Several experiments have reported single-qubit-gate error rates below 0.1%, and two-qubit error rates below 1%, coming close to the “error rate threshold” of certain Quantum Error Correction (QEC) schemes.
The biggest issue is that the quantum state of a qubit register can be controlled only up to a finite precision, due to either fluctuating experimental parameters or to the uncontrolled interaction with the environment. Remarkably, this does not forbid the implementation of a quantum computer because errors can be corrected to some extent by redundantly encoding the information into “logical” qubits that consist of several “physical” qubits. In the recent years the control accuracy of few qubit registers progresses in a variety of physical systems : trapped ions, superconducting circuits, spins, ... Several experiments have reported single-qubit-gate error rates below 0.1%, and two-qubit error rates below 1%, coming close to the “error rate threshold” of certain Quantum Error Correction (QEC) schemes. Experiments involving 10 to 50 qubits are planned for the near future.