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Homework Help: Computers: Programming Languages: Logic Gates using Interference of Light
by Soumyadip Rakshit
A diode is a device that allows current to flow in one direction and does not allow it to flow in the opposite direction. So, if we apply a forward bias, current flows and while on the reverse bias it does not. But that is an ideal diode we are talking about. As the present technology stands the ideal diode is only of theoretical importance, yet to be realised in the real world.
So is it possible to construct a diode with a Zero cut-in voltage, Zero ac/dc resistance, Zero leakage current, Zero reverse bias current, and above all ... instant switching. As we better our technologies we will, no doubt, get closer to that Zero mark but would we ever achieve the perfect Zero ... Probably not. The very fact that we have a physical medium for the diode hinders us in our quest for excellence.
In the digital world of computers the most important use of a transistor is in switching. Switching from 0 to 1 and back. We give in an input voltage and accordingly get a high or low voltage as output. So do we have the perfect switch which instantaneous changes from high to low and vice versa requiring no time in between switching. Do we have the perfect transistor switch. The answer again is 'No'.
The basic fact here is that we are working on perfecting something, which can be made thus much perfect and no more. So why don't we change our perspective and work on an entirely different setup. Why don't we start afresh?
The fastest thing known on earth is light. In order to achieve the fastest, we should work with the fastest. We already use light to transfer data, so why not work with it on the data. We make the light equivalent of a transistor and work directly with the light to get our 1’s and 0’s, our NANDs and NORs.
If we can work directly with light and get the output as Light' or 'No light' for 1 and 0, we can reach the ultimate in speed and efficiency. There will be no movement of electrons, there will be no junction transistor, there will be no voltage required to bias it. There will be no heat produced, minimal space and power required. And above all, the speed will be that of light. This is what we call perfection ... the real ideal.
When two rays of light meet we may get bright light or we may even get darkness. That is the theory of interference of light. So if we can adjust the input light rays such that they can give variable results depending on what other light they interact with, we can achieve our aim.
For example we take two light inputs and give the resultant output according to the XOR gate.
Source 1 and 2 are coherent light sources. The output is taken at the point on the interference fringe pattern where the two sources interfere destructively producing darkness.
When both the light sources are OFF, there is no light at the output. When any and only one of them is ON, there is no interference and we get light at the output. When both the sources of light are ON they interfere and we get no light at the output.
Thus our XOR gate is made.
| A |
B |
C |
| 0 |
0 |
0 |
| 0 |
1 |
1 |
| 1 |
0 |
1 |
| 1 |
1 |
0 |
Now if we take the output (C) at the centre where there is light due to constructive interference of S1 and S2 we get the OR gate.
| A |
B |
C |
| 0 |
0 |
0 |
| 0 |
1 |
1 |
| 1 |
0 |
1 |
| 1 |
1 |
1 |
If we make Source 1 fixed and ON and give one input to Source 2 we get our inverter Circuit ... the NOT gate.
From these gates we can make the NOR, Buffer and the EX-NOR gates. Also we can make the AND and NAND gates using the De Morgan’s formula (A’+B’)’ = A.B Since we have both the NOT and the OR gates made previously we can combine them together to get our desired result.
Now let us see what have we achieved through the above gates.
The sources of light are the input signals. There are no physical bodies involved in the operations. It has a minimal production of heat (only from the light sources) It needs minimal power for functioning as only little power is required for the light sources. The entire setup is in vacuum (since light travels the fastest in vacuum). It can be made as small as required since neither vacuum nor light needs any space. It is as fast as light can travel in vacuum.
Homework Help: Computers: Programming Languages
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