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Why classical gates are irreversible?

  1. Jul 30, 2009 #1
    Hi everybody.
    I give you an example to clarify my question:
    a NAND gate is irreversible becuase you can not find inputs from outputs and this is because of one bit is lost in output ( Antropy will increase ).the number of inputs and output are not the same in NAND gate.
    ok......now my question is that we can consider one of input as output so we can make it reversible.why is'nt this true?

    I have graduated in physics. thanks
    Last edited: Jul 30, 2009
  2. jcsd
  3. Jul 30, 2009 #2


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    Staff: Mentor

    There is a direction associated with the amplification that is used for the logic gate function. Look at the equivalent circuit for a logic gate, and it is apparent that the inputs are high-impedance controls (like FET gates), and the outputs are low-impedance drivers.
  4. Jul 30, 2009 #3


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    Note that it IS of course possible to build reversible computers (in the computational sense) using classical components.
    As far as I know there are no practical applications, but reversible comouters have been used as "toy" systems for a long time (this is one reason why quantum computing took off so quickly, much of the theory for reversible gates has been around for a long time).
  5. Jul 30, 2009 #4
    thanks for your replies.
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