2023

Preserving a qubit during adjacent measurements at a few micrometers distance

Sainath Motlakunta, Nikhil Kotibhaskar, Chung-You Shih, Anthony Vogliano, Darian Mclaren, Lewis Hahn, Jingwen Zhu, Roland Hablützel, Rajibul Islam.

**Abstract:** Protecting a quantum object against irreversible accidental measurements from its surroundings is necessary for controlled quantum operations. This becomes especially challenging or unfeasible if one must simultaneously measure or reset a nearby object’s quantum state, such as in quantum error correction. In atomic systems - among the most established quantum information processing platforms - current attempts to preserve qubits against resonant laser-driven adjacent measurements waste valuable experimental resources such as coherence time or extra qubits and introduce additional errors. Here, we demonstrate high-fidelity preservation of an ‘asset’ ion qubit while a neighboring ‘process’ qubit is reset or measured at a few microns distance. We achieve <1E-3 probability of accidental measurement of the asset qubit while the process qubit is reset, and <4E-3 probability while applying a detection beam on the same neighbor for experimentally demonstrated fast detection times, at a distance of 6 μm or four times the addressing Gaussian beam waist. These low probabilities correspond to the preservation of the quantum state of the asset qubit with fidelities above 99.9% (state reset) and 99.6% (state measurement). Our results are enabled by precise wavefront control of the addressing optical beams while utilizing a single ion as a quantum sensor of optical aberrations. Our work demonstrates the feasibility of in-situ state reset and measurement operations, building towards enhancements in the speed and capabilities of quantum processors, such as in simulating measurement-driven quantum phases and realizing quantum error correction.