The problem of bit-flip
In quantum computing, bit-flip errors are still a problem, but so is something called a 'phase-flip' error. That's when the error flips the phase relationship between '0' and '1' in the superposition state -- a '0+1' turns into a '0-1' or vice versa.
"Say you have a wave -- maybe sine wave or cosine wave," Chow says. "If you have these two waves and you jostle them together, the sound might add together and constructively interfere. They'd be in phase. If you jostle them opposite each other, the sound might cancel due to destructive interference. In this way, they're out of phase."
To correct errors in quantum computing, you need to be able to detect bit-flip errors and phase-flip errors simultaneously. But until now, it has only been possible to address one type of quantum error at a time. That's especially problematic because quantum information is very fragile -- all existing qubit technologies lose their information when interacting with matter and electromagnetic radiation.
To break down that hurdle, the IBM researchers developed a quantum bit circuit, based on a square lattice of four superconducting qubits on a chip roughly 6mm on a side, rather than the linear array of qubits that researchers have used in the past. The researchers say they used a variety of techniques to measure the states of two independent syndrome (measurement) qubits, which each reveal one aspect of the quantum information stored on the other two qubits (called code or data qubits).
Chow says that because these qubits can be designed and manufacturing using standard silicon fabrication techniques -- metal on silicon -- once a handful of superconducting qubits can be manufactured reliably and repeatedly and controlled with low error rates, there should be no fundamental obstacle to demonstrating error correction in larger lattices of qubits. From there, one of the next milestones may be the creation of quantum algorithms.
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