Ghost imaging reconstructs an object by correlating measurements from two light beams: one that interacts with the object and one that doesn't. The technique is remarkable because the spatial resolution comes from the beam that never touched the object. But conventional ghost imaging requires two physical arms — two detectors, two optical paths, careful alignment.
Single-arm field-correlation ghost imaging achieves quantitative dynamic phase mapping with one arm (arXiv:2603.21648). The correlations that ghost imaging exploits exist within a single spatially incoherent beam. Instead of correlating two beams, the method correlates the field at one spatial point with the intensity at another point in the same beam. The object modulates the field, and the correlation analysis recovers both amplitude and phase information.
The “quantitative” and “dynamic” qualifiers matter. Quantitative means the phase values are calibrated — not just relative phase differences but absolute phase shifts. Dynamic means the measurement can track phase changes in time, not just static objects. The combination enables real-time monitoring of transparent objects that change their optical thickness.
The single-arm geometry eliminates the stability requirements of two-arm interferometry. Vibrations, thermal drift, and alignment errors that would destroy a conventional interferometric measurement don't affect the single-arm configuration because both the signal and the reference traverse the same path. Common-mode noise cancels automatically.
The structural insight: the correlations used for ghost imaging were never really between two beams. They were between two measurements of the same underlying field. The two-arm geometry made this explicit by physically separating the measurements. The single-arm geometry reveals that the separation was conceptual, not physical — you can extract the same correlations from spatially separated measurements within a single beam. The ghost never needed two bodies. The correlation lives in the field, not in the apparatus.