Quantum-nondemolition state detection and spectroscopy of single trapped molecules

Abstract

Trapped atoms and ions are among the best controlled quantum systems which find widespread applications in quantum information, sensing and metrology. For molecules, however, 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 with homonuclear molecules that decouple from black-body radiation. Here, a quantum-non-demolition protocol on single trapped N + 2 molecules is demonstrated. The spin-rovibronic state of the molecule is detected with more than 99% fidelity and the position and strength of a spectroscopic transition in the molecule are determined, both without destroying the molecular quantum state. The present method lays the foundations for new approaches to molecular precision spectroscopy, for state-to-state chemistry on the single-molecule level and for the implementation of molecular qubits.The impressive advances achieved in the control of ultracold trapped atoms and ions on the quantum level are now increasingly being transferred to molecular systems. Cold, trapped molecules have been created by, e.g., binding ultracold atoms via Feshbach resonances [1] and photoassociation [2, 3], molecular-beam slowing [4], direct laser cooling [5,6] and sympathetic cooling [7,8]. The trapping of the cold molecules enables experiments with long interaction times and thus paves the way for new applications such as studies of ultracold chemistry [9] and precision spectroscopic measurements which aim, e.g., at a precise determination of fundamental physical constants [10] and their possible time variation [11,12] as well as tests of fundamental theories which reach beyond the standard model [13,14].The complex energy level structure and the absence of optical cycling transitions in most molecular systems constitute a major challenge in the state preparation, laser cooling, state detection and coherent manipulation of molecules. Molecular ions trapped in radiofrequency ion traps which are sympathetically cooled by simultaneously trapped atomic ions [7,8] have proven a promising route for overcoming these obstacles. Recently, their rotational cooling and state preparation has been achieved [15][16][17][18], precision measurements of quantum electrodynamics and fundamental constants have been performed [10,19], the first studies of dipole-forbidden spectroscopic transitions in the mid-infrared spectral domain have been reported [20] and state-and energy-controlled collisions with cold atoms have been realized [21,22]. However, in order to reach the same exquisite level of control on the quantum level for a single molecule which can be achieved with trapped atoms [23], new methodological developments are required. In this context, the most promising route for achieving ultimate quantum control of molecular ions in trap experiments is constituted by quantum-logic protocols [24] in which a cotrapped atomic ion acts as a probe for th...

From megahertz to terahertz qubits encoded in molecular ions: theoretical analysis of dipole-forbidden spectroscopic transitions in N2+

“…However, rovibrational mixing, which is not explicitly apparent in the effective Hamiltonian approach taken here, introduces a non-zero overlap between different vibrational states. 49 Therefore, the first term in eqn (20) allows for vibrational transitions according to eqn (12).…”

“…All values are in SI units. Since S kl in eqn (22) and 23is the square of the polarization-independent transition moment that was defined in eqn (12), the Einstein A coefficients slightly differ from their regular definitions as they explicitly depend on the Zeeman levels.…”

“…These include molecular-beam slowing and trapping, [1][2][3] direct laser cooling, [4][5][6] assembly from ultracold atoms, 7 and sympathetic cooling. 8,9 In this context, experiments in which single molecular ions are co-trapped with single atomic ions [10][11][12][13][14][15] show excellent prospects for achieving the long-standing goal of gaining full control over the quantum state and dynamics of single isolated molecules. In these experiments, a quantum-logic approach 16 is pursued in which the co-trapped atomic ion is used to cool the external motion of the molecular ion to the quantum ground state and to nondestructively detect its internal quantum state.…”

“…In these experiments, a quantum-logic approach 16 is pursued in which the co-trapped atomic ion is used to cool the external motion of the molecular ion to the quantum ground state and to nondestructively detect its internal quantum state. Coherent Rabi and Ramsey spectroscopy, 11,13 quantum-non demolition state detection 10,12,15 and atom-molecule entanglement 14 have recently been demonstrated.…”

“…28,31 The newly developed methods for non-destructive detection and coherent manipulation of molecular ions promise an increase of several orders of magnitude in the experimental duty cycle. 12,32 This increase will result in a markedly improved spectroscopic sensitivity and, therefore, precision. Another exciting aspect of this technology is the implementation of molecular qubits which can be used for applications in quantum computation, 33 simulation, 34 metrology, 35 and communication.…”