[论文解读] Searching for dark matter with optical atomic clocks

Piotr Wcisło, Daniel Lisak+5 more|arXiv (Cornell University)|May 18, 2016

Advanced Frequency and Time Standards被引用 3

一句话总结

本文提出一种新颖方法，利用光学原子钟通过测量相对于本地参考的共同模式频率变化，来搜索瞬态暗物质-标准模型（DM-SM）耦合，无需长距离相位相干链路。该方法即使在仅一天的测量时间内，也设定了迄今为止最严格的实验室约束，对DM-SM耦合强度的限制，且对传感器间距不敏感，适用于远距离和共置的时钟。

ABSTRACT

One of the most fundamental questions of modern physics is the existence of yet unknown forms of matter and interactions. The total mass density of the Universe appears to be dominated by some hypothetical dark matter (DM). However, beyond its gravitational interaction at galactic scale, little is known about the DM nature and properties. One possibility is that it has a form of stable topological defects built from light scalar fields which, for nonzero DM-SM coupling, would result in transient variations of fundamental constants. Optical atomic clocks, highly sensitive to variations of the fine-structure constant, seem to be natural candidates for such searches. Here we demonstrate the first experimental constraint on the strength of transient DM-SM coupling determined with optical atomic clocks. Instead of measuring the phase difference between two distant clocks we determine a common component of their readouts. We show that our constraint, even for one-day measurement, greatly exceeds previous laboratory and astrophysical limits and already reaches the ultimate level expected to be achievable with a constellation of GPS atomic clocks. In contrast to the previous proposal the sensitivity of our approach does not depend on the separation between the sensors, hence it may be applied for both distant and non-separated clocks. We demonstrate that searching for transient DM-SM couplings does not require the recording of the absolute frequency variations of the optical clock transition but only its variation with respect to a local frequency reference, like an optical cavity. It considerably simplifies the experiments with distant clocks, since it removes the need of Earth-size-long phase-noise compensated optical fiber links. The proof of principle reported in this Letter gives a simple and practical recipe for interpreting the readouts of existing optical atomic clocks.

研究动机与目标

• 开发一种利用现有光学原子钟检测瞬态暗物质-标准模型耦合的实用方法。
• 克服先前方法所需长基线光纤链路中相位噪声带来的挑战。
• 在无需遥远位置间绝对频率稳定性的前提下，实现对暗物质信号的灵敏探测。
• 证明相对于本地参考的共同模式频率变化足以检测瞬态DM-SM耦合。
• 建立对DM-SM耦合强度的约束，其严格程度超过以往的实验室和天体物理限制，且实验开销极小。

提出的方法

• 该方法利用两个或更多光学原子钟读出信号中的共同分量，而非测量远距离时钟之间的相位差。
• 其依赖于将光学钟跃迁的频率变化与本地频率参考（如光学腔）进行比较，而非追踪绝对频率漂移。
• 通过聚焦于相对于本地参考的相对变化，该方法消除了对远距离时钟之间长距离、相位噪声补偿的光纤链路的需求。
• 该分析基于检测精细结构常数的瞬态变化，若暗物质通过轻量标量场与标准模型耦合，将导致此类变化。
• 该技术设计为与现有光学钟基础设施兼容，仅需对标准时钟读数进行重新处理。
• 灵敏度源自对时钟信号中共同模式涨落的统计分析，假设瞬态DM相互作用为随机信号模型。

实验结果

研究问题

• RQ1 光学原子钟是否能在无需遥远传感器之间相位相干链路的情况下，检测瞬态暗物质-标准模型耦合？
• RQ2 是否可以通过测量相对于本地参考的相对频率变化，而非依赖绝对频率稳定性，实现对DM-SM耦合的高灵敏度探测？
• RQ3 使用该方法在一天测量时间内，可实现对瞬态DM-SM耦合强度的何种约束？
• RQ4 该方法的灵敏度如何随测量时间与时钟数量变化？是否超越了以往的实验室和天体物理限制？
• RQ5 该方法是否可在共置和广泛分离的光学钟上应用而灵敏度不降低？

主要发现

• 该方法实现的瞬态暗物质-标准模型耦合强度约束，即使在仅一天的测量时间内，也超越了以往的实验室和天体物理限制。
• 该方法的灵敏度与时钟之间的距离无关，使其适用于共置和广泛分离的光学钟。
• 该技术无需记录光学钟跃迁的绝对频率变化，仅需相对于本地参考的相对变化。
• 该方法通过消除对地球尺度、相位噪声补偿的光纤链路的需求，简化了远距离时钟实验。
• 原理验证表明，现有光学原子钟可仅通过数据处理的微小修改，直接重新用于暗物质搜索。
• 所实现的约束已接近由GPS原子钟星座所能达到的极限灵敏度，显示出未来高度可扩展的潜力。