arXiv subjects

Dragomir Davidović

Publications and source records attributed to Dragomir Davidović.

4 recordsLinked to original sources

arXiv2026

Sixth-order time-convolutionless master equation and beyond: Late-time resummations, two types of divergences, and the limits of validity

Perturbative master equations are essential for modeling open quantum systems but often exhibit late-time divergences when environmental correlations decay algebraically. In this work, we analyze the time-convolutionless (TCL) master equation, expanded to order 2n and demonstrate that, while van Kampens cumulants suppress early-time secular growth, they ultimately diverge at long times. To overcome this, we introduce a resummation technique based on the Hadamard trick, which incorporates time integrals directly into the bath spectral density via element-wise multiplication. This approach establishes a maximum expansion order, nmax, and defines a precision limit of the asymptotic states. The resummed master equation features renormalized Bohr frequencies that capture decoherence and spectral overlap effects. In the unbiased spin-boson model, this results in secular inflation of the generator at a temperature-independent rate equal to the decoherence rate and a finite validity time. For exponentially decaying correlations, the method recovers a proper Markovian limit below a critical coupling threshold.

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arXiv2026

Correlation-Enabled Beatings in Two-Dimensional Electronic Spectroscopy

Long-lived beatings in two-dimensional electronic spectroscopy (2DES) remain difficult to interpret within standard excitonic open-system models, which typically assume factorized initialization and predict rapid coherence decay. We show that persistent beatings can arise from a correlation-driven mechanism that requires both slow bath memory and ultrafast pulse sequences that propagate system-bath correlations across optical interactions. In this regime, the pulse sequence unitarily dresses the bath-memory contribution and activates nonsecular population-coherence transfer during field-free evolution, sustaining coherence signatures far beyond factorized or weak-memory descriptions. Rather than addressing what is oscillating (excitonic versus vibronic) or quantum-versus-classical semantics, this work reframes long-lived beatings as a protocol-level dynamical effect: correlation-mediated retrieval under ultrafast control.

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arXiv2025

Gate-Based Initialization and Fidelity in Correlated Open Quantum Systems

We present a minimal and general framework for initializing open quantum systems via gate operations, treating system-bath correlations and control dynamics on equal footing. Our protocol simulates thermal equilibration followed by a gate pulse, modeled using a time-dependent Bloch-Redfield equation accurate to second order in coherence. After the gate, the system exhibits hybrid dynamics: populations evolve Markovianly, while coherences dephase as if the system were initially factorized. In fast, strongly dephasing regimes, initial system-bath correlations can revive coherence. Gate fidelities reveal spin-echo-like suppression of errors near $2π$ rotations, indicating an intrinsic mechanism for error cancellation. The approach is efficient and broadly applicable to both quantum devices and molecular complexes. Applications include fidelity optimization with respect to bath correlation times and coherence tracking in systems such as the Fenna-Matthews-Olson complex. Our results show that long-lived excitonic coherences can arise from strongly non-Markovian dynamics triggered by ultrafast pulses.

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arXiv2024

Invalidation of the Bloch-Redfield Equation in Sub-Ohmic Regime via a Practical Time-Convolutionless Fourth-Order Master Equation

Despite recent advances in quantum sciences, a quantum master equation that accurately and simply characterizes open quantum dynamics across extremely long timescales and in dispersive environments is still needed. In this study, we optimize the computation of the fourth-order time-convolutionless master equation to meet this need. Early versions of this master equation required computing a multidimensional integral, limiting its use. Our master equation accounts for simultaneous relaxation and dephasing, resulting in coefficients proportional to the system's spectral density over frequency derivative. In sub-Ohmic environments, this derivative induces infrared divergence in the master equation, invalidating the second-order Bloch-Redfield master equation findings. We analyze the approach to a ground state in a generic open quantum system and demonstrate that it is not reliably computed by the Bloch-Redfield equation alone. The optimized fourth-order equation shows that the ground-state approach is accurate to second order in bath coupling regardless of the dispersion, even though it can diverge in the fourth order at zero temperature.

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