The company set up a pair of dual-rail qubits, each containing two coupled resonators. Between them was a tunable coupler that could be switched on or off. Depending on the state of this coupler, the two qubits would either operate independently or interact. Practically, this involves setting the coupler so that one qubit (termed the control qubit) partially occupies the coupler, allowing it to interact with the second.
Running an entangling gate turned out to be extremely simple: Activate the connection, wait, and then shut it off again. The “wait” portion would not even give you time to check your watch; it’s about 200 nanoseconds, with the entire entanglement taking 500 ns. “We can do these operations in a few hundred nanoseconds, so these are fast operations,” Lanting said. “So not only do you have kind of the high fidelity of the dual rail devices, but you are producing fast entangling operations.”
Critically, it had the same hierarchy of error types. Photon loss rates were about 0.5 percent per entanglement, and they were five times more common than any other type of error. “Bit-flips are practically non-existent at the 10−6 level,” the paper says. Most of the losses affected the source qubit, which is also allowed to partially enter the coupler.
“The addition of a fast and high-fidelity entangling operation completes the toolbox of gates and operations for dual-rail cavity qubits,” Lanting said.
Work to be done
That doesn’t mean D-Wave’s work is done, of course. To begin with, the devices showed rising error rates as more operations were done. “As the number of gates increases, both the fidelity and purity of the final state exhibit an unexpected approximately quadratic decrease,” the paper acknowledges. “We propose the cause to be drift in calibration parameters or fluctuations in the frequency of the coupling transmon on timescales of experimental shot averaging.”