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AIP conference proceedings

Laser-cooled atomic ions as probes of molecular ions

Kenneth R. Brown, C. Ricardo Viteri, Craig Robert Clark, James E. Goeders, Ncamiso B. Khanyile, Grahame Vittorini

Published Jan 1, 2015• 0 citations
Publisher

Abstract

Trapped laser-cooled atomic ions are a new tool for understanding cold molecular ions. The atomic ions not only sympathetically cool the molecular ions to millikelvin temperatures, but the bright atomic ion fluorescence can also serve as a detector of both molecular reactions and molecular spectra. We are working towards the detection of single molecular ion spectra by sympathetic heating spectroscopy. Sympathetic heating spectroscopy uses the coupled motion of two trapped ions to measure the spectra of one ion by observing changes in the fluorescence of the other ion. Sympathetic heating spectroscopy is a generalization of quantum logic spectroscopy, but does not require ions in the motional ground state or coherent control of the ion internal states. We have recently demonstrated this technique using two isotopes of Ca+ [Phys. Rev. A, 81, 043428 (2010)]. Limits of the method and potential applications for molecular spectroscopy are discussed.

DOI
10.1063/1.4906702

Related papers

Sympathetic heating and cooling of trapped atomic and molecular ions

Craig R. Clark

Laser-cooled atomic ions have led to an unprecedented amount of control over the quantum states of matter. The Coulombic interaction allows for information to be transferred between neighboring ions, and this interaction can be used to entangle qubits for logic operations in quantum information processors. The same procedure for logic operations can be used for high resolution atomic spectroscopy, and is the basis for the most accurate atomic optical clocks to date. This thesis describes how laser-cooled atomic ions can impact physical chemistry through the development of molecular ion spectroscopy techniques and the simulation of magnetic systems by ion trap quantum computers. \n\t \n\tA new technique developed for spectroscopy, Sympathetic Heating Spectroscopy (SHS), takes advantage of the Coulombic interaction between two trapped ions: the control ion and a spectroscopy ion. SHS uses the back action of the interrogating laser to map spectroscopy ion information onto the Doppler shift of the control ion for measurement. SHS only requires Doppler cooling of the ions and fluorescence measurement and represents a simplification of quantum logic spectroscopy. This technique is demonstrated on two individual isotopes of calcium: Ca-40(+) for cooling and Ca-44(+) as the spectroscopy ion. \n \n\tHaving demonstrated SHS with atomic ions, the next step was to extend the technique by loading and characterizing molecular ions. The identification of an unknown molecular ion is necessary and can be achieved by monitoring the change in motion of the two ion crystal, which is dependent on the molecular ion mass. The motion of two trapped ions is described by their normal modes, which can be accurately measured by performing resolved sideband spectroscopy of the S(1/2)-D(5/2) transition of calcium. The resolved sidebands can be used to identify unknown ions (atomic and molecular) by calculating the mass based on the observed value in axial normal mode frequencies. Again, the trapped molecular ion is sympathetically cooled via the Coulombic interaction between the Ca-40(+) and the unknown molecular ion. The sensitivity of SHS could be improved by implementing sympathetic sideband cooling and determining the heating by measuring single quanta of motion. \n\t \n\tThe ultimate limit of control would be the development of an ion trap quantum computer. Many theoretical quantum computing researchers have made bold claims of the exponential improvement a quantum computer would have over a classical computer for the simulation of physical systems such as molecules. These claims are true in principle for ideal systems, but given non-ideal components it is necessary to consider the scaling due to error correction. An estimate of the resource requirements, the total number of physical qubits and computational time, required to compute the ground state energy of a 1-D quantum Transverse Ising Model (TIM) of N spin-1/2 particles, as a function of the system size and the numerical precision, is presented. This estimate is based on analyzing the impact of fault-tolerant quantum error correction in the context of the quantum logic array architecture. The results show that a significant amount of error correction is required to implement the TIM problem due to the exponential scaling of the computational time with the desired precision of the energy. Comparison of this result to the resource requirements for a fault-tolerant implementation of Shor's quantum factoring algorithm reveals that the required logical qubit reliability is similar for both the TIM problem and the factoring problem.

•Jan 6, 2012•0 citations

Sympathetic Heating Spectroscopy: Probing Molecular Ions with Laser-Cooled Atomic Ions

Kenneth R. Brown

Sympathetic heating spectroscopy measures atomic and molecular ion spectra by observing the heating of a laser-cooled control ion. Limits of the method are tested using two isotopes of calcium. Applications for molecular ions are discussed.

Frontiers in Optics•Jan 1, 2010•0 citations

Detection of single-ion spectra by Coulomb-crystal heating

Craig Robert Clark, James E. Goeders, Yatis Dodia +2 more

The coupled motion of ions in a radiofrequency trap has been used to connect the frequency-dependent laser-induced heating of a sympathetically cooled spectroscopy ion with changes in the fluorescence of a laser-cooled control ion. This technique, sympathetic heating spectroscopy, is demonstrated using two isotopes of calcium. In the experiment, a few scattered photons from the spectroscopy ion are transformed into a large deviation from the steady-state fluorescence of the control ion. This allows us to detect an optical transition where the number of scattered photons is below our fluorescence detection limit. Possible applications of the technique to molecular ion spectroscopy are briefly discussed.

arXiv:1003.1428•Physical Review A•Apr 29, 2010•31 citations

Resolved Sideband Spectroscopy for the Detection of Weak Optical Transitions

James E. Goeders

This thesis reports on the setup of a new ion trap apparatus designed for experiments with single ⁴⁰Ca⁺ ions to perform molecular spectroscopy. The calcium ion is laser cooled, allowing for sympathetic cooling of the nonfluorescing molecular ion. The aim of these experiments is to explore loading and identifying molecular ions in RF-Paul traps, as well as developing new spectroscopic tools to measure transitions of molecular ions via the fluorescence of co-trapped ⁴⁰Ca⁺ ions. Ground state cooling of a mixed ion pair is implemented as a first step towards increasing the sensitivity of our technique to the level necessary to measure transitions with low scattering rates (like those present in molecular ions). Doppler cooling on the S(1/2)->P(1/2) transition of the calcium ion results in the formation of a Coulomb crystal, the behavior of which may be used to infer properties of the molecular ion. Following cooling, sideband spectroscopy on the narrow S(1/2)->D(5/2) quadrupole transition of calcium may be used to identify the mass of single molecular ions. This method is verified via a non-destructive measurement on ⁴⁰CaH⁺ and ⁴⁰Ca¹⁶O⁺. The normal modes of the Coulomb crystal can also be used to extract information from the target ion to the control ion. By driving the blue side of a transition, laser induced heating can be put into the two ion system, which leads to changes in fluorescence of the ⁴⁰Ca⁺ ion, first demonstrated with two Ca⁺ isotopes. Increasing the sensitivity of this technique requires ground state cooling of both the ⁴⁰Ca⁺ ion and the ion of interest, enabling the transfer of the ion's motional state into the ground state with high probability. This thesis demonstrates ground state cooling of the atomic ion and sympathetic cooling of a second ion (⁴⁴Ca⁺). Once in the ground state, heating of the Coulomb crystal by scattering photons off of the spectroscopy ion can be measured by monitoring the resolved motional sidebands of the S(1/2)->D(5/2) transition of ⁴⁰Ca⁺, allowing for spectral lines to be inferred. Future experiments will investigate this technique with molecular ions.

•Jul 2, 2013•2 citations

Experimental techniques for cold chemistry and molecular spectroscopy in an ion trap

Kevin Sheridan

A range of experimental techniques for application in reaction studies between ionic and neutral atoms/molecules and high resolution spectroscopy experiments with sympathetically cooled molecular ions are presented. A novel ion trap loading scheme using the photo-ionisation of atoms generated by the pulsed laser ablation of a solid calcium target has been characterised. We have identified the range of ablation laser fluences that must be used in order to produce a flux of neutral calcium atoms, which is a prerequisite for isotope selective ion trap loading. Calcium ions are trapped and laser cooled in a linear radio-frequency ion trap. We have developed a spectroscopy scheme that allows the entire fluorescence spectrum of trapped ions to be rapidly collected with high precision while maintaining a low ion temperature and good ion localisation throughout interrogation. The scheme has been demonstrated by measuring the saturation intensity of the calcium ion 4S1/2?4P1/2 transition. We have developed a novel scheme to measure the secular motion of trapped ions and demonstrated the application of the technique to ion-neutral collision reaction experiments. Employing pulsed excitation and Doppler velocimetry, we have measured the centre-of-mass mode frequency of single ions as well as large ion crystals with a frequency precision better than 2x10-3 within an interrogation time on the order of seconds. This method has been used to measure the mass of ions and observe charge exchange collisions between trapped calcium isotopes. In particular, we have measured the 44Ca++40Ca!40Ca++44Ca reaction cross section and demonstrated the single-event resolution of the technique. Finally, we have developed a novel all-optical broadband scheme for exciting, amplifying and measuring the secular motion of ions in the trap. Oscillation induced by optical excitation has been coherently amplified to control and measure the ion's secular motion. Requiring only a single interrogation laser, the ion's oscillation amplitude can be precisely controlled. The application of this technique to non-destructive spectroscopy of trapped molecular ions is discussed.

•Oct 3, 2013•0 citations

Spectroscopy of Molecular Ions in Coulomb Crystals

Aaron T. Calvin, Kenneth R. Brown

In this Perspective, we examine the use of laser-cooled atomic ions and sympathetically cooled molecular ions in Coulomb crystals for molecular spectroscopy. Coulomb crystals are well-isolated environments that provide localization and long storage times for sensitive measurements of weak signals and cold temperatures for precise spectroscopy. Coulomb crystals of molecular and atomic ions enable the detection of single-photon molecular ion transitions at a range of wavelengths by a change in atomic ion fluorescence at visible wavelengths. We give an overview of the state of the art from action spectroscopy to quantum logic spectroscopy for a wide range of molecular transitions from rotational sublevels separated by 10 –7 cm –1 to rovibronic transitions at 25 000 cm –1 . We emphasize how this system allows for unparalleled control of the molecular ion state for precision spectroscopy with applications in astrochemistry and fundamental physics. We conclude with an outlook of the use of this control in cold molecular ion reactions.

The Journal of Physical Chemistry Letters•Sep 13, 2018•19 citations
Trapped laser-cooled atomic ions are a new tool for understanding cold molecular ions. The atomic ions not only sympathetically cool the molecular ions to millikelvin temperatures, but the bright atomic ion fluorescence can also serve as a detector of both molecular reactions and molecular spectra. We are working towards the detection of single molecular ion spectra by sympathetic heating spectroscopy. Sympathetic heating spectroscopy uses the coupled motion of two trapped ions to measure the spectra of one ion by observing changes in the fluorescence of the other ion. Sympathetic heating spectroscopy is a generalization of quantum logic spectroscopy, but does not require ions in the motional ground state or coherent control of the ion internal states. We have recently demonstrated this technique using two isotopes of Ca+ [Phys. Rev. A, 81, 043428 (2010)]. Limits of the method and potential applications for molecular spectroscopy are discussed.
Laser-cooled atomic ions have led to an unprecedented amount of control over the quantum states of matter. The Coulombic interaction allows for information to be transferred between neighboring ions, and this interaction can be used to entangle qubits for logic operations in quantum information processors. The same procedure for logic operations can be used for high resolution atomic spectroscopy, and is the basis for the most accurate atomic optical clocks to date. This thesis describes how laser-cooled atomic ions can impact physical chemistry through the development of molecular ion spectroscopy techniques and the simulation of magnetic systems by ion trap quantum computers. \n\t \n\tA new technique developed for spectroscopy, Sympathetic Heating Spectroscopy (SHS), takes advantage of the Coulombic interaction between two trapped ions: the control ion and a spectroscopy ion. SHS uses the back action of the interrogating laser to map spectroscopy ion information onto the Doppler shift of the control ion for measurement. SHS only requires Doppler cooling of the ions and fluorescence measurement and represents a simplification of quantum logic spectroscopy. This technique is demonstrated on two individual isotopes of calcium: Ca-40(+) for cooling and Ca-44(+) as the spectroscopy ion. \n \n\tHaving demonstrated SHS with atomic ions, the next step was to extend the technique by loading and characterizing molecular ions. The identification of an unknown molecular ion is necessary and can be achieved by monitoring the change in motion of the two ion crystal, which is dependent on the molecular ion mass. The motion of two trapped ions is described by their normal modes, which can be accurately measured by performing resolved sideband spectroscopy of the S(1/2)-D(5/2) transition of calcium. The resolved sidebands can be used to identify unknown ions (atomic and molecular) by calculating the mass based on the observed value in axial normal mode frequencies. Again, the trapped molecular ion is sympathetically cooled via the Coulombic interaction between the Ca-40(+) and the unknown molecular ion. The sensitivity of SHS could be improved by implementing sympathetic sideband cooling and determining the heating by measuring single quanta of motion. \n\t \n\tThe ultimate limit of control would be the development of an ion trap quantum computer. Many theoretical quantum computing researchers have made bold claims of the exponential improvement a quantum computer would have over a classical computer for the simulation of physical systems such as molecules. These claims are true in principle for ideal systems, but given non-ideal components it is necessary to consider the scaling due to error correction. An estimate of the resource requirements, the total number of physical qubits and computational time, required to compute the ground state energy of a 1-D quantum Transverse Ising Model (TIM) of N spin-1/2 particles, as a function of the system size and the numerical precision, is presented. This estimate is based on analyzing the impact of fault-tolerant quantum error correction in the context of the quantum logic array architecture. The results show that a significant amount of error correction is required to implement the TIM problem due to the exponential scaling of the computational time with the desired precision of the energy. Comparison of this result to the resource requirements for a fault-tolerant implementation of Shor's quantum factoring algorithm reveals that the required logical qubit reliability is similar for both the TIM problem and the factoring problem.
The coupled motion of ions in a radiofrequency trap has been used to connect the frequency-dependent laser-induced heating of a sympathetically cooled spectroscopy ion with changes in the fluorescence of a laser-cooled control ion. This technique, sympathetic heating spectroscopy, is demonstrated using two isotopes of calcium. In the experiment, a few scattered photons from the spectroscopy ion are transformed into a large deviation from the steady-state fluorescence of the control ion. This allows us to detect an optical transition where the number of scattered photons is below our fluorescence detection limit. Possible applications of the technique to molecular ion spectroscopy are briefly discussed.
This thesis reports on the setup of a new ion trap apparatus designed for experiments with single ⁴⁰Ca⁺ ions to perform molecular spectroscopy. The calcium ion is laser cooled, allowing for sympathetic cooling of the nonfluorescing molecular ion. The aim of these experiments is to explore loading and identifying molecular ions in RF-Paul traps, as well as developing new spectroscopic tools to measure transitions of molecular ions via the fluorescence of co-trapped ⁴⁰Ca⁺ ions. Ground state cooling of a mixed ion pair is implemented as a first step towards increasing the sensitivity of our technique to the level necessary to measure transitions with low scattering rates (like those present in molecular ions). Doppler cooling on the S(1/2)->P(1/2) transition of the calcium ion results in the formation of a Coulomb crystal, the behavior of which may be used to infer properties of the molecular ion. Following cooling, sideband spectroscopy on the narrow S(1/2)->D(5/2) quadrupole transition of calcium may be used to identify the mass of single molecular ions. This method is verified via a non-destructive measurement on ⁴⁰CaH⁺ and ⁴⁰Ca¹⁶O⁺. The normal modes of the Coulomb crystal can also be used to extract information from the target ion to the control ion. By driving the blue side of a transition, laser induced heating can be put into the two ion system, which leads to changes in fluorescence of the ⁴⁰Ca⁺ ion, first demonstrated with two Ca⁺ isotopes. Increasing the sensitivity of this technique requires ground state cooling of both the ⁴⁰Ca⁺ ion and the ion of interest, enabling the transfer of the ion's motional state into the ground state with high probability. This thesis demonstrates ground state cooling of the atomic ion and sympathetic cooling of a second ion (⁴⁴Ca⁺). Once in the ground state, heating of the Coulomb crystal by scattering photons off of the spectroscopy ion can be measured by monitoring the resolved motional sidebands of the S(1/2)->D(5/2) transition of ⁴⁰Ca⁺, allowing for spectral lines to be inferred. Future experiments will investigate this technique with molecular ions.
A range of experimental techniques for application in reaction studies between ionic and neutral atoms/molecules and high resolution spectroscopy experiments with sympathetically cooled molecular ions are presented. A novel ion trap loading scheme using the photo-ionisation of atoms generated by the pulsed laser ablation of a solid calcium target has been characterised. We have identified the range of ablation laser fluences that must be used in order to produce a flux of neutral calcium atoms, which is a prerequisite for isotope selective ion trap loading. Calcium ions are trapped and laser cooled in a linear radio-frequency ion trap. We have developed a spectroscopy scheme that allows the entire fluorescence spectrum of trapped ions to be rapidly collected with high precision while maintaining a low ion temperature and good ion localisation throughout interrogation. The scheme has been demonstrated by measuring the saturation intensity of the calcium ion 4S1/2?4P1/2 transition. We have developed a novel scheme to measure the secular motion of trapped ions and demonstrated the application of the technique to ion-neutral collision reaction experiments. Employing pulsed excitation and Doppler velocimetry, we have measured the centre-of-mass mode frequency of single ions as well as large ion crystals with a frequency precision better than 2x10-3 within an interrogation time on the order of seconds. This method has been used to measure the mass of ions and observe charge exchange collisions between trapped calcium isotopes. In particular, we have measured the 44Ca++40Ca!40Ca++44Ca reaction cross section and demonstrated the single-event resolution of the technique. Finally, we have developed a novel all-optical broadband scheme for exciting, amplifying and measuring the secular motion of ions in the trap. Oscillation induced by optical excitation has been coherently amplified to control and measure the ion's secular motion. Requiring only a single interrogation laser, the ion's oscillation amplitude can be precisely controlled. The application of this technique to non-destructive spectroscopy of trapped molecular ions is discussed.
In this Perspective, we examine the use of laser-cooled atomic ions and sympathetically cooled molecular ions in Coulomb crystals for molecular spectroscopy. Coulomb crystals are well-isolated environments that provide localization and long storage times for sensitive measurements of weak signals and cold temperatures for precise spectroscopy. Coulomb crystals of molecular and atomic ions enable the detection of single-photon molecular ion transitions at a range of wavelengths by a change in atomic ion fluorescence at visible wavelengths. We give an overview of the state of the art from action spectroscopy to quantum logic spectroscopy for a wide range of molecular transitions from rotational sublevels separated by 10 –7 cm –1 to rovibronic transitions at 25 000 cm –1 . We emphasize how this system allows for unparalleled control of the molecular ion state for precision spectroscopy with applications in astrochemistry and fundamental physics. We conclude with an outlook of the use of this control in cold molecular ion reactions.