# Can anybody explain the modulation of radio waves in terms of photons?

1. Aug 9, 2011

### sriecewit

Hi All,

I wonder if the modulation of RF waves (Amplitude Modulation, Frequency Modulation, and Phase Modulation) can be explained in terms of photons.

Thanks,
Srini

2. Aug 9, 2011

### xts

Sure:
AM - you are sending either small or large number of photons in a unit of time.
FM - you are sending photons of such or another frequency
PM - at some moment you stop sending coherent photons like previously, and start sending the ones of the same frequency, but shifted in phase

3. Aug 9, 2011

### sriecewit

Can you elaborate the third one a bit?
More specifically, with respect to BPSK modulation, where the photons needs to be transmitted continuously. What difference does the photos experience when phase transition happens?

Srini

4. Aug 10, 2011

### xts

Exactly the same, as for classical electromagnetism - the phase gets shifted, in case of BPSK by 180°. Time-dependent factor of the photon wavefunction is $e^{-i(\omega t+\Theta_0)}$, at the phase transition you stop sending coherent photons with factor $e^{-i\omega t}$ and start sending ones with $e^{-i(\omega t+\pi)}$

5. Aug 10, 2011

### sriecewit

Are photons associated with the phase property? Aren't they just energy packets, whose energy varies as the frequency times the constant?
Let me put it in this way. What all parameters does specify a photon?

6. Aug 10, 2011

### f95toli

This is a complicated question. Single photons do not have a definite phase (the phase for any number state is undefined). However, other states (coherent states etc) DO have definite phases. Even the concept of a photon is a bit tricky to define "properly" without a good understanding of field theory (=advanced quantum mechanics).
Anyway, the point is that you can't really talk about properties of photons, but you CAN talk about properties of fields.