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Quantum computing uses the principles of quantum mechanics to encode and process data, which means that it could one day solve computational problems that are intractable with today’s computers. While quantum computers work with bits, which represent a 0 or a 1, quantum computers use quantum bits, or qubits, the basic units of quantum information.
“With applications ranging from drug discovery to optimization and simulations of complex biological systems and materials, quantum computing has the potential to reshape vast areas of science, industry and society,” says Professor Vincenzo Savona, Director of the Center for Quantum Science and Engineering at EPFL.
Unlike classical bits, qubits can exist in a “superposition” of both 0 and 1 states simultaneously. This allows quantum computers to explore multiple solutions simultaneously, which could make them significantly faster at certain computational tasks. However, quantum systems are delicate and susceptible to errors caused by interactions with their environment.
“Developing strategies to protect either qubits from this or to detect and correct errors once they have occurred is critical to enabling the development of large-scale, fault-tolerant quantum computers,” Savona says. Together with EPFL physicists Luca Gravina and Fabrizio Minganti, they have taken a significant step forward by proposing a ‘Schrdinger’s cat critical code’ for advanced resilience to errors. The study introduces a new coding scheme that could revolutionize the reliability of quantum computers.
What is a “critical Schrdinger cat code”?
In 1935, physicist Erwin Schrdinger proposed a thought experiment as a critique of the prevailing understanding of quantum mechanics at the time of the Copenhagen interpretation. In Schrdinger’s experiment, a cat is placed in a sealed box with a flask of poison and a radioactive source. If a single atom of the radioactive source decays, the radioactivity is detected by a Geiger counter, which then shatters the flask. The venom is released, killing the cat.
According to the Copenhagen view of quantum mechanics, if the atom is initially in superposition, the cat will inherit the same state and be in a superposition of living and dead. “This state exactly represents the notion of a quantum bit, realized on a macroscopic scale,” says Savona.
In past years, scientists have been inspired by Schrdinger’s cat to build an encoding technique called “Schrdinger’s cat code”. Here, the 0 and 1 states of the qubit are encoded on two opposite phases of an electromagnetic field oscillating in a resonant cavity, analogous to the dead or alive states of the cat.
“Schrdinger cat codes have been done in the past using two distinct approaches,” Savona explains. “One takes advantage of anharmonic effects in the cavity, the other relies on carefully engineered cavity losses. In our work, we bridged the two by operating in an intermediate regime, combining the best of both worlds. Although previously thought to be unsuccessful, this regime hybrid results in advanced error suppression capabilities.” The central idea is to operate near the critical point of a phase transition, which is what the “critical” part of the critical cat code refers to.
Critical cat code has an additional benefit: It shows exceptional resistance to errors from random frequency shifts, which often pose significant challenges to operations involving multiple qubits. This solves a major problem and paves the way for devices with several interacting qubits – the minimum requirement for building a quantum computer.
“We’re domesticating the quantum cat,” Savona says. “Operating in a hybrid regime, we have developed a system that surpasses its predecessors, which represents a significant leap forward for cat qubits and quantum computing as a whole. The study is a milestone on the road towards building better quantum computers and showcases EPFL’s dedication to advancing the field of quantum science and unlocking the true potential of quantum technologies.”
The results are published in the journal PRX Quantum.
Luca Gravina et al, Critical Schrdinger Cat Qubit, PRX Quantum (2023). DOI: 10.1103/PRXQuantum.4.020337
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