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Review of Scientific Instruments

A compact and fast radio-frequency source for efficient Raman sideband cooling

Liren Pang, Zhiyu Ma, Biao Wang, Rui Gong, Songquan Wei, Hongli Liu, W. H. Yuan, Ke Deng, Jie Zhang, Z. H. Lu

Published Dec 1, 2024• 3 citations
Publisher

Abstract

A compact and fast radio-frequency (RF) source developed for Raman sideband cooling (RSBC) in trapped ion and cold atom experiments is presented. The source is based on direct digital synthesizer, advanced real-time infrastructure for quantum physics, and field programmable gate array. The source has a frequency switching speed of 40 ns and can output continuous μs-level time sequences for RSBC. The maximum output frequency of the source is 1.4 GHz. The RF source is capable of pre-writing data for eight channels. As a demonstration, the RF source is applied to our 25Mg+-27Al+ ion pair optical clock experiment. In order to cool down the ion pair to the vibrational ground state, two-order RSBC is applied to the 25Mg+ ion. As a result, the ion pair motions in the three X, Y, and Z directions are all cooled to the vibrational ground state efficiently, demonstrating the feasibility of this technique. The developed RF source can be widely implemented for other cold atom experiments.

DOI
10.1063/5.0235305

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A compact and fast radio-frequency (RF) source developed for Raman sideband cooling (RSBC) in trapped ion and cold atom experiments is presented. The source is based on direct digital synthesizer, advanced real-time infrastructure for quantum physics, and field programmable gate array. The source has a frequency switching speed of 40 ns and can output continuous μs-level time sequences for RSBC. The maximum output frequency of the source is 1.4 GHz. The RF source is capable of pre-writing data for eight channels. As a demonstration, the RF source is applied to our 25Mg+-27Al+ ion pair optical clock experiment. In order to cool down the ion pair to the vibrational ground state, two-order RSBC is applied to the 25Mg+ ion. As a result, the ion pair motions in the three X, Y, and Z directions are all cooled to the vibrational ground state efficiently, demonstrating the feasibility of this technique. The developed RF source can be widely implemented for other cold atom experiments.
A radio-frequency (RF) source designed for cold atom experiments is presented. The source uses AD9858, a direct digital synthesizer, to generate the sine wave directly, up to 400 MHz, with sub-Hz resolution. An amplitude control circuit consisting of wideband variable gain amplifier and high speed digital to analog converter is integrated into the source, capable of 70 dB off isolation and 4 ns on-off keying. A field programmable gate array is used to implement a versatile frequency and amplitude co-sweep logic. Owing to modular design, the RF sources have been used on many cold atom experiments to generate various complicated RF sequences, enriching the operation schemes of cold atoms, which cannot be done by standard RF source instruments.
We present a radio frequency source based on the AD9914 direct digital synthesizer chip, with a frequency range up to 1.4 GHz. The phase noise at an output frequency of 1 GHz is lower than −110 dBc/Hz at a 10 kHz offset. Through an optimized design of a field-programmable gate array, we have achieved the theoretical maximum modulation rate of AD9914 and developed multiple functional modules for cold atom experiments to implement modulation of frequency, phase, and amplitude. The modulation update rate reaches 146 MHz. The phase modulation response latency is 158 ns, and the amplitude modulation response latency is 95.1 ns. We have applied the RF source to a sideband Pound–Drever–Hall locking experiment and achieved frequency stabilization, demonstrating its suitability for cold atom experiments that require precise and complex laser control.
An experimental study of sideband laser cooling of trapped ions in miniature radio-frequency traps is presented. In+{\mathrm{In}}^{+}In+ ions are laser cooled by exciting the narrow intercombination line ${5s}^{2}{}^{1}{S}_{0}\ensuremath{\rightarrow}5s5p{}^{3}{P}_{1}$, which shows resolved sidebands due to the vibrational motion of the ions in the trap. The influence of the micromotion at the frequency of the oscillating trapping field is investigated. Heating processes through coupling with the trapping field occur for certain regions of negative laser detuning. A method of bichromatic sideband cooling is introduced that combines efficient cooling from high temperatures with the possibility of reaching low mean vibrational quantum numbers 〈n〉<1〈n〉<1〈n〉<1 by using narrow optical transitions. This technique allows recording of high-resolution excitation spectra of the cooling transition in the Lamb-Dicke regime and thus determination of 〈n〉〈n〉〈n〉. Single ions as well as two-ion Coulomb crystals have been cooled to temperatures below $100 \ensuremath{\mu}\mathrm{K}$, and population of the ground state of motion to more than 50% over long time scales is demonstrated. These values are close to the lower limit for the detection of vibrational excitation with the present method.
Sideband cooling is a key technique for improving the performance of optical atomic clocks by preparing cold atoms and single ions into the ground vibrational state. In this work, we demonstrate detailed experimental research on pulsed Raman sideband cooling in a 171Yb optical lattice clock. A sequence comprised of interleaved 578 nm cooling pulses resonant on the 1st-order red sideband and 1388 nm repumping pulses is carried out to transfer atoms into the motional ground state. We successfully decrease the axial temperature of atoms in the lattice from 6.5 μK to less than 0.8 μK in the trap depth of 24 μK, corresponding to an average axial motional quantum number 〈 nz 〉 < 0.03. Rabi oscillation spectroscopy is measured to evaluate the effect of sideband cooling on inhomogeneous excitation. The maximum excitation fraction is increased from 0.8 to 0.86, indicating an enhancement in the quantum coherence of the ensemble. Our work will contribute to improving the instability and uncertainty of Yb lattice clocks.
We report a detailed investigation on near-ground state cooling of one and two trapped atomic ions. We introduce a simple sideband cooling method for confined atoms and ions, using RF radiation applied to bare ionic states in a static magnetic field gradient, and demonstrate its application to ions confined at secular trap frequencies, ωz≈2π×117 kHz. For a single 171Yb+ ion, the sideband cooling cycle reduces the average phonon number, n from the Doppler limit to n= 0.30(12). This is in agreement with the theoretically estimated lowest achievable phonon number in this experiment. We extend this method of RF sideband cooling to a system of two 171Yb+ ions, resulting in a phonon number of n= 1.1(7) in the centre-of-mass mode. Furthermore, we demonstrate the first realization of sympathetic RF sideband cooling of an ion crystal consisting of two individually addressable identical isotopes of the same species.
We present a compact field-programmable gate array (FPGA) based pulse sequencer and radio-frequency (RF) generator suitable for experiments with cold trapped ions and atoms. The unit is capable of outputting a pulse sequence with at least 32 transistor-transistor logic (TTL) channels with a timing resolution of 40 ns and contains a built-in 100 MHz frequency counter for counting electrical pulses from a photo-multiplier tube. There are 16 independent direct-digital-synthesizers RF sources with fast (rise-time of ∼60 ns) amplitude switching and sub-mHz frequency tuning from 0 to 800 MHz.