what determines the direction of a single photon Photon has momentum and energy.assume i am creating a nuclear fusion by fusing hydrogen nuclei into helium and a photon is created.Now which direction does it(single photon) moves or travel?can we direct that single photon to any specific direction?
i am not a physics student,but please help me understand.
 A: You cannot direct an individual photon, only the probability of the photon going into a given direction. 
In interactions that you are describing, a complicated one, at the center of mass, the probability distribution will depend from which level the photon emerged, if it comes from a quadrupole or higher moment charge distribution of the quark charges in the nucleons. There is no way to define a direction of the charge distributions for the individual quarks that will finally fuse, and thus the probability is uniform anyway. 
As  a special experiment,  one might set up  polarized hydrogen/protons so that a spin direction would give a direction against which to study whether the probability of photons is higher in some direction than another. In an experiment where the two protons are are two beams colliding, there exists also the beam direction against which the photons will have a probability distribution. This distribution might have a directionality given by the charge distributions in the protons.
It is easier to study photon production from atoms because, in contrast to quarks, the atomic levels that electrons occupy are known and thus the probability distribution of the emitted photons can be calculated. Electrons are accessible to measurement and controllable in the lab.  Always it is probabilities we have, because that is what quantum mechanical calculations give us. In this case we can have collective directionality: Setups can be made that induce coherence in huge numbers of photons and then they can be directed by appropriate conditions. See for example the laser.
