Quantum Saturation of the Electro‐Optic Effect

Future quantum computing architectures require electro‐optic materials that maintain a strong, stable performance at cryogenic temperatures. In conventional electro‐optic materials, large electro‐optic coefficients are often confined to narrow temperature windows near structural phase transitions, where small changes in temperature lead to large changes in the electro‐optic response. Using thermodynamic analysis, phase‐field simulations, experimental growth, and cryogenic optical measurements, we show that quantum fluctuations can be harnessed to overcome this trade‐off. By tuning the ferroelectric phase boundaries down to 0 K, quantum fluctuations induce a saturation regime in which a large electro‐optic response becomes nearly temperature‐independent below 25 K. We demonstrate that the phase boundaries can be tuned through either strain in BaTiO 3 or through chemical composition in Ba 1‐ x Ca x TiO 3 , leading to a large, temperature‐insensitive, cryogenic electro‐optic response, r eff = 2850 pm V −1 (experiment) and r 51 = 2900 pm V −1 (theory) for BaTiO 3 on GdScO 3 and r 51 = 1898 pm V −1 (theory) for Ba 0.77 Ca 0.23 TiO 3 . These values exceed the room temperature performance of bulk BaTiO 3 r 51 = 1600 pm V −1 and surpass the cryogenic performance of BaTiO 3 ‐on‐Si, r eff = 200 pm V −1 , by over an order of magnitude. These findings establish a general design principle for engineering high‐performance electro‐optic materials for cryogenic applications.