For quantum computing, is 99% accuracy enough?
(IEEESpectrum) Teams of researchers at Delft University of Technology in the Netherlands, Riken in Japan, and the University of New South Wales (UNSW) in Sydney have developed different quantum devices in silicon with operational fidelity greater than 99 percent. Their work was published separately in Nature on 20 January. The first two teams use the spins of two electrons to achieve their results, while the researchers in Australia use the spins of two nuclei and one electron.
John Martinis, a professor of physics at the University of California, Santa Barbara, and the former chief architect of Google’s Sycamore quantum processor, commenting on the breakthrough, notes that a 99 percent fidelity “represents the threshold for error correction, which has been a long-term goal. Of course, the fidelity needs to be improved, but this research is an important step to show that qubits can be controlled accurately.”
Andrea Morello, professor at UNSW’s School of Electrical Engineering and Telecommunications, explains that his group achieved a 1-quantum bit (qubit) nuclear spin operation with up to 99.95 percent fidelity, and a 2-qubit operation in a 3-qubit system with up to 99.37 accuracy. He adds that the advantage of using nuclei spins is that they have extremely high fidelity. “We demonstrated in 2015 that we could get 99.99 percent quantum-bit operation accuracy using nuclei spin in silicon because the nuclei don’t interact with the rest of the world.” What’s more, such isolation enabled the nuclei to preserve their quantum information for a remarkable 35 seconds—“an eternity when compared to the hundred microseconds or so obtained by the Google and IBM superconducting quantum computers,” Morello says.
In addition to producing universal nuclear operations with 99 percent-plus fidelity, Morello says rotating the electron can also be used to create an entangled state of all three qubits. This is important because the electron can then be entangled with other electrons, which themselves can entangle their own nuclei, something that can be repeated over and over. “So this is like a doorway leading to a scalable quantum computer,” says Morello.
Morello acknowledges the challenges that lie ahead. He points out that to build a completely error-protected logical qubit that can encode one quantum bit of information will require an additional few thousand quantum bits connected together in such a way that whenever an error occurs (for instance, the accidental rotation of one of the spins), the error can be detected and corrected.