Critical Schrödinger Cat Code: Quantum Computing Breakthrough for Better Qubits:
What is a "critical Schrödinger cat code?"
In 1935, physicist Erwin Schrödinger proposed a thought experiment as a critique of the prevailing understanding of quantum mechanics at the time – the Copenhagen interpretation. In Schrödinger'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 poison 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 find itself in a superposition of alive and dead. "This state represents exactly the notion of a quantum bit, realized at the macroscopic scale," says Savona.
In past years, scientists have drawn inspiration from Schrödinger's cat to build an encoding technique called "Schrödinger's cat code." Here, the 0 and 1 states of the qubit are encoded onto two opposite phases of an oscillating electromagnetic field in a resonant cavity, similar to the dead or alive states of the cat.
"Schrödinger cat codes have been realized in the past using two distinct approaches," explains Savona. "One leverages anharmonic effects in the cavity, the other relying 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 believed to be unfruitful, this hybrid regime results in enhanced error suppression capabilities." The core idea is to operate close to the critical point of a phase transition, which is what the 'critical' part of the critical cat code refers to.
The critical cat code has an additional advantage: it exhibits exceptional resistance to errors that result from random frequency shifts, which often pose significant challenges to operations involving multiple qubits. This solves a major problem and paves the way to the realization of devices with several mutually interacting qubits – the minimal requirement for building a quantum computer.
"We are taming the quantum cat," says Savona. "By 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 toward building better quantum computers, and showcases EPFL's dedication to advancing the field of quantum science and unlocking the true potential of quantum technologies.
Journal Reference:
Luca Gravina, Fabrizio Minganti, Vincenzo Savona. Critical Schrödinger Cat Qubit [open], PRX Quantum (DOI: 10.1103/PRXQuantum.4.020337)
