Low-frequency gravitational wave detection via double optical clocks in space

We propose a Doppler tracking system for gravitational wave detection via double optical clocks in space (DOCS). In this configuration two spacecrafts (each containing an optical clock) are launched to space for Doppler shift observations. Compared to the similar attempt of gravitational wave detection in the Cassini mission, the radio signal of DOCS that contains the relative frequency changes avoids completely noise effects due for instance to troposphere, ionosphere, ground-based antenna and transponder. Given the high stabilities of the two optical clocks (Allan deviation  ∼$4.1\times {10}^{-17}$ @ 1000 s), an overall estimated sensitivity of $5\times {10}^{-19}$ could be achieved with an observation time of 2 yr, and would allow to detect gravitational waves in the frequency range from  ∼10⁻⁴ Hz to  ∼10⁻² Hz. (paper)