A prototype differential atom interferometer for fundamental physics

Key Points:

• The AION project demonstrated laser phase noise cancellation in a differential atom interferometer using fermionic 87Sr atoms, reaching the standard quantum limit (SQL) even with several radians of synthetic laser phase noise, a key milestone toward long-baseline gravitational wave detection.
• Long-baseline atom interferometers operating in the mid-frequency gravitational wave band (~0.1–10 Hz) can fill the frequency gap between terrestrial (LIGO, Virgo, KAGRA) and space-based (LISA) detectors, enabling observation of intermediate-mass black hole mergers and prolonged inspiral phases of solar-mass mergers for multi-messenger astronomy.
• The experiment used two spatially separated 87Sr atom interferometers interrogated by a common clock laser, exploiting common-mode rejection to suppress laser phase noise that would otherwise overwhelm the gravitational wave signal, verified by a maximum-likelihood analysis extracting differential phase with SQL-limited precision.
• Controlled sinusoidal signals injected as phase modulations were successfully recovered under high laser noise conditions, demonstrating the system's capability to detect coherent oscillatory signals such as those expected from gravitational waves or ultralight dark matter.
• While technical challenges remain—including scaling to longer baselines, increasing atom numbers, and implementing large momentum transfer techniques—the integration of atomic clock technology with atom interferometry opens new avenues for fundamental physics, including gravitational wave detection, dark matter searches, and precision tests of fundamental constants.