A new design for a broadband detector of gravitational radiation relies on two atom interferometers separated by a distance L. In this scheme, only one arm and one laser are used for operating the two atom interferometers. The innovation here involves the fact that the atoms in the atom interferometers are not only considered as perfect test masses, but also as highly stable clocks. Atomic coherence is intrinsically stable, and can be many orders of magnitude more stable than a laser.
Consider a detector configuration with two ensembles of atoms separated by a distance L, in which only a single laser beam is used to operate them. The laser interrogates the atoms similarly to how a local oscillator laser interacts with atoms in an optical clock. The results give the phase differences between the laser and the highly coherent atomic internal oscillations. As the laser phase fluctuations enter into the responses of the two phase difference measurements at times separated by the one-way-light-time, L (units in which the speed of light c = 1), it can be shown that the laser phase fluctuations can be exactly cancelled (while retaining the gravitational wave signal) by applying time-delay interferometry (TDI) to the phase measurement data.
The fundamental limitation of a one-arm Doppler measurement configuration (such as that of interplanetary spacecraft tracking experiments) is determined by the frequency stability of the “clock” that defines the frequency of the electromagnetic link. The most stable clocks are presently optical atomic clocks, which have shown stabilities of about 10–17 over 1,000-second integration time. This is accomplished by frequency-locking a highly stabilized laser to an atomic transition as an ideal passive frequency standard. The intrinsic atomic coherence is only limited by its natural lifetime. External perturbations cause additional frequency fluctuations, which may be controlled to a level of 10–18 and lower. These considerations imply that atoms might be used directly as ideal local reference oscillators for gravitational wave detection.
One of the key requirements in interferometric gravitational wave experiments is for the local reference frames to be as inertially free as possible. This is to reduce any non-gravitational forces and local gravitational disturbances that can cause changes in the laser phase. Ground-based interferometers achieve a high level of seismic isolation of their mirrors by using either passive or active isolation systems. Space-based detectors instead, such as LISA (Laser Interferometer Space Antenna), achieve inertial isolation by using highly sophisticated drag-free test masses. Although, in principle, one could trap atoms in such test masses, it is more practical to rely on laser-cooled atoms in ultra-high vacuum as alternative drag-free test masses, and to directly use them as reference sensors.