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...

The quantum noise of the vacuum limits the achievable sensitivity of quantum sensors. In non-classical measurement schemes the noise can be reduced to overcome this limitation. However, schemes based on squeezed or Schrödinger cat states require alignment of the relative phase between the measured interaction and the non-classical quantum state. Here we present two measurement schemes on a trapped ion prepared in a motional Fock state for displacement and frequency metrology that are insensitive to this phase. The achieved statistical uncertainty is below the standard quantum limit set by quantum vacuum fluctuations, enabling applications in spectroscopy and mass measurements.

A cryogenic radio-frequency ion trap system designed for quantum logic spectroscopy of highly charged ions is presented. It includes a segmented linear Paul trap, an in-vacuum imaging lens and a helical resonator. We demonstrate ground state cooling of all three modes of motion of a single 9 Be + ion and determine their heating rates as well as excess axial micromotion. The trap shows one of the lowest levels of electric field noise published to date. We investigate the magnetic-field noise suppression in cryogenic shields made from segmented copper, the resulting magnetic field stability at the ion position and the resulting coherence time. Using this trap in conjunction with an electron beam ion trap and a deceleration beamline, we have been able to trap single highly charged Ar 13+ (Ar XIV) ions concurrently with single Be + ions, a key prerequisite for the first quantum logic spectroscopy of a highly charged ion.

We measured the isotope shift in the 2 S1 /2 → 2 P3 /2 (D2) transition in singly-ionized calcium ions using photon recoil spectroscopy. The high accuracy of the technique enables us to resolve the difference between the isotope shifts of this transition to the previously measured isotopic shifts of the 2 S1 /2 → 2 P1 /2 (D1) line. This so-called splitting isotope shift is extracted and exhibits a clear signature of field shift contributions. From the data we were able to extract the small difference of the field shift coefficient and mass shifts between the two transitions with high accuracy. This J-dependence is of relativistic origin and can be used to benchmark atomic structure calculations. As a first step, we use several ab initio atomic structure calculation methods to provide more accurate values for the field shift constants and their ratio. Remarkably, the high-accuracy value for the ratio of the field shift constants extracted from the experimental data is larger than all available theoretical predictions.

Efficient preparation and detection of the motional state of trapped ions is important in many experiments ranging from quantum computation to precision spectroscopy. We investigate the stimulated Raman adiabatic passage (STIRAP) technique for the manipulation of motional states in a trapped ion system. The presented technique uses a Raman coupling between two hyperfine ground states in 25 Mg + , implemented with delayed pulses, which removes a single phonon independent of the initial motional state. We show that for a thermal probability distribution of motional states the STIRAP population transfer is more efficient than a stimulated Raman Rabi pulse on a motional sideband. In contrast to previous implementations, a large detuning of more than 200 times the natural linewidth of the transition is used. This approach renders STIRAP suitable for atoms in which resonant laser fields would populate nearby fluorescing excited states and thus impede the STIRAP process. We use the technique to measure the wavefunction overlap of excited motional states with the motional ground state. This is an important application for force sensing applications using trapped ions, such as photon recoil spectroscopy, in which the signal is proportional to the depletion of motional ground state population. Furthermore, a determination of the ground state population enables a simple measurement of the ionʼs temperature.OPEN ACCESS RECEIVED has a Gaussian shape, whereas the frequency is varied linearly in time across an atomic resonance. RAP has been used in optical qubits on carrier transitions for robust internal state preparation [27][28][29][30], and on sideband transitions, to prepare Fock [31] and Dicke [32,33] states. The STIRAP technique is typically realized in Λ-systems and relies on an adiabatic evolution from an initial to a final state without populating a short-lived intermediate state. It is usually implemented using Gaussian-shaped intensity profiles of two laser pulses with a fixed frequency difference that are delayed with respect to each other in time. It has been demonstrated for population transfer [34] and the generation of Dicke states [35], and suggested for efficient qubit detection of single ions [36] and Doppler-free efficient state transfer in multi-ion crystals [37].Here we demonstrate STIRAP between hyperfine qubit states in 25 Mg + involving a change in the motional state. The coupling strength of such sideband transitions is strongly dependent on the initial motional state of the ion [38]. We used the insensitivity of STIRAP to the coupling strength to perform a complete population transfer of motionally excited states to determine the motional ground state population. For thermal states the ground state population is a direct measure for the temperature [39]. Using this approach, good agreement with the expected Doppler cooling temperature is found.We implement STIRAP using a large detuning, which is in contrast to the near resonant STIRAP transfer typically discussed in the literature [25,40]. In this si...