Computing Library › Quantum Logic Gates
Quantum Logic Gates
Fredkin Gate (CSWAP)
The Fredkin gate is a controlled-SWAP — it exchanges two targets only when the control is |1⟩, a reversible, conservative logic primitive.
Circuit symbol
Action on basis states
| input | output |
|---|---|
| |100⟩... controls swap of targets | conditional SWAP |
How it works
CSWAP is universal for reversible classical computation and is the heart of the SWAP test — a routine that estimates the overlap ⟨ψ|φ⟩ between two quantum states, used across quantum machine learning.
In code (Qiskit)
python
qc = QuantumCircuit(3)
qc.cswap(0, 1, 2) # Fredkinthree-qubitreversibleswap-test