Source-verifiedLevel 3
Fredkin Gate (CSWAP)
The Fredkin gate (Controlled-SWAP) swaps two target qubits only when the control qubit is |1⟩.
What it means
The Fredkin gate is a 3-qubit gate that conditionally swaps the second and third qubits based on the first (control) qubit.When the control is |0⟩, the targets are unchanged; when |1⟩, they are swapped.It is universal for classical reversible computation and can be used as a comparator.The Fredkin gate preserves the Hamming weight of the input and can be decomposed into Toffoli and CNOT gates.Everyday analogy
Like a railroad switch that only activates when the signal (control) is on.
Common misconceptions
- Fredkin is NOT the same as Toffoli — Fredkin swaps while Toffoli flips.
- Fredkin gate preserves Hamming weight, which Toffoli does not.
Key takeaways
- Conditionally swaps two qubits based on control.
- Universal for classical reversible computation.
- Preserves Hamming weight.
Check your understanding
What does the Fredkin gate do when control = |1⟩?
- A.Flips both targets
- B.Swaps both targets
- C.Does nothing
- D.Entangles targets
Show the answer
Answer: B. Swaps both targets
Why: The Fredkin gate swaps the two target qubits when control is |1⟩.
Builds on
Primary source: Fredkin & Toffoli, Conservative logic, Int. J. Theor. Phys. 21, 219 (1982), doi:10.1007/BF01857727
Learn it hands-on
This concept is part of a 46-level curriculum with an interactive simulator and Lumen, a tutor whose answers are verified before you see them. Levels 1–5 are free.