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arXiv:2012.03430

Prospects for 10^{-18} Instability Laser Referenced on Thermal Atomic Ensembles

Haosen Shang, Duo Pan, Zhang, Xiaogang, Xiaobo Xue, Tiantian Shi, Jingbiao Chen

Published Dec 7, 2020• 2 citations
arXivHTMLPDF

Abstract

A thermal atomic ensemble-based laser source with superior frequency stability is proposed that relies on the accumulated contributions from an abundance of nonzero-transverse-velocity atomic ensembles. Compared with the traditional case in which only atoms with near-zero transverse velocities are utilized, the amplitude of the optical Ramsey fringes for a thermal calcium beam can be dramatically enhanced by three orders of magnitude or more, thus, the signal-to-noise ratio can be improved 33-fold. Based on the recent results of atomic interferometry-based laser stabilization, a quantum projection noise-limited frequency instability less than 2E-17/tau^0.5 is feasible. Such an ultrastable laser has promising applications in diverse areas, including metrology and astronomy.

DOI
10.48550/arxiv.2012.03430

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A thermal atomic ensemble-based laser source with superior frequency stability is proposed that relies on the accumulated contributions from an abundance of nonzero-transverse-velocity atomic ensembles. Compared with the traditional case in which only atoms with near-zero transverse velocities are utilized, the amplitude of the optical Ramsey fringes for a thermal calcium beam can be dramatically enhanced by three orders of magnitude or more, thus, the signal-to-noise ratio can be improved 33-fold. Based on the recent results of atomic interferometry-based laser stabilization, a quantum projection noise-limited frequency instability less than 2E-17/tau^0.5 is feasible. Such an ultrastable laser has promising applications in diverse areas, including metrology and astronomy.
A laser with superb frequency stability plays an important role in fundamental physics research and advanced technologies. Frequency references are usually utilized to stabilize the laser, such as high-finesse cavities and atomic ensembles, of which thermal atomic beams are attractive because of their balanced performance between simplicity and stability. However, due to the transverse-velocity distribution in the thermal atomic beams, most atoms (generally >99>99%>99) are wasted when interacting with the laser. Here, we propose a laser referenced on a version of velocity-grating Ramsey-Bord\'e atom interferometry with greatly improved atom utilization and thus superior laser frequency stability. Compared with a conventional atomic interferometry applied to stabilize the laser, the proposed system in principle generates optical Ramsey fringes with the amplitude enhanced by 1000-fold or more. Such a configuration has promising applications in diverse areas, including precision measurements and quantum metrology.
This dissertation details the developments of a calcium thermal beam atom interferometer designed to act as an ultra-stable frequency reference over a broad range of timescales. The obtained fractional frequency instabilities of the final apparatus reached ≤ 10−16 after 10's of seconds of averaging without the aid of vibration isolation or significant apparatus temperature stabilization. The vastly superior stability performance of this calcium system compared to other thermal atomic references was facilitated by the enormous atom numbers from the thermal beam, which can be efficiently addressed with Bord\'e-Ramsey spectroscopy on the well-studied (4s2)1S0-(4s4p)3P1 (m=0) clock transition in neutral 40Ca. This method yields interrogation laser frequency dependent interference fringes with a linewidth of 1.6 kHz. This measurement is further enhanced with an electron-shelving detection scheme used for state detection after the interferometry process, causing each excited state atom to emit many hundreds of photons in the detection region. The exceptional instability results from this atom interferometer highlight an underutilized realm of precision measurement science based on relatively simple thermal ensembles. An intuitive understanding of the Bordé-Ramsey process is presented and relevant experimental techniques explained such that this dissertation should prove useful to a broad readership.
We demonstrate a means of increasing the signal-to-noise ratio in a Ramsey-Bordé interferometer with spatially separated oscillatory fields on a thermal atomic beam. The S01↔P13 intercombination line in neutral Ca40 is used as a frequency discriminator, with an extended cavity diode laser at 423 nm probing the ground state population after a Ramsey-Bordé sequence of 657 nm light-field interactions with the atoms. Evaluation of the instability of the Ca frequency reference is carried out by comparison with (i) a hydrogen-maser and (ii) a cryogenic sapphire oscillator. In the latter case the Ca reference exhibits a square-root Λ variance of 9.2×10−14 at 1 s and 2.0×10−14 at 64 s. This is an order-of-magnitude improvement for optical beam frequency references, to our knowledge. The shot noise of the readout fluorescence produces a limiting square-root Λ variance of 7×10−14/√τ, highlighting the potential for improvement. This work demonstrates the feasibility of a portable frequency reference in the optical domain with 10−14 range frequency instability.
Atomic beam optical clocks offer continuous operation, simplified structure, and excellent short- to mid-term stability, making them promising candidates for next-generation optical flywheel clocks. However, the atomic utilization efficiency in current thermal beam optical clocks remains below 1%, limiting their further performance enhancement. Therefore, maximizing the utilization of atoms within the atomic beam has become a critical problem. In this work, we propose an innovative nanosecond Ramsey spectroscopy for Ca beam optical clocks, which enables the full utilization of atoms with all transverse velocities. The spectral amplitude is enhanced 2300 times compared to conventional Ca beam optical clocks, and the frequency stability can reach the quantum projection noise limit of 1.2×10−17/τ1.2\times 1{0}^{-17}/\sqrt{\tau }1.2×10−17/τ​ in theory, comparable to the best cold-atom optical clocks. This approach paves the way for high-precision, transportable flywheel optical clocks.
Laser Frequency Stabilization at ≤10−16\leq 10^{-16}≤10−16 from a Thermal Atomic bBeam
We demonstrate laser frequency stabilization at the 10-16fractional frequency level in one second through spectroscopy on a 650 °C calcium atomic beam. The calcium beam is interrogated with a cavity stabilized 657 nm laser arranged in a Ramsey-Borde configuration. A second laser at 431 nm reads off the final atomic state populations while allowing for high signal-to-noise. Limitations of this method are being explored and, and, based on current noise analysis, there is potential to achieve fractional frequency stabilities in the 10-17range or below.