Silicon-based qubits take a big leap forward

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      Two papers take a similar approach to improving the performance of qubits based on quantum dots. One is from a group of researchers based at the Delft University of Technology, and the other is primarily from Japan’s RIKEN, with some collaborators at Delft. Both groups used silicon with wiring on it to create a quantum dot that trapped a single electron. The spin of the trapped electron was used as the basis for the qubit. And both groups took a similar approach, testing their gate under a wide range of conditions to identify the ones that tended to produce errors and then operating the qubit in a way that avoided those errors.

      In the work at Delft, entangling the two qubits was done by manipulating the quantum dots so that the wave functions of the trapped electrons overlapped. After optimizing the use of the hardware, the researchers found that both the single-qubit and two-qubit gate operations have a fidelity rate of over 99.5 percent. That’s above the threshold needed for getting the most commonly considered form of quantum error correction to work.

      To show that the qubits are actually useful, the researchers use their two-qubit setup to calculate the ground state energy of molecular hydrogen. This calculation is relatively easy to do on classical hardware, so the results could be checked.

      The RIKEN group did something similar and generally found that speeding up operations had a major effect on error rates. Again, managing this problem produced gates with a fidelity of 99.5 percent, well above the threshold needed for error correction. To show that the gates worked, the team implemented a couple of quantum computing algorithms and showed that they were completed with a success rate in the area of 97 percent.

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