# Talks

Quantum Internet - The Certifiable Road Ahead

Stephanie Wehner, *QuTech, Delft University of Technology, Netherlands*

In this talk, we will identify stages towards the development a full blown quantum internet. Each stage is characterized by a specific increase in network functionality at the expense of a matching increase in experimental difficulty.. We highlight some known application protocols that can be realized in the specific stages, and discuss simple tests to certify that a particular stage as been attained. Finally, we highlight some recent results that may be of interest to the community.

Quantum internet, routing protocols, and anonymous transmission

Iordanis Kerenidis, *CNRS Paris, France*

We will provide a model for quantum networks that enables us to use classical routing techniques to efficiently create entanglement between any two nodes in the network. We will also show new protocols for anonymous transmission of classical and quantum messages in a quantum network.

Asynchronous quantum clock synchronization by entanglement purification

Jonathan P. Dowling, *Louisiana State University, USA*

We examine the problem of performing asynchronous quantum clock synchronization between

distant parties. Previously a protocol for synchronizing clocks using shared entanglement was proposed by Jozsa and co-workers, but it was pointed out that in general there will be an unknown phase in the state which introduces an error to the synchronized time. We show that in fact entanglement purification can in fact remove the unknown phase, despite prior arguments to suggest the contrary. The key step is to performing random bilateral rotations, which creates a Werner state in the singlet state for the local basis choice. We show that the scheme produces a singlet state in the presence of differing basis conventions for Alice and Bob, an overall time offset in the execution of the purification algorithm, and the presence of a noisy channel. This allows for clock synchronization to be performed without either party having a common time reference. We discuss implementations of the protocol via shared entanglement between space-to-space and space-to-ground downlinks.

Certifying graph states and their use in quantum networks

Damian Markham, *CNRS, Université Pierre et Marie Curie, France*

We discuss the certification and application of graph states for quantum networks. We present a simple protocol for certifying graph states using stabilisers, which we apply to secret sharing, delegated computation, certified generation of random operations, and quantum tomography.

Quantum communication networks: fundamental rate limits, resource allocation and protocol design

Saikat Guha, *University of Arizona, USA*

In this talk, I will set up the problem of quantum communications, i.e., generating shared entanglement and/or transmitting qubits reliably between multiple user pairs or groups at the highest possible rates, which are connected via optical links in some network topology with quantum-processing-capable "repeater" nodes. I will first review some recent information-theoretic results on quantifying fundamental performance limits based solely on the transmission losses of the network's links, but with no other constraints imposed on resources made available at the nodes or on the specific communication protocols employed. Next, I will discuss detailed performance evaluation of some simple repeater protocols that include various device imperfections and explicit error correction strategies, for supporting a single flow on a linear repeater chain. I will then describe recent results on how simple "routing" strategies can help multiple simultaneous flows attain rates that exceed what is possible with each repeater time-sharing its local action between optimally assisting individual flows. Finally, I will discuss various open problems on: (a) quantum network design, i.e., where to place repeaters given resource constraints and network performance needs, (b) resource allocation, i.e., how to provision resources (e.g., memories, spectral or temporally multiplexed channels) dynamically to flows based on network traffic, and (c) protocol design, i.e., designing explicit recipes for repeater nodes to decide their local quantum (processing and measurement) actions that may or may not have the knowledge of global network topology and may only need to act based upon local link-state knowledge. I hope this talk will help develop a common language for discussing fundamental limits, specific protocols, and new thoughts to be elaborated in the individual talks to follow in this session.

Scaling of resources in different repeater protocols

Wolfgang Dür, *University of Innsbruck, Austria*

We compare the reachable fidelities and required resources for different repeater protocols. We use the reachable (private) fidelity as a benchmark, and are in particular interested in achievable rates and overheads in terms of number of storage qubits or parallel channels. We concentrate on scaling properties of required resources and the required times for long-distance pair generation, which leads to rates per channel (or storage qubits) as a key quantity. We discuss how spatial resources can be traded for temporal resources to a certain extend, and and investigate the possibility of multiplexing in different schemes.

We show that a novel repeater protocol based on hashing outperforms previous schemes as overheads (per channel) are constant and do not scale with the distance, in contrast to polynomial or polylogarithmical scaling of previous proposals. We also provide a comparison for fixed number of links, thereby demonstrating that the scaling advantage also manifests in significantly larger rates for continental or intercontinental distances. The crucial feature that allows this is the usage of deterministic one-way entanglement purification (with constant yield) in a non-nested fashion.

Martin Roetteler, *Microsoft Research*

We design protocols for communication between parties that are connected by a network of quantum channels. We assume that there is no prior entanglement between any of the parties, but that classical communication is free. The task is to transfer an unknown quantum state from a source subsystem to a target subsystem under the constraints imposed by the network. We establish a connection t classical network coding and then show how this allows to convert any classical solution into a quantum protocol.

Base on joint work with Hirotada Kobayashi, Francois Le Gall, Harumichi Nishimura (https://arxiv.org/abs/0908.1457), and Niel de

Beaudrap (https://arxiv.org/abs/1403.3533).

Ultimate performance of repeater-assisted quantum communications

Stefano Pirandola, *University of York, England*

We study the ultimate rates for transmitting quantum information, distributing entanglement, and generating secret keys via quantum repeaters, from the basic scenario of a single repeater chain to an arbitrarily-complex quantum communication network, where systems may be routed through single or multiple paths. Combining methods from quantum information and network theory, we derive single-letter upper bounds for the end-to-end capacities that are achievable by the most general adaptive protocols of quantum and private communication. Most importantly, we establish these capacities under fundamental noise models, including bosonic loss which is the most important for optical communications. Depending on the routing, optimal strategies can efficiently be found by solving the widest path or the maximum flow problem suitably extended to the quantum setting.

Based on https://arxiv.org/abs/1601.00966

Implementation of quantum repeater and networks

Kae Nemoto, *National Institute of Informatics, Japan*

Quantum repeater designs have been categorized in three generations. Though repeater designs in each category could achieve arbitrary distance communication in theory, such systems will require high accuracy in their physical devices unrealistic to the current technology. Small quantum repeater systems are expected to be realized in the near future, and they will not be capable to fully treat errors. We review the properties of the typical designs for each generation, and examine how we implement and scale up such quantum communication systems. Analyzing the performance of the systems, we discuss advantages and disadvantages of quantum repeaters and networks.

From theory to practice: What it takes for quantum repeaters to prove useful?

Mohsen Razavi, *University of Leeds, England*

One measure of usefulness for a new quantum communications system is whether it can outperform existing technologies in certain aspects. In the case of QKD and quantum repeaters, one can ask if we can implement, with today’s technology, a quantum repeater system that provides higher secret key generation rates over a distance region. In this talk, I will go over a series of proposals for implementing the simplest possible repeater structure: a repeater with only one middle node. Furthermore, we only use quantum memories in that middle node, and the two end users are only equipped with BB84-like encoders. This will simplify to some extent certain requirements of building such systems. By considering different structures suitable for different types of memories that can be used in such a setup, I will provide some qualitative and quantitative insight into what possible routes are the most promising in a timeframe of the next 5-10 years. It turns out that for implementing even the simplest repeater structure, with the performance criterion mentioned above, a host of devices, in addition to the quantum memories, need to be working at the edge of, or slightly above, their current state of the art. This would highlight the importance of accounting for quality, in addition to quantity, of employed devices in estimating the cost of quantum repeater networks.

Towards multi-node quantum networks: progress and challenges

Ronald Hanson,** ***QuTech and Kavli Institute of Nanoscience, Delft University of Technology, Netherlands*

Future quantum networks connecting multi-qubit nodes via quantum-optical channels may harness the unique features of entanglement in a range of exciting applications, such as quantum computation and simulation, secure communication, enhanced metrology for astronomy and time-keeping as well as fundamental tests of nature.

Diamond spins associated with NV centers are promising building blocks for such a network as they combine a coherent electron-optical interface (similar to that of trapped atomic qubits) with a local register of robust and well-controlled nuclear spin qubits. A central challenge is to merge the established techniques for remote entanglement generation [1] with the storage and processing of previously generated entangled states [2]. Novel insights into the relevant intra-node mechanisms of decoherence and crosstalk as well the development of new techniques [3] have recently enabled entanglement distillation on a pair of spatially separated two-qubit nodes [4].

The next major milestone will be the realization of multi-node quantum networks. A key requirement for this goal is that the entangling rate between nodes exceeds the decoherence rate within the nodes. Here I will present an overview of the ongoing work and related challenges, with the specific target of realizing a multi-node network wired by quantum entanglement.

References

[1] W. Pfaff et al., Science 345, 532 (2014); B. Hensen et al., Nature 526, 682 (2015).

[2] J. Cramer et al., Nature Comm. 7, 11526 (2016).

[3] A. Reiserer et al., Phys. Rev. X 6, 021040 (2016).

[4] N. Kalb, A. Reiserer, P.C. Humphreys et al., Science 356, 928 (2017).

Christopher Monroe, *JQI and U. Maryland, USA*

Trapped atomic ions are standards for quantum information science, acting as qubits that have unsurpassed levels of quantum coherence, can be replicated and scaled with the atomic clock accuracy, and allow near-perfect measurement. Gates between proximal qubits can be mediated by modulating the Coulomb repulsion between ions with control laser beams, allowing the local qubit connectivity graph to be reconfigured and optimally adapted to a given task. Moreover, connections between nodes of trapped ion crystals can be made with photonic protocols acting between subsets of ions in each collection. This allows a modular quantum architecture to be established for uses in quantum communication and large-scale quantum computation.

An integrated diamond nanophotonics platform for quantum-optical networks

Ruffin Evans, *Harvard University, USA*

I will present a new platform for scalable quantum nanophotonics based on silicon-vacancy (SiV) color centers coupled to diamond nanodevices. The SiV has the unprecedented combination of long spin coherence times and nearly lifetime-limited optical linewidths while maintaining compatibility with nanofabrication techniques. First, I will discuss recent work on extending the coherence time of the SiV to beyond 10 ms by cooling the system down to below 100 mK. By placing SiV centers inside diamond photonic crystal cavities, we realize a quantum optical switch controlled by a single color center. We control the switch using SiV metastable states and observe switching at the single-photon level. Raman transitions are used to realize a single-photon source with a tunable frequency and bandwidth in a diamond waveguide. By measuring intensity correlations of indistinguishable Raman photons emitted into a single waveguide, we observe quantum interference resulting from the superradiant emission of two entangled SiV centers.

Towards a Solid-State Quantum Repeater based on Rare-Earth doped Crystals

Mikael Afzelius, *University of Geneva, Switzerland*

The ability to generate multimode quantum correlations between photons and excitations in matter is a key resource for quantum repeaters and quantum networks. Solid-state devices are gaining interest as potential quantum nodes, as these open up possibilities for device miniaturization and component integration. A large variety of solid-state systems are currently being investigated, among those single color centers or defects, ensembles of ions and mechanical oscillators. Rare-earth ions in crystals have a rather unique set of properties that makes them suitable as multimode quantum nodes, such as large optical bandwidths combined with long optical and spin coherence times.

Quantum repeater: Quis es et unde venis

Gerhard Rempe, *Max Planck Institute of Quantum Optics*

Groundbreaking experiments with single quantum bits of light and matter have shown that it is possible to build elementary quantum networks in which quantum states can either be directly transferred or indirectly teleported between quantum network nodes by means of photons. In order to detect photon loss and implement a success herald, a nondestructive photon-tracking mechanism has been realized. It is intrinsically deterministic and has subsequently been applied to demonstrate an efficient quantum memory for flying photons, a quantum gate between a stationary atom and a flying photon as well as a quantum gate between two flying photons. Most recently, the nondestructive atom-photon interaction mechanism has been used to carve entanglement between two nearby atoms and trigger a fast quantum gate between them. In parallel, the memory time of a network node has been extended by several orders of magnitude to reach record-high values. All these achievements together seem to provide a unique toolbox for the realization of an elementary quantum repeater. The main challenges include the integration of the different capabilities into a common platform, the exploration of practical architectures and the identification of suitable protocols, all with the goal to increase the quantum communication bit rate beyond that of a simple lossy quantum channel.

Quantum networking based on ensembles of neutral atoms

Alex Kuzmich, *University of Michigan, Ann Arbor*

Qubits encoded into ensembles of neutral atoms trapped in optical lattices can store quantum information for many seconds. Dozens or even hundreds of qubits can be encoded into a single atom trap by use of memory multiplexing. The atomic qubits can be either converted into, or entangled with, photonic qubits. The wavelength of the photonic qubits is determined by the resonance frequencies of the atomic transitions. These typically are in the visible or the near-infrared, however telecom wavelengths become accessible by employing collective cascade emission or wavelength conversion in auxiliary atomic ensembles. Quantum gates between pairs of atomic qubits, or an atomic and a photonic qubit, or a pair of photonic qubits, can be realized by employing transitions to highly excited atomic Rydberg states. All of these elements have been demonstrated in separate experiments. The outstanding challenge is to improve performance metrics and to achieve integration of all these capabilities.

From free-space to all-fibered light-matter interfaces: combining nanophotonics and cold atoms as a new route

Julien Laurat, *Laboratoire Kastler Brossel, UPMC, Paris, France*

In the general context of quantum information networks, my group focuses on the manipulation of light at the single-photon level using large ensembles of cold neutral atoms. In a free-space implementation, we reported for instance the quantum storage of OAM-encoded qubits and a multiple-degree-of-freedom memory for structured light, targeting thereby an enhanced information coding density. Most recently, we also push the overall storage-and-retrieval efficiency close to 70%. These achievements offer now an efficient node for future tests of complex quantum network functionalities. In this talk, I will discuss these multiplexed realizations and then focus on a new route: the coupling of cold atom arrays with 1D nanoscale waveguides. This challenging combination appears as promising alternative to free-space focusing and a strong pathway to build light-matter interfaces thanks to the tight confinement of the light. It enables longer interaction length, large optical depth, and potentially non-linear interactions at very low power level. I will highlight some recent results, including the realization of an all-fibered memory and the heralding of a single collective excitation in this platform. The ability to generate single photons, store them and control their transport in 1D waveguides coupled to atom arrays would allow for novel all-fibered quantum network capabilities and many-body effects emerging from long-range interactions.

Ultrafast long-distance quantum communication with static linear optics

Peter van Loock, *University of Mainz, Germany*

The first and most common approach to (fiber-based) quantum communication across large distances, circumventing the effect of an optical transmission loss exponentially growing with distance, is the quantum repeater. However, the standard quantum repeater based on local quantum memories and two-way classical communication is rather slow, producing fairly low rates and requiring long-lasting memories. An obvious remedy here is to replace quantum error detection (as employed in a standard quantum repeater in the form of entanglement purification) by quantum error correction. Based on existing ideas for such encoded quantum repeaters, we will discuss the possibility of implementing ultrafast long-distance quantum communication with linear optics. For this purpose, we propose a projection measurement onto encoded Bell states with a static network of linear optical elements that enables one to acquire sufficient syndrome information from a transmitted encoded qubit in a non-destructive fashion. By increasing the size of the quantum error correction code, both the linear-optics Bell measurement efficiency and the photon-loss tolerance can be enhanced at the same time. As a result, all-optical quantum communication over large distances with communication rates similar to those of classical communication is, in principle, possible based on local state teleportations using optical sources of encoded Bell states, fixed arrays of beam splitters, and photon detectors. We also consider an extension of our scheme from loss to fault tolerance including error sources beyond transmission losses such as depolarizing effects and detector inefficiencies (losses and dark counts). Other issues to be covered are possible improvements via enhanced linear optical Bell measurements on the level of the physical qubits and the supposedly most demanding part of our scheme, namely the generation of the encoded light states.

Wide area quantum communication in China - from Beijing-Shanghai quantum backbone network to "Mucius" quantum satellite

Cheng-Zhi Peng, *University of Science and Technology of China*

Recently, China has launched the first quantum science experimental satellite "Mucius" and finished the construction of Beijing-Shanghai quantum backbone network towards a space-earth integrated quantum communication network. In this talk, I will introduce these two big projects and report their recent developments, especially the recent published results of satellite-to-earth quantum entanglement distribution, quantum key distribution, and earth-to-satellite quantum teleportation.

Entanglement distribution over global distances

Khabat Heshami, *NRC Ottawa, Canada*

Entanglement over global distances can enable tests of fundamental physics, probing effects of gravity on quantum phenomena, and monitoring space-time variations of fundamental constants. These are in addition to the practical applications of long-distance entanglement distribution in secure communications and blind or distributed quantum computation. I will discuss entanglement distribution over up to 20,000 kms based on quantum repeaters with satellite links. This requires satellites equipped with sources of entangled photons and ground stations with quantum memories and quantum non-demolition detection. Finally, I will briefly describe an approach to achieve quantum non-demolition detection of photonic qubits.

Quantum repeaters based on room temperature atomic ensembles

Anders Sørensen, *Niels Bohr Institute, University of Copenhagen, Denmark*

For real life applications of quantum repeaters it will be desirable to have systems at room temperature. Furthermore system should preferably be easy miniaturised and easy to produce to allow for parallel operation. I will discuss how this is achievable with spin coated atomic micro cells. A particular problem for these systems is that the atoms move in and out of the laser beams controlling the interactions. Traditionally experiments have dealt with this issue by having sufficiently short memory times so that atoms do not have time to leave the interaction region. For quantum repeaters where long storage times are required, however, this technique cannot be used. By employing narrow optical filter cavities it is instead possible to operate in the opposite regime of “motional averaging” where the interaction time is long compared to the time it takes the atoms to traverse the atomic cell. I will present the theory behind this motional averaging and how it can be used for quantum repeaters. Furthermore I will discuss the first step towards realising quantum repeaters based on motional averaging.

Telecom-band frequency conversion interfaces using nanophotonic resonators

Kartik Srinivasan, *Center for Nanoscale Science and Technology, National Institute of Standards and Technology, USA*

Efficient, low-noise spectral translation of a quantum state of light, or quantum frequency conversion, may be an important resource for quantum repeaters and networks. I will describe our development of chip-integrated silicon nitride nanophotonic resonators for connecting near-infrared and visible wavelength sources with the telecommunications band. In a first class of devices, the four-wave-mixing Bragg scattering process is exploited, where the application of two continuous-wave pump fields results in spectral translation of an input signal by a range given by the difference in the pump frequencies. Experiments demonstrating bidirectional conversion between the 980 nm and 1550 nm bands with >60% conversion efficiency will be presented, and application to quantum dot single-photon sources will be discussed. Extensions to other systems, such as the nitrogen vacancy center in diamond, will also be considered. Finally, a second approach for connecting the visible and telecommunications bands, based on entanglement swapping using engineered photon pair sources, will be discussed.

Scalable ensemble-based repeater architectures for multipartite states

Christine Muschik, *University of Innsbruck, Austria*

The vision to develop quantum networks entails the realisation of multi-user applications, which requires the generation of long-distance multipartity-entangled states. We are currently witnessing rapid experimental progress in building prototype networks, calling for new design concepts to guide future developments in this fast-moving field. We address this question in a concrete and implementation-oriented manner by proposing a robust and experimentally feasible scheme for distributing three-party entangled states of GHZ type, that can be used for clock synchronisation, and for quantum secret sharing and quantum voting. Our intrinsically two-dimensional approach is based on atomic or solid state ensembles and provides a robust architecture with built-in error filtering mechanisms. While our scheme is inspired by current (one-dimensional) ensemble-based repeater experiments, it does not suffer from the same problems associated with photon losses and finite quantum memory times. Taking realistic error sources and imperfections into account, we provide an efficient design for future experiments with a clear perspective in terms of scalability.

Towards a device-independent certification of building blocks of quantum networks

Nicolas Sangouard, *Department of Physics, University of Basel, Switzerland*

How can one certify the proper functioning of quantum networks? In this talk, we will present a bottom up approach in which the building blocks of quantum networks are certified separately. We will focus on Bell tests as they provide certification techniques that are independent of the actual implementations. We will show first results and on-going investigations on how these Bell tests can be used to detect quantum correlations between hundreds of qubits or to certify the proper functioning of devices that are needed to process quantum information.