The mean photon number, also known as the average photon number or the expectation value of photon number, is a quantity used to describe the average number of photons in a given system or state. It is denoted by the symbol ⟨n⟩.
The mean photon number can take any positive value, including values less than one. It represents the average number of photons present in a specific state or mode. However, it's important to note that the mean photon number does not have to be an integer; it can be a fractional value.
In quantum mechanics, the concept of mean photon number arises from the probabilistic nature of light and its quantum behavior. Photons can exhibit characteristics such as superposition and interference, and their properties are described by quantum states.
In certain scenarios, such as weak light sources or attenuated signals, the mean photon number can be less than one. This means that, on average, there are fewer than one photon present in the system or mode being considered. It indicates a low photon count or a low intensity of light.
For example, in quantum optics, when dealing with coherent states or squeezed states, the mean photon number can be less than one. Coherent states represent classical-like states of light with a well-defined phase, and their mean photon number can be any positive real number, including fractions or values less than one.
In summary, the mean photon number is a statistical measure that represents the average number of photons in a given system or state. It can take values less than one, indicating situations with low photon counts or attenuated light sources.