Hi ! My name is Mayukhmali Das. I am currently pursuing Electronics and Telecommunication Engineering from Jadavpur University. As we all know that Christopher Nolan's movie TENET is going to be released this year. It deals with Time reversal. Now the question arises ; Will our normal computers function when time is reversed ? The answer is----- most probably not ; Even though most of the gates used ; are Mosfet based now a days, Still our traditional computers are not reversible. This is due to entropy of information. Entropy of information decreases when it goes through gates like And, Or, Xor, Nand and Nor. This is because there are two inputs but we get only one output. Besides gates like And gate, Xor gate and others are not logically reversible. As for an output of 0 ; 'And' gate can have 3 different combinations of inputs. We know that Entropy = nT(ln2) where n is the number of states. Thus entropy decreases while passing through these gates . So ; in normal computers entropy decreases in the direction of time. So in reverse time ; entropy will increase in the direction of time. Keep in mind that ; here I am talking about entropy of system and not universe. Thus we define the arrow of time in the direction of decreasing information entropy and increasing entropy of the universe ( sum total of the entropy of information + heat generated ). To make a computer work in reverse time ; there must be reversible logic and no change in information entropy. Then it will matter less ; whether time runs forward or backward. I have used CNOT ; Toffoli and Pauli-X Gates to make this 4-bit Adder.
I am using a quantum computation simulator for this project. I can give inputs by constantly the X-rotation of the qubits in the Bloch sphere. The initial region of simultaneous black and white dots signify the input bits. If we are adding two numbers ; abcd and efgh ; then the inputs are d, h ( half adder ) ; c, g (full adder); b, f (full adder); a, e ( full adder ) respectively from top to bottom. The outputs are represented by black dots which when read from bottom to top forms the number. This is just the beggining of my project. My project Digital TENET is focused on building a quantum microprocessor which will be time reversible.
Consider the addition of 1011 and 1010 ;
This is the input ; 10110011 from top to bottom. All the initial inputs were | 0 > which are converted as required into | 1 > using X rotation.
This the 4-bit adder circuit that I made. It uses CNOT ; Toffoli and Pauli-X Gates.
This the output ; which is from bottom to top 10101 ; the first bit is the carry over of the last adder.
Let us take a different example ; let's add ---- 1001 and 0111 ;
You can see clearly that the output when read from bottom to top is 10000. This 4-bit adder circuit will work when time is reversed. It keeps the entropy of information constant as well as all the gates used are logically reversible.
Check this video out --------- it shows how the output changes ; for automatically changing inputs using X-rotation.
Thank you
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