is a single photon a wave? Is it a wave packet? How could it split and interfere?
How could a wave packet, with many frequencies, be a photon with one freq.?
Thank you very much.
 A: It's better to know about wave particle duality before going to your question.  
Lets know what Broglie says in his noble lecture (December 12, 1929):    
This is the extracted passage which makes an attempt to say the importance of both wave and particle nature. 

The existence of a granular structure of light and of other radiations was
  confirmed by the discovery of the photoelectric effect. If a beam of light or
  of X-rays falls on a piece of matter, the latter will emit rapidly moving electrons.
  The kinetic energy of these electrons increases linearly with the frequency
  of the incident radiation and is independent of its intensity. This
  phenomenon can be explained simply by assuming that the radiation is composed
  of quanta hv capable of yielding all their energy to an electron of the
  246 1929 L.DE BROGLIE
  irradiated body: one is thus led to the theory of light quanta proposed by
  Einstein in 1905 and which is, after all, a reversion to Newton’s corpuscular
  theory, completed by the relation for the proportionality between the energy
  of the corpuscles and the frequency. A number of arguments were put
  forward by Einstein in support of his viewpoint and in 1922 the discovery
  by A. H. Compton of the X-ray scattering phenomenon which bears his
  name confirmed it. Nevertheless, it was still necessary to adopt the wave
  theory to account for interference and diffraction phenomena and no way
  whatsoever of reconciling the wave theory with the existence of light corpuscles
  could be visualized.
  As stated, Planck’s investigations cast doubts on the validity of very small
  scale mechanics. Let us consider a material point which describes a small trajectory
  which is closed or else turning back on itself. According to classical
  dynamics there are numberless motions of this type which are possible complying
  with the initial conditions, and the possible values for the energy of
  the moving body form a continuous sequence. On the other hand Planck
  was led to assume that only certain preferred motions, quantized motions, are
  possible or at least stable, since energy can only assume values forming a
  discontinuous sequence. This concept seemed rather strange at first but its
  value had to be recognized because it was this concept which brought Planck
  to the correct law of black-body radiation and because it then proved its
  fruitfulness in many other fields. Lastly, it was on the concept of atomic motion
  quantization that Bohr based his famous theory of the atom; it is SO
  familiar to scientists that I shall not summarize it here.
  The necessity of assuming for light two contradictory theories-that of
  waves and that of corpuscles - and the inability to understand why, among
  the infinity of motions which an electron ought to be able to have in the
  atom according to classical concepts, only certain ones were possible: such
  were the enigmas confronting physicists at the time I resumed my studies of
  theoretical physics. 

In the following passage Broglie predicts the existence of corpuscle accompanied by wave.   

When I started to ponder these difficulties two things struck me in the main.
  Firstly the light-quantum theory cannot be regarded as satisfactory since it
  defines the energy of a light corpuscle by the relation W = hv which contains
  a frequency v. Now a purely corpuscular theory does not contain any
  element permitting the definition of a frequency. This reason alone renders
  it necessary in the case of light to introduce simultaneously the corpuscle
  concept and the concept of periodicity.
  On the other hand the determination of the stable motions of the electrons
  in the atom involves whole numbers, and so far the only phenomena in
  which whole numbers were involved in physics were those of interference
  and of eigenvibrations. That suggested the idea to me that electrons themselves
  could not be represented as simple corpuscles either, but that a periodicity
  had also to be assigned to them too.
  I thus arrived at the following overall concept which guided my studies:
  for both matter and radiations, light in particular, it is necessary to introduce
  the corpuscle concept and the wave concept at the same time. In other words
  the existence of corpuscles accompanied by waves has to be assumed in all
  cases. However, since corpuscles and waves cannot be independent because,
  according to Bohr’s expression, they constitute two complementary forces
  of reality, it must be possible to establish a certain parallelism between the
  motion of a corpuscle and the propagation of the associated wave. The first
  objective to achieve had, therefore, to be to establish this correspondence.

The following passage is very much interesting and much related to your question. So, pay little more attention.  

We shall content ourselves here by considering the general significance of
  the results obtained. To sum up the meaning of wave mechanics it can be
  stated that: "A wave must be associated with each corpuscle and only the
  study of the wave’s propagation will yield information to us on the successive
  positions of the corpuscle in space". In conventional large-scale mechanical
  phenomena the anticipated positions lie along a curve which is the trajectory
  in the conventional meaning of the word. But what happens if the wave
  does not propagate according to the laws of optical geometry, if, say, there
  are interferences and diffraction? Then it is no longer possible to assign to the
  corpuscle a motion complying with classical dynamics, that much is certain.
  Is it even still possible to assume that at each moment the corpuscle occupies
  a well-defined position in the wave and that the wave in its propagation carries
  the corpuscle along in the same way as a wave would carry along a cork?
  These are difficult questions and to discuss them would take us too far and
  even to the confines of philosophy. All that I shall say about them here is that
  nowadays the tendency in general is to assume that it is not constantly possible
  to assign to the corpuscle a well-defined position in the wave. I must
  restrict myself to the assertion that when an observation is carried out enabling
  the localization of the corpuscle, the observer is invariably induced
  to assign to the corpuscle a position in the interior of the wave and the
  probability of it being at a particular point M of the wave is proportional to
  the square of the amplitude, that is to say the intensity at M. 

Answer to your question lies in the following passage.  

If we consider a cloud of
  corpuscles associated with the same wave, the intensity of the wave at each
  point is proportional to the cloud density at that point (i.e. to the number of
  corpuscles per unit volume around that point). This hypothesis is necessary
  to explain how, in the case of light interferences, the light energy is concentrated
  at the points where the wave intensity is maximum: if in fact it is
  assumed that the light energy is carried by light corpuscles, photons, then
  the photon density in the wave must be proportional to the intensity. 

By my understanding from the Broglie lecture, photons won't split. Photons are associated with wave, which superimpose constructively and destructively to form interference pattern. And the points of maximum intensity corresponds to the region where the photon of the wave are more likely to be present.   
If any correction (or wrong understanding) here, I will be happy to know.  
A: You are confusing terms. Photons have energies, and waves have frequencies.
Generally (and not so accurately), a wave function can be expressed as a superposition of mutually orthogonal wave function, called eigen-functions, each associated with an eigen-value (energy in your case), so that when a measurement (yours, or some interaction with an appropriate macroscopical body), the wave function will collapse to one of the energy eigen-functions, and the corresponding eigen-value is the energy you will measure.
