Inevitably, giant stars at the end of their lives collapse under the gigantic force of gravity and turn into black holes. We could ask if there is a way to delay this process, perhaps postpone the death of the star. While investigating "anti-aging therapy" of giant star, researchers at the Center for the Theoretical Physics of the Universe, within the Institute for Basic Science (IBS) conceptualized an ideal material that could store data for an exceptionally longer time than current short-lived devices, bringing new hints for future quantum memory technologies.

Archaeologists have been able to discover and often decipher the messages carved and written in clay tablets, stones or papers long left by ancient civilizations. These specimens were successful in surviving into the 21st century. So the question here is whether our digital messages will survive too in pristine condition for thousands of years from now? The production of new digital information is bigger than ever before but the silicon-based devices come with an expiration date: around three to five years for hard disks, five to ten years for flash storage devices, CDs and DVDs. Unfortunately, all our priceless memories stored as digital photos, videos and digitalized documents are not likely to become available for our descendants unless we are diligent enough to frequently copy them to new devices. Overcoming this limitation is one of the biggest challenges faced by scientists today. "We all die, but we want to slow down the aging process so that we can live longer, much longer than now. The same goes for our digital data, we want to prolong their existence," explains Soo-Jong Rey, director of the Field, Gravity, and Strings Group at the Center for the Theoretical Physics of the Universe.

Going quantum is the best way to harness the many facets of the nanoscale world. This lets us exploit the strange property of "quantum entanglement" whereby coherent structures can be formed at these small scales. In these scales, the fundamental quantum principle was raised by Rolf Landauer back in 1961. He discovered that heat and information are intimately connected. Processing data generates heat and, for this reason, information deteriorates and cannot be stored forever. Now with digital miniaturization, we are bringing technology to its quantum limits. Information is stored in smaller quantum scale devices, against its natural tendency to spread out, and therefore generating even more heat.

Needless to say, decline and decay are part of life, as it all boils down to energy transfer. It is the same phenomenon that causes a hot coffee's temperature to become as same as the room temperature when it encounters with a cool mug and air. Energy is transferred from the coffee to the mug, and eventually to the air. Energy tends to dissipate, unless it is shielded and confined. This exchange process that reduces the temperature of the coffee is ultimately connected to a quantum information process that physicists call "scrambling" at the ultimate quantum scale. As the word suggests, scrambling involves the mixing of energy and information where the originals cannot be retrieved. It's like when yolk and white become unrecognizable once they become scrambled egg.

In order to keep the coffee warm for longer, it would be necessary to shield it from any other cool materials or substances. In the case of memory devices, to keep the device working for longer, electrons or atoms bearing energy or information of quantum units should not interact with other electrons and atoms, and they need to be isolated as much as possible. The confinement is created by other atoms that form a barrier. A long time ago, Phil Anderson proved that this atom-built barrier perfectly works if our world was one-dimensional, such as a line. Imagine having atoms in a line and putting an obstacle in the middle to keep them far apart. However, if they move in a two-dimensional flat land or in a three-dimensional material, this issue is notoriously complicated. Although the semiconductor industry is specialized in controlling these barriers, atoms can always find paths to move around or jump and reach their neighbors.

To complicate the issue even further, it was discovered that electrons move together as clusters, called strongly correlated systems or many-body systems. As scientists tend to isolate single atoms and electrons and prevent them from interacting with each other, holding the reins of a cluster of them is even more challenging.

In order to find an idealized system that is localized and correlated at the same time, the IBS research team relied on an exotic concept called supersymmetry. "In supersymmetry, each particle has a partner. For example, each electron pairs with a selectron of the same energy and mass. Because of these pairings, the system can be solved with pen and paper without the need for a computer simulation no matter how many particles you have," describes Rey.

Using the mathematical principles of supersymmetry, the scientists conceptualized an ideal material with the right structural organization that could store quantum data for an exceptionally long time, exponentially longer than the current memory devices.

The material they envision has a special architecture of energy levels for its electrons. Energy levels can be imagined as the floors in a hotel. However, the shape of the hotel looks different depending on the type of the atom. The more energy the electron has, the higher floor it occupies. So electrons involved in data storage would occupy the top floors. Using this analogy, the hotel for silicon has a shape similar to an upside-down pyramid with rooms available in each floor. Electrons with data on the top floor can easily exchange their energy or data with electron on the lower floors. In this way, they swap rooms with other electrons by transferring energy or data. Scrambling will occur after the continuous swapping of rooms.

The hotel proposed by Rey's research team tapers quickly as it climbs taller. In this hotel, most of the electrons are on the first floor because very few rooms are available in the higher floors. Since there are not any rooms available upstairs, electrons cannot interact with each other, and therefore cannot swap rooms. In this way, data from the electrons in the top floors are not lost as time passes. Eventually, the scrambling process will happen but it would take an exponential time.

▲ Figure 1: Analogy to compare the properties of different materials for data storage. The energy level of electrons can be represented as floors of a hotel occupied by electrons. In the case of the "Silicon Hotel," shown on the computer screen, there are several rooms available in each floor. Electrons rich with data on the top floors can easily exchange their energy and data with electron on the lower floors. The more of interactions the materials have, the shorter the lifespan for data storage. Instead, the "Ideal Hotel" does not have any rooms available upstairs so that the electrons cannot interact with each other as well as swap rooms. Eventually, there would be some exchanges, but it would take a very long time. A material with this type of energy levels would store information for much longer than the current silicon-based devices. (credits: modified from Freepiks.com)

The scientist said, "The second law of thermodynamics states that the entropy cannot decrease, but it does not mention how much time it takes for an ordered state to become chaotic. So the game is about longevity, to prolong it as much as possible. Eventually, the hotel will collapse as it is unavoidable for the entropy to become the ultimate winner but we want to make sure that such victory comes only after a very long time."

Although a material with such energy levels does not exist yet, this new understanding can guide material scientists and memory device engineers on how to develop superior memory storage devices that will fit to this concept and that could replace silicon.

Going back to the "large stars' anti-aging therapy," in the same way as it is theoretically possible to design a material for longer digital storage, scientists are wondering if it is possible to point at precise criteria to delay the decay of large stars. In other words could delay the formation of black holes? For now, only future research will be able to answer this.

The study was published in the Journal of High Energy Physics.

Letizia Diamante