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{{Short description|Quantum computing technique}}
**Uncomputation** is a technique, used in reversible circuits, for cleaning up temporary side effects on [ancilla bit|ancilla bits] so they can be re-used <ref>{{cite arXiv |eprint=1504.05155}}</ref>.▼
[[File:Using Toffoli Gates and Ancilla Bits to make a Not Gate with many controls.png|thumb|400px|Creating a logical conjunction of the five controls out of [[Toffoli gate]]s and ancilla bits. Uncomputation is used to restore the ancilla bits to their original states before finishing.]]
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Uncomputation is important to quantum computing. Whether or not intermediate effects have been uncomputed affects how states interfere with each other when measuring results <ref>{{cite arXiv |eprint=quant-ph/0209060}}</ref>.▼
▲Uncomputation is
The process is primarily motivated by the [[principle of implicit measurement]],<ref>{{cite book |last1=Nielsen |first1=Michael A. |last2=Chuang |first2=Isaac L. |title=Quantum computation and quantum information |date=2010 |publisher=Cambridge University Press |___location=Cambridge |isbn=978-1107002173 |edition=10th Anniversary}}</ref>{{page needed|date=January 2025}} which states that discarding a register during computation is physically equivalent to measuring it. Failure to uncompute garbage registers can have unintentional consequences. For example, if we take the state <math></math> <math>
\frac{1}{\sqrt 2}(|0\rangle|g_0\rangle + |1\rangle|g_1\rangle)
</math> where <math>g_0</math> and <math>g_1</math> are garbage registers. Then, if we do not apply any further operations to those registers, according to the principle of implicit measurement, the entangled state has been measured, resulting in a collapse to either <math>|0\rangle|g_0\rangle</math> or <math>|1\rangle|g_1\rangle</math> with probability <math>\frac{1}{2}</math>. What makes this undesirable is that wave-function collapse occurs before the program terminates, and thus may not yield the expected result.
==References==
{{Reflist}}
{{Quantum-stub}}
[[Category:Quantum information science]]
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