Laser fractional frequency instability at mathbf{4times 10⁻¹⁷} with a room temperature optical reference cavity

Ultrastable lasers play a key role in optical frequency metrology, setting measurement speed and ultimately impacting both stability and accuracy of optical frequency standards. Here we demonstrate laser fractional frequency instability at ${4}\times10^{-17}$ and laser frequency linewidth of $12\,$mHz full width at half maximum, employing a 68 cm long optical reference cavity operating at room temperature. To the best of our knowledge, both frequency instability and linewidth are the lowest ever reported for a room temperature system. This work highlights that state-of-the-art frequency stability and spectral purity are achievable at room temperature, making them accessible to a broader range of users.