Past Talks

Abstract

A quantum network distributes quantum entanglements between remote nodes, which is key to many quantum applications. However, unavoidable noise in quantum operations could lead to both low throughput and low quality of entanglement distribution. This paper aims to address the simultaneous exponential degradation in throughput and quality in a  buffered multi-hop quantum network. Based on an end-to-end fidelity model with worst-case (isotropic) noise, we formulate the high-fidelity remote entanglement distribution problem for a single source-destination pair, and prove its NP-hardness. To address the problem, we develop a fully polynomial-time approximation scheme for the control plane of the quantum network, and a distributed data plane protocol that achieves the desired long-term throughput and worst-case fidelity based on control plane outputs. To evaluate our algorithm and protocol, we develop a discrete-time quantum network simulator. Simulation results show the superior performance of our approach compared to existing fidelity-agnostic and fidelity-aware solutions.

Abstract

An important task in the quantum information setting is to compress a quantum information source. Although Schumacher compression provides a way to perform lossless compression at the rate of the source's von Neumann entropy, the task of lossy compression below the entropy limit is an open question. In this talk, we will consider the lossy quantum source coding problem, with the aim of compressing a given quantum source at a rate below its von Neumann entropy. Using a standard Hamming code-based construction, we demonstrate the need for a new formulation. To develop a new formulation, we use the duality connections present in the classical setting, and thereby propose a new formulation for the lossy quantum source coding problem. This formulation differs from the existing quantum rate-distortion formulation in two aspects. Firstly, we use a global error constraint as opposed to the sample-wise local error criterion used in the standard rate-distortion setting. Secondly, instead of a distortion observable, we employ the notion of a Petz recovery map. Using these, we characterize the asymptotic performance limit in terms of the single-letter coherent information of the given map. This characterization represents a breakthrough in the framework for quantum compression since it is the first-ever single-letter description of the performance limit of a lossy quantum compression problem. It also demonstrates a stark duality with the corresponding quantum channel coding problem and also enjoys a strong equivalence connection with the lossless Schumacher compression. We will provide various examples, shedding light on the formulation and characterization of the performance limit.

Abstract

The Illinois Express Quantum Network (IEQNET) is a program to realize metropolitan-scale quantum networking over deployed optical fiber using currently available technology. IEQNET consists of multiple sites that are geographically dispersed in the Chicago metropolitan area. Each site has one or more quantum nodes (Q-Nodes) representing the communication parties in a quantum network. Q-Nodes generate or measure quantum signals such as entangled photons and communicate the measurement results via standard classical signals and conventional networking processes. The entangled photons in IEQNET nodes are generated at multiple wavelengths and are selectively distributed to the desired users via transparent optical switches. 

In this talk I 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. Specifically, SDN decouples the control and data planes, with the control plane being entirely implemented in the classical domain. I will also discuss the IEQNET processes that address issues associated with synchronization, calibration, network monitoring, and scheduling.

Abstract

Quantum Information Networks (QINs) attract increasing interest, as they enable connecting quantum devices over long distances, thus greatly enhancing their intrinsic computing, sensing, and security capabilities. The core mechanism of a QIN is quantum state teleportation, consuming quantum entanglement, which can be seen in this context as a new kind of network resource. 

In this talk, I will firstly discuss use cases per activity sector. Then, I will present a high-level architecture of a generic QIN, before focusing on the architecture of the Space segment, with the aim of identifying the main design drivers and critical elements. Finally, I will explain the Thales Alenia Space roadmap to developing the first QINs and detail the already concluded first step, the design and numerical simulation of a Space-to-ground entanglement distribution demonstrator.

Abstract

Network Utility Maximization (NUM) is a powerful mathematical framework that can be used to design and analyse classical communication protocols. NUM has enabled the development of distributed algorithms for solving the resource allocation problem, while at the same time providing certain guarantees, e.g., that of fair treatment, to the users of a network. In this talk, I will discuss our recent work on extending the notion of NUM to quantum networks, and introduce three quantum utility functions – each incorporating a different entanglement measure. The aim of the study is both to gain an understanding of some of the ways in which quantum users may perceive utility, as well as to explore structured and theoretically-motivated methods of simultaneously servicing multiple users in distributed quantum systems. Using our quantum NUM constructions, we develop an optimization framework for networks that use the single-photon scheme for entanglement generation, which enables us to solve the resource allocation problem while exploring rate-fidelity tradeoffs within the network topologies that we consider.  We find that our utility functions result in contrasting behaviors which provide some ideas regarding the suitability of quantum network utility definitions to different quantum applications. 

Abstract

The recent developments in quantum technology and understanding of the quantum nature of information have sparked the possibility of information technology that could outperform the classical one in presence of quantum resources, such as entanglement. Studying entanglement is hence essential for our understanding of such diverse areas as quantum optics, condensed matter physics and even high energy physics. Among all possible entangled states, k-uniform (k-UNI) and absolutely maximally entangled (AME) states, also called perfect tensors, have attracted much attention for a wide range of tasks such as multipartite teleportation, quantum secret sharing, and quantum networks. In this talk, I will discuss the class of known k-UNI and AME states by introducing a method for explicitly constructing such states that combines classical error correcting codes and qudit graph states. Furthermore, we see that at least for a large subset of k-UNI and AME, the states are inequivalent under stochastic local operations and classical communication. This subset is defined by an iterative procedure for constructing a hierarchy of k-UNI and AME graph states. 

Abstract

Quantum repeater is the key technology enabler for long-distance quantum communication. To date, most of the existing quantum repeater protocols are designed based on specific quantum codes or graph states. In this talk, I present a general framework for all-photonic one-way quantum repeaters based on the measurement-based error correction, which can be adapted to any Calderbank-Shor-Steane code including the recently discovered quantum low density parity check (QLDPC) codes. I also discuss a novel decoding scheme, where the error correction process is carried out at the destination based on the accumulated data from the measurements made across the network. This procedure not only outperforms the conventional protocols with independent repeaters but also simplifies the local quantum operations at repeaters. I will show a few examples including the [[48,6,8]] generalized bicycle code (as a small but efficient QLDPC code) which has an equally good performance while reducing the resources by at least an order of magnitude.

Abstract

In this paper, we propose and investigate a complementary technique to reduce EP generation latency—to pre-distribute EPs over certain (pre-determined) pairs of network nodes; these pre-distributed EPs can then be used to generate EPs for the requested pairs, when needed, with lower generation latency. For such an pre-distribution approach to be most effective, we need to address an optimization problem of selection of node- pairs where the EPs should be pre-distributed to minimize the generation latency of expected EP requests, under a given cost constraint. In this paper, we appropriately formulate the above optimization problem and design two efficient algorithms, one of which is a greedy approach based on an approximation algorithm for a special case. Via extensive evaluations over the NetSquid simulator, we demonstrate the effectiveness of our approach and developed techniques; we show that our developed algorithms outperform a naive approach by up to an order of magnitude.

Abstract

Quantum networks are known to offer unprecedented capabilities for secure communication tasks, remote and distributed quantum processing, and sensing. Nearly all of the current efforts focus on fiber-based terrestrial networks. I'll discuss current research aimed at extending quantum networks to mobile platforms, including satellites, and some of the concomitant new challenges and opportunities.

Abstract

The continuous quadratures of a single mode of the light field present a promising avenue to encode quantum information. By virtue of the infinite dimensionality of the associated Hilbert space, quantum states of these continuous variables (CV) can enable higher communication rates compared to single photon-based qubit encodings. Quantum repeater protocols that are essential to extend the range of quantum communications at enhanced rates over direct transmission have also been recently proposed for CV quantum encodings. Here we present a quantum repeating switch for CV quantum encodings that caters to multiple communication flows. The architecture of the switch is based on quantum light sources, detectors, memories, and switching fabric, and the routing protocol is based on a Max-Weight scheduling policy that is throughput optimal. We present numerical results on an achievable bipartite entanglement request rate region for multiple CV entanglement flows that can be stably supported through the switch. We elucidate our results with the help of exemplary 3-flow networks.

Abstract

Quantum communication networks (QCNs) are emerging as a promising technology that could constitute a key building block in future communication networks in the 6G era and beyond. In order to enhance the performance of QCNs, the available quantum resources must be carefully optimized and allocated so as to satisfy the requirements of the different users. Among those resources, entanglement is a key element that allows for data transmission between different nodes. However, if the entanglement generation rates in the QCNs are not optimized, then some of these valuable resources will be discarded and lost. Here, we consider the problem of optimizing the entanglement generation rates and their distribution over a quantum memory in a heterogenous QCN. This problem is posed as a mixed integer nonlinear programming optimization problem whose goal is to efficiently utilize the available quantum memory to maximize the users' satisfaction. Extensive simulations are conducted to evaluate the effectiveness of the proposed system. 

Abstract

The quantum technology revolution brings with it the promise of a quantum internet. A new — quantum — network stack will be needed to account for the fundamentally new properties of quantum entanglement. The first realisations of quantum networks are already happening and research interest in quantum network protocols has started growing. I will present a proposal for a quantum network architecture with a particular focus on the network protocol stack layer responsible for end-to-end entanglement connectivity and a software architecture based on a programmable quantum data plane.

Abstract

Large-scale quantum networks with thousands of nodes require scalable network protocols and physical hardware to realize. In this work, we introduce packet switching as a new paradigm for quantum data transmission in both future and near-term quantum networks. We propose a classical-quantum data frame structure and explore methods of frame generation and processing. Further, we present conceptual designs for a quantum reconfigurable optical add-drop multiplexer to realize the proposed transmission scheme. Packet switching allows for a universal design for a next generation Internet where classical and quantum data share the same network protocols and infrastructure. In this new quantum networking paradigm, entanglement distribution, as with quantum key distribution, is an application built on top of the quantum network rather than as a network designed especially for those purposes. For analysis of the network model, we simulate the feasibility of quantum packet switching for some preliminary models of quantum key and entanglement distribution. Finally, we discuss how our model can be integrated with other network models toward a realization of a quantum Internet.

Abstract

A quantum network allows for reliable transfer of quantum information between two distant nodes. The standard approach is to establish entanglement between the two nodes, so that they can perform teleportation. To do so, nodes usually establish a chain of entangled pairs between the source and the destination. Then, they perform swapping across the path, in order to create the desired entangled pair between the endpoints of the path. The process of selecting the appropriate chain of nodes for this operation is called quantum routing, for which several algorithms have been proposed.

Many of the existing quantum routing algorithms are conservative during the swapping process: they wait until all of the entangled pairs along the path are ready, and then start the swapping process. In this talk, we argue that we can act more opportunistically by starting the swapping process as soon as possible. We demonstrate the positive effects of opportunism by showing that it decreases the waiting time, and increases the transmission rate in a line network. Furthermore, we provide simulations demonstrating the increased efficiency caused by opportunism in more general networks. Finally, we mention a more general framework, in which opportunism can create a dynamic range of options for designing quantum networks.

Abstract

Quantum computing (QC) is poised to impact diverse fields such as chemistry, material science and machine learning. In recent years, we have witnessed rapid progress in building QC systems, but there is a huge gap between the resource requirements of useful QC applications and the hardware that is buildable now. My research seeks to close the applications-to-devices resource gap in QC by developing quantum computer architecture and compilation techniques.

In this talk, I will present a cross-cutting, full-stack design philosophy for building the QC execution stack. Using this approach, I will show examples of cross-cutting optimizations that bridge the resource gap, from the top with noise-adaptive compiler techniques to optimize application requirements, and from the bottom via system architectures efficiently exploiting scarce QC resources. These optimizations offer two to four orders of magnitude improvement in execution fidelity, compared to existing compilers and architectures, and are important to enable progress towards larger QC systems. In response to my work, several industry vendors have included noise-adaptivity and its extensions as part of their toolflows and adjusted device architecture to expose native operations and hardware characterization data to software.

Abstract

Greenberger-Horne-Zeilinger (GHZ) states play an important role in error-correction architectures for distributed quantum computation. The GHZ protocols that we consider follow from a heuristic dynamic programming algorithm that optimizes over a large class of protocols that create and purify GHZ states. All protocols use a common framework based on measurements of non-local stabilizer operators of the target GHZ state, where each non-local measurement consumes another (non-perfect) entangled state as a resource. 

We apply our algorithm in the context of a distributed surface code, where each of the nodes is a noisy diamond defect center. In a distributed surface code, Bell and GHZ states can be used to non-locally measure the stabilizers of the code and detect errors. As a metric to compare GHZ protocols we use the effective measurement fidelity in case the GHZ state is used to measure a surface code stabilizer. This allows us to find good GHZ protocols for distributed surface codes with present-day and near-future diamond defect center models and parameters. We use these GHZ protocols to calculate distributed surface code error thresholds. 

This is joint work with Runsheng Ouyang, Paul Möller and David Elkouss.

Abstract

We consider here a set of near-term implementable distillation protocols. These protocols distill n to k pairs by using bilocal Clifford operations, a single round of communication and a possible final local operation depending on the observed measurement outcomes. In the case of permutationally invariant depolarising noise on the input states, we find a correspondence between these distillation protocols and graph codes. We leverage this correspondence to find provably optimal distillation protocols in this class for several tasks. This correspondence allows us to investigate use cases for so-called non-trivial measurement syndromes. Furthermore, we detail a recipe to construct the circuit used for the distillation protocol given a graph code. We use this to find circuits of short depth and small number of two-qubit gates. We find that these found circuits perform comparable with circuits found based on genetic algorithms. Finally, we compare our found protocols with other methods for the teleportation of encoded states, where we find it is possible to achieve both improved rates and fidelities.

Abstract

In this talk, I will discuss several natural quantum problems and, in particular, how the problems change as the quantum resources change. I will show how to take an economics perspective to assign a "shadow price" to each quantum resource. To do this, I will use optimization theory and show that shadow prices are often given "for free" if you know where to look for them. No knowledge about economics, optimization theory, or quantum theory is needed for this talk. This is joint work with Gary Au (University of Saskatchewan).  

Abstract

Fidelity estimation is essential for the quality control of entanglement distribution networks. In this paper, we consider a setup in which nodes randomly sample a subset of the entangled qubit pairs for measurement and then estimate the fidelity of the unsampled pairs conditioned on the measurement outcome. The proposed estimation protocol, which performs local Pauli operators according to a predefined sequence, is implementation friendly. Despite its simplicity, this protocol achieves the lowest estimation error in the difficult scenario with arbitrary noise and no prior information. The analysis reveals the issue of excessive measurements, a counterintuitive phenomenon in which more measurements lead to less accurate estimation when the sampling ratio exceeds 0.5 in scenarios with arbitrary noise.

Abstract

Machine learning can help us in solving problems in the context of big-data analysis and classification, as well as in playing complex games such as Go. But can it also be used to find novel protocols and algo- rithms for applications such as large-scale quantum communication? Here we show that machine learning can be used to identify central quantum protocols, including teleportation, entanglement purification, and the quantum repeater. These schemes are of importance in long-distance quantum communication, and their discovery has shaped the field of quantum information processing. However, the usefulness of learn- ing agents goes beyond the mere reproduction of known protocols; the same approach allows one to find improved solutions to long-distance communication problems, in particular when dealing with asymmet- ric situations where the channel noise and segment distance are nonuniform. Our findings are based on the use of projective simulation, a model of a learning agent that combines reinforcement learning and decision making in a physically motivated framework. The learning agent is provided with a universal gate set, and the desired task is specified via a reward scheme. From a technical perspective, the learning agent has to deal with stochastic environments and reactions. We utilize an idea reminiscent of hierarchical skill acquisition, where solutions to subproblems are learned and reused in the overall scheme. This is of particular importance in the development of long-distance communication schemes, and opens the way to using machine learning in the design and implementation of quantum networks.

Abstract

Long-distance quantum communication relies on distribution of either single qubits or entangled photon pairs across large distances. However, the direct transmission distance is limited to around few hundred kilometres by exponential losses in optical fibres. Quantum repeaters that rely on heralded generation and storage of entanglement have been proposed to overcome this limit. Although they can surpass the direct transmission limit, they still fall short of providing a global coverage. On the other hand, distribution of entangled states from an orbiting satellite provides a significant improvement over the land-based fibre links [1]. Here, the communication range is mainly limited by the line-of-sight of the satellite which depends on its orbit (~2000 km for low Earth orbit, 1/3 of the globe for geostationary orbit). 

In order to provide a truly global coverage, we propose to combine the above two solutions in a single architecture: a quantum repeater in space. The network would consist of two types of satellites: one carrying an entangled photon pair source and the other quantum memories and Bell state measurement stations. Our work shows that up to 3 orders of magnitude faster entanglement distribution rate can be achieved for distances  km [2] when compared to ground-space hybrid networks. After presenting the details of this work, I will focus on the optimizations performed with a three-satellite QR link [3]. I will finally summarize our experimental efforts towards creating quantum memory systems suitable for space applications for such global quantum communication architectures. 

[1] J. S. Sidhu, S. Joshi, M. Gündoğan et. al. Advances in Space Quantum Communications, IET Quant. Comm. 2, 182 (2021) 

[2] M. Gündoğan, J. S. Sidhu, V. Henderson, L. Mazzarella, J. Wolters, D.K.L Oi and M. Krutzik, Proposal for space-borne quantum memories for global quantum networking, npj Quant. Inf. 7, 128 (2021)

[3] J. Wallnöfer, F. Hahn, M. Gündoğan, J. S. Sidhu, F. Krüger, N. Walk, J. Eisert and J. Wolters, Simulating quantum repeater strategies for multiple satellites, under review, arXiv:211015806 (2021)

Abstract

EPR-pairs are crucial in transmitting data qubits over an insecure quantum internet. For sending qubits reliably, a standard method is first to distribute EPR-pairs between a sender and a receiver. After sharing the EPR-pairs, the sender sends the data qubit using quantum teleportation. It shows the importance of EPR-pair distribution in a quantum internet. In this presentation, I will explain how to use the well-known multi-commodity flow-based technique to maximize the total achievable rates for distributing EPR-pairs among multiple source-destination pairs in a network of quantum repeaters while keeping a lower bound on the end-to-end fidelity as a requirement.

Abstract

Multi-photon graph states are a fundamental resource in quantum communication networks, distributed quantum computing, and sensing. These states can in principle be created determinis- tically from quantum emitters such as optically active quantum dots or defects, atomic systems, or superconducting qubits. However, finding efficient schemes to produce such states has been a long-standing challenge. Here, we present an algorithm that, given a desired multi-photon graph state, determines the minimum number of quantum emitters and precise operation sequences that can produce it. The algorithm itself and the resulting operation sequence both scale polynomially in the size of the photonic graph state, allowing one to obtain efficient schemes to generate graph states containing hundreds or thousands of photons.

Abstract

We consider the problem of secure packet routing at the maximum achievable rate in a Quantum key distribution (QKD) network. Assume that a QKD protocol generates symmetric private keys for secure communication over each link in a multi-hop network. The quantum key generation process, which is affected by noise, is assumed to be modeled by a stochastic counting process. Packets are first encrypted with the available quantum keys for each hop and then transmitted on a point-to-point basis over the communication links. A fundamental problem that arises in this setting is to design a secure and capacity-achieving routing policy that accounts for the time-varying availability of the quantum keys for encryption and finite link capacities for transmission. In this paper, by combining the QKD protocol with the Universal Max Weight (UMW) routing policy, we design a new secure throughput-optimal routing policy, called Tandem Queue Decomposition (TQD). TQD solves the problem of secure routing efficiently for a wide class of traffic, including unicast, broadcast, and multicast. One of our main contributions in this paper is to show that the problem can be reduced to the usual generalized network flow problem on a transformed network without the key availability constraints. Simulation results show that the proposed policy incurs a substantially smaller delay as compared to the state-of-the-art routing and key management policies. The proof of throughput-optimality of the proposed policy makes use of the Lyapunov stability theory along with a careful treatment of the key-storage dynamics.

Abstract

Quantum networks are complex systems formed by the interaction among quantum processors through quantum channels. Analogous to classical computer networks, quantum networks allow for the distribution of quantum computation among quantum computers. In this work, we describe a quantum walk protocol to perform distributed quantum computing in a quantum network. The protocol uses a quantum walk as a quantum control signal to perform distributed quantum operations. We consider a generalization of the discrete-time coined quantum walk model that accounts for the interaction between a quantum walker system in the network graph with quantum registers inside the network nodes. The protocol logically captures distributed quantum computing, abstracting hardware implementation and the transmission of quantum information through channels. Control signal transmission is mapped to the propagation of the walker system across the network, while interactions between the control layer and the quantum registers are embedded into the application of coin operators. We demonstrate how to use the quantum walker system to perform a distributed CNOT operation, which shows the universality of the protocol for distributed quantum computing. Furthermore, we apply the protocol to the task of entanglement distribution in a quantum network.

After discussing the protocol, I'll address how the logical behavior captured by quantum walks provides a way to define optimal quantum circuit distribution across a network.

Abstract

QuNetSim is a quantum-enabled network simulator that adds common quantum networking tasks like teleportation, superdense coding, sharing EPR pairs, etc, to aid in the development of quantum networking protocols. With QuNetSim, one can design and test robust quantum network protocols under various network conditions. In this presentation I will give an overview of what QuNetSim does and demonstrate some examples of how it can be used.