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arXiv:2004.11877 · Optics Letters

Stable, low-error, and calibration-free polarization encoder for free-space quantum communication

Marco Avesani, Costantino Agnesi, Andrea Stanco, Giuseppe Vallone, Paolo Villoresi

Published Jul 13, 2020• 46 citations
arXivHTMLPDF

Abstract

Polarization-encoded free-space quantum communication requires a quantum state source featuring fast modulation, long-term stability, and a low intrinsic error rate. Here we present a polarization encoder that, contrary to previous solutions, generates predetermined polarization states with a fixed reference frame in free-space. The proposed device does not require calibration either at the transmitter or at the receiver and achieves long-term stability. A proof-of-concept experiment is also reported, demonstrating a quantum bit error rate lower than 0.2% for several hours without any active recalibration.

DOI
10.1364/ol.396412

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Polarization-encoded free-space quantum communication requires a quantum state source featuring fast modulation, long-term stability, and a low intrinsic error rate. Here we present a polarization encoder that, contrary to previous solutions, generates predetermined polarization states with a fixed reference frame in free-space. The proposed device does not require calibration either at the transmitter or at the receiver and achieves long-term stability. A proof-of-concept experiment is also reported, demonstrating a quantum bit error rate lower than 0.2% for several hours without any active recalibration.
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Polarization modulation plays a key role in polarization-encoding quantum key distribution (QKD). Here, we report a new, to the best of our knowledge, polarization modulation scheme based on an inherently stable Sagnac interferometer. The presented scheme is free of polarization mode dispersion and calibration as well as insensitive to environmental influences. Successful experiments at a repetition frequency of 1.25 GHz have been conducted to demonstrate the feasibility and stability of the scheme. The measured average quantum bit-error rate of the four polarization states is as low as 0.27% for 80 consecutive minutes without any adjustment. This high-speed intrinsically stable polarization modulation can be widely applied to many polarization-encoding QKD systems, such as BB84, MDI, etc.
Satellite networks and quantum communications offer complementary opportunities for enhanced operations. Quantum communications provide security for the transmissions between satellites and ground stations; while the free-space link of satellite networks provide the potential of long distance transmission of quantum bits (qubit) for space communications. However, with the promising advantages of the two approaches, challenges remain to fully develop quantum-based satellite communications such as robust and reliable information detection which is difficult to achieve due to the movement of satellites. In this paper, a tracking algorithm is proposed for polarization-encoded quantum satellite communications where polarization states are used to determine the bit transfer between the transmitter and receiver. The polarization tracking is essential for the decoding of a qubit and the quantum key distribution (QKD). A practical channel model for free-space quantum communications is adopted in this paper. With the estimated polarization, a novel dynamic polarization compensation scheme is also proposed. The results show that our methods can accurately estimate the polarization, providing much lower quantum bit error rate (QBER) by compensation, as compared with the direct qubit detection without polarization tracking and compensation scheme.
Polarization-encoding provides a promising approach for the practical quantum key distribution (QKD) system due to the simple encoding and decoding method. Here we present a long-term stable polarization-encoding QKD with a real-time polarization control. The polarization of the signal photons in the optical fiber was maintained stable by using time-division-multiplexed reference pulses for feedback control. We implemented this technique in the QKD experiment in 50 km fiber to show that it could facilitate polarization-encoded quantum communication. The system operated stably for ∼460 min, and the quantum bit error rate was ∼5.27%. The raw key generating rate in this system was kept stable at ∼500 bits s -1 . The advantages and disadvantages of the previous polarization-control schemes were also rigorously analyzed.
There are several groups have implemented polarization encoding measurement-device-independent quantum key distribution (MDI-QKD). However, these groups use manual polarization controller to achieve the initial polarization state for encoders, and some of the encoders cannot fully satisfy the two bases for polarization encoding quantum key distribution. Here, we apply an all-fiber and calibration-free intrinsically stable polarization-modulated unit (APMU), which can maintain stable encoding for several hours without any calibration. A proof-of-principle demonstration of MDI-QKD based on our APMU is implemented with the four-intensity decoy state protocol. The proposed APMU can realize stable MDI-QKD encoding and simplify the operation for Alice and Bob, which makes the polarization encoding MDI-QKD more practical.