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Photonics Research

Measurement-free quantum feedforward control: theory and experiment

Maolin Luo, Heyun Tan, Xiaoqian Zhang, Fei Gao, Xiaoqi Zhou

Published May 22, 2026• 0 citations
Publisher

Abstract

The goal of quantum control is to enable robust quantum information processing in noisy environments. Quantum feedback control is one of the most established strategies, but its reliance on measurement and real-time feedback imposes intrinsic limitations from measurement backaction and latency. Here, we propose and experimentally realize a measurement-free quantum feedforward control scheme, together with a composite framework that sequentially combines quantum feedforward control and quantum feedback control. We demonstrate both theoretically and experimentally that quantum feedforward control effectively protects nonorthogonal quantum states without measurement, while the composite strategy achieves superior performance when measurement is available, which makes it a practical alternative to quantum feedback control. These results establish quantum feedforward control as an independent control approach beyond quantum feedback control and highlight the advantages of the composite scheme, providing a new route toward high-fidelity quantum information processing in noisy environments.

DOI
10.1364/prj.595791

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The goal of quantum control is to enable robust quantum information processing in noisy environments. Quantum feedback control is one of the most established strategies, but its reliance on measurement and real-time feedback imposes intrinsic limitations from measurement backaction and latency. Here, we propose and experimentally realize a measurement-free quantum feedforward control scheme, together with a composite framework that sequentially combines quantum feedforward control and quantum feedback control. We demonstrate both theoretically and experimentally that quantum feedforward control effectively protects nonorthogonal quantum states without measurement, while the composite strategy achieves superior performance when measurement is available, which makes it a practical alternative to quantum feedback control. These results establish quantum feedforward control as an independent control approach beyond quantum feedback control and highlight the advantages of the composite scheme, providing a new route toward high-fidelity quantum information processing in noisy environments.
Quantum feedback control (QFBC) and quantum feedforward control (QFFC) are two of the major techniques for protecting two nonorthogonal qubit states against decoherence. In this paper, we propose a quantum composite control scheme for protecting such states, where QFBC and QFFC are combined. Note that the combination is deliberately devised, other than simply one plus one (i.e., the trivial combination). We show the performance of our scheme is better than that of both the previous control schemes and the trivially combined one for most states and noises in the sense that, for given success probability, our scheme can achieve the biggest fidelity, and for given fidelity, our scheme has the biggest success probability. Furthermore, our scheme can be experimentally implemented with current technologies.
A goal of the emerging field of quantum control is to develop methods for quantum technologies to function robustly in the presence of noise. Central issues are the fundamental limitations on the available information about quantum systems and the disturbance they suffer in the process of measurement. In the context of a simple quantum control scenario-the stabilization of nonorthogonal states of a qubit against dephasing-we experimentally explore the use of weak measurements in feedback control. We find that, despite the intrinsic difficultly of implementing them, weak measurements allow us to control the qubit better in practice than is even theoretically possible without them. Our work shows that these more general quantum measurements can play an important role for feedback control of quantum systems.
Measurements in quantum mechanics cannot perfectly distinguish all states and necessarily disturb the measured system. We present and analyze a proposal to demonstrate fundamental limits on quantum control of a single qubit arising from these properties of quantum measurements. We consider a qubit prepared in one of two nonorthogonal states and subsequently subjected to dephasing noise. The task is to use measurement and feedback control to attempt to correct the state of the qubit. We demonstrate that projective measurements are not optimal for this task, and that there exists a nonprojective measurement with an optimum measurement strength which achieves the best trade-off between gaining information about the system and disturbing it through measurement backaction. We study the performance of a quantum control scheme that makes use of this weak measurement followed by feedback control, and demonstrate that it realizes the optimal recovery from noise for this system. We contrast this approach with various classically inspired control schemes.
Recent progress in quantum physics has made it possible to perform experiments in which individual quantum systems are monitored and manipulated in real time.The advent of such new technical capabiMies provides strong motivation for the development of theoretical and experimental methodologies for quantum feedback control.The availability of such methods would enable radically new approachesto experimentalphysics in the quantum realm.Likewise, the investigationof quantumfeedback control will introduce crucialnew considerationsto control theory, such as the uniquely quantum phenomena of entanglement and measurement backaction.The extension of established ana2ysistectilques from control theory into the quantum domain may also provide new insight into the dynamics of complex quantum systems.We anticipate that the successful formulation of an input-output approach to the analysis and reduction of large quantum systems could have very general applications in non-equilibrium quantum statistical mechanics and in the nascent field quantum information theory...
Quantum computers hold immense promise for solving problems intractable for classical machines, yet their practical realization is hindered by the fragility of quantum states. Quantum Error Correction (QEC) is indispensable for protecting quantum information from environmental noise and operational imperfections. Traditional QEC schemes often rely on measurement-based feedback, which involves performing measurements, processing the classical outcomes, and then applying corrective operations. This process introduces latency, consumes significant resources, and can itself be a source of errors due to the invasive nature of quantum measurement and the classical-quantum interface. Coherent feedback control offers a compelling alternative by allowing the controller to directly interact with the quantum system using quantum fields, without intermediate measurement and classical processing. This maintains the coherence of the quantum information throughout the feedback loop, potentially leading to more robust and faster error correction. This paper explores the theoretical framework and potential advantages of employing coherent feedback control for autonomous QEC. We review existing QEC paradigms, delve into the principles of coherent feedback, and propose how this approach can be integrated into autonomous QEC architectures. Specifically, we investigate its application to bosonic codes and stabilizer codes, discussing the engineering challenges and the prospects for achieving truly fault-tolerant quantum computation by minimizing the classical overhead and enhancing real-time error suppression.
Progress in quantum physics has made it possible to perform experiments in which individual quantum systems are monitored and manipulated in real time. The advent of such new technical capabilities provides strong motivation for the development of theoretical and experimental methodologies for quantum feedback control. The availability of such methods would enable radically new approaches to experimental physics in the quantum realm. Likewise, the investigation of quantum feedback control will introduce crucial new considerations to control theory, such as the uniquely quantum phenomena of entanglement and measurement back-action. The extension of established analysis techniques from control theory into the quantum domain may also provide new insight into the dynamics of complex quantum systems. We anticipate that the successful formulation of an input-output approach to the analysis and reduction of large quantum systems could have very general applications in nonequilibrium quantum statistical mechanics and in the nascent field of quantum information theory.