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Frontiers in Optics

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, Giuseppe Vallone, Paolo Villoresi

Published Jan 1, 2023• 0 citations
Publisher

Abstract

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.

DOI
10.1364/fio.2023.jtu5a.78

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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.
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.
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.
Measurement-device-independent quantum key distribution (MDI-QKD) is proved to be able to eliminate all potential detector side channel attacks. Combining with the reference frame independent (RFI) scheme, the complexity of practical system can be reduced because of the unnecessary alignment for reference frame. Here, based on polarization multiplexing, we propose a time-bin encoding structure, and experimentally demonstrate the RFI-MDI-QKD protocol. Thanks to this, two of the four Bell states can be distinguished, whereas only one is used to generate the secure key in previous RFI-MDI-QKD experiments. As far as we know, this is the first demonstration for RFI-MDI-QKD protocol with clock rate of 50 MHz and distance of more than hundred kilometers between legitimate parties Alice and Bob. In asymptotic case, we experimentally compare RFI-MDI-QKD protocol with the original MDI-QKD protocol at the transmission distance of 160 km, when the different misalignments of the reference frame are deployed. By considering observables and statistical fluctuations jointly, four-intensity decoy-state RFI-MDI-QKD protocol with biased bases is experimentally achieved at the transmission distance of 100km and 120km. The results show the robustness of our scheme, and the key rate of RFI-MDI-QKD can be improved obviously under a large misalignment of the reference frame.
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.
Abstract Robust implementation of quantum key distribution requires precise state generation and measurements, as well as the choice of an optimal encoding to minimize channel disturbance. Time‐bin encoding represent a good candidate for fiber links as birefingence does not perturb this kind of states whereas stable and low‐error encoders and decoders are available for polarization encoding. Here a cross‐encoded scheme where high accuracy quantum states are prepared through a self‐compensating, calibration‐free polarization modulator and transmitted using a polarization‐to‐time‐bin converter is presented. The receiver performs time‐of‐arrival measurements in the key‐generation basis and converts qubits back to polarization encoding for measurements in the control basis. Temporal synchronization between the transmitter and receiver is performed with a qubit‐based method that does not require additional hardware to share a clock reference. The system is tested in a 12‐h run and demonstrates good and stable performance in terms of key and quantum bit error rates. The flexibility of this approach represents an important step toward the development of hybrid networks with both fiber‐optic and free‐space links.
Quantum key distribution (QKD) is moving from research laboratories towards applications. As computing becomes more mobile, cashless as well as cardless payment solutions are introduced. A possible route to increase the security of wireless communications is to incorporate QKD in a mobile device. Handheld devices present a particular challenge as the orientation and the phase of a qubit will depend on device motion. This problem is addressed by the reference frame independent (RFI) QKD scheme. The scheme tolerates an unknown phase between logical states that vary slowly compared to the rate of particle repetition. Here we experimentally demonstrate the feasibility of RFI QKD over a free-space link in a prepare and measure scheme using polarization encoding. We extend the security analysis of the RFI QKD scheme to be able to deal with uncalibrated devices and a finite number of measurements. Together these advances are an important step towards mass production of handheld QKD devices.