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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Frequency Locking and Power Enhancement by Coherent Addition of a Clock Laser for a Portable Multi-40Ca+ Optical Clock

Akira Nagano, U. Tanaka, Kazuhiro Hayasaka

Published May 15, 2023• 0 citations
Publisher

Abstract

Summary—We construct a frequency locking system for a portable multi-ion optical clock and develop a clock laser whose power is enhanced by the coherent addition method. We reduce the feedback cycle time down to 50 ms, which realizes short-term stabilization with a commercial confocal cavity. To excite the clock transition at 729 nm of multiple ions, we perform coherent addition of 1458-nm optical amplifier outputs and obtain 100 mW at 729 nm by second-harmonic generation.

DOI
10.1109/eftf/ifcs57587.2023.10272102

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Summary—We construct a frequency locking system for a portable multi-ion optical clock and develop a clock laser whose power is enhanced by the coherent addition method. We reduce the feedback cycle time down to 50 ms, which realizes short-term stabilization with a commercial confocal cavity. To excite the clock transition at 729 nm of multiple ions, we perform coherent addition of 1458-nm optical amplifier outputs and obtain 100 mW at 729 nm by second-harmonic generation.
We present a simple, compact, and efficient scheme for integrated multiple wavelength stabilization and continuous operation of a transportable 40Ca+ optical clock using a multi-channel cavity. The fractional frequency instability of 729 nm clock laser is ∼ 1.5 ×10−15 at 10 s with an approximate linewidth of 1 Hz. Meanwhile, frequency fluctuations of all the other lasers are less than ± 330 kHz/day. The one-day stability of this clock is measured as ∼ 5 ×10−17 through 72 h continuous operation. This scheme is potentially useful for the realization of next-generation transportable optical clocks and other metrological systems.
Research on the development of the optical frequency standard based on trapped and cold 40 Ca + with the 4s 2 S 1/2 -3d 2 D 5/2 clock transition at 729 nm is reported. A single calcium ion was trapped and laser cooled in the Paul trap and stay in trap for more than 15 days. The linewidth of a 729 nm laser was reduced to less than 10 Hz by locking to a cavity for longer than 50 hours uninterruptedly. The overall systematic uncertainty of the clock transition has been characterized to be better than 6.5×10 −16 . The absolute frequency of the clock transition was measured at 10 −15 level using an optical frequency comb referenced to a Hydrogen maser, which was calibrated to the SI second through the global positioning system (GPS). The frequency value was 411042129776393.0(1.6) Hz after the correction of the systematic shifts.
We report on the progress toward an indium ion optical frequency standard with an expected accuracy of 10-18. A115In+has been successfully prepared by sympathetic cooling with two laser-cooled40Ca+ions. The clock laser at 237 nm has been constructed as the forth harmonic of an amplified diode laser with a linewidth smaller than 0.9 Hz. After beam shaping and intensity stabilization a power over 10 mW is available for the clock transition (1S0-3P0) interrogation. Observation of the clock transition is in progress by employing the electron shelving method using the1S0-3P1transition at 230 nm.
Future applications of portable 40Ca+ optical clocks require reliable magnetic field stabilization to improve frequency stability, which can be achieved by implementing an active and passive magnetic field noise suppression system. On the one hand, we have optimized the magnetic shielding performance of the portable optical clock by reducing its apertures and optimizing its geometry; on the other hand, we have introduced an active magnetic field noise suppression system to further suppress the magnetic field noise experienced by the ions. These efforts reduced the ambient magnetic field noise by about 10000 times, significantly reduced the linewidth of the clock transition spectrum, improved the stability of the portable 40Ca+ optical clock, and created the conditions for using portable optical clocks in non-laboratory magnetic field environments. This active magnetic field suppression scheme has the advantages of simple installation and wide applicability.
Stabilization of a laser to an optical-clock transition has been demonstrated using Ramsey's phase-modulation technique. The technique was implemented using a 674 nm laser and a component of the 5 s 2S1/2 → 4d 2D5/2 optical clock transition in a single 88Sr+ ion. The lock performance observed was consistent with the short-term stability of the local oscillator. Compared with existing frequency-modulation stabilization schemes, this technique offers advantages for use in single-ion optical clocks through its possible increased resolution and the ability to conveniently compensate known systematic shifts.
As one of the most accurate instruments in history, the optical clock will be used as the measuring equipment for the next generation of seconds. The demand for miniaturization of optical clock is progressively urgent. In this paper, a multi-channel radio frequency module with a 20% volume of the commercial module is designed and implemented for the transportable 40Ca+ ion optical clock. Based on the double crystal oscillator interlocking technique，1 GHz low-phase noise reference source is developed for direct digital synthesis. By simulation and optimization of the signal link design, a frequency range of the low-phase noise RF signal can reach 0-400 MHz with a 4 μHz resolution. Through two-stage power amplified with different kinds of filters, it can get an output power up to +33 dBm (2 W) at 100 MHz with a 25 dBc/Hz phase noise lower than the commercial module at 1 Hz, and its third harmonic suppression ratio has been reduced by more than 20 dBm at the frequency point of 300 MHz. This multi-channel RF module is used for the power stability and timing control test of 729 nm clock laser, to meet the requirements of the transportable 40Ca+ optical clock. Without this, this module can also be applied to other quantum systems such as quantum absolute gravimeter, quantum gyro and quantum computer.