Robust and Scalable Quantum Measurement

Accurate and reliable quantum bit (qubit) measurement is essential for performing error correction on a quantum computer. A superconducting qubit is typically measured by probing a corresponding resonator, whose decay rate affects both the rate that the qubit is measured and the rate at which the qubit decays. In state-of-the-art experiments, however, the resonator decay rate is quite sensitive to system imperfections, having significant variation across a chip. This variation degrades the reliability and performance of quantum error correction codes.

We show that this variation is a byproduct of conventional ways to provide directionality to the readout pulse. Instead, we propose a new form of readout that simultaneously preserves the directionality of readout while making the resonator decay rate robust to system imperfections. The key innovation is to make a resonator which emits photons preferentially in one direction across its full bandwidth. We design and fabricate such a resonator and demonstrate readout fidelity of 99.0% with high directionality.

Our work presents a new direction for quantum processors by making their reliability and performance more robust to system imperfections, as opposed to simply trying to make the system closer to perfect. Further research in this direction will be critical to realizing more reliable, modular design of qubit readout. This will be essential to the scaling of superconducting quantum computers.