All About Quantum Bits and How to Fix
This work from Adrian Cho simplifies understanding quantum bits and how to keep them from being so erroneous. Recommendreading from the source. Because Quantum is Coming. Qubit
The biggest flipping challenge in quantum computing
+ Manipulating individual qubits can introduce errors, and unless that error rate falls below a certain level, then entangling more qubits with the original one only adds more noise to the system, says Maika Takita, a physicist at IBM. “To demonstrate anything you have to get below that threshold,” she says. The ancillary qubits and other error-correction machinery add even more noise, and once those effects are included, the necessary error threshold plummets further. To make the scheme work, physicists must lower their error rate to less than 1%. “When I heard we achieved an 3% error rate, I thought that was great,” Takita says. “Now, it needs to be much lower.”
Error correction also requires twiddling with qubits repeatedly. That makes the process more demanding than quantum supremacy, which involved measuring all the qubits just once, says Marissa Giustina, a physicist with Google. Error correction “requires you to measure and measure and measure over and over again in a cycle, and that has to be done quickly and reliably,” she says.
+ If some experts question the significance of Google’s quantum supremacy experiment, all stress the importance of quantum error correction. “It is really the difference between a $100 million, 10,000-qubit quantum computer being a random noise generator or the most powerful computer in the world,” says Chad Rigetti, a physicist and co-founder of Rigetti Computing. And all agree with Kuperberg on the first step: spreading the information ordinarily encoded in a single jittery qubit among many of them in a way that maintains the information even as noise rattles the underlying qubits. “You’re trying to build a ship that remains the same ship, even as every plank in it rots and has to be replaced,” explains Scott Aaronson, a computer scientist at the University of Texas, Austin.
+ Although a handful of qubits would suffice to demonstrate the principle of quantum error correction, in practice physicists will have to control huge numbers of them. To run Shor’s algorithm well enough to factor, say, a number 1000 bits long—roughly the size used in some internet encryption schemes—they’ll need to maintain logical qubits with a part-in-1-billion error rate. That may require entangling a grid of 1000 physical qubits to safeguard a single logical qubit, researchers say, a prospect that will take generations of bigger and better quantum computing chips.
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