Stern–Gerlach detection of neutral-atom qubits in a state-dependent optical lattice

Qubit state measurements are an essential part of any quantum computer, constituting the readout. Accurate measurements are also an integral component of one-way quantum computation and of error correction, which is needed for fault-tolerant quantum computation¹. Here, we present a state measurement for neutral-atom qubits based on coherent spatial splitting of the atoms’ wavefunctions. It is reminiscent of the Stern–Gerlach experiment², but carried out in light traps. For around 160 qubits in a three-dimensional array, we achieve a measurement fidelity of 0.9994, which is roughly 20 times lower error than in previous measurements of neutral-atom arrays^3,4. It also greatly exceeds the measurement fidelity of other arrays with more than four qubits, including those with ion and superconducting qubits^5,6. Our measurement fidelity is essentially independent of the number of qubits measured, and since the measurement causes no loss, we can reuse the atoms. We also demonstrate that we can replace atoms lost to background gas collisions during the experiment⁷.