Session R31: Long Distance Quantum Communication

Live

Quantum Key Distribution (QKD) provides a unique solution for the distribution of secret keys used for information authentication and encryption. QKD has the potential to yield several benefits including protecting critical energy delivery infrastructure which needs long-term authentication and encryption solutions. A real-world scenario for the practical use of this technology can be applied to the distribution of network keys between the operations center and electrical substations. We successfully demonstrated a trusted-node QKD network comprised of three diverse QKD systems working autonomously on an electric grid and collected data over a period of 28 hours. The interoperability of these devices extends the physical distance for which they can distribute secret keys, covering larger territory than a single system operating on its own. The network demonstrated the generation of quantum secret keys between the operations center and several electrical utility substations. This was the world’s first demonstration relaying secret keys between energy delivery infrastructure.   

Live

The realization of large-scale quantum networks hinges on the capability to reliably store quantum entanglement while generating further links. The nitrogen vacancy centre in diamond is a promising system for this role, hosting both a spin-photon interface for remote entanglement and magnetic coupling to nuclear spin qubits. Recent experiments have shown deterministic delivery of entanglement between two network nodes [1], and high-fidelity control of a 10-qubit system [2]. However, combining these advances is challenging due to always-on electron-nuclear couplings which cause nuclear spin decoherence during remote entanglement generation.

In this work, we utilize isotopic engineering of diamond to improve the performance of a nuclear spin quantum network memory by two orders-of-magnitude over the state of the art. Importantly, we find that memory robustness is now limited by stochastic ionisation of the NV rather than by the direct hyperfine coupling alone, and investigate techniques to overcome this challenge. Our work paves the way for advanced quantum network protocols using spins in diamond.

[1] - P. C. Humphreys, et al., Nature 558, 268-273 (2018)
[2] - C. E. Bradley, et al., Phys. Rev. X 9, 031045 (2019)   

Live

We introduce various measures of forward classical communication for bipartite quantum channels. Since a point-to-point channel is a special case of a bipartite channel, the measures reduce to measures of classical communication for point-to-point channels. As it turns out, these reduced measures have been reported in prior work of Wang et al. on bounding the classical capacity of a quantum channel. As applications, we show that the measures are upper bounds on the forward classical capacity of a bipartite channel. The reduced measures are upper bounds on the classical capacity of a point-to-point quantum channel assisted by a classical feedback channel. Some of the various measures can be computed by semi-definite programming.   

Live

Recent experimental breakthroughs in satellite quantum communications have opened up the possibility of creating a global quantum internet using satellite links. This approach appears to be particularly viable in the near term, due to the lower attenuation of optical signals from satellite to ground, and due to the currently short coherence times of quantum memories. In this work, we propose a global-scale quantum internet consisting of a constellation of orbiting satellites that provides a continuous, on-demand entanglement distribution service to ground stations. We develop a technique for determining optimal satellite configurations with continuous coverage that balances the total number of satellites and entanglement-bit (ebit) rates. We then determine various optimal satellite configurations for a polar-orbit constellation, and we analyze the resulting satellite-to-ground loss and achievable ebit rates for multiple ground station configurations. We also provide a comparison between these ebit rates and the rates of ground-based quantum repeater schemes. Overall, our work provides the theoretical tools and the experimental guidance needed to make a satellite-based global quantum internet a reality.   

Live

Quantum repeaters enable surpassing the fundamental distance limit that quantum communication schemes can cover. However, realistic hardware parameters make their realization a challenge. Theoretical analysis and optimization of repeater schemes can lower the barrier for their realization. Our contributions to this are threefold. First, we provide an efficient algorithm for computing the probability distribution of the time until the first entangled state between the end nodes of a repeater chain is delivered, as well as the state's average fidelity. This improves upon the exponential runtime of existing algorithms. Next, as application, we use the algorithm for optimizing the available secret key rates for repeater schemes including a cut-off condition, which mitigates the effect of memory decoherence. We find that the use of the optimal cut-off lowers the parameter threshold for which secret key can be generated. Lastly, we use our insights gained to improve existing analytical bounds on the mean entanglement distribution rate. Our contributions thus serve as useful tools for the design and realization of long-distance quantum communication networks.   

Live

Prototypes of metropolitan optical quantum networks based on different technologies are currently under construction around the world. Interconnecting these heterogeneous quantum networks, scaling the number of users, and increasing communication distances requires developing a network architecture with a control plane that is robust, decentralized, and secure. We describe how the modularized design of the Simulator of QUantum Network Communication (SeQUeNCe) we developed allows comparisons of alternative architecture designs that assign different roles to layers in the control protocol stack. We also demonstrate the practical use of the simulator by comparing the impact of factors such as memory allocation policies and classical control message latencies on overall network throughput. Finally, we also show our preliminary results with scaling the SeQUeNCe simulator to 100+ node simulations by optimizing quantum state representations and utilizing supercomputers at the Argonne Leadership Computing Facility.   

Live

We develop models of quantum network protocols and simulate their behavior in metropolitan optical quantum networks equipped with repeater nodes. We build models of quantum repeaters equipped with atomic quantum memories and use the Barrett-Kok protocol to generate and distribute entanglement between nearest pairs of repeaters. An entanglement swapping protocol is used to extend the entanglement to any pair of network nodes. Finally, we also model the BBPSSW purification protocol to mitigate errors in entanglement generation and swapping procedures. We use the Simulator of QUantum Network Communication (SeQUeNCe), an open-source tool that our team introduced, to quantify performance of these protocols in a model of the Chicago metropolitan quantum network. We simulate six network configurations using different hardware parameters, such as different quantum memory management policies and assumptions about classical message latencies. Our simulation results illustrate that high latency of control messages can significantly limit the performance of the network, as well as the impact of quantum memory parameters on the overall performance.   

Live

Quantum networks are important for distributed quantum computing, sensor networks and quantum cryptography. While small-scale, optical fiber-based quantum networks appear to be ready for near-term deployment, there remains important challenges in establishing large-scale quantum networks. One fundamental practical problem arises from the way optical components, such as the fiber transmission channel, dynamically vary the polarization and arrival time of photons due to temperature fluctuations. Since quantum networks rely on photon interference, precise control of the properties of photons is necessary for deploying a functional quantum network. Brute-force algorithms are often used to sequentially calibrate each degree of freedom however this approach becomes increasingly challenging for lossy and long-distance networks where the photon detection rate is low. In this talk, we present a Bayesian optimization algorithm for automating the calibration of single-photon states. To validate our approach, we present a proof-of-concept demonstration of the optimization of an experimental Hong-Ou-Mandel measurement scheme. We envision that our methodology opens up the possibility of fast and reliable calibration of quantum optical experiments for a wide variety of applications.   

Live

The Illinois Express Quantum Network (IEQNET) is a program to realize a metro-scale quantum network testbed over deployed fiber using currently available technology. IEQNET consists of multiple sites (Q-nodes) that are geographically dispersed in the Chicago metropolitan area. Q-nodes generate or measure quantum signals such as entangled photons and communicate the results via conventional communication channels. The entangled photons in IEQNET nodes are generated at multiple wavelengths, and are selectively distributed to the desired users via optical switches. We will describe the network architecture of IEQNET, including the Internet-inspired layered hierarchy that leverages software-defined-networking (SDN) technology to perform traditional wavelength routing and assignment between the Q-nodes. Using SDN allows us to decouple the control and data planes, with the control plane being entirely classical. An important goal of IEQNET is to investigate to which extent the control plane can coexist with the data plane using the same fiber lines. We envision IEQNET to aid in developing robust and practical quantum networks by demonstrating metro-scale quantum communication tasks such as entanglement distribution and quantum-state teleportation.   

Live

Quantum teleportation, one of the most interesting predictions of quantum theory, has been widely investigated since its seminal demonstrations over 20 years ago. This is due to its connections to fundamental physics, and its central role in the realization of quantum information technology such as quantum computers and networks. Using fiber-coupled devices, including state-of-the-art low-noise superconducting nanowire single photon detectors and off-the-shelf optics, we achieve quantum teleportation of time-bin qubits with fidelities ≥90% at the telecommunication, and quantum memory-compatible, wavelength of 1536.5 nm. Further, to demonstrate the compatibility of our setup with deployed quantum networks, we teleport qubits over 22 km of single-mode fiber while transmitting qubits over an additional 22 km of fiber. Our system, which forms part of the Caltech and Fermilab quantum networks, provides a foundation for a high-fidelity quantum internet using practical devices.   

Live

Quantum repeaters (QRs) have been proposed as a solution for distributing entangled photons over large distances. The demands that system level performance metrics (data rate, fidelity of entanglement etc.) of a QR enabled link impose upon the hardware components of such a link is an area of great interest and can be used to guide applied materials and device design towards these objectives. This has become more important as the list of candidates for quantum technologies has increased as the physical realization of each qubit or detector technology brings its own set of advantages and disadvantages. We present a framework that uses a modular model of QR and highlights the trade-offs that exist between technological components. The reported values of these components, as present in the literature, are discussed, and accompanying analysis lays out the performance metrics that would be needed for these candidates to find application in a QR. We also present a method that takes advantage of idle repeaters to get a modest improvement in rates.   

On Demand

In recent years, the first experiments in demonstrating quantum communication technologies on board satellites, both large and small, have occurred. These experiments have shown that in principle, Quantum Key Distribution (QKD) using the satellites as a trusted-node for building a global quantum-safe communication system is possible. However, what if we do not want to use trusted-nodes? And what are the steps needed for a satellite-assisted quantum internet? In this talk, I will review the challenges and suggest some requirements to enable such a network to be built.