Ancilla bit

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Lua error in package.lua at line 80: module 'strict' not found. An ancilla bit is an extra bit which has a secondary role in a logic circuit.

File:NOT gate with 5 controls constructed from 4 Toffoli gates and 3 ancilla bits.png
Using three ancilla bits and four Toffoli gates to construct a NOT gate with 5 controls. The ancilla bits end up trashed because the effects on them were not uncomputed.

A trivial use for ancilla bits is downgrading complicated gates into simple gates. For example, by placing controls on ancilla bits known to be ON, a Toffoli gate can be used as a controlled NOT gate or a NOT Gate.

A single ancilla bit is necessary and sufficient for universal classical reversible computation.[1] Additional ancilla bits are not necessary for universality, but the extra workspace can allow for simpler circuit constructions that use fewer gates.[citation needed]

In quantum computing, quantum catalysis uses ancilla qubits to store entangled states that enable tasks that would not normally be possible with local operations and classical communication (LOCC).[2] Quantum computers also use ancilla bits for quantum error correction.[3]

References

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  2. Lua error in package.lua at line 80: module 'strict' not found.
  3. Lua error in package.lua at line 80: module 'strict' not found.


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