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Chinese Physics Letters

A Potassium Atom Four-Level Active Optical Clock Scheme

Shengnan Zhang, Yanfei Wang, Tong-Gang Zhang, Wei Zhuang, Jingbiao Chen

Published Apr 1, 2013• 21 citations
Publisher

Abstract

We present an active optical clock scheme with a four-level quantum potassium system. We calculate the population probabilities of each state using the density matrix. At the steady state, ρ 33 and ρ 55 are equal to 8.3% and 3.5%, respectively, and the population inversion between the 5S 1/2 and 4P 3/2 states is built up in the thermal potassium cell with a 404.7 nm pumping laser. According to the mechanism of the active optical clock, under the action of the 404.7 nm pumping laser, the scheme can output a 1252.2 nm quantum-limited-linewidth laser, which can be directly used as an active optical frequency standard.

DOI
10.1088/0256-307x/30/4/040601

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We present an active optical clock scheme with a four-level quantum potassium system. We calculate the population probabilities of each state using the density matrix. At the steady state, ρ 33 and ρ 55 are equal to 8.3% and 3.5%, respectively, and the population inversion between the 5S 1/2 and 4P 3/2 states is built up in the thermal potassium cell with a 404.7 nm pumping laser. According to the mechanism of the active optical clock, under the action of the 404.7 nm pumping laser, the scheme can output a 1252.2 nm quantum-limited-linewidth laser, which can be directly used as an active optical frequency standard.
Since the concept of active optical clock has been proposed, the experimental research of Rb four-level active optical clock has never been carried out, but it has great value in research and application. First the Rb atoms are pumped from ground state 5S1/2to 6P3/2excited state by a narrow-linewidth 420 nm laser, and then decay to lower energy levels by spontaneous transitions, and immediately are pumped by pumping laser again. This process continues until the population inversion is established between 6 S and 5P states. Once the population inversion is built up, a Fabry-Perot type resonator, at a bad cavity regime whose cavity mode linewidth is much wider than that of the laser gain profile, is used to realize 1367 nm active optical clock laser output. As Rb atom 1367 nm 5P-6S excited state transition is in the communication E band, it can be used as a narrow-linewidth communication band optical frequency standard, which greatly improves the communication rate and accuracy.
An active optical clock can provide laser light source with potential much narrower linewidth than that of conventional cavity stabilized laser for optical clocks since many cavity noises, including the Brownian thermal noise, can be reduced by orders of magnitude. In this report, a scheme of active optical clock based on Sr atomic beam is proposed. The experimental work of population inversion has been done. A 689 nm laser is used to realize the population inversion between (5s5p)3P1state and (5s2)1S0state by the method of adiabatic rapid passage. A pumping rate of 59% is obtained. In order to establish the population inversion more effectively, an experimental scheme is proposed with a 483 nm pumping laser. A 687 nm laser is used for verification of pumping effectiveness. The atoms left in ground state after 689 nm laser and 687 nm laser pumping is about 7%, which may mainly result from87Sr isotope.
Two feasible schemes are proposed by laser cooling of alkali atoms to realize the narrow-linewidth, continual superradiant lasing. Utilizing the D1 gray molasses to realize a cloud of cold Cs atoms for the two-level Faraday active optical clock (AOC), the effect of Doppler broadening on the transmission bandwidth of Faraday atomic filter will be effectively suppressed to the level of natural linewidth. Thus, the influence of mechanical or thermal vibrations of cavity mirrors on the clock laser will be decreased by about 102 times, which is expected to further narrow the laser linewidth by two orders of magnitude compared with the thermal atomic Faraday AOC to Hz level. Moreover, the four-level AOC based on the narrow linewidth cooling of Cs atoms using the 6S-7P transition is proposed. The 455 nm laser at 6S1/2-7P3/2transition plays two roles simultaneously, one is the cooling laser for a magneto-optical trap (MOT), another is the pumping laser for population inversion of 1470 nm clock transition, which is helpful to improve the compactness of the system. In the four-level structure, the frequency of pumping laser is far detuning from that of clock laser, therefore, the light shift resulting from the 455 nm laser is negligible. This four-level AOC based on cold atoms will further reduce the collision shift in thermal atom system to achieve a high-precision optical frequency reference.
After being pumped from the 5 s 1/2 ground state to the 6 p 1/2 state, the population inversion between 6 s 1/2 and 5 p 1/2,3/2 can be established for a rubidium four-level active optical clock. We calculate the ac Stark shift due to lattice trapping laser which dominates the frequency shift of clock transition in a lattice trapped rubidium four-level active optical clock. Several magic wavelengths are found, which can form desired optical lattice trapping potential. By choosing a proper intensity and linewidth of the trapping laser, the fractional frequency uncertainty of clock transition due to the ac Stark shift of the trapping laser, is estimated to be below 10 −18 .
Active optical clocks, a new class of optical atomic clocks, work through stimulated emission lasing, a unique mechanism that is distinct from laser absorption spectroscopy of conventional optical clocks. With coherent weak optical feedback from a Fabry-Perot resonator operating in the bad-cavity regime, active optical clocks sustain a lasing oscillation at an atomic transition frequency. They are used directly as quantum optical frequency standards with expected stability better by about two orders of magnitude than that of the best clocks. This review starts with a simplified account of historical developments and the limitations of critical techniques in optical atomic clocks. After an introduction of the basic mechanism underlying active optical clocks, we discuss the most recent experimental results for several configurations and experimental set-ups of active optical clocks. These include those based on a 2-level atomic beam and 3-level laser-cooled trapped-atom system, those based on a 4-level atom scheme, and the Faraday active optical clock. Moreover, following the successful lasing and preliminary results on a Faraday active optical frequency standard with Faraday atomic filter using thermal Cs cell, we describe an experimental scheme of a Faraday active optical clock with a Faraday atomic filter using magic-lattice-trapped Sr atoms at the 689-nm transition, and Faraday optical clocks operating in the good-cavity regime. We conclude with a comprehensive comparison of the different active optical clocks, prospects for active Faraday optical clocks, and potential applications of active optical clocks.