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