Figure 3

Proposed Michelson-type setup for EPR-entanglement generation. Two test mass mirrors are suspended using fibers (small circles), as is the platform that supports all other components. Each test mass is used as the joined end mirror of a Michelson interferometer. This configuration balances the light beams' dc radiation pressure forces, which reduces the transient response when the laser is switched on. The two mirrors are located close to each other, which improves the setup's sensitivity to gravitationally induced state reduction [3, 4, 5, 6, 30]. The dashed light beams inside the mirrors depict light paths that are guided through bore holes. Altogether, there are still just two optical readouts, which can both be operated close to an interferometer dark fringe to reduce the light power on the photodiodes. The mirrors have at least one convex surface, are coated on both sides, and are operated as off-resonant cavities, which reduces the number of coating layers whose thermally excited motion is directly sensed by the light beams [61]. Changing the laser frequency and/or the mirror temperature allows one to change the mirror transmission. Nonzero transmission is helpful to excite a Sagnac mode for aligning the setup [62, 63]. Stabilizing the mirror etalons to off-resonance might be realized by Pound-Drever-Hall locking [59] and a phase modulation frequency at about half the etalons' free spectral range. In general, the mirror etalons will have different lengths, but for reasonable finesse values the off-resonance conditions should always be simultaneously achievable. To increase the optomechanical coupling, power- and signal-recycling mirrors (PRM, SRM) are used to establish cavities [59, 60]. The cavities have rather large bandwidths, such that information about the optomechanical state can still be efficiently detected at the balanced homodyne detectors for entanglement swapping and state conditioning.