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European physicists propose huge underground gravitational-wave laboratory

14 Nov 2019 Michael Banks
Image of the Laboratoire Souterrain à Bas Bruit
Music to their ears: One site option for the European Laboratory for Gravitational and Atom-interferometric Research (ELGAR) is the Laboratoire Souterrain à Bas Bruit in southern France. (Courtesy: CC BY-SA 4.0/GAllegre)

Physicists from across Europe have revealed plans for a huge underground gravitational-wave observatory that, if funded, could be operational by the mid-2030s. The European Laboratory for Gravitational and Atom-interferometric Research (ELGAR) could be located in either France or Italy and would cost around €200m to build. Those involved in the project have now applied for European funding to carry out a detailed design and costing for the facility.

Gravitational waves are ripples in space-time that were predicted over 100 years ago by Albert Einstein. In 2015 the twin Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) in the US along with the Virgo gravitational-wave detector in Italy detected the first gravitational-wave signal and since then tens of such events have been spotted. The observation and pinpointing of such gravitational waves is expected to be boosted in the coming years by the recent completion of Japan’s KAGRA observatory, which is the world’s first underground gravitational-wave observatory to use cryogenic mirrors.

ELGAR is the first large-scale instrument that relies solely on quantum technologies and the only project of research infrastructure in Europe based on matter-wave interferometry

Benjamin Canuel

Rather than detecting gravitational waves by bouncing laser beams off mirrors as carried out by aLIGO, Virgo and KAGRA, ELGAR would instead use atom interferometry. This involves splitting an atom beam – rubidium atoms in ELGAR’s case — in half and allowing both halves to travel for a certain distance before being recombined to look for differences in their paths. A slightly longer path would result from a tiny curvature in space-time that could be caused by a passing gravitational wave.

Atom interferometers tend to be more sensitive at low frequency than their laser counterparts as atomic beams travel more slowly. “The technology for ELGAR is already mature,” says Benjamin Canuel from the Photonics, Numerial and Nanosciences Laboratory (LP2N) at the Institut d’Optique Graduate School in Bordeaux, who is coordinating the ELGAR proposal. “Many technological bricks of the ELGAR detector are now available in lab experiments but an ambitious R&D programme is required to benefit from those techniques in a large research infrastructure.”

Plugging the gap

ELGAR would feature two 32 km long arms that each would contain 80 atom “gradiometers” that are separated by 200 m. The gradiometers would measure the relative difference in the positions of the atoms beams as they pass through.  This set-up would allow researchers to detect gravitational waves in the 0.1–10 Hz frequency range, which would be emitted, for example, by medium-size black-hole binaries. These black holes have masses between 100 and one million solar masses and are elusive but crucial to explain whether supermassive black holes formed from the expansion of small black holes, from the merger of multiple smaller black holes, or possibly from other scenarios.

This frequency range would allow researchers to plug a gap in observations given that ground-based detectors like LIGO cover the frequency range from around 10 Hz to 10 000 Hz while the LISA space-based observatory, would, if launched in the 2030s, study gravitational waves between 0.1 mHz to 0.1 Hz.

Three possible sites that have been picked for ELGAR – the Laboratoire Souterrain à Bas Bruit (LSBB) in southern France and two former mines in the Mediterranean island of Sardinia. The LSBB is currently the location for the €12m Matter–wave laser Interferometric Gravitation Antenna (MIGA) — a demonstrator atom interferometer being built by a consortium of 17 French institutions and featuring a 150 m-long optical cavity. MIGA will carry out precision measurements of gravity as well as applications in geosciences and fundamental physics.

“The choice of ELGAR’s location will be another important goal of the design study that should give a precise methodology for site comparison and characterization, and could also eventually consider other sites in Europe,” Canuel told Physics World.

The ELGAR proposal is similar to one announced earlier this year by physicists in China. Known as the Zhaoshan Long-baseline Atom Interferometer Gravitation Antenna – Gravitational Waves (ZAIGA-GW), their facility, if built, would consist of three 1 km-long tunnels in the shape of an equilateral triangle with each arm being an independent atom interferometer. Costing 1.5 billion yuan, it would aim to detect gravitational waves in the 0.1–10 Hz frequency range and could be later upgraded to 3 or 10 km arms.

The team behind the ELGAR proposal come from six European Union countries and they are now applying for funding to carry out a complete design study for the facility including a full cost analysis. “ELGAR is quite unique in Europe,” adds Canuel. “It is the first large-scale instrument that relies solely on quantum technologies and the only project of research infrastructure in Europe based on matter-wave interferometry.”

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