High-fidelity quantum control relies on accurate models of driven dynamics. We examine this re- quirement for single-qubit gates in superconducting transmons by comparing control pulses derived from the standard Duffing approximation and from a Hamiltonian constructed by diagonalizing the transmon eigenbasis. Using the same correction-pulse construction for both models, we show that correction fields derived from the Duffing approximation can substantially reduce the gate error pre- dicted by that model while remaining less effective when combined with an independently calibrated baseline pulse in the diagonalized-transmon model. In the fast-gate regime, such transferred correc- tions can even fail to improve over the uncorrected diagonalized-transmon baseline. We show that small model-dependent differences in both the energy spectrum and the representation of the drive operator can compound during driven evolution, resulting in different predicted error generators and correction pulses. A mismatch in the accumulated AC Stark phase provides one illustrative di- agnostic of this dynamical model dependence. We further demonstrate that the model Hamiltonian informs the choice of control framework: Omitting relevant leakage pathways or higher-order error channels can lead to an overly restricted correction strategy. Including these channels motivates an extended correction framework that improves the gate performance using the same physical control resources.
The open-loop optimization of quantum dynamics using gradient-based quantum optimal control methods involves calculating the time-ordered propagator and its gradient. In this Letter, we present a unifying framework for gradient-based quantum optimal control with respect to any general pulse parameterization by deriving the formal solution from first principles. For the case of unitary propagators, we derive a series expansion involving time-independent commutators and time-dependent coefficients, significantly reducing the number of matrix exponentials needed to compute the gradient. The expansion highlights the connection between derivatives of the propagator and operator evolution in the Heisenberg picture. The method is particularly suited for simulating optimal control tasks in quantum systems with local interactions, which is a common situation in large multi-qubit platforms. We compare the computational cost required for the series with the Gradient Optimization of Analytic conTrols (GOAT) method, and, focusing on the problem of preparation of a GHZ state, demonstrate more than an order of magnitude speedup for a qubit ladder and a chain geometry.
Ashutosh Mishra, Elena Lupo, Frank K. Wilhelm et al.· 0 citations
We investigate the simultaneous control of quantum parameter-estimation precision and non-Markovian memory effects in a driven two-qubit system that is collectively coupled to a leaky cavity. Using a dressed-state formalism, we derive analytical solutions for identical and non-identical qubits and analyze how classical driving and detuning affect metrological performance. Classical driving enhances the quantum Fisher information (QFI) by sustaining coherence and shifting effective transitions away from cavity resonance, thereby suppressing decoherence in both weak- and strong-coupling regimes. The steady-state QFI is further improved by positive classical-field detuning, which reduces spectral overlap with the lossy reservoir. To characterize memory effects, we examine the QFI-flow and the Breuer–Laine–Piilo (BLP) trace-distance measure. Our results reveal a nontrivial control trade-off: increasing the Rabi frequency suppresses non-Markovian information backflow, whereas increasing qubit detuning enhances memory effects by promoting system–environment correlations. Despite their different definitions, the QFI-flow and BLP measures yield consistent qualitative behavior in this system. These findings demonstrate that classical-field parameter tuning empowers researchers to optimize estimation precision and non-Markovianity in dissipative cavity-QED, with direct applicability to superconducting circuit platforms.
Ghasem Zare Firozi, A. Mortezapour, Borhan Arghavani Nia et al.· AVS Quantum Science· 0 citations
