Superposition – where a qubit can hold a value of 0, 1 or both simultaneously – is fundamental to quantum computers. However, two types of qubit errors are possible. A bit flip error flips a 0 to a 1 and vice versa. However, phase flip errors flip the sign of the phase relationship between 0 and 1 and both types of error must be detected in order for quantum error correction to function properly.
"Until now, researchers have been able to detect bit flip or phase flip quantum errors, but never the two together," said Jay Gambetta, a manager in IBM's Quantum Computing Group. "Previous work, using linear arrangements, only looked at bit flip errors. Our four qubit results take us past this hurdle by detecting both types of quantum errors and can be scalable to larger systems, as the qubits are arranged in a square lattice as opposed to a linear array."
The IBM Research team used two independent syndrome – or measurement – qubits, which reveal one aspect of the quantum information stored on two other qubits – code, or data, qubits. One syndrome qubit shows whether a bit flip error occurred to either code qubit, while the other syndrome qubit shows whether a phase flip error occurred.
IBM's quantum bit circuit houses the four superconducting qubits on a chip roughly 6mm square which can be designed and manufactured using standard silicon fabrication techniques.
According to IBM, once a handful of superconducting qubits can be manufactured reliably and repeatedly, and controlled with low error rates, there will be no fundamental obstacle to demonstrating error correction in larger lattices of qubits.
