Let say we would like to perform such experiment: Let we have 2 small/mobile atomic clocks on a hard rod. (AC1)==============================================(AC2) Distance between clocks about 2 meters. There is no any electrical wire connections between these two clocks for synchronization. This is important for this experiment. The clocks themselves must have necessary accuracy and stability during the time of the experiment. Lets say during a few minutes. Both clocks have light emitter (L1 and L2) (AC1)[L1]=========>=========[R]===========<=========[L1](AC2) and there is some receiver R in the middle of the rod. Periodically both clocks sends short light impulses to the receiver. We may adjust frequency of (AC1) so that [R] would see/measure equal frequency of light impulses coming from both sides. Now we may adjust position of the receiver [R] so that both impulses would come simultaneously. So that [R] will not see any phase shift between light impulses. Now lets put some very tiny plate |P| between [L1] and [R] (AC1)[L1]=====|P|====>========[R]===========>=========[L1](AC2) so that it would generate light delay about 10^-13 seconds. I need info about equipment capable to detect such delay. Is it realistic to achieve such accuracy with currently available equipment? What biggest problems may arise here? Thank you in advance for your minds.