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IEEE Journal of Quantum Electronics

Experimental Shot Noise Measurement Using the Imperfect Detection—A Special Case for Pulsed Laser

Anju Rani, Jayanth Ramakrishnan, Tanya Sharma, Pooja Chandravanshi, Ayan Biswas, Ravindra Pratap Singh

Published Aug 24, 2023• 1 citation
Publisher

Abstract

Measuring the quantum fluctuations of a laser source is the first task in performing continuous variable quantum key distribution protocols. The quantum fluctuations of the source are measured using balanced homodyne detection. In this paper, we have measured the shot noise of a pulsed laser using imperfect homodyne detection. The imperfections accounted for in the detection process are a delay between the homodyne output arms and also due to the selection of the pulse integration window larger as well as smaller than the photo-current pulse width during the analysis. We have analyzed the imperfect detection results for two different experimental layouts, and a comparative study has been performed. From our analysis, it is evident that these imperfections play a significant role in balanced homodyne detection and must be optimized properly. Our results indicate that balanced homodyne detection can be performed using limited resources, which paves the way for easy experimental realization of optical homodyne tomography and continuous variable quantum key distribution in a laboratory setting.

DOI
10.1109/jqe.2023.3308263

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Measuring the quantum fluctuations of a laser source is the first task in performing continuous variable quantum key distribution protocols. The quantum fluctuations of the source are measured using balanced homodyne detection. In this paper, we have measured the shot noise of a pulsed laser using imperfect homodyne detection. The imperfections accounted for in the detection process are a delay between the homodyne output arms and also due to the selection of the pulse integration window larger as well as smaller than the photo-current pulse width during the analysis. We have analyzed the imperfect detection results for two different experimental layouts, and a comparative study has been performed. From our analysis, it is evident that these imperfections play a significant role in balanced homodyne detection and must be optimized properly. Our results indicate that balanced homodyne detection can be performed using limited resources, which paves the way for easy experimental realization of optical homodyne tomography and continuous variable quantum key distribution in a laboratory setting.
Abstract Single photon detectors are indispensable for secure communication in quantum cryptography. Shot noise, a fundamental limit to photodetection occurs due to discreteness of photons. Shot noise plays an important role in telecommunications and optical science because of its property of setting a practical limit to the signal-to-noise ratios. In order to implement BB84 protocol, there is a need to assess appropriate lower bound on experimental errors caused during secure key distribution between Alice and Bob. Shot noise limit can be measured using balanced homodyne detection. In this paper, balanced homodyne detection for measuring shot noise of 1550 nm laser has been implemented and validated. Keywords : Shot Noise, Balanced Homodyne detection, 1550nm, laser, signal Cite this Article Shikhar Singh. Shot Noise for 1550 nm Laser using Balanced Homodyne Detection. Research & Reviews: Journal of Physics . 2020; 9(1): 47–50p.
Abstract The time-domain balanced homodyne detector (TBHD) is critical component in continuous-variable quantum key distribution (CVQKD). The shot-noise-limited attribute of a TBHD depends on the whole spectrum of detector, in contrast to a frequency-domain balanced homodyne detector that focuses only on part of the spectrum. We propose a method for accurate calibration of the shot-noise-limited attribute of the TBHD by overlapping the gain lines of the TBHD and an integrated photodetector. The overlapping of two lines indicates that the detector is indeed shot noise limited, which is crucial to the security of CVQKD. Experimentally, we use this method to obtain a calibration error of ±1‰. Theoretical analysis showed that the CVQKD performance is very sensitive to calibration errors of shot noise, and proper countermeasures should be employed to eliminate this issue. The presented methods are expected to contribute to CVQKD security and TBHD standardization.
The research of quantum noise in the interaction of laser and atoms plays a critical role in quantum optical experiments. The measurement of quantum noise is mainly based on the balanced homodyne detection method. Here, we reported a balanced homodyne detector with low noise, high gain, and high common-mode rejection ratio (CMRR) at the wavelength of 795 nm. By optimizing the selection of components and printed circuit board (PCB) layout, a minimum electronic noise of −96.7 dBm is obtained. Over the range of 1 MHz to 5 MHz, we get a flat CMRR above 59 dB and a linear gain regime, which can enable a shot noise measurement with the laser power of100 μw100~\mu \text{w}100 μw. The developed detector can be used in quantum coherent experiments with a weak probe laser of power down to tens of microwatts.
We discuss a balanced homodyne detection scheme with imperfect detectors in the framework of the operational approach to quantum measurement. We show that a realistic homodyne measurement is described by a family of operational observables that depends on the experimental setup, rather than a single field quadrature operator. We find an explicit form of this family, which fully characterizes the experimental device and is independent of a specific state of the measured system. We also derive operational homodyne observables for the setup with a random phase, which has been recently applied in an ultrafast measurement of the photon statistics of a pulsed diode laser. The operational formulation directly gives the relation between the detected noise and the intrinsic quantum fluctuations of the measured field. We demonstrate this with two examples: the operational uncertainty relation for the field quadratures, and the homodyne detection of suppressed fluctuations in photon statistics.
Continuous variable-quantum key distribution (CV-QKD) based on coherent detection has potential to be a cost-effective solution to deploy secure keys over optical networks. This is due to its compatibility with mature optical communication technologies, such as homodyne and heterodyne detection schemes. Nevertheless, realistic implementations of CV-QKD systems suffer from imperfections in the practical devices used to generate and detect the coherent states. Those imperfections deteriorate the performance of the CV-QKD system and can be exploited by an eavesdropper to steal key information without being detected. In this work, we discuss the impact of practical device imperfections on the excess noise of the CV-QKD and on the achievable transmission distance. We also analyze the impact of detection imperfections on the measurement of the shot noise, and its impact on CV-QKD system security.
Imperfections in practical detectors, including limited detection efficiency, and inherent electronic noise, can seriously decrease the transmission distance of continuous-variable measurement-device-independent quantum key distribution systems. Owing to the difficulties inherent in realizing a high-efficiency fiber homodyne detector, challenges still exist in continuous-variable measurement-device-independent quantum key distribution system implementation. We offer an alternative approach in an attempt to solve these difficulties and improve the potential for system implementation. Here, a novel practical detector modeling method is utilized, which is combined with a one-time shot-noise-unit calibration method for the purpose of system realization. The new modeling method benefits greatly from taking advantage of one-time shot-noise-unit calibration methods, such as measuring electronic noise and shot noise directly to a novel shot-noise unit, so as to eliminate the statistical fluctuations found in previous methods; this makes the implementation of such systems simpler, and the calibration progress more accurate. We provide a simulation of the secret key rate versus distance with different parameters. In addition, the minimal detection efficiency required at each distance, as well as the contrast between the two methods, are also shown, so as to provide a reference in terms of system realization.