Ghost frequency-time combs: quantum coherence in classical and entangled laser beams

Introduction: Proceeding from the recently discovered ghost frequency-time comb (GFC) of 50% contrast found through intensity-intensity correlation measurements of multi-cavity-mode continues-wave (CW) laser beams, this report further extends this analysis to a novel type of entangled coherent state, or entangled laser beams, capable of producing GFC of 100% contrast, described as the “quantum ghost frequency-time comb” (QGFC). Despite being in continuous-wave operation, both classical and entangled laser beams produce sharp periodic comb-like correlation functions. Why is there a “nonlocal” and periodic correlation between the two distant locally and independently measured CW laser beams? Can the GFC and QGFC phenomena be interpreted as correlations between statistical intensity fluctuations? For the QGFC specifically, this question becomes particularly acute: How do “zero-coincidences” occur in the measurement of CW laser beams? Why do “zero-coincidences” occur periodically? This report address these questions. Although both GFC and QGFC are observed, or more precisely, calculated from locally measured intensity fluctuations, the 50% contrast GFC is caused by an incoherent superposition of quantum probability amplitudes of all possible randomly paired cavity modes, while the 100% contrast QGFC is caused by a coherent superposition of quantum probability amplitudes of all possible entangled pairs of cavity-modes, where these amplitudes contribute to a nonlocal joint photodetection event. Experimental observations of GFC and QGFC may finally resolve the long-standing debate between classical statistics and quantum coherence. Besides their fundamental significance, bright GFC and QGFC make important contributions to the fields of nonlocal positioning, time transfer and precision spectroscopy. High-brightness entangled laser beams may also open up new applications that might not be realizable by using entangled photon pairs at the single-photon level.