Optimal control for fast and highfidelity quantum gates in coupled superconducting flux qubits
Abstract
We apply the quantum optimal control theory based on the Krotov method to implement singlequbit X and Z gates and twoqubit cnot gates for inductively coupled superconducting flux qubits with fixed qubit transition frequencies and fixed offdiagonal qubitqubit coupling. Our scheme shares an advantage with other directcoupling schemes in that it requires no additional coupler subcircuit and control lines. The control lines needed are only for the manipulation of individual qubits (e.g., a timedependent magnetic flux or field applied on each qubit). The qubits are operated at the optimal coherence points and the gate operation times (less than 1 ns for singlequbit gates and ∼2 ns for cnotgates) are much shorter than the corresponding qubit decoherence time. A cnot gate or other general quantum gates can be implemented in a single run of the pulse sequence rather than being decomposed into several singlequbit and several entangled twoqubit operations in series by composite pulse sequences. Quantum gates constructed via our scheme are all with very high fidelity (very low error) as our optimal control scheme takes into account the fixed qubit detuning and fixed twoqubit interaction as well as all other timedependent magneticfieldinduced singlequbit interactions and twoqubit couplings. The effect of leakage to higherenergylevel states and the effect of qubit decoherence on the quantumgate operations are also discussed.
 Publication:

Physical Review A
 Pub Date:
 July 2014
 DOI:
 10.1103/PhysRevA.90.012318
 arXiv:
 arXiv:1406.7707
 Bibcode:
 2014PhRvA..90a2318H
 Keywords:

 03.67.Lx;
 85.25.Cp;
 02.30.Yy;
 Quantum computation;
 Josephson devices;
 Control theory;
 Quantum Physics;
 Condensed Matter  Mesoscale and Nanoscale Physics;
 Condensed Matter  Superconductivity
 EPrint:
 4 figures, accepted by Phys. Rev. A