[Soul Surfer]

I don't think that it is possible to modulate a single photon with a complex signal but this does bring out a question that I have never found a satisfactory answer to that does relate to this question and that is

Does an individual photon posess bandwidth?

The bandwith of a communication signal is part of the measure of the amount of signal that it can carry.  The other part is the signal to noise ratio of the environment in which the information is being carried  and this too limits the amount of information that a single photon can carry.  the total equation involves a time-bandwidth product where the broader the bandwidth the fiber the detail you can observe  this again is limited by the quantum mechanical uncertainty principle.

To return to my original question it is probably easiest to think in terms of light photons.  A spectrum line that is emiited very rapidly by an atom or molecule has a greater uncertainty of frequency (bandwidth) than one that is emitted slowly.  Lasers can be very narrow band and highly coherent.
You cam measure this bly looking at the coherence length of a spectrum line ir even resolve it using a very fine resolution spectrometer.  but this involves many photons.  It is known that interference patterns effectivley exist if only one photon passes through an interferometer at a time. but is it possible to know the bandwidth of a single photon?

[syhprum]

Let us go back to the original question

"Can more than one bit be transmitted on a single photon?"

What can we learn from a SINGLE photon, If it was one of a series we could derive a lot of information from its relative spacing to others but it is not!
If we had a polarisation reference we could derive information from its polarisation although perhaps we could measure to what extent it was linear or circular I have my doubts on this point whether this is possible with a single photon.
we could measure its energy here we would be on firmer ground as it might well be emitted from a very well defined source such as an electron/positron annihilation when it would have set out with a well defined energy.
From this we can measure its velocity relative to us but as velocity is a vector quantity there is still ambiguity so all in all I do not think that a great deal of information can be derived from a SINGLE photon!   
 

[Atomic-S (OP)]

The Schroedinger equation which governs the state function of a particle, being a differential equation of a certain type, permits as a solution any linear combination of specific solutions. On this basis, just about any wave shape can be accommodated for a single particle. The conclusion seems inescapable, then, that a single photon can indeed possess bandwidth. And, it may be that when a modulated signal travels a great distance so that the receiver has access to only a very few photons, these photons are not, as one might first suppose, each possessing of an individual energy and wavelength that differs from those of other photons, but that each photon may possess an exact copy of all the energies and frequencies inherent in the modulated waveform. (Whether this is true or not I cannot say with certainty -- maybe someone with greater mathematical knowledge of this question knows.)

[Atomic-S (OP)]

Then again (to make this subject even more confusing): If an attempt is made to measure the energies of the photons in such a signal, every measurement will yield but one energy for the photon -- an inevitable consequence of quantum mechanics.  On what basis, then, could we say that the photon had bandwidth to begin with. Of course, the measurement of energy on all the photons coming through would yield a spectrum of energies,  from which collectively one could infer bandwidth. But did each photon individually have bandwith to begin with? And then we could ask, is there actually any physical difference between a system of many photons each possessing the modulated signal, and a system of many photons each possessing only one frequency but which all together produce an overall modulated signal?  Of course to answer this question, we must have some understanding of in what mathematical way a collection of photons combine, and this would require us to get into some messy math.

[lightarrow]

Can more than one bit be transmitted on a single photon?

Were you referring to this:
http://www.spaceref.com/news/viewpr.rss.html?pid=21727 ?

[Atomic-S (OP)]

It would appear that the future is here already.