An Atom Interferometer Observatory and Network (AION)
for the exploration of Ultra-Light Dark Matter and Mid-Frequency Gravitational Waves

AION logo

What is AION

The project proposes to construct and operate a next-generation Atomic Interferometric Observatory and Network (AION) in the UK that will enable the exploration of properties of dark matter as well as searches for new fundamental interactions. In addition, it will provide a pathway towards detecting gravitational waves from the very early Universe and astrophysical sources in the mid-frequency band ranging from several mHz to a few Hz, which is mostly unexplored as yet.

Video simulation of the view which would be seen by a close observer, of the final merger of GW150914, showing the distortion of the star-field from gravity as the black holes orbit and merge. Attribution: Simulating eXtreme Spacetimes (SXS) Project.

Introduction to AION

We outline a staged plan to build a set of atom interferometers with progressive baselines up to a minimum of 10 m, which will pave the way for 100 m and eventually km-scale detectors in the future. The proposed quantum sensors are based on the superposition of atomic states and designs are taking advantage of features used by the world 's best atomic clocks in combination with established techniques for building inertial sensors. This programme will make the UK a leader in the exploitation of the enormous physics potential of this frequency band with several options for ground-breaking discoveries. It will also develop the foundation for future science with ultra-sensitive quantum sensors and lay the foundations for a new and potentially highly disruptive class of applications of precision measurement in surveying and prospecting.

The full AION programme consists of 4 stages. The first develops existing technology and the infrastructure for the 100 m detector, and produces detailed plans and assessment of performance before moving to Stage 2. The second stage, which is not part of this funding request, builds, commissions and exploits the 100 m detector and prepares a design study for the km-scale. The final two stages, full-scale terrestrial (kilometre-scale – Stage 3) and satellite-based (thousands of kilometres scale – Stage 4) detectors are the objectives of the continuing programme.

The science case has broad applications to fundamental physics and aligns well with the highest priorities in the UK and international science communities. It has unparalleled sensitivity to the physics of space-time and its distortion between the sensors. The detector is highly sensitive to time-varying signals that could be caused by ultra-light particles. The discovery of such particles and their associated fields would both reveal the nature of the, as yet undiscovered, dark matter and blueprint a novel method to probe the associated theoretical frameworks. There are several such candidate particles including string axions, relaxions and moduli that are able to produce a signal in the frequency range 100 nHz – 10 Hz [2]. In the case of dark-sector physics, like the search for ultra-light dark matter candidates or other hidden dark-sector fields, experiments in this frequency range have the potential to explore a large mass region (10-13 – 10-23 eV) with unprecedented sensitivity. These hidden sector particles could play a crucial role in particle physics beyond the Standard Model, astrophysics, and cosmology.

The detector will also be sensitive to other new fundamental interactions. This sensitivity arises because these interactions, mediated by new light particles, will affect the quantum sensors differently from any backgrounds expected from the Standard Model and gravity. Gravitational waves will induce time-varying signals in the sensors [4] opening a new window on the cosmos that lies between the Advanced LIGO and LISA experiments. It is expected that the gravitational wave spectrum from 0.1 Hz - 10 Hz will be mostly free of continuum foreground noise from astrophysical sources (such as white dwarf confusion noise). AION connects the fundamental research areas of particle physics and gravitational wave physics. It will enable studies of many of the fundamental puzzles of our Universe, like dark matter as well as processes occurring in the early Universe such as potential gravitational wave signals from inflationary fields, as predicted in Higgs cosmology, for example.

The AION science programme is summarized as follows:

This third theme offers long-term, high scientific value outputs and is a cross-over between traditional particle physics, astrophysics and the physics of the early Universe and opens a new dimension to multi-messenger observations. In particular, with the eventual construction of the km‑scale atom interferometer detector, new gravitational wave sources will become observable. AION will establish the large-scale interferometric infrastructure and techniques in the UK and provide UKRI with the ability to complement the observational breadth of LISA, Advanced LIGO and the proposed Einstein Telescope, crucially allowing complete coverage of the frequency spectrum. The 100 m baseline atom interferometer would, on its own, be sensitive to high (O(103)) solar mass events. At the 1 km scale, sources such as neutron star binaries or black hole mergers will be observed in the mid-frequency band, and then may be observed later by LIGO after the merger has passed to higher frequencies. Such joint observations will be a powerful new source of information, giving a prediction of the time and location of a merger event in LIGO – potentially months before it occurs. An example of the sensitivities that future atom interferometer detectors could reach, both land- and space-based devices, is shown in Figure 1. It is based on the design study of the MAGIS Collaboration [see below] and shows the complementarity of the different approaches to cover the gravitational wave spectrum from 103 Hz to 10-3 Hz.

From the outset this project will benefit strongly from close collaboration on an international level with the US initiative, MAGIS-100 [6], which pursues a similar goal of an eventual km-scale atom interferometer. MAGIS-100 was initiated in 2017 by physicists from Stanford, UC Berkley, NIU, FNAL, the University of Liverpool and the DoE Office of HEP. We would like to emphasize that collaboration with AION by the MAGIS experiment has already been endorsed by the community at Stanford and Fermilab, presenting the UK with an immediate window of scientific opportunity. In addition to being a vital ingredient of our short and mid-term objectives, the UK-US collaboration will serve as the test bed for full-scale terrestrial (kilometre-scale) and satellite-based (thousands of kilometres scale) detectors and build the framework for global scientific leadership in this area. The AION programme would reach its ultimate sensitivity by operating two detectors simultaneously, one in the UK and one in the US, thereby enabling unique physics opportunities not accessible to either detector alone.

The AION Core Team

The AION core team currently consists of 21 members from eight different UK institutions.

The AION project core team members

The team is charged with assembling a complete funding proposal and the effort is organised as a Work Package 3 under the Quantum Sensor for Fundamental Physics consortium. AION was also selected as priority project in the context of the “Big Ideas” call from STFC in 2018.

The AION Core Team meets regularly and meeting coordinates and agendas can be accessed here.

In addition to the AION core team, there are several other members of the UK science community interested in this project. In First AION Workshop more than 60 participants from various different UK institutions attended this event.

The AION Physics Case in a Nutshell

The proposed programme has broad applications to fundamental physics and aligns very well the highest priorities in the UK and international science communities. It has exquisite sensitivity to the physics of space-time and its distortion between the sensors. The detector is highly sensitive to time-varying signals that can be caused by ultra-light particles. The discovery of such particles and their associated fields would both reveal the nature of the, as yet undiscovered, dark matter and blueprint a novel method to probe the associated theoretical frameworks. Furthermore, gravitational waves will induce time-varying signals in the sensor opening a new window on the cosmos that lies between Advanced LIGO and the LISA experiments with exciting opportunities for precision studies of various gravitational wave sources as well as totally new discoveries.

The AION project connects the fundamental research areas of particle physics and gravitational wave physics and it will enable studies of many of the fundamental puzzles of our universe, like dark matter as well as processes occurring in the early universe such as potential gravitational wave signals from inflationary fields as, for example, predicted in Higgs cosmology.

The AION Physics programme is summarized as follows:
  1. to build the instrument to explore these well motivated ultra-light dark matter candidates several orders of magnitude beyond current bounds;
  2. to open the door to a new generation of generic precision searches for new particles and their associated fields complementing those performed at collider facilities;
  3. to explore mid-frequency band gravitational waves from astrophysical sources and from the very early universe.

This third theme offers long-term, high scientific value outputs and is a cross-over between traditional particle physics, astrophysics and the physics of the early universe and opens a new dimension to multi-messenger observations. AION will take a technology developed for ultra-cold atomic physics and particle physics and deploy it for wider scientific goals. In particular, with an eventual construction of the km-scale Atom Interferometer detector, new gravitational wave sources will become observable. AION will establish the large-scale interferometric infrastructure and techniques in the UK and provide UKRI with the ability to complement the observational breadth of LISA, Advanced LIGO and the proposed Einstein Telescope, allowing complete coverage of the frequency spectrum. The 100m baseline atom interferometer would, on its own, be sensitive to high (~103) solar mass events. At the 1km scale, sources such as neutron star binaries or black hole mergers will be observed in the mid-frequency band, and then may be observed later by Advanced LIGO after the merger has passed to higher frequencies. Such joint observations will be a powerful new source of information, giving a prediction of the time and location of a merger event in Advanced LIGO – potentially months before it occurs.

In the following we show a few sensitivity examples for AION physics exploration. Further details on the AION physics case will be made available when the funding proposal is finalised:

Dark Matter

Plot of sensitivity of AION to scalar DM interactions
Figure 1: The sensitivity of AION to scalar DM interactions with electrons (left), photons (middle) and the Higgs portal (right).

Gravitational Waves

topics discussed below.
Comparison between AION and other experiments
Figure 2: Comparison of the strain measurements possible with AION and other experiments, showing their sensitivities to BH mergers of differing total masses at various redshifts. Also shown is the possible gravitational gradient noise (GGN) level for a 1-km detector.