Page 74 - Introduction to Information Optics

1.8. Quantum Mechanical Channel 59

Fig. 1.18. Photon channel capacity as a function of signal powers, for various values of thermal
noise temperature T. Dashed lines represent the classical asymptotes of Eq. (1.203).

Then the photon channel capacity is limited by the quantum statistic; that is,
1/2
c ~!L(»Y (1.203)
^quant
'
In 2 \3hJ ' '
However, if the signal-to-noise ratio is low (hv « kT), the photon channel
capacity reduces to the classical limit:

(1.204)
V--rla,

Figure 1.18 shows a plot of the photon channel capacity as a function of signal
energy. We see that at the low signal-to-noise ratio (hv « kT) regime, the
channel capacity approaches this classical limit. However, for high signal-to-
noise ratio (hv » kT), the channel capacity approaches the quantum limit of Eq
(1.203), which offers a much higher transmission rate.