How do you detect a photon without consuming it? In double slit experiment, they say they can put a detector after the slit so they know exactly which slit the photon has passed.
But if you successfully detect a photon, isn't the photon also consumed in the detection process?
Because the detection process is basically energy exchange.
A photon has a quantum energy. Any energy exchange will consume the photon itself.
So the detected photons are consumed and would never reach the wall.
All photons that have reached the wall are the ones not detected.
Am I correct? 
 A: This statement is wrong:

A photon has a quantum energy. 

A photon may, when produced, carry a quantum of energy off a quantized bound state of a system, like an atom, a molecule  or a particle resonance, like a pi0. It may also be produced by other processes, as Bremsstrahlung , synchrotron radiaton and in metal antennas, because of the existence of electronic bands, a continuous spectrum comes out.
The correct statement is: a "photon may take away a quantum of energy". A photon  can be of any energy whatsoever.

Any energy exchange will consume the photon itself.

Only if the exchange happens with a bound atomic or molecular system it is necessary that a quantum of energy ( +/- the Heisenberg uncertainty) has to be absorbed, and only the correct frequency photons will disappear in the interaction.
Otherwise there exists elastic scattering  where only the angle of incidence changes, and Compton scattering and Raleigh scattering. In these processes the photon may lose energy or gain energy and thus just change frequency. ( also red shifts and blue shifts appear in various fields and kinematical frameworks). These interactions happen with  various  collective fields and there is no quantized spectrum.
So in principle one might design a double slit experiment where the photon will interact minimally (within the dimensions of the problem) so as to see what happens. They have already done that with electrons, demonstrating that it is the change in the boundary conditions that destroys the pattern, the electrons still reach the screen.
