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European Quantum Electronics Conference (EQEC)

Quantum key distribution with coherent states at telecom wavelength

Jérôme Lodewyck, Thierry Debuisschert, Rosa Tualle-Brouri, Philippe Grangier

Published Jan 18, 2006• 1 citation
Publisher

Abstract

A complete set-up implementing quantum key distribution (QKD) with coherent states at 1550 nm, entirely made of fibre optics and standard telecom components such as integrated-optics modulators, couplers and fast InGaAs photodiodes is built. The whole set-up is fully interfaced and includes a synchronous automatic data processing software. It is designed to operate with continuous variable error correction and privacy amplification algorithms. The present acquisition electronics limits the repetition rate to 1 MHz, but optical components can go as fast as 10 GHz. Therefore, one can envision an extension of this device yielding up to 100 MHz rates.

DOI
10.1109/eqec.2005.1567467

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A complete set-up implementing quantum key distribution (QKD) with coherent states at 1550 nm, entirely made of fibre optics and standard telecom components such as integrated-optics modulators, couplers and fast InGaAs photodiodes is built. The whole set-up is fully interfaced and includes a synchronous automatic data processing software. It is designed to operate with continuous variable error correction and privacy amplification algorithms. The present acquisition electronics limits the repetition rate to 1 MHz, but optical components can go as fast as 10 GHz. Therefore, one can envision an extension of this device yielding up to 100 MHz rates.
Quantum key distribution (QKD) is the first commercial quantum technology operating at the level of single quanta and is a leading light for quantum-enabled photonic technologies. However, controlling these quantum optical systems in real world environments presents significant challenges. For the first time, we have brought together three key concepts for future QKD systems: a simple high-speed protocol; high performance detection; and integration both, at the component level and for standard fibre network connectivity. The QKD system is capable of continuous and autonomous operation, generating secret keys in real time. Laboratory and field tests were performed and comparisons made with robust InGaAs avalanche photodiodes and superconducting detectors. We report the first real world implementation of a fully functional QKD system over a 43 dB-loss (150 km) transmission line in the Swisscom fibre optic network where we obtained average real-time distribution rates over 3 hours of 2.5 bps.
We have developed a continuous-variable quantum key distribution (CV-QKD) system that employs discrete quadrature-amplitude modulation and homodyne detection of coherent states of light. We experimentally demonstrated automated secure key generation with a rate of 50 kbps when a quantum channel is a 10 km optical fibre. The CV-QKD system utilises a four-state and post-selection protocol and generates a secure key against the entangling cloner attack. We used a pulsed light source of 1550 nm wavelength with a repetition rate of 10 MHz. A commercially available balanced receiver is used to realise shot-noise-limited pulsed homodyne detection. We used a non-binary LDPC code for error correction (reverse reconciliation) and the Toeplitz matrix multiplication for privacy amplification. A graphical processing unit card is used to accelerate the software-based post-processing.
Abstract Quantum key distribution (QKD) is one quantum technology that can provide secure encryption keys for data transmission. The secret key rate (SKR) is a core performance indicator in QKD, which directly determines the transmission rate of enciphered data. Here, for the first time, we demonstrate a high-key-rate Gaussian-modulated continuous-variable QKD (CV-QKD) using telecom optical components. The framework of CV-QKD over these components is constructed. Specifically, the high-rate low-noise Gaussian modulation of coherent states is realized by a classical optical IQ modulator. High-baud low-intensity quantum signals are received by an integrated coherent receiver under the shot-noise limit. A series of digital signal processing algorithms are proposed to achieve accurate signal recovery and key distillation. The system can yield a high asymptotic SKR of 10.37 Mbps within 20 km standard telecom fiber, and the secure distance can exceed 100 km. This result confirms the feasibility of CV-QKD with state-of-the-art performance using telecom optical components. Besides, due to the ease of integrating these discrete components, it provides a high-performance and miniaturized QKD solution for the metropolitan quantum network.
Our system is based upon a Mach-Zender interferometer using phase modulation. Photons are generated by a 1.55 pm DFB pulsed laser diode operating with a repetition rate of 2 MHz. The pulses are strongly attenuated so that on average 0. I photons/clock cycle leaves the senders apparatus. Phase modulators. controlled by custom electronics, in the two interfering arms were used to encode the bit information. The signal is multiplexed with pulses from a 1.3 pm clock laser which serves as a timing reference. Cooled InGaAs avalanche photodiodes operating in Geiger mode are used to detect the single photons. It is imperative for operation over long fibres that the dark count rate in the single photon detector is as low as possible. Our detectors typically have a dark count probability of 3.2~10.~ er gate, along with a detection efficiency of 12% at 1.55 pm. This yields a Noise Equivalent Power of I .OXIO-'~ WHz- , whcch IS one of the lowest reported to date. R ' . The interferometer displayed a single photon interference fringe visibility between 99.1% and 99.7% for fibre lengths up to 65km. For longer fibre lengths the visibility falls, but remains around 96% at IOlkm, see Figure 1. The QBER, shown in Figure 2, remains virtually constant at around 3.6% for fibre length up to 50km, suggesting that the dominant contribution derives from inaccuracy in the phase modulation. Beyond 50km the QBER increases with fibre length, due to dark count noise. At 101 km, the QBER averaged over a 2-minute key transfer is typically 7. I %. This error rate is below the 1 I .5% limit, thus allowing us to perform error correction and privacy amplification to form a secure key. The sifted bit rate (shown in Figure 2) decreases with increasing fibre length at a rate of -0.20 dB/km, close to the specified value. At IOOkm the average bit rate was typically 15 Hz, sufficient to allow a cryptographic key to be transferred in several tens of seconds. 100
Quantum computers threaten current secure communication methods, prompting the emergence of new ideas to ensure communication resistant to this threat. Quantum Key Distribution (QKD) is one such technology leveraging quantum physics for secure symmetric key distribution. We, therefore, present an experimental setup with accompanying programs for initialization, calibration, and data processing designed for implementing QKD based on the BB84 protocol, using lasers at wavelengths of 810 nm. In our approach, Alice and Bob do not need to have synchronized watches nor share a synchronizing signal: their alignment in time is achieved by minimization of the bit error rate (BER). The protocol has been successfully executed at up to 8.3 dB loss, equivalent to 2.7 km of G.652.D telecom fiber, commonly used in current communications. Achieved BER is 1.0% at 0.7 dB loss and rises to 1.7% for 8.3 dB loss. While usual QKD solutions prefer expensive IR components and detectors in the telecom wavelength range, our study shows the viability of using affordable near-infrared (NIR) optics and silicon SPAD-based detectors for shorter distances.