Fundamental charge noise in electro-optic photonic integrated circuits

Abstract

Understanding thermodynamical measurement noise is of central importance for electrical and optical precision measurements. These range from semiconductor sensors, in which the Brownian motion of charge carriers poses limits, to optical reference cavities for atomic clocks or gravitational wave detection, which are limited by thermo-refractive and thermo-elastic noise. Here we find that charge-carrier density fluctuations give rise to a noise process in electro-optic photonic integrated circuits. We show that the noise exhibited by lithium niobate and lithium tantalate photonic integrated microresonators feature a frequency scaling to the power of −1.2, deviating from thermo-refractive noise theory. This noise is consistent with thermodynamical charge noise, which leads to electrical field fluctuations that are transduced via the strong Pockels effects of electro-optic materials. Our results establish electrical Johnson–Nyquist noise as the fundamental limitation for electro-optic integrated photonics, crucial for determining performance limits for both classical and quantum devices. Thermal agitation of charge carriers, known as Johnson noise, is the dominant noise in electronic circuits. Now it has also been observed as a key noise source in integrated electro-optic photonic circuits, posing challenges for future applications.