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arXiv:quant-ph/0602044 · Physical Review A

Electrodynamically trapped Yb+ ions for quantum information processing

Chr. Balzer, A. Braun, Th. Hannemann, Chr. Paape, M. Ettler, Werner Neuhauser, C. Wunderlich

Published Apr 28, 2006• 76 citations
arXivHTMLPDF

Abstract

Highly efficient, nearly deterministic, and isotope selective generation of ${\mathrm{Yb}}^{+}$ ions by one- and two-color photoionization is demonstrated. State preparation and state selective detection of hyperfine states in $^{171}\mathrm{Yb}^{+}$ is investigated in order to optimize the purity of the prepared state and to time-optimize the detection process. Linear laser-cooled ${\mathrm{Yb}}^{+}$ ion crystals confined in a Paul trap are demonstrated. Advantageous features of different previous ion trap experiments are combined, while at the same time the number of possible error sources is reduced by using a comparatively simple experimental apparatus. This opens a new path toward quantum state manipulation of individual trapped ions, and in particular, to scalable quantum computing.

DOI
10.1103/physreva.73.041407

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Highly efficient, nearly deterministic, and isotope selective generation of Yb+{\mathrm{Yb}}^{+}Yb+ ions by one- and two-color photoionization is demonstrated. State preparation and state selective detection of hyperfine states in 171Yb+^{171}\mathrm{Yb}^{+}171Yb+ is investigated in order to optimize the purity of the prepared state and to time-optimize the detection process. Linear laser-cooled Yb+{\mathrm{Yb}}^{+}Yb+ ion crystals confined in a Paul trap are demonstrated. Advantageous features of different previous ion trap experiments are combined, while at the same time the number of possible error sources is reduced by using a comparatively simple experimental apparatus. This opens a new path toward quantum state manipulation of individual trapped ions, and in particular, to scalable quantum computing.
Trapped ions are currently one of the most promising architectures for realising the quantum information processor. The long lived internal states are ideal for representing qubit states and, through controlled interactions with electromagnetic radiation, ions can be manipulated to execute coherent logic operations. In this thesis an experiment capable of trapping Yb+ ions, including 171Yb+, is presented. Since ion energy can limit the coherence of qubit manipulations, characterisation of an ion trap heating rate is vital. Using a trapped 174Yb+ ion a heating rate consistent with previous measurements of other ion species in similar ion traps is obtained. This result shows abnormal heating of Yb+ does not occur, further solidifying the suitability of this species for quantum information processing. Efficient creation, and cooling of trapped ions requires exact wavelengths for the ionising, cooling and repump transitions. A simple technique to measure the 1S0 ? 1P1 transition wavelengths, required for isotope selective photoionisation of neutral Yb, is developed. Using the technique new wavelengths, accurate to 60 MHz, are obtained and differ from previously published results by 660 MHz. Through a simple modification the technique can also predict Doppler shifted transition frequencies, which may be required in non-perpendicular atom-laser interactions. Using trapped ions, the 2S1=2 ? 2P1/2 Doppler cooling and 2D3/2 ? 2D[3/2]1/2 repump transitions are also measured to a greater accuracy than previously reported. Many experiments require wavelengths which can only be obtained using complex expensive laser systems. To remedy this a simple cost effective laser is developed to enable laser diodes to be operated at sub zero temperatures, extending the range of obtainable wavelengths. Additional diode modulation capabilities allow for the manipulation of atoms and ions with hyperfine structures. The laser is shown to be suitable for manipulating Yb+ ions by cooling a diode from 372 nm to 369 nm and simultaneously generating 2.1 GHz frequency sidebands. Coherent manipulation such as arbitrary qubit rotations, motional coupling and ground state cooling, are required for trapped ion quantum computing. Two photon stimulated Raman transitions are identified as a suitable technique to implement all of these requirements and an investigation into implementing this technique with 171Yb+ is conducted. The possibility of exciting a Raman transition via either a dipole or quadrupole transitions in 171Yb+ is analysed, with dipole transitions preferred because quadrupole transitions are found to be too demanding experimentally. An inexpensive setup, utilising a dipole transition, is designed and tested. Although currently limited the setup shows potential to be an inexpensive, high fidelity method of exciting a Raman transition.
We demonstrate the use of trapped ytterbium ions as quantum bits for quantum information processing. We implement fast, efficient state preparation and state detection of the first-order magnetic field-insensitive hyperfine levels of 171Yb+^{171}\mathrm{Yb}^{+}171Yb+, with a measured coherence time of 2.5s2.5\phantom{\rule{0.3em}{0ex}}\mathrm{s}2.5s. The high efficiency and high fidelity of these operations is accomplished through the stabilization and frequency modulation of relevant laser sources.
Trapped ions in linear Paul traps are largely isolated from interaction with the environment. This property of trapped ions make them a primary choice for quantum technology. \nOver the last decade, trapped atomic ions in linear radio frequency Paul traps have shown to be an important tool to implement quantum algorithms. The scalability of linear ion \ntraps is required to handle large numbers of qubits, in order to implement useful quantum computation. Advance micro-fabrication technology allows the realisation of scalable ion traps. Further developments of micro-trap designs, for the purpose of scalable quantum technology, requires inter-disciplinary investigations of ion traps. Micro-scale ion trap designs typically require a versatile experimental setup. The first part of this thesis describes such an experimental setup including a chip bracket that can host macroscopic ion traps as well as advanced symmetric and asymmetric ion trap chips with up to 90 control electrodes. The system provides versatile optical access for both type of traps and the vacuum chamber is designed in a way so the ion traps can be replaced within a short amount of time. To test the working of the setup, a macroscopic ion trap with an ion-electrode distance of 310±10 μm is used to trap ytterbium ions (Yb+). The trap is characterised by measuring the heating rate, (n•), and spectral noise density SE(ω). A \nphotoionisation technique is used to ionise the different isotopes of Yb in our trap. Isotope selective photoionisation requires exact measurements of 1So↔1P1 transition for the different Yb isotopes. A technique to measure these resonant frequencies is described. \nThis technique works by observing and aligning fluorescence spots and by using this technique, the 1So↔1P1 transition frequencies for stable isotopes of Yb were measured with an accuracy of 60 MHz. These new measured transition frequencies for stable Yb isotopes differ from previously published work by 660 MHz. Furthermore, this technique can also be used to obtain the transition frequencies at different laser-atomic beam angles, typical for non-perpendicular laser-atomic beam angles. The second part of this thesis discusses the optimisation of surface trap geometries as they are being used to implement scalable ion trap designs which consist of a large number of trapping zones. The trap depth in surface traps is low compared to symmetric traps of similar dimensions. How to optimise the trap geometry to achieve maximum trap depth for a given ion height above the trap electrodes, is discussed. Fast and adiabatic ion shuttling operations in one dimension as well as ion separation and recombination processes are important for many quantum information implementations. The maximum speed of separation of trapped ions for adiabatic shuttling operations depends on the secular frequencies, the trapped ion experiences in the process. It will be shown how such ion trap structures have to be designed for fast ion separation process and linear shuttling. Numerical results of adiabatic shuttling operations for trapped ions in such trap structures are also presented.
High-Resolution Spectroscopy of 173^{173}173Yb+^{+}+ Ions
Compared to other stable isotopes of Yb+\rm{Yb}^+Yb+, 173Yb+^{173}\rm{Yb}^+173Yb+ has a richer hyperfine structure, which leads to more favorable clock transitions, spectroscopic techniques for probing new physics, and more sophisticated quantum computing architectures. However, to date, its electronic spectrum remains poorly characterized. Here, we report on efficient laser cooling, state preparation, and detection of a single trapped 173Yb+^{173}\rm{Yb}^+173Yb+ ion. The previously unobserved 2 ⁣S1/2→2 ⁣D3/2^2\!S_{1/2} \rightarrow {}^2\!D_{3/2}2S1/2​→2D3/2​ electric quadrupole transition at 436 nm is coherently excited, and the isotope shift between 171Yb+^{171}\rm{Yb}^+171Yb+ and 173Yb+^{173}\rm{Yb}^+173Yb+ on this transition is determined with an uncertainty of 1.4 Hz. Using microwave spectroscopy, we resolve the hyperfine structure (HFS) of the 2 ⁣D3/2{}^2\!D_{3/2}2D3/2​ state with a relative uncertainty below 10−810^{-8}10−8. From the HFS measurement data, we infer for 173{}^{173}173Yb a nuclear magnetic octupole moment Ω=−0.062(8) (b×μN)Ω= -0.062(8)\,({\rm b} \times μ_N)Ω=−0.062(8)(b×μN​) with uncertainty reduced by more than 2 orders of magnitude compared to previous studies. The data also allow us to determine hyperfine anomalies for the 2 ⁣S1/2{}^2\!S_{1/2}2S1/2​ and 2 ⁣D3/2{}^2\!D_{3/2}2D3/2​ states.
Abstract Efficient cooling of trapped ions is one of the key problems in the development of optical clocks and quantum sensors, in quantum computations, and many other fields. We numerically analyze the motion of linear Yb + crystals and optimized trapping potential to achieve a uniform spatial distribution of ions. Using a compact robust laser system we demonstrate Doppler cooling of a 174 Yb + isotope in a quadrupole Paul trap. This is an important step towards laser cooling of 171 Yb + ions, which will be used in single-ion optical clocks and for the realization of high-fidelity quantum gates.
For quantum-information processing (QIP) with trapped ions, the isotope 43Ca+{^{43}\text{C}\text{a}}^{+}43Ca+ offers the combined advantages of a quantum memory with long coherence time, a high-fidelity readout, and the possibility of performing two-qubit gates on a quadrupole transition with a narrow-band laser. Compared to other ions used for quantum computing, 43Ca+{^{43}\text{C}\text{a}}^{+}43Ca+ has a relatively complicated level structure. We discuss how to meet the basic requirements for QIP and demonstrate ground-state cooling, robust state initialization, and efficient readout for the hyperfine qubit with a single 43Ca+{^{43}\text{C}\text{a}}^{+}43Ca+ ion. A microwave field and a Raman light field are used to drive qubit transitions, and the coherence times for both fields are compared. Phase errors due to interferometric instabilities in the Raman field generation do not limit the experiments on a time scale of 100 ms. We find a quantum-information storage time of many seconds for the hyperfine qubit.