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Physical Review Letters

Experimental Measurement-Device-Independent Verification of Continuous-Variable Entanglement

Xutong Wang, Jing Wen Fu, Jietai Jing

Published Sep 10, 2025• 1 citation
Publisher

Abstract

Quantum entanglement serves as an essential resource for quantum information technology. In many realistic quantum information tasks, it is necessary to reliably verify entanglement even when the measurement devices are unreliable. In this Letter, we experimentally demonstrate measurement-device-independent verification of continuous-variable (CV) entanglement. We show that the conventional CV entanglement witness may falsely identify a separable state as a CV entangled state when the measurement devices are unreliable. To eliminate such need for trusted measurement devices, we implement a measurement-device-independent entanglement witness for verifying CV entanglement and demonstrate its resilience against local oscillator power and phase modifications. Our Letter sheds light on advancing high-security quantum information processing and its practical implementation in quantum networks.

DOI
10.1103/rk3t-69tb

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Quantum entanglement serves as an essential resource for quantum information technology. In many realistic quantum information tasks, it is necessary to reliably verify entanglement even when the measurement devices are unreliable. In this Letter, we experimentally demonstrate measurement-device-independent verification of continuous-variable (CV) entanglement. We show that the conventional CV entanglement witness may falsely identify a separable state as a CV entangled state when the measurement devices are unreliable. To eliminate such need for trusted measurement devices, we implement a measurement-device-independent entanglement witness for verifying CV entanglement and demonstrate its resilience against local oscillator power and phase modifications. Our Letter sheds light on advancing high-security quantum information processing and its practical implementation in quantum networks.
The reliable detection of entanglement plays a crucial role in the construction of quantum networks. The conventional entanglement witness (EW) method has high requirements for measurement device and relies on reliable implementation of the measurement. With unreliable measurement device, EW process can be easily attacked by eavesdroppers, which can lead to incorrect entanglement detection results. Therefore, a feasible and secure measurement device independent entanglement witness (MDIEW) method is desired for constructing quantum networks. Here, we detect the continuous variable entanglement by a MDIEW in a quantum network. It is demonstrated that the conventional EW method can be affected when the local oscillator intensity is changed, while the MDIEW method can detect entanglement between users without being affected. Our results provide a trustworthy method to detect entanglement, which is an important step for constructing secure quantum networks.
We experimentally demonstrated a protocol for measurement device independent entanglement witness (MDI-EW) presented in [1]. A measurement device independent (MDI) protocol will allow two parties (Alice and Bob) to verify that they share an entangled state via the help of trusted input states without making any assumption on the measurement device using only the statistics of the output of the measurement device. Verification of entanglement between states is important as entangled states are a commonly used resource in several aspects of quantum information science such as quantum key distribution. Experimental implementation of MDI-EW in discrete variables have previously been demonstrated in [2], [3].
Entanglement, the essential resource in quantum information processing, should be witnessed in many tasks such as quantum computing and quantum communication. The conventional entanglement witness method, relying on an idealized implementation of measurements, could wrongly conclude a separable state to be entangled due to imperfect detections. Inspired by the idea of a time-shift attack, we construct an attack on the conventional entanglement witness process and demonstrate that a separable state can be falsely identified to be entangled. To close such detection loopholes, based on a recently proposed measurement-device-independent entanglement witness method, we design and experimentally demonstrate a measurement-device-independent entanglement witness for a variety of two-qubit states. By the new scheme, we show that an entanglement witness can be realized without detection loopholes.
Secure and reliable certification of quantum resources is a fundamental challenge in the advancement of next-generation quantum technologies, particularly as devices become more complex and integrated into practical applications, where parts of the system may be untrusted or inaccessible by users. Addressing this challenge requires certification methods that rely on minimal assumptions and limited trust while still faithfully and reliably verifying the quantum resources in question. For infinite-dimensional bosonic systems, existing certification methods rely on fully trusted and well-calibrated measurement systems, leaving significant security vulnerabilities open. In this work, we present the first experimental demonstration of measurement-device-independent (MDI) certification schemes for infinite-dimensional bosonic systems, where the certification process is conducted using entirely untrusted measurement devices and assuming only the trusted preparation of coherent states. Specifically, we implement schemes for the MDI certification of continuous-variable (CV) entanglement and the operation of an elementary optical CV quantum memory. We leverage techniques of Bayesian metrology and exploit the practicality and accessibility of Gaussian quantum optics to achieve secure and efficient certification. These results demonstrate the potential of the MDI framework to enhance trust in quantum technologies with applications in quantum communication and quantum computing.
Entanglement, a critical resource for quantum information processing, needs to be witnessed in many practical scenarios. Theoretically, witnessing entanglement is by measuring a special Hermitian observable, called an entanglement witness (EW), which has non-negative expected outcomes for all separable states but can have negative expectations for certain entangled states. In practice, an EW implementation may suffer from two problems. The first one is reliability. Due to unreliable realization devices, a separable state could be falsely identified as an entangled one. The second problem relates to robustness. A witness may not be optimal for a target state and fail to identify its entanglement. To overcome the reliability problem, we employ a recently proposed measurement-device-independent entanglement witness scheme, in which the correctness of the conclusion is independent of the implemented measurement devices. In order to overcome the robustness problem, we optimize the EW to draw a better conclusion given certain experimental data. With the proposed EW scheme, where only data postprocessing needs to be modified compared to the original measurement-device-independent scheme, one can efficiently take advantage of the measurement results to maximally draw reliable conclusions.
Abstract A proper quantum memory is argued to consist in a quantum channel which cannot be simulated with a measurement followed by classical information storage and a final state preparation, i.e. with an entanglement breaking (EB) channel. The verification of quantum memories (non-EB channels) is a task in which an honest user wants to test the quantum memory of an untrusted, remote provider. This task is inherently suited for the class of protocols with trusted quantum inputs, sometimes called measurement-device-independent (MDI) protocols. Here, we study the MDI certification of non-EB channels in continuous variable (CV) systems. We provide a simple witness based on adversarial metrology, and describe an experimentally friendly protocol that can be used to verify all non Gaussian incompatibility breaking quantum memories. Our results can be tested with current technology and can be applied to test other devices resulting in non-EB channels, such as CV quantum transducers and transmission lines.