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The Journal of Chemical Physics

Photon-trap spectroscopy of mass-selected ions in an ion trap: Optical absorption and magneto-optical effects

Akira Terasaki, T. Majima, Tamotsu Kondow

Published Dec 18, 2007• 44 citations
Publisher

Abstract

A novel experimental technique has been developed to observe a trace of optical absorption of free mass-selected ions. The technique combines a linear radio-frequency ion trap with a high-finesse optical cavity to perform cavity ring-down spectroscopy (photon-trap spectroscopy for generality), where the storage lifetime of photons in the cavity provides a sensitivity high enough to probe the trapped ions. Absorption spectra of the manganese ion Mn(+) are presented, showing hyperfine structures for the (7)P(2,3,4)<--(7)S(3) transitions in the ultraviolet range. Implementation of a solenoidal magnet allows us to observe the Zeeman splitting and the Faraday rotation as well.

DOI
10.1063/1.2822022

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A novel experimental technique has been developed to observe a trace of optical absorption of free mass-selected ions. The technique combines a linear radio-frequency ion trap with a high-finesse optical cavity to perform cavity ring-down spectroscopy (photon-trap spectroscopy for generality), where the storage lifetime of photons in the cavity provides a sensitivity high enough to probe the trapped ions. Absorption spectra of the manganese ion Mn(+) are presented, showing hyperfine structures for the (7)P(2,3,4)<--(7)S(3) transitions in the ultraviolet range. Implementation of a solenoidal magnet allows us to observe the Zeeman splitting and the Faraday rotation as well.
This thesis describes experimental work towards developing a trapped ion quantum information processor. An existing ion trap apparatus was capable of trapping and laser-cooling single ions or small ion strings of 40 Ca+, and had been used for studies of quantum jumps and natural lifetime measurements in Ca. This thesis describes improvements in this apparatus, which have allowed the stability and the flexibility of experimental control of the ions to be greatly increased. This enabled experiments to read out the spin state of a single trapped ion, and to load ions with isotope selectivity through photoionization. The optical systems were improved by installation of new lasers, optical reference cavities, and a system of acousto-optic modulators for laser intensity switching and frequency control. The photon counting for fluorescence detection was improved, and a new photon time-of-arrival correlation circuit developed. This has permitted rapid and more sensitive detection of micromotion, and hence cancellation of stray fields in the trap. A study of resonant circuits in the low RF, high voltage (10 MHz, 1 kV) regime was carried out with a view to developing a new RF supply for the Paul trap with reduced noise and increased power. A new supply based on a helical resonator was built and used to trap ions. This technique has reduced noise and will permit higher secular frequencies to be attained in the future. A magnetic field B in the ion trap is used to define a quantization axis, and in one series of experiments was required to be of order 100 G to provide a substantial Zeeman splitting. A set of magnetic field coils to control the size and direction of B is described. The design of these posed some problems owing to an unforseen issue with the vacuum chamber. In short, it is magnetizable and acts to first approximation like a magnetic shield. The field coils had to be sufficiently substantial to produce the desired field at the ion even in the presence of this shielding effect, and dark resonance (and other) spectra with Zeeman splitting were obtained to calibrate the field using the ion as a probe. Finally, the thesis describes the successful loading of the ion trap by laser photoionization from a weak atomic beam. This involved two new lasers at 423 nm and 389 nm. Saturated absorption spectroscopy of neutral calcium is first described, then transverse excitation of an atomic beam in our vacuum chamber is used to identify all the main isotopes of calcium and confirm their abundances in our source (a heated sample of natural calcium). Finally, photoionization is used to load the trap. This has three advantages over electron-impact ionization. By avoiding an electron gun, we avoid charging of insulating patches and subsequent electric field drift as they discharge; the flux in the atomic beam and hence calcium (and other) deposits on the electrodes can be greatly reduced; and most importantly, the photoionization is isotope selective. Evidence is presented which suggests that even with an non-enriched source, the rare isotope 43 Ca can be loaded with reasonable efficiency. This isotope is advantageous for quantum information experiments for several reasons, but chiefly because its ground state hyperfine structure can act as a stable qubit.
We have developed a general method to perform optical pumping by a pulsed laser with the aid of an optical cavity (cavity-assisted optical pumping). Optical pumping is achieved by repetitive interaction of a single laser pulse with a target material in the cavity. This method is demonstrated for manganese ions, Mn+{\text{Mn}}^{+}Mn+, stored in a linear radio-frequency ion trap; about 108{10}^{8}108 ions are spin polarized by a 5-ns laser pulse via ${^{7}P}_{J}\ensuremath{\leftarrow}{^{7}S}_{3}$ (J=3J=3J=3 or 4) transition in the ultraviolet region. The linewidth of the pulsed light source is broad enough to transfer the populations of lower hyperfine levels to the highest one (F=11/2)(F=11/2)(F=11/2) in the 7S3{^{7}S}_{3}7S3​ ground state; the nuclear spin is polarized as well.
We have used stored ion methods to improve resolution and sensitivity in optical spectroscopy. Single atomic ions have been confined by electric and magnetic fields, cooled by laser radiation pressure to temperatures on the order of 1 mK, and probed spectroscopically with narrowband lasers. The absorption resonance of a single Hg + ion has been observed by a decrease in the transmitted light intensity. An ultraviolet transition in Hg + has been observed with a linewidth of only 30 kHz. Quantum jumps to and from metastable levels of Hg + have been observed and used to determine radiative decay rates and to infer the existence of photon antibunching. Quantum jumps have also been observed in single Mg + ions.
Cold trapped ions have many applications in quantum information science and precision metrology. In this thesis, we present progress toward two objectives involving ions confined to linear RF traps: the strong coupling of Ba+ ions with a high finesse optical cavity, and the observation of an optical nuclear transition in 229Th3+. In pursuit of the first objective, a novel high-temperture vapor cell for the spectroscopy of neutral barium was constructed. Using this vapor cell, a new technique for isotope-selective photoionization loading of Ba+ in an ion trap was developed. In pursuit of the second objective, techniques ultimately to be used in creating, trapping, and observing 229Th3+ are studied using 232Th3+. Ion traps are loaded with 232Th3+ via laser ablation of thorium targets. 232Th3+ is detected optically using laser-induced fluorescence and electronically using a channel electron multiplier. A technique for creating ablation targets from trace quantities of thorium nitrate is presented. The primary loss mechanisms of Th3+, charge exchange and chemical reactions, are studied.
The combination of an ion trap with a high finesse optical cavity is an ideal system for the investigation of strong coupling cavity quantum electrodynamics, and allows the observation of a number of interesting quantum phenomena. To achieve the small mode volumes required without impairing the ion trapping small traps with a short ion electrode distance are needed. Two microscopic linear rf ion traps have been developed and built to accommodate experimental cavities of lengths of several 100 microns. The first trap design, the 'sandwich' trap, was successfully used to trap 40Ca+ - ions for several hours. It was characterised extensively including a measurement of the heating rates of the ions in the trap. Spectroscopy measurements of the cooling transition, as well as the two repumping transitions were carried out. The second trap design, the 'alumina' trap, also successfully trapped 40Ca+ - ions, and a full characterisation of this trap was made. The experimental cavity was installed at a preliminary cavity length distance of 3.7 mm. The cavity characteristics were examined. Finally the trapped ions were overlapped with the cavity mode by adjusting the trap minimum position along the trap axis via dc voltages and the vertical position of the cavity. To progress further a locking scheme for the cavity length as well as a single - photon detection setup are necessary. To achieve strong coupling a reduction of the cavity length will have to be made.