking into the material's behavior to show the existence of a new phase of matter called an 'excitonic insulator', which has been experimentally challenging to find since it was first theorized in the 1960s.

"This excitonic insulating phase looks in many ways like a very normal insulator, but one way to distinguish between an unusual and ordinary insulator is to see exactly how long it takes for it to become a metal," said Rao. "For normal matter, going from an insulator to metal is like melting an ice cube. The atoms themselves move positions and rearrange, making it a slow process. But in an excitonic insulator, this could happen very fast because the atoms themselves do not need to move to switch phases. If we could find a way to measure how fast this transition occurs, we could potentially unmask the excitonic insulator."

To do these experiments, the researchers used a sequence of very short laser pulses to first perturb the material and then measure how its reflection changed. At room temperature, they found that when Ta2NiSe5 was struck by a strong laser pulse, it exhibited signatures of the metallic state immediately, becoming a mirror on a timescale faster than they could resolve. This provided strong evidence for the excitonic insulating nature of Ta2NiSe5.

"Not only does this work remove the material's camouflage, opening up further studies into its unusual quantum mechanical behavior, it also highlights this material's unique capability of acting as an ultrafast switch," said first author Hope Bretscher, also from the Cavendish Laboratory. "In fact, for the optical switch to be effective, not only must it transition quickly from the insulating to the metallic phase, but the reverse process must also be fast.

"We found that Ta2NiSe5 returned to an insulating state rapidly, much faster than other candidate switch materials. This ability to go from mirror to window, to mirror again, make it extremely enticing for computing applications."

"Science is a complicated and evolving process--and we think we've been able to take this discussion a step forward. Not only we can now better understand the properties of this material, but we also uncovered an interesting potential application for it," said co-author Professor Ajay Sood, from the Indian Institute of Science in Bangalore.

"While practically producing quantum switches with Ta2NiSe5 may still be a long way off, having identified a new approach to the growing challenge of computer's speed and energy use is an exciting development," said Rao.

Michigan Tech physicists catch electrons in action in an antiferromagnetic nanowire

Tyler O'Neal, Staff Editor SYSTEMS

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.