Research Article

Quantum-nondemolition state detection and spectroscopy of single trapped molecules

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Science  13 Mar 2020:
Vol. 367, Issue 6483, pp. 1213-1218
DOI: 10.1126/science.aaz9837

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Reading a molecule without destroying it

Achieving efficient quantum control of ultracold molecular systems may open opportunities in molecular precision spectroscopy, quantum information, and related fields. Sinhal et al. report a quantumnondemolition protocol for the detection of the spin-rovibronic state of a single trapped cold molecular ion co-trapped with an atomic ion. They show that monitoring the motion of Ca+ after coherent motional excitation of the Ca+-N2+ string makes it possible to detect the N2+ state without destroying either the molecule or the state itself. The procedure can be repeated multiple times while preserving the high readout fidelity.

Science, this issue p. 1213


Trapped atoms and ions, which are among the best-controlled quantum systems, find widespread applications in quantum science. For molecules, a similar degree of control is currently lacking owing to their complex energy-level structure. Quantum-logic protocols in which atomic ions serve as probes for molecular ions are a promising route for achieving this level of control, especially for homonuclear species that decouple from blackbody radiation. Here, a quantum-nondemolition protocol on single trapped  N2+ molecules is demonstrated. The spin-rovibronic state of the molecule is detected with >99% fidelity, and a spectroscopic transition is measured without destroying the quantum state. This method lays the foundations for new approaches to molecular spectroscopy, state-to-state chemistry, and the implementation of molecular qubits.

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