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arXiv:2501.07232 · Optica

Protecting collective qubits from non-Markovian dephasing

Antoine Covolo, Valentin Magro, Mathieu Girard, Sébastien Garcia, Alexei Ourjoumtsev

Published Jul 23, 2025• 3 citations
arXivHTMLPDF

Abstract

In quantum technologies, qubits encoded as collective atomic or solid-state excitations present important practical advantages, such as strong directional coupling to light. Unfortunately, they are affected by inhomogeneities between the emitters, which make them decay into dark states. In most cases, this process is non-Markovian. Through a simple formalism, we unveil a regime where this decay is suppressed by a combination of driving and non-Markovianity. We experimentally demonstrate this “driving protection” using a Rydberg superatom, making its coherent Rabi oscillations last about 14 times longer than the free lifetime of the collective qubit.

DOI
10.1364/optica.563676

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In quantum technologies, qubits encoded as collective atomic or solid-state excitations present important practical advantages, such as strong directional coupling to light. Unfortunately, they are affected by inhomogeneities between the emitters, which make them decay into dark states. In most cases, this process is non-Markovian. Through a simple formalism, we unveil a regime where this decay is suppressed by a combination of driving and non-Markovianity. We experimentally demonstrate this “driving protection” using a Rydberg superatom, making its coherent Rabi oscillations last about 14 times longer than the free lifetime of the collective qubit.
Preventing quantum entanglement from decoherence effect is of theoretical and practical importance in the quantum information processing technologies. In this regard, we consider the entanglement dynamics of two identical qubits where the qubits which are coupled to two independent (Markovian and/or non-Markovian) as well as a common reservoir at zero temperature are further interacted with a classical driving laser field. Then, we study the preservation of generated two-qubit entanglement in various situations using the concurrence measure. It is shown that by applying the classical driving field and so the possibility of controlling the Rabi frequency, the amount of entanglement of the two-qubit system is improved in the off-resonance condition between the qubit and the central cavity frequencies (central detuning) in both non-Markovian and Markovian reservoirs. While the central detuning has a constructive role, the detuning between the qubit and the classical field (laser detuning) affects negatively on the entanglement protection. The obtained results show that long-living entanglement in the non-Markovian reservoir is more accessible than in the Markovian reservoir. We demonstrate that, in a common reservoir non-zero stationary entanglement is achievable whenever the two-qubit system is coupled to the reservoir with appropriate values of relative coupling strengths.
Collective qubits between atomic ground and Rydberg states can be converted, on-demand, into single photons, making them well-suited for scalable quantum network-type protocols. We demonstrate long-lived many-body Rabi oscillations and multi-particle entanglement, as well as study the dynamics of interaction-induced dephasing for collective Rydberg qubits held in a state-insensitive optical lattice trap. Both excitation blockade and spin-wave dephasing can contribute to suppression of multiple excitations, allowing for deterministic preparation of collective atomic qubits, single photons, and atom-photon entanglement for quantum information processing.
We introduce and demonstrate a scheme for eliminating the inhomogeneous dephasing of a collective quantum state.The scheme employs off-resonant fields that continuously dress the collective state with an auxiliary sensor state, which has an enhanced and opposite sensitivity to the same source of inhomogeneity.We derive the optimal conditions under which the dressed state is fully protected from dephasing when using either one or two dressing fields.The latter provides better protection, circumvents qubit phase rotation, and suppresses the sensitivity to drive noise.We further derive expressions for all residual, higherorder sensitivities.We experimentally study the scheme by protecting a collective excitation of an atomic ensemble, where inhomogeneous dephasing originates from thermal motion.Using photon storage and retrieval, we demonstrate complete suppression of inhomogeneous dephasing and, consequently, a prolonged memory time.Our scheme may be applied to eliminate motional dephasing in other systems, improving the performance of quantum gates and memories with neutral atoms.It is also generally applicable to various gas, solid, and engineered systems, where sensitivity to variations in time, space, or other domains limits possible scale-up of the system.
Quantum systems are affected by interactions with their environments, causing decoherence through two processes: pure dephasing and energy relaxation. For quantum information processing it is important to increase the coherence time of Josephson qubits and other artificial two-level atoms. We show theoretically that if the coupling between these qubits and a cavity field is longitudinal and in the ultrastrong-coupling regime, the system is strongly protected against relaxation. Vice versa, if the coupling is transverse and in the ultrastrong-coupling regime, the system is protected against pure dephasing. Taking advantage of the relaxation suppression, we show that it is possible to enhance their coherence time and use these qubits as quantum memories. Indeed, to preserve the coherence from pure dephasing, we prove that it is possible to apply dynamical decoupling. We also use an auxiliary atomic level to store and retrieve quantum information.
This study investigates the performance and ergotropy protection of open collective quantum batteries subject to superradiant decay. By employing a passive spectral detuning strategy within an intermediate cavity, an optimal detuning value (Δ∗Δ^*Δ∗) is analytically derived and numerically verified to spectrally isolate the system and protect quantum coherence, achieving up to 1088% ergotropy improvement for single qubits and superextensive collective advantage for N≥3N \ge 3N≥3. Our analysis resolves a "non-Markovian paradox," revealing that maximizing ergotropy does not strictly require non-Markovian memory; rather, suppressing environmental memory via detuning optimally preserves coherence, which serves as the fundamental resource. Survival maps across different environments demonstrate that thermal noise dissipates coherence more severely than telegraph noise. Finally, we establish that collective amplification of the effective coupling (geff=gN)g_{\rm eff} = g\sqrt{N})geff​=gN​) inevitably drives large qubit arrays into the ultra-strong coupling regime, providing a quantitative ceiling NmaxN_{\rm max}Nmax​ on the validity of the Tavis-Cummings description and the current ergotropy protection protocol.
Finding strategies to preserve quantum resources in open systems is nowadays a main requirement for reliable quantum-enhanced technologies. We address this issue by considering structured cavities embedding qubits whose transition frequency is modulated by a driving field. We first focus on a single-qubit system to determine the optimal modulation parameters and the qubit-cavity coupling regime allowing a long-time preservation of both coherence and quantum Fisher information. We relate this behavior to the inhibition of the qubit effective decay rate rather than to stronger memory effects (non-Markovianity) of the system. We then use these findings to efficiently control resources like entanglement, discord and coherence in a system of two separated qubits. We discuss the feasibility of the system and show that individual qubit frequency modulation can produce quantum resource lifetimes orders of magnitude longer than their natural (uncontrolled) decay times. These results provide new useful insights towards efficient experimental strategies against decoherence.