  Recent progress in quantum information has led to the start of several large
national and industrial efforts to build a quantum computer. Researchers are
now working to overcome many scientific and technological challenges. The
program's biggest obstacle, a potential showstopper for the entire effort, is
the need for high-fidelity qubit operations in a scalable architecture. This
challenge arises from the fundamental fragility of quantum information, which
can only be overcome with quantum error correction. In a fault-tolerant quantum
computer the qubits and their logic interactions must have errors below a
threshold: scaling up with more and more qubits then brings the net error
probability down to appropriate levels ~ $10^{-18}$ needed for running complex
algorithms. Reducing error requires solving problems in physics, control,
materials and fabrication, which differ for every implementation. I explain
here the common key driver for continued improvement - the metrology of qubit
errors.
