Charting the circuit QED design landscape using optimal control theory

Abstract

With recent improvements in coherence times, superconducting transmon qubits have become a promising platform for quantum computing. They can be flexibly engineered over a wide range of parameters, but also require us to identify an efficient operating regime. Using state-of-the-art quantum optimal control techniques, we exhaustively explore the landscape for creation and removal of entanglement over a wide range of design parameters. We identify an optimal operating region outside of the usually considered strongly dispersive regime, where multiple sources of entanglement interfere simultaneously, which we name the quasi-dispersive straddling qutrits regime. At a chosen point in this region, a universal gate set is realized by applying microwave fields for gate durations of 50 ns, with errors approaching the limit of intrinsic transmon coherence. Our systematic quantum optimal control approach is easily adapted to explore the parameter landscape of other quantum technology platforms.

Amplification of the parametric dynamical Casimir effect via optimal control

“…We model the qubit-field interaction by the Rabi Hamiltonian [21], with a time-dependent modulation of the qubit frequency. We use and compare two strategies to optimize the visibility of the DCE: a multi step heuristic method employing bang-bang switches of the qubit frequency from the off-resonance regime to the resonance regime and back out of resonance, and optimal control theory which has been proven to be able to successfully control circuit quantum electrodynamics processes [2,[22][23][24][25][26][27][28]. In particular, we employ the dressed chopped random basis algorithm (dCRAB) which has been already applied successfully to various theoretical and experimental atomic and condensed matter control problems to meet various control goals, including state-transfer, gate synthesis, observable control, and fast quantum phase transition crossing [29][30][31][32][33][34][35].…”

Fast Holonomic Quantum Computation on Superconducting Circuits With Optimal Control

“…Based on the current experimental technique, the strong capacity coupling between transmon qubits has been achieved experimentally on superconducting circuits. [ 60–63 ] Here, we consider the case that two transmon qubits are capacitively coupled simultaneously to an auxiliary transmon by the capacity coupling. As shown in Figure 1c, the auxiliary transmon with frequency dispersively coupled to both qubits with frequencies .…”

Precise single-qubit control of the reflection phase of a photon mediated by a strongly-coupled ancilla–cavity system

“…The reflection coefficient R q (ω) is effectively controlled by the qubit state if ε>Ω?Γ i , κ. Note that we must also assume sufficient anharmonicity of the transmon energy levels to avoid excitation of the qubit transmon to higher states [48,49] and undesired couplings among ancilla levels due to the strong classical microwave drive Ω. This puts requirements on the anharmonicities α i ?Ω, ò of the transmons.…”

“…The coupling between transmons is taken as ò=150 MHz and the other parameters are optimized numerically as before, specifically the bandwidth and EIT microwave strength, which are in the 10 MHz and 100 MHz range, respectively, but increase with g. Error rates are limited to interaction times of hundreds of ns, similar to circuit-QED two-qubit gates which have similar limitations. Thus, qubit-photon gates can achieve similar intrinsic fidelities as qubitqubit gates [49], with photon traveling losses likely to be the most severe bottleneck for scalable technology in the foreseeable future.…”