© 2026 Aqora Quantum S.A.S.

Terms Privacy Legal Notice

arXiv:2408.17304 · Optica Quantum

A Passive and Self-Characterizing Cross-Encoded Receiver for Reference-Frame-Independent Quantum Key Distribution

Massimo Giacomin, Francesco B. L. Santagiustina, Giuseppe Vallone, Paolo Villoresi, Costantino Agnesi

Published Jun 20, 2025• 2 citations

Abstract

Quantum Key Distribution (QKD) promises to revolutionize the field of security in communication, with applications ranging from state secrets to personal data, making it a key player in the ongoing battle against cyber threats. Reference-Frame-Independent (RFI) QKD aims to simplify QKD implementations by allowing to reduce the requirements of alignment on a shared reference frame. This is done by performing two mutually unbiased measurements on the control states. In this work, we present a novel fully passive receiver for time-bin encoded RFI-QKD. Conversion of time-bin to polarization is employed to perform the required quantum measurement in a fully passive manner. Furthermore, to overcome experimental errors, we retrieved a complete description of our measurement apparatus by employing a recently introduced Quantum Detector Self-Characterization technique, without performing tomographic studies on the detection stage. In fact, the security analysis carried out in this work uses experimentally retrieved Positive Operator Valued Measurements, which consider our receiver defects, substituting the ideal expected operators and thus increasing the overall level of secrecy. Lastly, we conducted a proof-of-principle experiment that validated the feasibility of our method and its applicability to QKD applications.

DOI: 10.1364/opticaq.552059

Related papers

Free-Running Analysis for a Fully-Passive Self-Characterizing Reference-Frame-Independent Quantum Key Distribution Receiver

Massimo Giacomin, Francesco B. L. Santagiustina, Giuseppe Vallone +2 more

Quantum Key Distribution (QKD) is a strategy for securely generating cryptographic keys between two authenticated users. However, in real-world QKD applications, maintaining a consistent reference frame between users is crucial, typically requiring complex systems to control quantum properties of photons, as polarization or phase. To overcome these challenges, a free-running approach to increase the level of secrecy of the distilled key rate, with a fully passive and self-characterizing receiver, is presented. This approach exploits the phase invariance of the time-bin (TB) strategy for the key distribution together with the robustness of the polarization encoding for the state measurement. Furthermore, the presented receiving apparatus implements a self-characterization procedure, in order to reconstruct the experimental POVMs describing the real measurement procedure implemented by the utilized receiver, taking into account all the unavoidable imperfections. The results provided in this work confirm the maturity of RFI-based quantum key distribution protocols without the need of implementing actively compensating systems. This kind of receivers offer less expensive solutions in terms of costs and complexity for a variety of quantum network implementations in environmental noisy quantum networks.

International Conference on Quantum Communications, Networking, and Computing (QCNC)•Mar 31, 2025•0 citations

Experimental study of four-state reference-frame-independent quantum key distribution with source flaws

Jipeng Wang, Hongwei Liu, Haiqiang Ma +1 more

Reference-frame-independent quantum key distribution (RFI-QKD) has been proposed as a method to avoid the requirement of real-time alignment of reference frames. However, the source flaw has not been considered in most of the previously reported works. In this paper, based on the analysis of Wang et al. [Phys. Rev. A 92, 042319 (2015)], we perform a proof-of-concept experiment to demonstrate the loss-tolerant RFI-QKD by constructing a time-bin phase-encoded system and quantifying its source flaws. Moreover, the finite-key analysis is taken into account while estimating the secure key rate. Our experimental demonstration makes a positive contribution to the realization of quantum cryptography in a complex and imperfect real-world environment.

Physical Review A•Mar 7, 2019•30 citations

A Receiver for Reference-Frame-Independent Quantum-Key-Distribution Based on Hybrid Polarization and Time-Bin Encoding

Massimo Giacomin, Costantino Agnesi, Francesco B. L. Santagiustina +2 more

We propose a novel receiver scheme for RFI-QKD based on a hybrid conversion of time-bin encoded qubits into polarization. This allows us to obtain stable transmission of time-of-arrival information whereas the phase is retrieved via free-space polarization measurements. A prototype is developed and preliminary experimental data are presented.

Frontiers in Optics•Jan 1, 2023•0 citations

Fully passive reference frame independent quantum key distribution

Jia‐Wei Ying, Shi‐Pu Gu, Xing-Fu Wang +2 more

Reference-frame-independent quantum key distribution (RFI QKD) significantly alleviates alignment requirements for reference frame in practical quantum communication systems. While the original protocol requires Alice to prepare six quantum states in $Z$, $X$, and $Y$ bases, its reliance on active modulation introduces inherent side-channel vulnerabilities from device imperfections. We address this security limitation by integrating a fully passive source into the RFI framework. In this paper, we propose a fully passive RFI QKD protocol. Our protocol avoids active modulation entirely, suppressing side-channel risks through passive quantum state generation. Moreover, by making full utilization of the quantum states generated by the fully passive source, we enhance the secure key rate of fully passive protocol. We establish a system model to analyze the performance of the protocol. Through the optimization of post-selection intervals and intensities, we obtain the maximum secure key transmission rate of the protocol. Under ideal circumstances, the secure key transmission rate of our protocol can reach more than 50% of that of the ideal QKD. Under practical conditions, we have considered the finite-length effect. When pulse number generated by the source reaches $10^{12}$, the maximum communication distances of the protocol can reach 167 km and 136 km with a reference frame misalignment of $0 $ and $45^{\circ} $ respectively. We believe that our protocol can contribute to the development of practical QKD systems.

arXiv:2504.15528•Apr 22, 2025•0 citations

Field demonstration of time-bin reference-frame-independent quantum key distribution via an intracity free-space link

Huan Chen, Jipeng Wang, Bang-Ying Tang +3 more

Free-space quantum key distribution (QKD) based on mobile platforms, such as satellites, drones, and vehicles, is considered a promising way to overcome the rate-distance limit without a quantum repeater. Real-time reference frame calibration is required in most recent implemented polarization encoded QKD systems due to the relative motion between sender and receiver. Although active compensations can be used to calibrate the reference frame, doing so increases the complexity of the system and reduces the key rate. To overcome this problem, the reference-frame-independent (RFI) QKD was proposed in which fixed deviations of the reference frame between the two parties are tolerated automatically. In this Letter, we report the experimental implementation of a time-bin encoded RFI QKD in an urban environment through free space. The quantum bit error rate for key-distill is as low as 1% over a 2 km free-space link with a total equivalent loss of 31.5 dB. Our demonstration shows that a stable RFI QKD can be implemented in the free-space channel.

Optics Letters•Apr 27, 2020•34 citations

Fully passive received quantum access network based on reference-frame-independent quantum key distribution

Xiaodong Fan, Yuehua Chen, Tianxiang Zhan +10 more

Information-theoretically secure quantum access network (QAN) provides a multiple-user structure for large-scale key distribution. However, active reference-frame compensation and clock synchronization require extra devices and signals, increasing the complexity of the system and time/resource consumption. In this work, a fully passive received QAN comprising both free-space and fiber channels is demonstrated. Based on an improved reference-frame-independent quantum key distribution (RFI-QKD) protocol and qubit-synchronization method, Bob only passively receives quantum signals from all Alices with various wavelengths and laser repetition rates. Extra classical signals for reference frame calibration and clock synchronization are not required, so that the QAN can be asynchronous. Furthermore, RFI-QKD with a 7 km free-space channel in an urban environment is first fully demonstrated with a positive key rate, and the maximal distance over a 250 km fiber channel for four-state RFI-QKD is also achieved. The experimental results show that our fully passive received QAN is robust, cost-effective, and flexible, paving its application in the future quantum network.

npj Quantum Information•Jun 1, 2026•0 citations

Quantum Key Distribution (QKD) promises to revolutionize the field of security in communication, with applications ranging from state secrets to personal data, making it a key player in the ongoing battle against cyber threats. Reference-Frame-Independent (RFI) QKD aims to simplify QKD implementations by allowing to reduce the requirements of alignment on a shared reference frame. This is done by performing two mutually unbiased measurements on the control states. In this work, we present a novel fully passive receiver for time-bin encoded RFI-QKD. Conversion of time-bin to polarization is employed to perform the required quantum measurement in a fully passive manner. Furthermore, to overcome experimental errors, we retrieved a complete description of our measurement apparatus by employing a recently introduced Quantum Detector Self-Characterization technique, without performing tomographic studies on the detection stage. In fact, the security analysis carried out in this work uses experimentally retrieved Positive Operator Valued Measurements, which consider our receiver defects, substituting the ideal expected operators and thus increasing the overall level of secrecy. Lastly, we conducted a proof-of-principle experiment that validated the feasibility of our method and its applicability to QKD applications.

Quantum Key Distribution (QKD) is a strategy for securely generating cryptographic keys between two authenticated users. However, in real-world QKD applications, maintaining a consistent reference frame between users is crucial, typically requiring complex systems to control quantum properties of photons, as polarization or phase. To overcome these challenges, a free-running approach to increase the level of secrecy of the distilled key rate, with a fully passive and self-characterizing receiver, is presented. This approach exploits the phase invariance of the time-bin (TB) strategy for the key distribution together with the robustness of the polarization encoding for the state measurement. Furthermore, the presented receiving apparatus implements a self-characterization procedure, in order to reconstruct the experimental POVMs describing the real measurement procedure implemented by the utilized receiver, taking into account all the unavoidable imperfections. The results provided in this work confirm the maturity of RFI-based quantum key distribution protocols without the need of implementing actively compensating systems. This kind of receivers offer less expensive solutions in terms of costs and complexity for a variety of quantum network implementations in environmental noisy quantum networks.

Reference-frame-independent quantum key distribution (RFI-QKD) has been proposed as a method to avoid the requirement of real-time alignment of reference frames. However, the source flaw has not been considered in most of the previously reported works. In this paper, based on the analysis of Wang et al. [Phys. Rev. A 92, 042319 (2015)], we perform a proof-of-concept experiment to demonstrate the loss-tolerant RFI-QKD by constructing a time-bin phase-encoded system and quantifying its source flaws. Moreover, the finite-key analysis is taken into account while estimating the secure key rate. Our experimental demonstration makes a positive contribution to the realization of quantum cryptography in a complex and imperfect real-world environment.

Reference-frame-independent quantum key distribution (RFI QKD) significantly alleviates alignment requirements for reference frame in practical quantum communication systems. While the original protocol requires Alice to prepare six quantum states in

Z

,

X

, and

Y

bases, its reliance on active modulation introduces inherent side-channel vulnerabilities from device imperfections. We address this security limitation by integrating a fully passive source into the RFI framework. In this paper, we propose a fully passive RFI QKD protocol. Our protocol avoids active modulation entirely, suppressing side-channel risks through passive quantum state generation. Moreover, by making full utilization of the quantum states generated by the fully passive source, we enhance the secure key rate of fully passive protocol. We establish a system model to analyze the performance of the protocol. Through the optimization of post-selection intervals and intensities, we obtain the maximum secure key transmission rate of the protocol. Under ideal circumstances, the secure key transmission rate of our protocol can reach more than 50% of that of the ideal QKD. Under practical conditions, we have considered the finite-length effect. When pulse number generated by the source reaches

10^{12}

, the maximum communication distances of the protocol can reach 167 km and 136 km with a reference frame misalignment of $0 $ and $45^{\circ} $ respectively. We believe that our protocol can contribute to the development of practical QKD systems.

Free-space quantum key distribution (QKD) based on mobile platforms, such as satellites, drones, and vehicles, is considered a promising way to overcome the rate-distance limit without a quantum repeater. Real-time reference frame calibration is required in most recent implemented polarization encoded QKD systems due to the relative motion between sender and receiver. Although active compensations can be used to calibrate the reference frame, doing so increases the complexity of the system and reduces the key rate. To overcome this problem, the reference-frame-independent (RFI) QKD was proposed in which fixed deviations of the reference frame between the two parties are tolerated automatically. In this Letter, we report the experimental implementation of a time-bin encoded RFI QKD in an urban environment through free space. The quantum bit error rate for key-distill is as low as 1% over a 2 km free-space link with a total equivalent loss of 31.5 dB. Our demonstration shows that a stable RFI QKD can be implemented in the free-space channel.

Information-theoretically secure quantum access network (QAN) provides a multiple-user structure for large-scale key distribution. However, active reference-frame compensation and clock synchronization require extra devices and signals, increasing the complexity of the system and time/resource consumption. In this work, a fully passive received QAN comprising both free-space and fiber channels is demonstrated. Based on an improved reference-frame-independent quantum key distribution (RFI-QKD) protocol and qubit-synchronization method, Bob only passively receives quantum signals from all Alices with various wavelengths and laser repetition rates. Extra classical signals for reference frame calibration and clock synchronization are not required, so that the QAN can be asynchronous. Furthermore, RFI-QKD with a 7 km free-space channel in an urban environment is first fully demonstrated with a positive key rate, and the maximal distance over a 250 km fiber channel for four-state RFI-QKD is also achieved. The experimental results show that our fully passive received QAN is robust, cost-effective, and flexible, paving its application in the future quantum network.