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The Importance of Time and Frequency Reference in Quantum Astronomy and Quantum Communications

Tommaso Occhipinti, Paolo Zoccarato, Ivan Capraro, Pietro Bolli, Filippo Messina, G. Naletto, Paolo Villoresi, C. Barbieri

Published Nov 1, 2007• 1 citation
OpenAlex

Abstract

Very accurate and stable time tagging capabilities are fundamental for Quantum Astronomy and Quantum Key Distribution. The main task of Quantum Astronomy is to find particular signatures of different astrophysical emission mechanisms or scattering processes by measuring the statistics of the arrival time of each incoming photon. This line of research will be particularly important with future extremely large telescopes. On the other hand, Quantum Key Distribution (QKD) assures a secure cryptographic key sharing between optical transmitters and receivers through the synchronous exchange of quantum states (e.g., single polarized photons). \nBoth technologies need to detect the arrival times each photon with very high temporal resolution in order to discriminate the signal photons from the background ones. In this article, we present the activities of our research group on Quantum Astronomy and Quantum Key Distribution, taking into account their very strict requirements. \nFor Quantum Astronomy, we have developed an instrument called AquEYE capable of time-tagging each incoming single photon using as detectors four Single Photon Avalanche Diodes (SPAD). In this experiment, we need to maintain an absolute time scale reference with a maximum error phase less than 1 ns for measurements lasting more than 30 minutes. For the development phase of our instrument, we used an available rubidium oscillator disciplined by a GPS receiver. This system will supply the reference clock and trigger signal to our acquisition electronics, which are based on a time-to-digital converter. In the paper, we will present the more advanced solution we have under evaluation. \nAs we are also working in the realization of a QKD prototype, we will present our study on time and frequency stability of the local oscillators of the electronics. For the moment, we are using two simple quartzes, but in the article we will show the results of several mathematical simulations. With them, we will analyze the performance tradeoffs in terms of the final cryptographic key rate, through different synchronization techniques and frequency reference sources.

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Very accurate and stable time tagging capabilities are fundamental for Quantum Astronomy and Quantum Key Distribution. The main task of Quantum Astronomy is to find particular signatures of different astrophysical emission mechanisms or scattering processes by measuring the statistics of the arrival time of each incoming photon. This line of research will be particularly important with future extremely large telescopes. On the other hand, Quantum Key Distribution (QKD) assures a secure cryptographic key sharing between optical transmitters and receivers through the synchronous exchange of quantum states (e.g., single polarized photons). \nBoth technologies need to detect the arrival times each photon with very high temporal resolution in order to discriminate the signal photons from the background ones. In this article, we present the activities of our research group on Quantum Astronomy and Quantum Key Distribution, taking into account their very strict requirements. \nFor Quantum Astronomy, we have developed an instrument called AquEYE capable of time-tagging each incoming single photon using as detectors four Single Photon Avalanche Diodes (SPAD). In this experiment, we need to maintain an absolute time scale reference with a maximum error phase less than 1 ns for measurements lasting more than 30 minutes. For the development phase of our instrument, we used an available rubidium oscillator disciplined by a GPS receiver. This system will supply the reference clock and trigger signal to our acquisition electronics, which are based on a time-to-digital converter. In the paper, we will present the more advanced solution we have under evaluation. \nAs we are also working in the realization of a QKD prototype, we will present our study on time and frequency stability of the local oscillators of the electronics. For the moment, we are using two simple quartzes, but in the article we will show the results of several mathematical simulations. With them, we will analyze the performance tradeoffs in terms of the final cryptographic key rate, through different synchronization techniques and frequency reference sources.
The main argument of this thesis is the application of advanced techniques for the optimization of single photon communication and in general of single photon applications. The work is inserted in the contest of various projects that involve the departments of Information Engineering and Astronomy of the University of Padua. In particular my contribution has been the development of a quantum cryptography setup that we called QuAKE. The system has been designed and implemented in our labs and include in the hardware some advanced temporal and spatial filtering techniques. These features has been realized respectively with an ad hoc electronics and with an adaptive optics system, the latter developed entirely in our department. The high level software for quantum cryptography has been also implemented and many optimizations have been realized both in the logical design and in the single algorithms. The last part of this thesis describes an astronomical instrument, called AquEYE, developed by our group and capable of time tagging single photons coming from celestial sources. In particular a description of the time and frequency distribution unit is given since this has been my contribution to the AquEYE instrument so far. The thesis is organized as follows: after an introduction to quantum cryp- tography (chapter 1), the QuAKE system is presented (chapter 2), the electronics and the optical setup are described (chapter 3) as well as the adaptive optics system (chapter 4), it follows a description of the results obtained testing the adaptive optics system outdoor and on the QuAKE system (chapter 5) and the description of the high level software and the related results (chapter 6), last a description on the timing and frequency unit of AquEYE is presented as well as some of the early results of the instrument (chapter 7).
Quantum key distribution allows for a provably secure transmission of cryptographic keys over an optical channel. Encoded polarization states or time-bin degree of freedom have been used for successful demonstrations. However, photon losses in long fibers, slow single photon detectors, and detector dark counts significantly limit the overall bit rate. Improving key throughput and reducing the overhead of key reconciliation remain as major challenges. Methods which utilize multiple time bins allow for multiple key bits to be encoded in a single photon, thus increasing the fidelity of transmitted keys and decreasing the overhead of key reconciliation in real-world conditions. Previous implementations of these methods required that Alice and Bob share a time reference by sharing a dedicated classical channel used for synchronization. This work presents a technique that allows two parties to exchange time-bin encoded photons without the need for synchronized time references. Our technique uses a framing protocol which allows Alice to encode a time reference along with a key which is determined by Alice before transmission. Security can be achieved by monitoring the visibility of a pair of Franson interferometers, using decoy pulses and measuring the round trip time between Alice and Bob. The bit rate of this technique is limited only by the recovery time of the detector and the speed of the modulation electronics. We experimentally demonstrate a raw bit rate of 5Mb/s over an optical channel with 55dB of loss, which is competitive with current research. We also demonstrate absolute timing synchronization with an accuracy of 20ps.
Clock synchronization is critical for maintaining low error rates in quantum key distribution. Here, we describe how a frequency mismatch between the transmitter and receiver clocks affects the quantum bit error rate in quantum key distribution, and derive from this model a simple synchronization algorithm together with clock stability requirements for practical operation. Our algorithm continuously compensates for both frequency mismatch and time-offset fluctuations directly from detection timestamps. It does not require a dedicated synchronization channel or auxiliary qubit sequence, converges from a large frequency mismatch within approximately one second of photon acquisition, and remains effective in low-photon-count regimes (more than 30 dB of channel loss) using standard hardware. We validate our approach by demonstrating successful key exchange over 100 km of fiber and continuous operation over 24 hours in a 16 km metropolitan network using commercial systems, with performance equivalent to using a service channel for clock synchronization.
In today’s world, a vast amount of sensitive information - from personal communications to financial transactions and government data - is constantly moving across platforms and networks. As a result, ensuring the security and privacy of this information has become a priority for individuals, organizations and governments alike. Quantum key distribution (QKD) offers a promising solution for secure communications, allowing the generation of identical keys between two entities separated in space. Although QKD systems have evolved considerably in recent years, there is still a need for optimization in various subsystems to make these technologies more efficient. One such area in need of improvement is the synchronization process, which is critical to the proper functioning of QKD systems. Taking this into account, this thesis investigates an effective synchronization technique with low computational complexity for QKD systems, aiming to strike a balance between efficiency and simplicity/cost, using the Qubit4Sync method as a model. The technique studied eliminates the need to prepare reference signals, as is the case with current systems operating in gated mode, and recovers the period using only the qubits similar to those exchanged during the QKD protocol. In this context, simulations were carried out using the Qubit4Sync method and the setup was adapted to work without the reference signal, with the detectors operating in free-running mode. The data from the setup used was processed and the simulation confirmed that once the dark counts are greater than the quibts detected, it becomes more difficult to recover the correct period. In the laboratory, the method was tested with various conditions of efficiency and dead-time of the single photon detectors. Due to the high optical noise of the channel, the recovery of the period was unsuccessful, as it was always equal to the value of the dead-time. After comparing the simulation with the practical results, it turned out that the noise in practice was higher than predicted. When investigating the setup, it was found that the extinction rate imposed by the Mach-Zehnder Modulator (MZM) was lower than expected. This may have contributed to the transmitter generating more optical noise than initially anticipated. Consequently, this factor may explain the practical results, since the detection rate of the detector is saturated at its dead-time value.
One of our major target is quantum key distribution (QKD,) which is closest to the practical use. Though QKD can be performed only with current technology, there still remain many thing to be improved. One of the most important devices that determine the system performance is photon detectors. It limits the transmission distance in optical fiber communication wavelength. A photon detector combining the two avalanche photon diodes (APD) has been demonstrated for qubit discrimination in 1550 nm. Spikes accompanied with the signals in gated-mode were canceled by balanced output from the two APDs. The balanced APD photon detector shows very low dark count rate (2x10-7) counts/pulse) with 10 % detection efficiency. We have also developed a highly stable interferometer on a planer-lightwave-circuit (PLC.) We have achieved single-photon interference over 150 km using time-division interferometers for quantum cryptography, which were composed of the two integrated-optic asymmetric Mach-Zehnder interferometers, and the balanced gated-mode photon detectors. The observed visibility was more than 80 % after 150-km transmission, which refers to a quantum bit error rate of less than 10 %. We will also describe temperature insensible QKD system and high speed (100 bps) key transmission over 40 km fiber.
Measurement-device-independent quantum key distribution (MDI-QKD) has been proven to protect legitimate users from attacks against measurement devices. The MDI-QKD requires that the two photons arriving at the instrument be indistinguishable. Precise time control is required to eliminate the distinguishability due to differences in photon arrival times. In the conventional methods, the time difference between photons is measured at a measuring instrument (Charlie), and a control signal is transmitted to the users (Alice and Bob). However, this method requires a long feedback loop, and the control may become unstable for long-distance transmission. This article proposes a method in which the photon arrival time difference is detected and controlled at Charlie. The reference signal for the time control is generated by an optical frequency comb in synchronization with the quantum signal. Therefore, the quantum signal photons can also be synchronized by synchronizing the reference signal pulses. A proof-of-principle experiment confirmed that the time synchronization accuracy required for protocol execution could be obtained. This proposal simplifies the implementation of the MDI-QKD.