Time Optimal Unitary Operations
  Extending our previous work on time optimal quantum state evolution, we formulate a variational principle for the time optimal unitary operation, which has direct relevance to quantum computation. We demonstrate our method with three examples, i.e. the swap of qubits, the quantum Fourier transform and the entangler gate, by choosing a two-qubit anisotropic Heisenberg model. 
Alberto Carlini, Akio Hosoya, Tatsuhiko Koike and Yosuke Okudaira