  Experimental tests for assessing the physical reality of the hypothetical
wave modes of quantum vacuum with zero-point energy are of fundamental
importance for quantum field theories and cosmology. Physical effects like the
Casimir effect have alternate description in terms of retarded interaction
between charged matter, due to quantum fluctuations of material dipoles.
However, there are simple quantum optical configurations where the hypothetical
quantum vacuum modes seem to assume an essential real role in the observable
quantum noise of optical signals. I present the logical and theoretical basis
of a decisive test that relies on the comparisons of balanced homodyne
detection with a novel differential scheme of balanced wave-front division
detection, when the two real optical beams at the detectors are derived from
one coherent beam as input. Both ideal and practical configurations of my
experimental test are discussed. Results from the experiments on balanced
detection, beam localization of optical beams, and atomic Bose-Einstein
condensates are used to reach definite conclusions against the reality of the
wave modes of quantum vacuum. It is shown that the entire quantum noise follows
consistently from the state reduction of quantum superpositions of
particle-number states at the point of detection, where the quantum measurement
is completed. This is consistent with the demonstrated applications of squeezed
light in interferometry and quantum metrology. This result achieves consistency
between quantum noise in quantum optics and observational cosmology based on
general relativity, by avoiding the wave modes of quantum vacuum with divergent
zero-point energy density. Generalization from the limited sphere of quantum
optics to general quantum field theories promises the complete solution to the
problem of a divergent cosmological constant.
