Christiane P. Koch

Engineering Strong Beamsplitter Interaction between Bosonic Modes via Quantum Optimal Control Theory

  1. Daniel Basilewitsch,
  2. Yaxing Zhang,
  3. S. M. Girvin,
  4. and Christiane P. Koch
In continuous-variable quantum computing with qubits encoded in the infinite-dimensional Hilbert space of bosonic modes, it is a difficult task to realize strong and on-demand interactions
between the qubits. One option is to engineer a beamsplitter interaction for photons in two superconducting cavities by driving an intermediate superconducting circuit with two continuous-wave drives, as demonstrated in a recent experiment. Here, we show how quantum optimal control theory (OCT) can be used in a systematic way to improve the beamsplitter interaction between the two cavities. We find that replacing the two-tone protocol by a three-tone protocol accelerates the effective beamsplitter rate between the two cavities. The third tone’s amplitude and frequency are determined by gradient-free optimization and make use of cavity-transmon sideband couplings. We show how to further improve the three-tone protocol via gradient-based optimization while keeping the optimized drives experimentally feasible. Our work exemplifies how to use OCT to systematically improve practical protocols in quantum information applications.
arxiv pdf

Charting the circuit QED design landscape using optimal control theory

  1. Michael H. Goerz,
  2. Felix Motzoi,
  3. K. Birgitta Whaley,
  4. and Christiane P. Koch
We use quantum optimal control theory to systematically map out the experimentally reachable parameter landscape of superconducting transmon qubits. With recent improvements in decoherence
times, transmons have become a promising platform for quantum computing. They can be engineered over a wide range of parameters, giving them great flexibility, but also requiring us to identify good regimes to operate at. Using state-of-the-art control techniques, we exhaustively explore the landscape for the potential creation and distribution of entanglement, for a wide range of system parameters and applied microwave fields. We find the greatest success outside the usually considered dispersive regime. A universal set of gates is realized for gate durations of 50 ns, with gate errors approaching the theoretical limit. Our quantum optimal control approach is easily adapted to other platforms for quantum technology.
arxiv pdf