High-fidelity state detection for mid-circuit measurement

Complex quantum algorithms in trapped-ion systems will involve frequency mid-circuit detection to identify and correct for errors. Short detection times are desirable for reducing both spontaneous decay errors and algorithm runtimes, but detection fidelity is limited by a system's fluorescent photon collection efficiency. We describe a cryogenic trapped-ion chamber with an in-vacuum objective mounted on a remotely controlled hexapod that allows for 99.9966(4)% detection fidelity in 50 us detection time. We describe the experiments used to estimate the photon collection efficiency and determine detection fidelity, and we detail the vibration isolation, magnetic field shielding, and universal microwave coil internal to the vacuum chamber.