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High Speed Homodyne Detector for Gaussian-Modulated Coherent-State Quantum Key Distribution

Yue-Meng Chi

Published Jan 1, 2009• 6 citations
OpenAlex

Abstract

We developed a high speed homodyne detector in the telecommunication wavelength region for a Gaussian-modulated coherent-state quantum key distribution experiment. We are able to achieve a 100 MHz bandwidth, ultra-low electronic noise and pulse-resolved homodyne detector. The bandwidth of this homodyne detector has reached the same order of magnitude of the best homodyne detectors reported. By overcoming photodiode response functions mismatch, choosing proper laser sources, ensuring the homodyne detector linearity and stabilizing the homodyne detection system, we demonstrate that the homodyne detector has a 10 dB shot-noise-to-electronic-noise ratio in the time domain at a local oscillator of 5.4*10^8 photons/pulse at a laser repetition rate of 10 MHz. With this homodyne detector, we expect to ncrease our GMCS QKD experiment speed by 100 times, which will improve the key generation rate by 1-2 orders of magnitude.

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Journal of the Optical Society of America B•Feb 1, 2018•19 citations
We developed a high speed homodyne detector in the telecommunication wavelength region for a Gaussian-modulated coherent-state quantum key distribution experiment. We are able to achieve a 100 MHz bandwidth, ultra-low electronic noise and pulse-resolved homodyne detector. The bandwidth of this homodyne detector has reached the same order of magnitude of the best homodyne detectors reported. By overcoming photodiode response functions mismatch, choosing proper laser sources, ensuring the homodyne detector linearity and stabilizing the homodyne detection system, we demonstrate that the homodyne detector has a 10 dB shot-noise-to-electronic-noise ratio in the time domain at a local oscillator of 5.4*10^8 photons/pulse at a laser repetition rate of 10 MHz. With this homodyne detector, we expect to ncrease our GMCS QKD experiment speed by 100 times, which will improve the key generation rate by 1-2 orders of magnitude.
We discuss the excess noise contributions of a practical balanced homodyne detector (BHD) in Gaussian-modulated coherent-state (GMCS) quantum key distribution (QKD). We point out that the key generated from the original realistic model of GMCS QKD may not be secure. In our refined realistic model , we take into account excess noise due to the finite bandwidth of the homodyne detector and the fluctuation of the local oscillator (LO). A high-speed BHD suitable for GMCS QKD in the telecommunication wavelength region is built and experimentally tested. The 3 dB bandwidth of the BHD is found to be 104 MHz and its electronic noise level is 13 dB below the shot noise at an LO level of 8.5 × 10 8 photons per pulse. The secure key rate of a GMCS QKD experiment with this homodyne detector is expected to reach Mbits s −1 over a few kilometers.
In Gaussian-modulated coherent-state quantum key distribution, the measurement of quadratures of coherent states is performed by using a homodyne detector. However, conventional detectors usually suffer from narrow bands. We present a method to design a high-speed shot-noise-limited balanced homodyne detector. A 300-MHz bandwidth detector is experimentally tested and the level of shot noise is 14 dB higher than the electronic noise. The results show that a detector with this method is potential to design a GHz bandwidth detector for continuous variable quantum key distribution at a low level of ratio of shot noise to electronic noise.
We evaluate the performance of Gaussian Modulated Coherent State (GMCS) Continuous-Variable Quantum Key Distribution (CV-QKD), in terms of secure key rate, at different detector bandwidths. We illustrate the advantages and limitations of high-speed CV-QKD systems using a general noise model in which we consider various noise sources and their dependence upon detection bandwidth. Experimentally, the feasibility of high speed CV-QKD is demonstrated by using a GHz bandwidth balanced homodyne detector (BHD) established with commercially available components. We provide secure key rates of transmitted and local local oscillator schemes under various clock rates.
In most implementations of quantum-key-distribution (QKD) protocols the secure keys originate from single-photon signals. However, due to the unavoidable channel losses and the low efficiencies of single photon detectors, the key generation rate of a single-photon QKD system is low. Recently, there has been a growing interest in the Gaussian-modulated coherent state (GMCS) QKD protocol because it can be implemented with conventional laser sources and high efficiency homodyne-detectors. Here, we present our experimental results with a fully fiber-based one-way GMCS QKD system. Our system employed a double Mach-Zehnder interferometer (MZI) configuration in which the weak quantum signal and the strong local-oscillator (LO) go through the same fiber between Alice and Bob. We employed two novel techniques to suppress system excess noise. First, to suppress the LO's leakage, an important contribution to the excess noise, we implemented a scheme combining polarization and frequency multiplexing, achieving an extinction ratio of 70dB. Second, to further minimize the system excess noise due to phase drift of the double MZI, the sender simply remaps her data by performing a rotation operation. Under a realistic model, the secure key rates determined with a 5km and a 20km fiber link are 0.3bit/pulse and 0.05bit/pulse, respectively. These key rates are significantly higher than that of a practical BB84 QKD system.
Low-noise, efficient, phase-sensitive time-domain optical detection is essential for foundational tests of quantum physics based on optical quantum states and the realization of numerous applications ranging from quantum key distribution to coherent classical telecommunications. Stability, bandwidth, efficiency, and signal-to-noise ratio are crucial performance parameters for effective detector operation. Here we present a high-bandwidth, low-noise, ultra-stable time-domain coherent measurement scheme based on balanced homodyne detection ideally suited to characterization of quantum and classical light fields in well-defined ultrashort optical pulse modes.
We present a high-speed time-domain shot-noise-limited balanced homodyne detector that is suitable for the detection of the quantum states of nanosecond pulsed optical fields. The detector can detect the quantum noise of individual nanosecond optical pulses at a repetition rate of 40 MHz and achieve a signal-to-noise ratio of above 14.5 dB at a local oscillator power of 9.9×107 photons per pulse. The stability of the detector is characterized via an Allan variance measurement, and superior stability is observed. The detector is especially applicable in the fields of quantum optics and quantum information, where high-speed time-domain detection of nanosecond pulsed optical field quadratures is required.