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Physics-Uspekhi

Methods of quantum logic in ion frequency standards, quantum computers, and modern spectroscopy

Ksenia Yu. Khabarova, Ilia V. Zalivako, N. Kolachevsky

Published Oct 1, 2022• 16 citations
Publisher

Abstract

Today, precise laser control of the quantum states of single ions cooled to low temperatures in traps ensures significant progress in the development of such physical areas as optical and microwave frequency standards, quantum computing, and accurate measurements of transition frequencies to confirm fundamental physical theories.Pioneering ideas about the possibility of using lasers in the development of frequency standards, expressed in the 1960s by the Nobel laureate N G Basov, have enjoyed rapid development: the relative accuracy of frequency standards has reached the 18th decimal place, and the experimentally demonstrated coherence time of narrow optical transitions amounts to tens of seconds.The paper presents a selective review, as well as the results of research at the Lebedev Physical Institute, in the area of using elements of quantum logic in the action of coherent laser pulses on single ions.Also discussed is the use of quantum logic methods in optical clocks based on the Al + ion and the multiply charged Al + ion, and also in quantum computers based on Ca + and Yb + ions.

DOI
10.3367/ufne.2022.10.039270

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Abstract Precision spectroscopy of atomic systems1 is an invaluable tool for the study of fundamental interactions and symmetries2. Recently, highly charged ions have been proposed to enable sensitive tests of physics beyond the standard model2–5 and the realization of high-accuracy atomic clocks3, 5, owing to their high sensitivity to fundamental physics and insensitivity to external perturbations, which result from the high binding energies of their outer electrons. However, the implementation of these ideas has been hindered by the low spectroscopic accuracies (of the order of parts per million) achieved so far6–8. Here we cool trapped, highly charged argon ions to the lowest temperature reported so far, and study them using coherent laser spectroscopy, achieving an increase in precision of eight orders of magnitude. We use quantum logic spectroscopy9, 10 to probe the forbidden optical transition in 40Ar13+ at a wavelength of 441 nanometres and measure its excited-state lifetime and g-factor. Our work unlocks the potential of highly charged ions as ubiquitous atomic systems for use in quantum information processing, as frequency standards and in highly sensitive tests of fundamental physics, such as searches for dark-matter candidates11 or violations of fundamental symmetries2.
Laser-cooled trapped atoms have long been recognized as potentially very accurate frequency standards for clocks. Ultimate accuracies of 10-18 to 10-19 appear possible, limited by the time-dilation of trapped ions that move at laser-cooled velocities. The Al+ ion is an attractive candidate for high accuracy, owing to its narrow electronic transition in the optical regime and low sensitivity to ambient field perturbations. Precision spectroscopy on Al+ is enabled by quantum information techniques. With Al+ “quantum-logic” clocks, the current accuracy of 8.6×10-18 has enabled a geo-potential-difference measurement that detected a height change of 37±17 cm due to the gravitational red-shift. We have also observed quantum coherence between two Al+ ions with a record Q-factor of 3.4×1016, and compared the Al+ resonance frequency to that of a single Hg+ ion to place limits on the temporal variation of the fine-structure constant.
This paper presents an overview of the Al+ optical clock, which is based on quantum logic spectroscopy and provides the most accurate frequency measurement ever reported. Dehmelt’s original design of the optical clock was not realized due to the difficulty in generating the vacuum ultraviolet radiation necessary for laser cooling and the ion’s quantum state detection. The problem was solved by sympathetically cooling the ion with another ion and detecting the quantum state using the ancillary ion. The method, which is called quantum logic spectroscopy, comes from the use of phonon qubit as a bus of the quantum state of the ions in the Cirac-Zoller ion-trap quantum computer. The clock can be simplifi ed by introducing direct excitation by vacuum ultraviolet radiation or another ion species. The clock’s stability will be improved more than ten times by a middle-term stabilized clock laser.
Transitions between discrete energy levels in well-isolated atomic systems are highly reproducible and therefore make ideal references for highly accurate frequency standards or clocks. In particular, a single laser-cooled ion confined in a radiofrequency ion trap closely approximates the spectroscopic ideal of a particle at rest in a perturbation-free environment. Narrow optical transitions in such systems are therefore very promising for the realization of optical clocks with accuracy around 2 orders of magnitude higher than current microwave primary standards. In this paper the fundamental aspects, current performance and applications of trapped ion optical clocks are reviewed and the potential for future improvements is considered.
The results are presented and the future prospects for research are discussed on the practical implementation of the simplest quantum register consisting of two ions trapped and cooled in a linear Paul trap. Physical mechanisms allowing manipulation of systems with high-Q transitions that can be used as new generation optical frequency standards are discussed. Mechanism of a sympathetic cooling for such systems and spectroscopy of highly forbidden “clock” transitions based on quantum logics are presented. Some prospects for the development of a nuclear frequency standard based on optical transition in thorium nucleus allowing increasing frequency uncertainty up to 10-20 ÷ 10-22 are also discussed.
Research progress of Al+^{+}+ ion optical frequency standard based on quantum logic technique
Optical atomic clocks have many applications in the definition of the time and the length, measurement of fundamental constants and their potential variations with time, testing of relativity, very long baseline interferometry, geodesy, and global navigation satellite systems. An optical clock consists of three parts: ultra-stable laser, frequency comb and trapped ions or atoms. The accuracy of optical atomic clocks can be affected by Doppler shift, Zeeman shift, Stark shift, collision shift, blackbody radiation (BBR) shift, etc. The BBR shift is very difficult to be characterized and controlled. Among all optical atomic clocks under current development, Al+^++ optical clock has the smallest BBR shift at a level of 4.0×10−194.0\times10^{-19}4.0×10−19, therefore the Al+^++ is a promising ion for high-accuracy optical frequency standard. In this article we give a review on the three parts of optical frequency standards, the basic working principle of Al+^++ ion optical frequency standard based on quantum logic technique, and research progress of an Al+^++ optical clock under development at Huazhong University of Science and Technology.