Abstract The type of system employing continuous variables, such as light field quadrature for signal modulation, is an important branch for quantum key distribution. Achieving a high key rate using quantum key distribution technology is beneficial for realizing more frequent key updates or enabling one-time pad encryption for high-speed communication tasks. However, the deficiencies of typical large-bandwidth balanced photodetectors prevent the continuous variable quantum key distribution from matching the speed of state-of-the-art classical optical communications. Previous attempts to improve the key rate have focused on designing and manufacturing high-speed balanced photodetectors. It is well known that phase-sensitive amplifiers also improve the performance of balanced photodetectors and thus the performance of key distribution, but the concept remains purely theoretical. Here, for the first time, we experimentally demonstrate a high-speed continuous-variable quantum key distribution operating with a 10 GHz balanced photodetector enhanced by phase-sensitive amplification and detection. A conjugated multi-mode continuous variable protocol is used to accommodate the phase-sensitive operations. The optical amplifier offers a 10 dB increase in the photodetector’s clearance (quantum noise over electrical noise) and improves the detection efficiency from 72% to 96%, thus enabling an overall 248.9 Mb/s key distribution rate through a 16.7 km field-deployed optical fiber.
Abstract The type of system employing continuous variables, such as light field quadrature for signal modulation, is an important branch for quantum key distribution. Achieving a high key rate using quantum key distribution technology is beneficial for realizing more frequent key updates or enabling one-time pad encryption for high-speed communication tasks. However, the deficiencies of typical large-bandwidth balanced photodetectors prevent the continuous variable quantum key distribution from matching the speed of state-of-the-art classical optical communications. Previous attempts to improve the key rate have focused on designing and manufacturing high-speed balanced photodetectors. It is well known that phase-sensitive amplifiers also improve the performance of balanced photodetectors and thus the performance of key distribution, but the concept remains purely theoretical. Here, for the first time, we experimentally demonstrate a high-speed continuous-variable quantum key distribution operating with a 10 GHz balanced photodetector enhanced by phase-sensitive amplification and detection. A conjugated multi-mode continuous variable protocol is used to accommodate the phase-sensitive operations. The optical amplifier offers a 10 dB increase in the photodetector’s clearance (quantum noise over electrical noise) and improves the detection efficiency from 72% to 96%, thus enabling an overall 248.9 Mb/s key distribution rate through a 16.7 km field-deployed optical fiber.
We develop and experimentally demonstrate a phase-sensitive continuous variable quantum key distribution system with improved secure key rate. This is achieved using multimode coherent states with phase-conjugated subcarrier modulation and phase-sensitive detection. The local oscillator for phase-sensitive detection is regenerated from a polarization-multiplexed carrier wave via optical injection locking. The proposed scheme has a higher classical information capacity at a given number of received photons and exhibits a higher secure key rate when applying the security analysis of the GG02 protocol. Experimental results confirm the higher secret key rate and better excess noise tolerance of the new scheme compared to the typical implementation of GG02.
Quantum key distribution (QKD) is a prominent application in the field of quantum cryptography, providing information-theoretic security for secret key exchange. The implementation of QKD systems on photonic integrated circuits (PICs) can reduce the size and cost of such systems and facilitate their deployment in practical infrastructures. To this end, continuous-variable (CV) QKD systems are particularly well-suited as they do not require single-photon detectors, whose integration is presently challenging. Here we present a CV-QKD receiver based on a silicon PIC capable of performing balanced detection. We characterize its performance in a laboratory QKD setup using a frequency multiplexed pilot scheme with specifically designed data processing allowing high modulation and secret key rates. The obtained excess noise values are compatible with asymptotic secret key rates of 2.4 Mbit/s and 220 kbit/s at an emulated distance of 10 km and 23 km, respectively. These results demonstrate the potential of this technology toward fully integrated devices suitable for high-speed, metropolitan-distance secure communication.
Nowadays, quantum communications provide a vast field of research in rapid expansion, with a huge potential impact on the future developments of quantum technologies. In particular, continuous variable systems, employing coherent-state encoding and quadrature measurements, represent a suitable platform, due to their compatibility with both the modulation and detection systems currently employed in standard fiber-optical communications. In this work, we address some relevant aspects of the field, and provide innovative results being also experimentally oriented. In particular, we focus on two relevant paradigms: quantum decision theory and continuous variable quantum key distribution (CVQKD). In the former case, we address the problem of coherent-state discrimination and design new hybrid receivers for binary phase-shift keying discrimination, obtaining a quantum advantage over conventional detection schemes, being also robust against typical experimental imperfections. In the latter scenario, we proceed in two different directions. On the one hand, we design new CVQKD protocols employing discrete modulation of coherent states, being a feasible solution compatible with the state of the art in optical communications technologies. On the other hand, we address the more fundamental problem of performing channel losses mitigation to enhance existing protocols, and investigate the role of optical amplifiers for the task. Finally, we make a first step towards a fully non-Gaussian CVQKD scheme by proposing, for the first time, the adoption of an optimized state-discrimination receiver, commonly adopted for quantum decision theory, within the context of CVQKD, obtaining a genuine quantum enhancement over conventional protocols.
Abstract Transmission losses through optical fibers are one of the main obstacles preventing both long-distance quantum communications and continuous-variable quantum key distribution. Optical amplification provides a tool to obtain, at least partially, signal restoration. In this work, we address a key distribution protocol over a multi-span link employing either phase-insensitive (PIA) or phase-sensitive (PSA) amplifiers, considering Gaussian modulation of coherent states followed by homodyne detection at the receiver’s side. We perform the security analysis under both unconditional and conditional security frameworks by assuming in the latter case only a single span of the whole communication link to be untrusted. We compare the resulting key generation rate (KGR) for both kinds of amplified links with the no-amplifier protocol, identifying the enhancement introduced by optical amplification. We prove an increase in the KGR for the PSA link in the unconditional scenario and for both PSA and PIA in the conditional security setting depending on position of the attack and the measured quadrature.
Quantum key distribution (QKD) is the fastest-growing and relatively mature technology in the field of quantum information, enabling information-theoretically secure key distribution between two remote users. Although QKD based on off-the-shelf telecom components has been validated in both laboratory and field tests, its high cost and large volume remain major obstacles to large-scale deployment. Photonic integration, featured by its compact size and low cost, offers an effective approach to addressing the above challenges faced by QKD. Here, we implement a high-performance, integrated local local oscillator continuous-variable (CV) QKD system based on an integrated silicon photonic transmitter and receiver. By employing a high-speed silicon photonic integrated in-phase and quadrature modulator, a low-noise and high-bandwidth silicon photonic integrated heterodyne detector, and digital signal processing, our CV-QKD system achieves a symbol rate of up to 1.5625 GBaud. Furthermore, the system achieves asymptotic secret key rates of 31.05 and 5.05 Mbps over 25.8 and 50.4 km standard single-mode fiber, respectively, using 8-phase-shift keying discrete modulation. Our integrated CV-QKD system with a high symbol rate and long transmission distance paves the way for the quantum secure communication network in metropolitan areas.
Abstract Self-referenced continuous-variable quantum key distribution (SR-CVQKD) protocols, which operate with a real local local oscillator (LLO) at the receiver side, provide the major advantage that they can defend against all attacks associated with the local oscillator. We propose a method to enhance the performance of the SR-CVQKD by adding an optical parametric amplifier into the receiver’s apparatus. Considering reverse reconciliation, the secret key rate against collective eavesdropping attacks is calculated. For the plug-and-play dual-phase-modulated coherent-states (DPMCS) SR-CVQKD protocol, we demonstrate that an ideal phase-sensitive amplifier (PSA) can enhance the efficiency of a homodyne detector, and the compensation ability of a realistic phase-insensitive amplifier (PIA) for heterodyne detection varies according to different values of N, which indicates the noise variance of the PIA. Moreover, these arrangements of optical amplifiers also have applications in one-way SR-CVQKD systems.