Quantum information with highly-accessible continuous variable state of light: local oscillators and long-range quantum networks (Conference Presentation)
Continuous variables (CV) have become important across all facets of quantum information. From quantum sensing to quantum computing to quantum key distribution, the benefits of deterministic quantum state generation clearly make a compelling case to seek full CV-based quantum information solutions from top to bottom. Long range quantum networks have become of interest for their potential use in all three quantum information scenarios: second generation, distributed quantum sensors over quantum networks, multi-user QKD protocols across long range networks, and distributed quantum computing. However, a long range CV quantum network is impossible without overcoming two major roadblocks. First, to enable quantum state measurement or tomography at network nodes, coherent detection is required, which itself requires sending a powerful local oscillator across the network. Sending such a local oscillator across long distances presents a practical limitation: it cannot coexist on the same network infrastructure as with quantum signals. Second, very long range networks require robust quantum states and third-generation quantum repeaters, which themselves require a nonGaussian gate in the CV world. We will present our recent results on long range CV network generation made possible by feed forward phase recovery schemes for “locally” generated local oscillators. In addition, we will present our work on deterministic quantum network generation with highly accessible, cost effective, integrated sources of quantum entanglement. Finally, in order to enable all applications across true quantum networks, a non Gaussian quantum gate is required. We will outline our proposed cubic phase gate and our experimental progress towards achieving this goal.
Continuous variables (CV) have become important across all facets of quantum information. From quantum sensing to quantum computing to quantum key distribution, the benefits of deterministic quantum state generation clearly make a compelling case to seek full CV-based quantum information solutions from top to bottom. Long range quantum networks have become of interest for their potential use in all three quantum information scenarios: second generation, distributed quantum sensors over quantum networks, multi-user QKD protocols across long range networks, and distributed quantum computing. However, a long range CV quantum network is impossible without overcoming two major roadblocks. First, to enable quantum state measurement or tomography at network nodes, coherent detection is required, which itself requires sending a powerful local oscillator across the network. Sending such a local oscillator across long distances presents a practical limitation: it cannot coexist on the same network infrastructure as with quantum signals. Second, very long range networks require robust quantum states and third-generation quantum repeaters, which themselves require a nonGaussian gate in the CV world. We will present our recent results on long range CV network generation made possible by feed forward phase recovery schemes for “locally” generated local oscillators. In addition, we will present our work on deterministic quantum network generation with highly accessible, cost effective, integrated sources of quantum entanglement. Finally, in order to enable all applications across true quantum networks, a non Gaussian quantum gate is required. We will outline our proposed cubic phase gate and our experimental progress towards achieving this goal.
Quantum key distribution (QKD) enables two remote parties to share encryption keys with security based on the laws of physics. Continuous-variable (CV) QKD with coherent states and coherent detection integrates well with existing telecommunication networks. Thus far, long-distance CV-QKD has only been demonstrated using a highly complex scheme where the local oscillator is transmitted, opening security loopholes for eavesdroppers and limiting potential applications. Here, we report a long-distance CV-QKD experiment with a locally generated local oscillator over a 100-kilometer fiber channel with a total loss of 15.4 decibels. This record-breaking distance is achieved by controlling the phase noise-induced excess noise through a machine learning framework for carrier recovery and optimizing the modulation variance. We implement the full CV-QKD protocol and demonstrate the generation of keys secure against collective attacks in the finite-size regime. Our results mark a substantial milestone for realizing CV quantum access networks with a high loss budget and pave the way for large-scale deployment of secure QKD.
Quantum information processing (QIP) opens new opportunities for high-performance computing, high-precision sensing, and secure communications. Among various QIP features, the entanglement is a unique one. To take full advantage of quantum resources, it will be necessary to interface quantum systems based on different encodings of information both discrete and continuous. The goal of this paper is to lay the groundwork for the development of a robust and efficient hybrid continuous variable-discrete variable (CV-DV) quantum network, enabling the distribution of a large number of entangled states over hybrid DV-CV multi-hop nodes in an arbitrary topology. The proposed hybrid quantum communication network (QCN) can serve as the backbone for a future quantum Internet, thus providing extensive long-term impacts on the economy and national security through QIP, distributed quantum computing, quantum networking, and distributed quantum sensing. By employing the photon addition and photon subtraction modules we describe how to generate the hybrid DV-CV entangled states and how to implement their teleportation and entanglement swapping through entangling measurements. We then describe how to extend the transmission distance between nodes in hybrid QCN by employing macroscopic light states, noiseless amplification, and reconfigurable quantum LDPC coding. We further describe how to enable quantum networking and distributed quantum computing by employing the deterministic cluster state concept introduced here. Finally, we describe how the proposed hybrid CV-DV states can be used in an entanglement-based hybrid QKD.
Building scalable and secure quantum networks with many users has a high application potential but also holds many practical challenges. A significant stride in this pursuit involves extending quantum key distribution, an information-theoretically secure method for establishing cryptographic keys between two distant users, from a point-to-point protocol implemented on direct optical connections to a quantum access network. Yet, realizations of quantum access networks have, so far, relied on probabilistic or time-sharing strategies. Here, we show theoretically and experimentally that a solution without these constraints can come from the exclusive features of continuous-variable systems. Based on coherent states, we propose continuous-variable quantum passive-optical-network (CV-QPON) protocols, enabling deterministic and simultaneous secret key generation among all network users. We achieve this by leveraging the inherent wave-like property of coherent states split at a beam splitter and electric-field quadrature measurements. We show two protocols with different trust levels assigned to the network users and experimentally demonstrate key generation in a quantum access network with 8 users, each with an 11 km span of access link. Depending on the trust assumptions about users, we reach 1.5 Mbits/s and 2.1 Mbits/s of total network key generation. Demonstrating the potential to expand the network's capacity to accommodate tens of users at a high rate, our CV-QPON protocols offer a pathway toward establishing low-cost, high-rate, and scalable quantum access networks using standard telecom technologies and directly exploiting the existing access network infrastructure.
Quantum key distribution (QKD) has been proven to promise unconditionally secure data communication between remote legitimate users，based on the fundamental laws of quantum mechanics. QKD has great potential to be commercialized in the market and implemented in securing critical institutions, such as financial infrastructures, smart Grid, and national defence. Most theoretical security analyses of QKD protocols are based on several idealistic assumptions. However, the practical limitations, such as device imperfections, may bring security loopholes and leave QKD systems vulnerable to malicious eavesdropping attacks. Since QKD systems will be used as the practical applications in the real-world, it is of great importance to study the practical security and practical implementations of QKD protocols. Compared with dedicated single-photon based discrete variable (DV) QKD protocols, continuous variable (CV) QKD systems have attracted more attentions in recent years, as they benefit from their compatibility with commercial off-the-shelf (COTS) telecommunication components and high key rate over metropolitan networks. However, the requirement of a co-transmitted local oscillator (LO) brings a number of issues that compromises secure key generation and becomes a major obstacle to the practical implementation of CVQKD. To overcome these problems, a new CVQKD protocol with a locally generated LO (LLO) at Bob has been proposed. Based on previous studies, this thesis presents a novel quantum attack based on a discovered security loophole of the LLO CVQKD protocol, and demonstrates two high performance and low complexity practical LLO CVQKD systems. 1. After exploring the phase recovery scheme and the phase noise model, the first quantum attack on LLO CVQKD system is proposed, that exploits the phase estimation error associated with the amplitude of the phase reference pulses, in a ‘reference pulse attack’. Under this attack, a new system noise model and a refined linear quantum states transmission model have been developed when the phase noise is partially trusted. The attack performance is evaluated in terms of excess noise manipulation and mutual information extraction over both trusted – referred to as realistic, and untrusted – referred to as paranoid, security models of phase noise. Effective countermeasures are also given to improve the practical security and performance of the LLO CVQKD system under the trusted phase noise model. 2. Aside from the practical security analysis, two practical LLO CVQKD implementations developed in the laboratory are demonstrated here. The first practical LLO CVQKD system is featured as high speed (500 MHz) and low complexity, realizing the world’s fastest predicted key rate of 26.9 Mbps over a 15 km optical fibre link. To increase the signal repetition rate, and hence the potential secure key rate, our system is equipped with high-performance, wideband devices and components designed to support high repetition rate operations. To reduce the system complexity and correct for any phase shift during transmission, reference pulses are interleaved with quantum signals at Alice. Customized monitoring software has been developed allowing all parameters to be controlled in real-time without any modification to the physical setup. The system-level noise model analysis is introduced at high bandwidth and a new ‘combined optimisation’ technique is proposed to simultaneously optimize system parameters to high precision. It also has the potential for an even faster implementation. 3. The second demonstrated system presents the feasibility of the first practical LLO CVQKD system. The system comprises integrated real-time data-processing units (DPUs) and commercial off-the-shelf (COTS) telecommunication components. Instead of using conventional expensive, high-specification, standalone laboratory equipment (e.g. oscilloscope and arbitrary waveform generator (AWG) used in most CVQKD systems), each DPU consists of a high-speed, multi-channel data acquisition module, and is embedded in a computer at both Alice and Bob to achieve real-time quantum data generation, acquisition and analysis. In addition, a hardware-based filter unit has been implemented to amplify and stabilise modulation signals, together with a system-level noise model analysis and a combined parameter optimisation method to enhance the system key rate. In laboratory conditions, the system has been shown to run continuously, and the asymptotic secure key rates are estimated as 1.37 Mbps and 509 kbps over 15 km and 25 km optical fibre links, respectively, through stable excess noise measurements for over 10 hours at a 20 MHz repetition rate. The measurement results open up the possibility of building up a commercialized LLO CVQKD system.
In this thesis we consider quantum communication as realised in the continuous variable quantum optical regime. The work presented includes results concerning quantum communication applications, such as entanglement distillation and quantum key distributions, as well as fundamental questions regarding measurement uncertainty and the boundary between classical and quantum correlations. In the first part of the thesis we investigate various protocols in which one engages in a trade-off between determinism and the quality of quantum correlations. In discrete variable quantum optics non-determinism is ubiquitous, with photon sources, detectors and various gates commonly operating probabilistically. In the continuous variable regime one has access to many quantum states and operations, including exotic effects such as entanglement, in a fully deterministic fashion and one might regard giving up this feature as highly undesirable. However it has been discovered that certain vital tasks such as entanglement distillation, indispensable for large scale quantum information networks, cannot be realised deterministically. We address precisely these limitations, firstly by conducting the most general analysis to date of a non-deterministic but heralded quantum amplifier that can circumvent the standard quantum limits, and distill Gaussian entanglement. We then analyse the security of long standing technique in continuous variable quantum key distribution, known as post-selection, which again allows the distillation of higher quality correlations. We provide a complete security proof in the asymptotic limit and show that there is in fact an relationship between this method and the noiseless amplifier. Furthermore, the protocol allows secure key to be generated for channel parameters where all previous CVQKD protocols fail. Thirdly we consider this equivalence between post-selection and heralded operations more generally, and conduct a detailed analysis of the post-selection based emulation of the noiseless amplifier. Turning to more fundamental issues, we consider links between several recently proposed entropic measures of quantum correlations and their potential operational links to continuous variable quantum key distribution. Finally we consider entropic uncertainty relations for continuous variables on infinite dimensional Hilbert spaces and apply these to one-sided device independent continuous variable quantum key distribution protocols.n
The quantum internet holds the potential to facilitate applications that are fundamentally inaccessible to the classical internet. Among its most prominent applications is quantum key distribution (QKD) networks, which connect two distant nodes to establish a secure key based on the principles of quantum mechanics. However, the subsequent extensive reliance on interferences in existing QKD protocols leads to the weak robustness of the system and the corresponding network. In this work, we propose a robust and cost-effective quantum network using the Kramers-Kronig receiver. We first propose a continuous-variable QKD protocol based on direct detection without interference, which achieves the recovery of quadrature components through the Kramers-Kronig relation. Subsequently, we have extended this protocol to continuous-variable quantum access networks, further highlighting the robustness and cost advantages of interference-free detection. The experimental results show that each user can achieve a secret key rate at 50 kbit/s within the access network range by using only one photodetector without interference structures. This scheme opens up new possibilities in establishing a robust and cost-effective quantum network, serving as a foundational element in the progress toward establishing a large-scale quantum internet.