Practice question · Multiple choice
A reversible circuit computes its result, then runs the same steps backwards to restore the inputs, keeping only a copy of the answer. What does Landauer's principle say about the energy this costs?
Hints
- The principle charges for erasing a bit, not for operating on one.
- Count how many bits are actually thrown away at the end.
Show the answer
B. In principle it can approach zero, because nothing is erased.
Why
The bound is on destroying information, not on computing, so the cost is per bit ERASED rather than per gate. Uncomputing returns the scratch bits instead of discarding them; only the copied answer cannot be returned.
Practise Information, Signals, and the Physics of Computing
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