Non-destructive state detection for quantum logic spectroscopy of molecular ions

Abstract

Precision laser spectroscopy 1 of cold and trapped molecular ions is a powerful tool for fundamental physics, including the determination of fundamental constants 2 , the laboratory test for their possible variation 3,4 , and the search for a possible electric dipole moment of the electron 5 . While the complexity of molecular structure facilitates these applications, the absence of cycling transitions poses a challenge for direct laser cooling 6 , quantum state control [7][8][9][10][11] , and detection. Previously employed state detection techniques based on photodissociation 12 or chemical reactions 13 are destructive and therefore inefficient, restricting the achievable resolution in laser spectroscopy. Here we experimentally demonstrate nondestructive state detection of a single trapped molecular ion through its strong Coulomb coupling to a well-controlled co-trapped atomic ion. An algorithm based on a state-dependent optical dipole force 14 (ODF) changes the internal state of the atom conditioned on the internal state of the molecule. We show that individual quantum states in the molecular ion can be distinguished by their coupling strength to the ODF and observe black-body radiationinduced quantum jumps between rotational states of a single molecular ion. Using the detuning dependence of the state detection signal, we implement a variant of quantum logic spectroscopy 15,16 of a molecular resonance. The state detection technique we demonstrate is applicable to a wide range of molecular ions, enabling further applications in state-controlled quantum chemistry 17 and spectroscopic investigations of molecules serving as probes for interstellar clouds 18,19 .One of the salient features of trapped ion systems is that the universal Coulomb interaction allows strong coupling of diverse quantum objects, such as different species of atomic ions or atomic and molecular ions. Being able to perform quantum logic operations e.g. in the form of gates 14,20,21 between the quantum objects has proven a powerful tool for quantum information processing and quantum simulations in such systems. It also allows combining the advantages of different atomic species. Quantum logic spectroscopy is one such application in which the high degree of control achieved over selected atomic ions is extended to species over which such control is lacking 15,16 . Here, we demonstrate for the first time quantum logic operations between a single molecular ion and a co-trapped atomic ion, making a wide range of molecular ions accessible to this highlydeveloped toolbox. The presented technique allows the investigation of single molecules in a well isolated system avoiding disturbance from the environment, which is the limiting factor in other implementations of single molecule spectroscopy such as surface enhanced Raman spectroscopy (SERS) 22 Quantum logic operations between atoms are based on state dependent forces often induced by laser fields. The same approach is applicable to molecular ions. The coupling is now distributed over many ro-vibrat...

High sensitivity to variation in the proton-to-electron mass ratio inO2+

“…While preliminary measurements could take place in a beam, trapping O + 2 and sympathetic cooling to a Coulomb crystal with co-trapped atomic ions would allow longer probe times and eliminate first-order Doppler shifts. Trapping only a few atoms and molecules could enable non-destructive detection via quantum logic spectroscopy [46,47]. Such detection could increase the dutycycle by reducing the need to reload ions and would reduce systematic effects associated with micromotion in a radiofrequency trap [48], though it may reduce the statistical limit because fewer molecules would be probed per experiment.…”

Chemistry With Controlled Ions

“…For this purpose, it is highly desirable to develop hyperfine-state preparation techniques for molecular ions [151] as a prerequisite for studies of hyperfine effects in cold reactions. Another intriguing prospect is the application of state-control techniques developed in the realm of quantum logic to molecular ions [152,153] which may ultimately also benefit chemical-reaction studies. So far, most experiments have focussed on small molecular systems, usually diatomics.…”

Sympathetic cooling of molecular ions with ultracold atoms

“…Experiments to probe the quantum coherence of the cold molecular ions are underway [71] and represent the first steps towards using molecular ions in quantum applications. Work is also ongoing to use the MOTion trap to study and simulate important nonequlibrium physics phenomenon.…”