Visibility and distinguishability quantify wave-ray duality: . We join them to polarization via the Polarization Coherence Theorem, a tight equality: .
© 2017 Optical Society of America
Centuries after Thomas Young’s famous double-slit interference experiment , quantification of coherence as a contextual resource (see ) is just now being examined (see [3,4]), and experimental evidence of polarization coherence in a previously unexplored context has been reported, exposing a new coherence triad .
The theorem of the title arises from the recognition that polarization is a two-party property, even in common usage. A polarized electorate means two things: (a) opinions within the population favor only a few political factions, and (b) this occurs within a specified background (the voting public, but not kindergarten school children). In much the same way, in optics a polarized field means that just one or two of the field’s intrinsic spin orientations are endowed with substantial field amplitude, so two independent degrees of freedom, spin and amplitude, are well correlated. Here we show how this leads to the new identity we refer to as the Polarization Coherence Theorem (PCT).
For simplicity we introduce the PCT in the most familiar context, Young’s double-slit scenario, as shown in Fig. 1. The combined amplitude of the light field arriving at the screen has a contribution from each of the two slits and :
By adopting the conventional small-angle and distant-screen treatment of the Young signals, the propagation factors from slits and to screen will be the same in magnitude but differ in phase (which is absorbed into amplitudes and ). Then the intensity at the screen is obtained with the expected sinusoidal interference term,
Fringe visibility for field is given as2).
When light is coming from only one of the two slits, then there can be no interference, and the origin of the light arriving at screen is known. Its intensity is distinguishable from the intensity observed when only the other slit is open. Consequently, the degree of distinguishability is also an indicator of a different coherence, and is defined as
One can easily check (Schwarz inequality) that , and conclude that and satisfy a strong constraint,
This coherence relation, here obtained without considering spin (ordinary polarization) at all, when interpreted in terms of wave/ray or wave/particle duality, occupies a central place in all discussions of duality and complementarity, and has been derived and re-derived by many investigators .
The Young scenario can also be discussed for an optical field exhibiting polarization unit vectors explicitly:7] or Wolf ) how to construct , the polarization coherence matrix for and its degree of polarization , indexed by spin orientations and . They are
Note that polarization also applies to field , despite the absence of directional unit vectors. We can say that is strongly “polarized” in the “direction” if has much greater magnitude than , and the reverse if . There is no intrinsic mathematical difference in the -polarization and -“polarization” examples, since and and the and functions are orthogonal vectors in their own vector spaces of spin and spatial mode. We will designate the and mode functions as “mode polarization vectors.” The field in Eq. (1) exhibits “mode polarization” in its mode space.
Thus we easily construct , the “mode polarization” coherence matrix for in Eq. (1), and its “degree of mode polarization” , indexed by “mode polarization” orientations and . They are3) and (4). In fact, we have just derived 10) and the multiply derived formula Eq. (5).
It should be obvious that the new identity in Eq. (10) arises equally quickly from Eq. (7) as from Eq. (8). That is, it is independent of the pair of degrees of freedom used to derive it. The identity in Eq. (10) is in fact a very general PCT. It says that the degree of polarization coherence for an optical field, in either the ordinary spin sense or a generalized mode sense, will support the famous duality inequality in Eq. (5). As such, Eq. (10) has unexpected implications for complementarity, which will have to be discussed elsewhere , as well as the single-photon version of the present result.
We are using analogies that have a firm relationship because they share the exact same vector space basis, and there is a clear connection between the two fields in Eq. (1) and Eq. (6) and thus between our two “polarizations.” Recall that the amplitudes are dependent on additional degrees of freedom of the light field, unspecified but labeled above. Similarly, the amplitudes and depend on spatial and temporal mode amplitudes, not specified, but in the relevant Young context they certainly refer to contributions coming from the spatial modes and , so we can expand and in terms of them:
Then the otherwise arbitrary field can be interpreted as just the projection of on an arbitrary intrinsic spin direction :
National Science Foundation (NSF) (PHY-1203931, PHY-1505189, PHY-1539859); Army Research Office (ARO) (W911NF-16-1-0162); Office of Naval Research (ONR) (N00014-14-1-0260).
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