In step forward for quantum computing hardware, physicist uncovers novel behavior in quantum-driven superconductors

The study is co-authored by Babak Seradjeh, Professor of Physics within the College of Arts and Sciences at Indiana University Bloomington, with theoretical physicists Rekha Kumari and Arijit Kundu of the Indian institute of Technology Kanpur. While the study is theoretical, the research team confirmed their results through numerical simulations. Published in Physical Review Letters, the world's premier physics journal, the research focuses on "Floquet Majorana fermions" and their role in a phenomenon called the Josephson effect, which could lead to more precise control of the dynamics of driven quantum systems.

Potentially advancing quantum computing

Developing a full-fledge quantum computer is hampered by a core issue: instability. This instability is mainly due to something called "quantum decoherence," wherein quantum bits, known as "qubits," lose their delicate quantum state due to interference from their environment -- such as temperature fluctuations or electromagnetic noise.

Qubits can be made using different physical systems, such as trapped ions, optical arrays, or superconductors -- materials that can conduct electricity with zero resistance without losing any energy, often at extremely low temperatures close to absolute zero. This makes quantum computers incredibly energy-intensive to keep cold, and thus stable, because when qubits aren't kept cold enough they become even more unstable, which means errors happen by larger amounts and more frequently.

One way to counter such errors is to look for "room-temperature superconductors," often referred to as the Holy Grail of superconductivity, because the cooling process is so costly and complex. If scientists could develop materials that exhibit superconductivity close to room temperature (approximately 20-25 degrees Celsius or 68-77 degrees Fahrenheit), it could revolutionize technology as we know it, eventually leading to lossless power transmission, exponentially faster and more energy-efficient electronics, and advanced cryptosecurity.

Professor Seradjeh and colleagues address the issue of decoherence in a different way -- by encoding the quantum information non-locally so that it is spread over a larger distance in space, and therefore making it immune to local noise and fluctuations.

What makes "Floquet Majorana Fermions" special for quantum computing?

Majorana fermions are named after Ettore Majorana, the Italian physicist who first proposed their existence in 1937 as subatomic particles that behave in unique ways; unlike most particles, Majorana fermions are their own antiparticles. (For every type of particle in the universe -- such as electrons and protons -- there exists a corresponding antiparticle with opposite charge and the same mass, and this symmetry between particles and antiparticles is a fundamental part of the structure of the universe.)

The mathematical physicist Alexei Kitaev realized in 2000 that Majorana fermions can exist not only as elementary particles, but also as quantum excitations in certain materials known as topological superconductors. These differ from regular superconductors in that a topological superconductor has unique, stable quantum states on its surface or edges that are protected by the material's underlying topology -- the way the electrons' motion is shaped at the quantum level.

These surface states make them resistant to disruptions, which is why they hold potential for developing more stable quantum computers. These special edge states behave just as Majorana fermions, which don't exist in regular superconductors. In theory, such Majorana fermions can be used to store quantum information nonlocally, thus providing a way to protect qubits from decoherence.

Professor Seradjeh and colleagues explored Majorana fermions in a specific context: superconductors that are "periodically driven," meaning, they are exposed to external energy sources that cycle on and off in a repeated pattern. This periodic driving alters the behavior of the Majorana fermions, transforming them into "Floquet Majorana fermions" (FMFs). Floquet Majorana fermions can exist in distinct states not possible without the periodic drive, changing based on their interaction with the cycling energy source. The periodic driving of the superconductor is key to maintaining the FMFs and the unusual patterns they create.

To produce an electric current in normal conductors between two points one needs to apply a voltage, which acts as the pressure that pushes electricity between two points. But because of a peculiar quantum tunneling process known as the "Josephson effect" current can flow between two superconductors without the need for an applied voltage. The FMFs influence this Josephson current in unique ways. In most systems, the current between two superconductors repeats itself at regular intervals. However, FMFs manifest themselves in a pattern of current that oscillates at half the normal rate, creating a unique signature that can help in their detection.

Tuning the current with new techniques

One of the key findings revealed by Seradjeh and colleagues' study is that the strength of the Josephson current -- the amount of electrical flow -- can be tuned using the "chemical potential" of the superconductors. Simply stated, the chemical potential acts as a dial that adjusts the properties of the material, and the researchers found that it could be modified by synching with the frequency of the external energy source driving the system. This could provide scientists a new level of control over quantum materials and opens up possibilities for applications in quantum information processing, where precise manipulation of quantum states is critical.

The discovery that Floquet Majorana fermions have unique properties that can be controlled through external drives could help pave the way for building quantum computers that are faster and more resistant to errors. These findings provide researchers worldwide with a roadmap for detecting and exploring new, controllable properties in driven quantum systems.