Schrödinger’s cat makes better qubits

Newswise — Quantum computing is a special type of computing that uses the rules of quantum physics to store and process information. It has the potential to solve complex problems that regular computers cannot handle. Regular computers use bits that can be either 0 or 1, but quantum computers use qubits, which are like super-powered bits that can hold more information.

According to Professor Vincenzo Savona, who leads the Center for Quantum Science and Engineering at EPFL, quantum computing has the potential to greatly impact various fields such as drug discovery, optimization, and simulating intricate biological systems and materials. Its capabilities could bring significant changes to science, industry, and society as a whole.

Unlike regular bits in classical computers, qubits can be in a special state called "superposition" where they can be both 0 and 1 at the same time. This unique property enables quantum computers to explore multiple solutions at once, potentially making them much faster for specific computational tasks. However, quantum systems are fragile and can easily be affected by their surroundings, leading to errors in their calculations.

Professor Vincenzo Savona emphasizes the importance of finding ways to safeguard qubits from errors or to identify and fix errors that arise. This is essential for building large-scale quantum computers that are robust and reliable. In collaboration with EPFL physicists Luca Gravina and Fabrizio Minganti, they have achieved a major breakthrough by introducing a concept called the "critical Schrödinger cat code." This new encoding method has the potential to greatly enhance the resilience of quantum computers and could revolutionize their reliability.

In 1935, physicist Erwin Schrödinger presented a hypothetical experiment to challenge the prevailing interpretation of quantum mechanics known as the Copenhagen interpretation. In his experiment, there is a sealed box containing a cat, a flask of poison, and a radioactive source. If a single atom of the radioactive source decays, it triggers a Geiger counter that breaks the flask, releasing the poison and causing the cat's death.

According to the Copenhagen interpretation of quantum mechanics, if the atom in Schrödinger's experiment is initially in a superposition, the cat would also be in a superposition state where it is simultaneously both alive and dead. Professor Vincenzo Savona explains that this particular state of the cat represents the concept of a quantum bit (qubit) extended to a larger, macroscopic scale.

Scientists have been inspired by Schrödinger's cat thought experiment to develop an encoding technique known as the "Schrödinger's cat code." In this technique, the 0 and 1 states of a qubit are encoded onto two different phases of an oscillating electromagnetic field within a resonant cavity. This encoding process is analogous to how the states of the cat (dead or alive) are represented.

Professor Vincenzo Savona explains that in the past, Schrödinger cat codes have been realized using two different approaches. One approach utilizes anharmonic effects in the resonant cavity, while the other relies on carefully designed cavity losses. In their recent work, they have combined these two approaches by operating in an intermediate regime, which combines the advantages of both methods. This hybrid regime, previously considered less promising, has shown improved capabilities in suppressing errors. The key concept behind the "critical" aspect of the critical cat code is to operate close to the critical point of a phase transition.

The critical cat code offers an additional advantage by demonstrating remarkable resilience against errors caused by random frequency shifts. These shifts can be problematic when dealing with multiple qubits and their interactions. Overcoming this challenge is a significant step towards building quantum computers with multiple interacting qubits, which is a fundamental requirement for practical quantum computing devices. The exceptional error resistance of the critical cat code helps pave the way for the realization of such devices.

Professor Vincenzo Savona expresses his excitement, stating, "We are taming the quantum cat." Through their work in the hybrid regime, they have created a system that surpasses previous achievements, representing a substantial advancement for cat qubits and the field of quantum computing in general. This study marks a significant milestone in the journey towards building more advanced quantum computers and demonstrates EPFL's commitment to advancing quantum science and unlocking the full potential of quantum technologies.