[Paper Review] Searching for dark matter with optical atomic clocks

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

Advanced Frequency and Time Standards3 citations

TL;DR

This paper demonstrates a novel method using optical atomic clocks to search for transient dark matter–Standard Model (DM-SM) couplings by measuring common-mode frequency variations relative to a local reference, bypassing the need for long-distance phase-coherent links. The approach sets the tightest laboratory constraint to date on DM-SM coupling strength, even with a one-day measurement, and is insensitive to sensor separation, enabling application to both distant and co-located clocks.

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.

Motivation & Objective

• To develop a practical method for detecting transient dark matter–Standard Model couplings using existing optical atomic clocks.
• To overcome the challenge of phase-noise in long baseline optical fiber links required by previous methods.
• To enable sensitive detection of dark matter signals without requiring absolute frequency stability across distant locations.
• To demonstrate that common-mode frequency variations relative to a local reference are sufficient for detecting transient DM-SM couplings.
• To establish a constraint on DM-SM coupling strength that exceeds previous laboratory and astrophysical limits with minimal experimental overhead.

Proposed method

• The method exploits the common component in the readouts of two or more optical atomic clocks, rather than measuring phase differences between distant clocks.
• It relies on comparing the frequency variation of the optical clock transition to a local frequency reference, such as an optical cavity, instead of tracking absolute frequency drifts.
• By focusing on relative variations with respect to a local reference, the approach eliminates the need for long, phase-noise-compensated optical fiber links between distant clocks.
• The analysis is based on detecting transient variations in the fine-structure constant, which would arise if dark matter couples to the Standard Model via light scalar fields.
• The technique is designed to be compatible with existing optical clock infrastructure, requiring only reprocessing of standard clock readouts.
• The sensitivity is derived from the statistical analysis of common-mode fluctuations in clock signals over time, assuming a stochastic signal model for transient DM interactions.

Experimental results

Research questions

• RQ1 Can optical atomic clocks detect transient dark matter–Standard Model couplings without requiring phase-coherent links between distant sensors?
• RQ2 Is it possible to achieve high sensitivity to DM-SM coupling by measuring relative frequency variations with respect to a local reference instead of absolute frequency stability?
• RQ3 What is the achievable constraint on the strength of transient DM-SM coupling using this method with a one-day measurement?
• RQ4 How does this method's sensitivity scale with measurement time and number of clocks, and does it surpass previous laboratory and astrophysical limits?
• RQ5 Can this approach be applied to both co-located and widely separated optical clocks without loss of sensitivity?

Key findings

• The method achieves a constraint on the strength of transient dark matter–Standard Model coupling that exceeds previous laboratory and astrophysical limits, even with a one-day measurement.
• The sensitivity of the approach is independent of the separation between clocks, enabling its use with both co-located and widely separated optical clocks.
• The technique does not require the recording of absolute frequency variations of the optical clock transition, only relative variations with respect to a local reference.
• The approach simplifies experiments with distant clocks by eliminating the need for Earth-scale, phase-noise-compensated optical fiber links.
• The proof-of-principle demonstrates that existing optical atomic clocks can be directly repurposed for dark matter searches with minimal modifications to data analysis.
• The constraint achieved approaches the ultimate sensitivity expected from a constellation of GPS atomic clocks, indicating high potential for future scalability.