<p>The above answer seems wrong to me: the basic idea is that logically reversible and physically reversible are distinct.</p>

<p>Consider a quantum computer (reversible by nature), in the memory you store qbits, after you do an operation, you store the qbits that are important, those qbits that are no longer useful after the operation can be expelled from the memory and discarded. (lost in the endless space of the universe) Although the operations are logically reversible, it is no longer necessary to store all the qbits and we deliberately lose some (thus not physically reversible). </p>

CC BY-SA 3.0
2012-01-31T23:52:06.230Z
2012-01-31T23:59:02.197Z
2012-01-31T23:59:02.197Z
Samuel Ranellucci
<p>You need workspace (registers set to 0) to read the user's input. Once you've read it, and completed the program, the user needs to erase the input and the output in order to restore the workspace. However, if you assume that the user wants to keep a copy of the input and the output, this is possible to do reversibly.</p>

<p>Similarly for the program, to erase the program from the computer there needs to be another copy stored somewhere.</p>

<p>If you don't allow any reversible operations between the user and the computer, and you insist that everything be done reversibly, then you can't load input into the computer, and you have a useless machine.</p>

CC BY-SA 3.0
2012-01-30T03:58:59.730Z
After reading this answer, I cannot help imagining a fastest computer in the world whose only issue is that it cannot receive input from user or produce output to user.
Tsuyoshi Ito
CC BY-SA 3.0
2012-01-30T14:33:05.237Z
Maybe it exists. Somewhere in the world there may be a diamond crystal in which the spins of the impurities interact in such a way that they just happen to form a universal quantum computer ...
Peter Shor 
CC BY-SA 3.0
2012-01-30T00:29:10.740Z
2012-01-30T00:29:10.740Z
Peter Shor 
<p>We are able to perform universal computations with the reversible model. Basically, during the computations, no information should be erased, so that no involved entropy increase would occur.</p>

<p>Are reversible computations alone sufficient to create a complete computer? The machine should be able to:</p>

<ul>
<li>load programs,</li>
<li>handle user input,</li>
<li>provide an output.</li>
</ul>

<p>I suspect that, for example reading user's input requires information deletion which cannot be avoided. </p>

CC BY-SA 3.0
2012-02-01T06:25:27.140Z
IMHO This isn't a research level question.
Pratik Deoghare
CC BY-SA 3.0
2012-02-01T09:31:24.503Z
@Pratik Deoghare This is actually a very non-trivial question. Peter Shor provides the outline of a possible answer below (they key part is "reversible operations between the user and the comptuer"), but the devil is in the details: modeling I/O is, as far as I know, an unsolved problem in reversible computing.
funkstar
CC BY-SA 3.0
2012-02-01T10:21:26.733Z
@funkstar Well, that reflects my naivety. :D Thanks!
Pratik Deoghare
<universal-computation><shannon-entropy>
Can reversible computations alone be used to create a computer?
Mooncer
# Can reversible computations alone be used to create a computer?

(Asked by: username_0 on 2012-01-30)

We are able to perform universal computations with the reversible model. Basically, during the computations, no information should be erased, so that no involved entropy increase would occur.

Are reversible computations alone sufficient to create a complete computer? The machine should be able to:

* load programs,
* handle user input,
* provide an output.

I suspect that, for example reading user's input requires information deletion which cannot be avoided.

**Tags:** universal-computation, shannon-entropy

> **Comments:**
> **username_1**: IMHO This isn't a research level question.  
> **username_2**: @username_1 This is actually a very non-trivial question. username_4provides the outline of a possible answer below (they key part is "reversible operations between the user and the comptuer"), but the devil is in the details: modeling I/O is, as far as I know, an unsolved problem in reversible computing.  
> **username_1**: @username_2 Well, that reflects my naivety. :D Thanks!  

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## 2 answers

### Answer by username_4 on 2012-01-30. Score: 9 | Accepted answer

You need workspace (registers set to 0) to read the user's input. Once you've read it, and completed the program, the user needs to erase the input and the output in order to restore the workspace. However, if you assume that the user wants to keep a copy of the input and the output, this is possible to do reversibly.

Similarly for the program, to erase the program from the computer there needs to be another copy stored somewhere.

If you don't allow any reversible operations between the user and the computer, and you insist that everything be done reversibly, then you can't load input into the computer, and you have a useless machine.

> **Comments:**
> **username_5**: After reading this answer, I cannot help imagining a fastest computer in the world whose only issue is that it cannot receive input from user or produce output to user.  
> **username_4**: Maybe it exists. Somewhere in the world there may be a diamond crystal in which the spins of the impurities interact in such a way that they just happen to form a universal quantum computer ...  

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### Answer by username_3 on 2012-01-31. Score: -3

The above answer seems wrong to me: the basic idea is that logically reversible and physically reversible are distinct.

Consider a quantum computer (reversible by nature), in the memory you store qbits, after you do an operation, you store the qbits that are important, those qbits that are no longer useful after the operation can be expelled from the memory and discarded. (lost in the endless space of the universe) Although the operations are logically reversible, it is no longer necessary to store all the qbits and we deliberately lose some (thus not physically reversible).


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