Title: Fundamental Limits of Entanglement-Assisted Covert Communication over the Bosonic Channels

Abstract: We study the fundamental limits of quantum-secure covert-communication over the lossy thermal-noise bosonic channels, the quantum-mechanical model for many practical channels. We show that entanglement assistance improves the scaling law for covert communication over these channels: instead of $L_{no-EA}\sqrt{n}-r_{no-EA}(n)$, $r_{no-EA}(n)=o(\sqrt{n})$, covert bits that can be reliably transmitted using $n$ modes of the bosonic channels without entanglement assistance, shared entanglement allows the reliable transmission of $L_{EA}\log(n)\sqrt{n}-r_{EA}(n)$, $r_{EA}(n)=o(log(n)\sqrt{n})$, covert bits using $n$ modes. We develop the expressions for covert capacities $L_{no-EA}$ and $L_{EA}$, as well as bounds for the remainder terms $r_{no-EA}(n)$ and $r_{EA}$. Finally, we show that our entanglement-assisted optical transceiver architecture captures the scaling gain for covert communication.

Title: Protocols in quantum networks: recent results and experimental challenges

Abstract: In this talk I will discuss applications of quantum networks going beyond quantum key distribution.
I will show that quantum networks can be used to perform distributed computing, both classical and quantum. Furthermore, I will present a recent experiment that shows quantum communication in a multipartite network and allows – besides security in the communication – keeping the identities of the participating parties secure.
I will conclude with a discussion of experimental challenges in realizing advanced networked quantum protocols.

Title: Network Coding for Efficient Vertical Handovers - The Scientific Method in Engineering

Abstract: In 2008, the Institute of Electrical and Electronics Engineers (IEEE) published its standard IEEE 802.21 for media-independent handover services. The main scope of this work was to design a technology agnostic mobility platform to perform vertical handovers between heterogeneous networks. What is the impact of contextualising this scientific question in the real engineering world? Actually, the value of the proposed novel architecture/protocol to efficiently perform vertical handovers is not only given by a theoretical analysis and practical tests (via emulation, simulation, testbeds, etc.). In fact, standardisation bodies, industry and market trends also impact on the value of scientific methods and solution in engineering research.

Title: Extending links in quantum networks – an overview on the research network Q.Link.X

Abstract: Quantum networks, i.e. local nodes consisting of stationary quantum bits able to process and store quantum information connected by “flying” qubits suited to transfer this information, are at the heart of concepts for long-range secure quantum communication and distributed quantum computing or quantum sensing. The main obstacle in establishing networks over large distances is the inherent loss of transmission channels, in particular for networks based on an infrastructure of optical fibers. The most feasible and promising route towards long distance quantum communication without sacrificing the physical security based on quantum mechanical concepts is based on quantum repeaters [1] to distribute entangled states to remote communication partners.The German research network “Quantum Links extended – Q.Link.X” investigates a bottom-up approach to realizing basic elements of fiber-based quantum repeaters (QR). The basic elements are QR-segments, i.e. heralded entanglement distribution between two nodes, and QR-cells, i.e. a node comprising two qubits sending out photons entangled with one of the qubits, respectively, and gate operations between the qubits to perform a Bell-state measurement, realizing a “single sequential quantum repeater” scheme [2]. The combination of QR-segments and QR-cells then allows for scaling up the quantum links. Q.Link.X investigates three different hardware platforms to realize these elements, i.e. trapped neutral atoms and ions, semiconductor quantum dots and color centers in diamond. The talk will present the basic concepts and an overview on recent experimental achievements in Q.Link.X.

[1] H.-J. Briegel et al., Phys. Rev. Lett. 81, 5932 (1998). DOI: 10.1103/PhysRevLett.81.5932
[2] D. Luong et al., Appl. Phys. B 122, 96 (2016). DOI: 10.1007/s00340-016-6373-4

Title: One-Way Quantum Repeater Based on Near-Deterministic Photon-Emitter Interfaces

Abstract: In a so-called one-way quantum repeater, quantum information is encoded in an error-correcting code to protect it against loss between the repeater stations. Compared to other repeater architectures, the one-way repeater does not require long-term quantum memories and can have a very high rate approaching the MHz-range over a 1000 km distance. In this talk, I will describe our recent proposal for a one-way repeater which can be constructed with as little as two stationary spin qubit and one quantum emitter per repeater station. This significantly increases the experimental feasibility and represents orders of magnitude decrease in spin-qubit resource compared to previous proposals. I will discuss potential implementations with diamond defect centers and semiconductor quantum dots efficiently coupled to photonic nanostructures and outline how such systems may be integrated into repeater stations.

Title: Post Shannon Quantum Communication and (unexpected) Links between Computing and Information Processing

Abstract: The talk will first introduce performance requirements and post Shannon communication tasks for future quantum communication networks. Following this, the post Shannon communication tasks "identification of messages" and "secure identification of messages" will be discussed in detail and corresponding capacities will be derived. It turns out that these communication tasks behave quite unexpectedly compared to "Shannon's message transmission" communication tasks. This also applies to "identification of messages" with feedback and entanglement assistance. Subsequently, it will be shown that important capacities are not Turing computable, i.e. they can never be calculated or simulated on digital hardware with performance guarantees. Continuing with these results, the second part of the talk will examine methods of information processing and physical theories regarding their computability on Turing machines. Some questions of computability and connections to Research Unit B, quantum simulation, and to Research Unit C, quantum computing, will be discussed. A large number of information processing tasks and physical theories will be identified that are not computable on Turing machines, i.e. ones that can never be simulated on a Turing machine with performance guarantees. For some of these problems, "implementations" on "ideal analog computers" are possible. Thus on the level of "abstract machine models" for computation it turns out that for these tasks the "ideal analog computer" is more powerful than the ideal digital computing model of "Turing Machine."

Title: Fault-tolerant Coding for Quantum Communication

Abstract: Designing encoding and decoding circuits to reliably send messages over many uses of a noisy channel is a central problem in communication theory. When studying the optimal transmission rates achievable with asymptotically vanishing error it is usually assumed that these circuits can be implemented using noise-free gates. While this assumption is satisfied for classical machines in many scenarios, it is not expected to be satisfied in the near term future for quantum machines where decoherence leads to faults in the quantum gates. As a result, fundamental questions regarding the practical relevance of quantum channel coding remain open. By combining techniques from fault-tolerant quantum computation with techniques from quantum communication, we initiate the study of these questions. We introduce fault-tolerant versions of quantum capacities quantifying the optimal communication rates achievable with asymptotically vanishing total error when the encoding and decoding circuits are affected by gate errors with small probability. Our main results are threshold theorems for the classical and quantum capacity: For every quantum channel T and every ϵ>0 there exists a threshold p(ϵ,T) for the gate error probability below which rates larger than C−ϵ are fault-tolerantly achievable with vanishing overall communication error, where C denotes the usual capacity. Our results are not only relevant in communication over large distances, but also on-chip, where distant parts of a quantum computer might need to communicate under higher levels of noise than affecting the local gates.

Title: Engineering single and N-photon emission from frequency resolved correlations

Abstract: Correlations in light resolved both in time and frequency provide valuable information about the level structure and dynamics of an emitting system and its capabilities as a quantum light source. First, I will review a generalization of Glauber's N-photon coherence functions to the frequency domain and present our “sensing method” to compute them and gain important insights [1,2]. Second, I will show how the information that these functions provide, lead to both fundamental understanding and technological applications in the epitome system of Quantum Optics, resonance fluorescence, a single two-level system emitter (qubit) driven by a laser:

In the low driving (Heitler) regime, this system was believed to provide an emission both perfectly antibunched and spectrally narrow (with a subnatural linewidth). We show that, when including measurement, these two properties are not compatible and propose a scheme which interferes the emission with an external laser to reconcile them again [3,4,5].

In the high driving (Mollow) regime, we propose schemes to create, by using a cavity to select and enhance emission from the system at precise frequencies, an N-photon emitter [6,7], possibly heralded, or two-mode squeezing.

References:
[1] Theory of frequency-filtered and time-resolved N-photon correlations, E. del Valle et al., Phys. Rev. Lett.109, 183601 (2012). DOI: 10.1103/PhysRevLett.109.183601
[2] Frequency-resolved Monte Carlo, J. C. López Carreño et al., Sci. Rep., 8, 6975 (2018). DOI: 10.1038/s41598-018-24975-y
[3] Joint subnatural-linewidth and single-photon emission from resonance fluorescence, J. C. López Carreño etal., Quantum Sci. Technol. 3, 045001 (2018). DOI: 10.1088/2058-9565/aacfbe
[4] Origin of Antibunching in Resonance Fluorescence, L. Hanschke et al. Phys. Rev. Lett. 125, 170402 (2020).DOI: 10.1103/PhysRevLett.125.170402
[5] Conventional and Unconventional Photon Statistics, E. Zubizarreta Casalengua et al., Laser Photonics Rev.1900279 (2020). DOI: 10.1002/lpor.201900279
Tuning photon statistics with coherent fields, E. Zubizarreta Casalengua et al. Phys. Rev. A 101, 063824(2020). DOI: 10.1103/PhysRevA.101.063824
[6] Emitters of N-photon bundles, C. Sánchez-Muñoz et al., Nature Photonics 8, 550 (2014). DOI: 10.1038/npho-ton.2014.114
[7] Filtering multiphoton emission from state-of-the-art cavity quantum electrodynamics, C. Sánchez Muñoz etal., Optica 5, 14 (2018). DOI: 10.1364/OPTICA.5.00001

Title: Teleportation with quantum microwaves

Abstract: We demonstrate the successful realization of unconditional quantum teleportation in the microwave regime over the distance of 42 cm by exploiting two-mode squeezing and analog feedforward. We generate squeezed and feedforward signals in the GHz regime by using superconducting Josephson parametric amplifiers. We realize quantum teleportation of coherent states with fidelities exceeding the no-cloning limit, thus, proving the unconditional security of the protocol. Furthermore, our experiments reveal the influence of the feedforward gain and entanglement strength on the teleportation fidelity in the presence of finite noise and losses. In the end, we demonstrate that quantum microwave communication is feasible over macroscopic distances in the cryogenic environment. Our results enable future implementations of microwave quantum local area networks and distributed quantum computing with superconducting circuits.

We acknowledge support by the German Research Foundation through the Munich Center for Quantum Science and Technology (MCQST), Elite Network of Bavaria through the program ExQM, EU Flagship project QMiCS (Grant No. 820505), and the German Federal Ministry of Education and Research (BMBF) via the project QUARATE (Grant No. 13N15380)

Title: QuNetSim - A Software Framework for Quantum Networks

Abstract: In this talk, I will introduce the QuNetSim software framework and explain its uses for developing novel quantum networking protocols.

Title: Entanglement assisted communication networks

Abstract: Quantum architectures may indeed play a significant role in futureclassical communication network, but in what precise way seems lessclear. In this talk, we will have a look at in what way genuinelymulti-partite features may come into play in this endeavour, beyondquantum-assisted point-to-point protocols. We ask how multi-partiterouting in quantum networks can be deviced [1]. Along the way, weaddress the old question of the manipulation of multi-partiteentangled states, a question for which we can offer technical progress[2]. Turning to more practical aspects, it has long been suggestedthat multi-partite schemes and entangled resources may offeradvantages in multi-partite cryptographic protocols, but it remains achallenge to pinpoint specific network coding advantages. We willinvestigate a family of secret sharing protocols in which such anadvantage can be proven [3]. If time allows, we will hint at even morepractically minded classical simulations of quantum repeater schemesfor communication networks [4] and aspects of their verification [5].

References
[1] Quantum network routing and local complementation, F. Hahn, A. Pappa, J. Eisert, npj Quant. Inf. 5, 76(2019). DOI: 10.1038/s41534-019-0191-6
[2] Rates of multi-partite entanglement transformations and applications in quantum networks, A. Streltsov, C.Meignant, J. Eisert, Phys. Rev. Lett. 125, 080502 (2020). DOI: 10.1103/PhysRevLett.125.080502
[3] Sharing classical secrets with continuous-variable entanglement: Composable security and network codingadvantage, N. Walk, J. Eisert, in preparation (2021).
[4] A classical simulation platform for quantum repeaters, J. Wallnöfer, N. Walk, F. Hahn, F. Krüger, J. Eisert,in preparation (2021).
[5] Quantum certification and benchmarking, J. Eisert, D. Hangleiter, N. Walk, I. Roth, D. Markham, R.Parekh, U. Chabaud, E. Kashefi, Nature Rev. Phys. 2, 382-390 (2020). DOI: 10.1038/s42254-020-0

Title: A programmable atom-photon quantum interface

Abstract: We are implementing a comprehensive set of single-atom-single-photon quantum interfaces that enable controlled generation, storage, transmission, conversion, and entanglement of photonic and atomic qubits in quantum networks. Such tools are required, for example, in quantum repeater protocols for reliable intermediate storage and long-range transmission of quantum information.

Specifically, we demonstrated a programmable ion-photon interface, employing controlled quantum interaction between a single trapped 40Ca+ ion and single photons [1,2]. Depending on the choice of input and output qubits, the interface protocol serves as an atom-to-photon or photon-to-atom qubit converter, or as a source of entangled atom-photon pair states.

The interface lends itself particularly to integrating Ca+ ions with entangled photon pairs from a resonant, narrowband spontaneous parametric down-conversion (SPDC) source [3,4]. We demonstrate high-fidelity transfer of entanglement from an SPDC photon pair to atom-photon pairs, as well as atom-to-photon quantum bit teleportation [5]. We also extend our quantum network toolbox into the telecom regime by quantum frequency conversion of ion-entangled photons [6]. The reverse conversion process from 1550-nm telecom photons to ion-resonant photons at 854 nm is also realized, with high fidelity after 40 km of fiber transmission [7].

[1] C. Kurz et al., Nat. Commun. 5, 5527 (2014). DOI: 10.1038/ncomms6527
[2] C. Kurz et al., Phys. Rev. A 93, 062348 (2016). DOI: 10.1103/PhysRevA.93.062348
[3] A. Lenhard et al., Phys. Rev. A 92, 063827 (2015). DOI: 10.1103/PhysRevA.92.063827
[4] J. Brito et al., Appl. Phys. B (2016), 122:36. DOI: 10.1007/s00340-015-6276-9
[5] S. Kucera et al., to be published.

Presentation #1

Title: Theory that Matters!

Abstract: This talk will motivate the need for contributions in theory and implementation for a given research field. By the example of past research activities the theory that matters approach is highlighted. As it will be explained in the talk, the approach will lead to meaningful research directions as well as leading to dissemination and exploitation potential.

Presentation #2

Title: Start Ups: The better research vehicle?

Abstract: By the example of three Start Ups it is shown that research can be carried out and monetized successfully. The three examples cover robotics, connectivity, and human-machine interaction.

Presentation #3

Title: Communication Networks: Quo Vadis?

Abstract: This talk will describe the evolution and revolution in communication networks by the example of "softwarization" and 5G technology. It will explain why the aforementioned (r)evolution was following the needs of industry or consumer market. Based on the current developments the talk also looks into the future, describing the rush for 6G technology and why we better should focus on more promising features and trends in communication networks such as quantum communication, molecular communication, or Post-Shannon.

Title: Designing Rate-Efficient Coded Quantum Networks

Abstract: Distributed quantum information stored across the nodes of a coded quantum network can help in quantum network recovery from node failures using encoded graph states. This problem is particularly interesting from the context of distributed quantum storage networks since a node loss leads to a mixed state post quantum decoherence, without any classical analogy. In this talk, I will present how rate-optimal quantum network code designs can be done, resilient to a single node failure using only local operations within the neighborhood of the lost node. These ideas can generalize and scale to arbitrary quantum node failures, useful for the design of next generation quantum networks. Discussions will be held on the theoretical and practical feasibility of the proposed ideas.

Title: Deployment of QKD with Commercial Transport Qquipment on Field Fiber

Abstract: Future-proofing current fibre networks with quantum key distribution (QKD) is an attractive approach to combat the ever growing breaches of data theft. This talk is about enabling long-term security for high-speed data. We give an overview of the QKD used cases. Furthermore, we compare information theoretic security and long-term security. We report on the real world QKD developments, Enabling long-term security for high-speed data. As an example of a link where QKD is used, we report on the UK regional network (the Cambridge quantum network).

Title: Infinite-fold enhancement in communications capacity using pre-shared entanglement

Abstract: Pre-shared entanglement can significantly boost communication rates in the regime of high thermal noise, and a low-brightness transmitter. In this regime, the ratio between the entanglement-assisted capacity and the Holevo capacity, the maximum reliable-communication rate permitted by quantum mechanics without any pre-shared entanglement as a resource, is known to scale as log(1/N_S), where N_S << 1 is the mean transmitted photon number per mode. This is especially promising in enabling a large boost to radio-frequency communications in the weak-transmit-power regime, by exploiting pre-shared optical-frequency entanglement, e.g., distributed by the quantum internet. In this paper, we propose a structured design of a quantum transmitter and receiver that leverages continuous-variable pre-shared entanglement from a downconversion source, which can harness this purported infinite-fold capacity enhancement---a problem that has been open for over a decade. Its implication to the breaking of the well-known square-root law for covert communications, with entanglement assistance, is discussed.

Title: The information capacity of entanglement-assisted quantum Gaussian measurements

Abstract: The talk is devoted to investigation of the entropy reduction and entanglement-assisted classical capacity (maximal information gain) of continuous variable quantum measurements. These quantities are computed explicitly for multimode Gaussian measurement channels of type 1 (generalized heterodyning) and type 2 (generalized homodyning). For this we establish a fundamental property of the entropy reduction of a measurement: under a restriction on the second moments of the input state it is maximized by a Gaussian state (providing an analytical expression for the maximum). In the case of one mode, the gain of entanglement assistance is investigated in detail.

Title: Towards a semiconductor based quantum repeater - Part 1

Abstract: One of the most promising routes toward long-distance quantum communication is based upon quantum repeaters (QR) that include intermediate stations equipped with quantum memories. Here, we discuss the common efforts of the semiconductor consortium within the BMBF financed network Quantum Link Extended (Q.Link.X) towards the QR.

Important pre-conditions for a quantum repeater are high-brightness quantum light sources, long coherent quantum memory times and efficient and high-fidelity quantum optical operations such as entanglement swapping. We will present the state-of-the art obtained in semiconductor technology achieved within the consortium on deterministic fabrication of quantum optical devices and interfaces, including tunable devices. High efficiency single photon sources with high indistinguishability has been obtained. Furthermore, recent progress towards a QR cell and QR segment at 900 nm will be presented as well as compete coherent control of a single quantum dot spin at 1,55 µm wavelengths.

We will also demonstrate swapping of entangled states between pairs of photons emitted by a single dot. Furthermore, we will report on the on-chip integration of a quantum dot (QD) microlens with a 3D-printed micro-objective in combination with a single-mode on-chip fiber coupler. The efforts to provide the relevant quantum photonic hardware emitting directly in the telecom C-Band (around 1.55 µm) will also be presented.

Title: High-dimensional entanglement for quantum communication

Abstract: Entanglement unlocks many applications in quantum communication, such as the highest possible level of security in quantum key distribution. As photons are inevitably lost or decohered over longer distances, it seems obvious that using the full spectrum of photonic degrees of freedom is desirable. In addition to more encodable bits per photon, entanglement in high dimensions also yields a surprising resistance to noise. This comes at the expense of more complicated measurements that in themselves can contribute to the overall noise in the data, leading to an interesting optimisation. In this talk I will discuss the origin of noise resistance and how it can be practically realised in different photonic implementations using entangled photons.

Title: Theory and practice of WiFi network development

Abstract: Systems Research in Wireless IEEE 8021.11 networks challenged years of wireless modelling work. Significant research results based on wireless channel and mac80211 modells did not hold in real WiFi networks. In this talk we present the Dev-Ops challenges in real productive wireless networks and the experimental effort needed to validate and analyse its systems performance. The example of mac802.11 rate control algorithms and its assumptions from theory are presented and compared to more robust resource allocation approaches. The importance of randomisation for robust resource allocation in wireless systems is is highlighted by concrete example, to provide an API for potential applications of randomized quantum protocols. The practical applicability to entangled quantum networks will be discussed.In this talk, we motivate the benefits of integrating systems research as early as possible, to validate theoretical results in testbed and network experiments.

Title: Cavity enhancement for efficient spin-photon interfaces

Abstract: Color centers in diamond provide a promising combination of properties for the realization of large-scale quantum networks. In particular, NV centers stand out due to their long electron spin coherence of up to seconds, their potential to harness individual 13C nuclear spins as even longer-lived multi-qubit memory register, and the availability of Fourier-limited spin-selective cycling transitions which enable single-shot readout. A key remaining challenge is to improve on the efficient extraction of coherent photons to enable spin-photon and spin-spin entanglement at higher rates.

In this talk I will summarize the recent progress on using microcavities to enhance the emission from color centers in diamond and compare approaches used for NV and SiV centers in diamond.

Title: Interfacing solid-state spins and photons for networked quantum technologies

Abstract: Further, solid-state systems promise scalability as they lend themselves to on-chip integration into nanophotonic resonators to boost the efficiency of the aforementioned interfaces [2, 3]. One of the grand challenges toward this goal is to identify suitable systems that show only little to no deterioration of spin-optical properties after integration into nanostructures.
This presentation starts with a brief summary on spin-photon interfaces based on diamond color centers and their limitations toward scalable networks. In the second part, we will introduce systems that promise scalability and integration due to their weak interaction with the local environment. In particular, this presentation focusses on silicon vacancy centers in semiconductor silicon carbide, for which we have recently shown spin-optical stabilities that are on-par with diamond defects [4]. We show a high-fidelity interface in which a single spin controls the properties of two-photon states with more than 90% fidelity [5]. Additionally, we show that the system can be integrated into nanophotonic waveguides with almost no degradation of the spin-optical quantum properties.
Those results represent a milestone toward CMOS compatible integration of optically-active solid-state spins into nanophotonic devices [6, 7]. In combination with recently demonstrated control over nuclear spin qubits [5, 8], this technology holds great promises toward the development of networked quantum technologies.

References
[1] Pompili, Matteo, et al. Realization of a multi-node quantum network of remote solid-state qubits. arXiv: 2102.04471 (2021).
[2] Optica 7, 1232 (2020). DOI: 10.1364/OPTICA.398628
[3] PRX Quantum 1, 020102 (2020). DOI: 10.1103/PRXQuantum.1.020102
[4] Nature Communications 10, 1954 (2019). DOI: 10.1038/s41467-019-09873-9
[5] Nature Communications 11, 2516 (2020). DOI: 10.1038/s41467-020-16330-5
[6] npj Quantum Information 6, 80 (2020). DOI: 10.1038/s41534-020-00310-0
[7] Nature Photonics 14, 330 (2020). DOI: 10.1038/s41566-019-0556-6
[8] Nature Materials 19, 1319 (2020). DOI: 10.1038/s41563-020-00802-6

Title: Privacy and Security in quantum network

Abstract: Multi-Party Quantum Computation, involves distrustful parties wishing to perform a joint computation without leaking information. This functionality has attracted a lot of attention as a potential killer-app for quantum networks through its ability to preserve privacy and integrity of the highly valuable computations that quantum computers would enable us to obtain. In this talk I present the latest progress towards bringing MPQC protocol to the actual real world. We improve on almost all known efficiency metrics and introduce new functionalities that may be of independentinterest for designing future protocols.

Title: Concepts behind high-tech entrepreneurial activities in the German market

Abstract: The first flights of Brothers Wright or Konrad Zuse’s Z3: Any historic entrepreneurial start had - and has - many obstacles to solve - and as often, upcoming obstacles are not predictable. What is predictable though, if we look back in history, is, that science drives economy - but sometimes it’s as well the economists driving science.

Thus, growth is a result of interdisciplinary network: Industry, government, Research - especially nowadays, when ROI forms part of the strategy budget and investors need fast results. History shows that both, an operation network but as well a public interest are needed, in any era, let it be an aristocracy or democracy, that doesn’t matter. Because research means progress. What counts, is connected co-work of economic and scientific power.
Pioneer mentality is the The lowest common denominator of politics, economy and science.

During my keynote I’ll let you dive deeper into the world and history of incorporating potential market perspectives and go-to-market strategies on the road from research to industry, especially focussing Germany, but as well the USA, the UK and more countries.

Title: Hybrid Photonic Integrated Circuits for Classical and Quantum Communications

Abstract: Hybrid photonic integration offers a way of realizing versatile components for a broad array of applications, especially in optical communications. This talk will discuss the current progress on hybrid photonic integrated circuits for quantum communications developed at Fraunhofer HHI as well as similarities and differences with similar components for classical communications. On this basis, possible pathways towards combined classical and quantum photonic integrated circuits for all-optical networks are proposed.

Title: NetSquid, a discrete event simulation platform for quantum networks

Abstract: We present NetSquid, a versatile discrete-event based simulation platform for designing scalable quantum networks and modular quantum computing systems. Its key features include the ability to accurately model the effects of time on the performance of non-ideal quantum systems, its modular approach to physical modeling, and its scalability. We delve into its inner-workings, including notable aspects of its architecture, and illustrate its effectiveness via several use case examples.

Title: Network and Software Architecture for the Quantum Internet

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 imminent 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 layer responsible for end-to-end entanglement connectivity and a software architecture based on a programmable quantum data plane.

Presentation #1

Title: Q.Link.X - Cooperation and Outreach

Abstract: Quantum technology is expected to produce significant impact on several branches of future information technology relevant for the society, including e.g. secure communication links. Simple point to point quantum links have become commercially available already some time ago. Only advanced concepts such as quantum networks, however, promise to make use of the full advantage of quantum technology, while the expertise on these concepts remains to date largely confined to the academic world and is furthermore distributed over numerous laboratories. Therefore, beyond the challenges posed by purely scientific and technological questions there are challenges to be met by the scientific community, the commercial world, and the society -- rendering efficient concepts of cooperation and outreach important. I will give reflections on this issue from the perspective of the academic world.

Presentation #2

Title: Opticlock – quantum technology almost ready for roll-out

Abstract: Late in 2016 a small number of pilot projects was initiated by the BMBF in order to boost interest in quantum technology both in the scientific community and the society. Opticlock was one of the 3 projects selected in an unusual procedure by the scientific community itself. Opticlock had proposed to take a single ion clock from an exuberant scientific laboratory set-up to a system ready for roll-out from the laboratory. The opticlock is now up and running continuously and living up to the expected specifications. I will review the scientific status and try to give some insight how a combination of six larger and smaller firms, two public research institutes, and two universities achieved a promising result.

Title: Towards a semiconductor based quantum repeater - Part 2

Abstract: One of the most promising routes toward long-distance quantum communication is based upon quantum repeaters (QR) that include intermediate stations equipped with quantum memories. Here, we discuss the common efforts of the semiconductor consortium within the BMBF financed network Quantum Link Extended (Q.Link.X) towards the QR. Important pre-conditions for a quantum repeater are high-brightness quantum light sources, long coherent quantum memory times and efficient and high-fidelity quantum optical operations such as entanglement swapping. We will present the state-of-the art obtained in semiconductor technology achieved within the consortium on deterministic fabrication of quantum optical devices and interfaces, including tunable devices. High efficiency single photon sources with high indistinguishability has been obtained. Furthermore, recent progress towards a QR cell and QR segment at 900 nm will be presented as well as compete coherent control of a single quantum dot spin at 1,55 µm wavelengths.

We will also demonstrate swapping of entangled states between pairs of photons emitted by a single dot. Furthermore, we will report on the on-chip integration of a quantum dot (QD) microlens with a 3D-printed micro-objective in combination with a single-mode on-chip fiber coupler. The efforts to provide the relevant quantum photonic hardware emitting directly in the telecom C-Band (around 1.55 µm) will also be presented.

Title: Quantum key distribution network with quantum candies

Abstract: In 1996 Biham Huttner and Mor presented a quantum key distribution (QKD) network relying on centers connected via teleportation lines. The Biham-Huttner-Mor protocol, i.e., the case of a network with just one center, was later on found to be very useful also for designing the first measurement-device-independent QKD (MDI-QKD).
Very recently, in a conference proceedings (TPNC'2020), we presented a surprisingly simple and intuitive model, which we name "Jacobs' quantum candies" to demonstrate the Bennett-Brassard 1984 (BB84) protocol for QKD, a model using only green and red candies, filled with either chocolate or vanilla cream. We also greatly extended Jacobs' quantum candies to enable entanglement-based protocols.
Here I'll show that our quantum candies are sufficient for presenting Biham-Huttner-Mor QKD network hence also MDI-QKD.

Title: Generation and detection of single photons

Abstract: Single photons are key ingredients for many applications in quantum technologies. In this talk, I will discuss the dynamics of generating single photons using semiconductor quantum dots and recent progress in the development of highly-efficient superconducting single photon detectors. Due to their excellent optical properties, such as fast emission rates and nearly transform-limited linewidth, semiconductor quantum dots are promising as single-photon sources. Here, the fundamental limits of the single-photon purity and photon indistinguishability for resonantly driven two-level systems and three-level ladder systems will be discussed, as well as the origin of antibunching under weak cw excitation. Superconducting single photon detectors have established themselves as the most promising system for the detection of single photons in the visible and near-infrared wavelength spectrum. However, understanding their operation in detail is crucial for further improving their performance metrics and integration into quantum photonic integrated circuits.
References
[1] K. A. Fischer, et al., Nature Physics 13, 649-654 (2017). DOI: 10.1038/nphys4052
[2] L. Hanschke et al., npj Quantum Information 4, 43 (2018). DOI: 10.1038/s41534-018-0092-0
[3] E. Schöll et al., Physical Review Letters 125, 233605 (2020). DOI: 10.1103/PhysRevLett.125.233605
[4] L. Hanschke et al., Physical Review Letters 125, 170402 (2020). DOI: 10.1103/PhysRevLett.125.1

Title: Information Theoretic Perspective on Quantum Repeaters

Abstract: We consider communication over a quantum broadcast channel with cooperation between the receivers. Through this setting, we provide an information-theoretic perspective on quantum repeaters. First, we observe that entanglement resources alone do not increase the achievable communication rates. By comparison with the recent results by Leditzki et al. Nat Commun 11, 1497 (2020), this observation reveals a violation of the BC-MAC duality between the broadcast channel with two receivers and the multiple-access channel with two transmitters. The next form of cooperation addressed is classical conferencing, where Receiver 1 can send classical messages to Receiver 2. We provide a regularized characterization of the classical capacity region and establish a single-letter formula for the special class of Hadamard broadcast channels. Given both classical conferencing and entanglement resources, Receiver 1 can teleport a quantum state to Receiver 2. This setting is intimately related to quantum repeaters, as the sender, Receiver 1, and Receiver 2 can be viewed as the transmitter, the repeater, and the destination receiver, respectively. When Receiver 1's sole purpose is to help the transmission to Receiver 2, the model reduces to the quantum primitive relay channel. We derive lower and upper bounds for each setting; and conclude with observations on the tradeoff between repeater-aided and repeaterless communication, and the bottleneck flow behavior of quantum repeaters.

Presentation #1

Title:

Erbium-doped crystals: A novel platform for QIP

Abstract: In spite of decade-long research into different physical systems, the demonstration of a scalable platform for quantum information processing remains an outstanding challenge. In this context, we investigate the use of erbium-doped silicon [1] and silicate [2,3] crystals. This novel experimental platform offers unique potential to overcome the main bottlenecks of other quantum hardware: First, erbium dopants can exhibit second-long coherence in a temperature range that is accessible with 4He cryocoolers [4]. Second, the optical transition of erbium is the narrowest spectral feature ever measured in a solid. Thus, frequency multiplexed addressing of individual dopants [5] gives access to an unprecedented qubit density as long as spin-spin interactions can be suppressed by dynamical decoupling [3]. Finally, erbium is the only emitter for which lifetime-limited optical coherence in the telecommunications frequency window has been demonstrated [2]. Here, loss in optical fibers is minimal, which is a prerequisite for global quantum networks. In addition, operating below the bandgap energy of silicon allows for complex nanophotonic circuits with low loss that can be fabricated by standard processes of the semiconductor industry. Our approach may thus enable the realization of scalable distributed quantum information processing based on well-established technology.
References
[1] L. Weiss, A. Gritsch, B. Merkel, and A. Reiserer, Optica 8, 40 (2021). DOI: 10.1364/OPTICA.413330
[2] B. Merkel, A. Ulanowski, and A. Reiserer, Phys. Rev. X 10, 041025 (2020). DOI: 10.1103/Phys-RevX.10.041025
[3] B. Merkel, P. C. Faria, N. H. Valencia, and A. Reiserer, arXiv: 2005.08822 (2020).
[4] M. Rani, M. P. Hedges, R. L. Ahlefeldt, and M. J. Sellars, Nat. Phys. 14, 50 (2018). DOI: 10.1038/nphys4254
[5] S. Chen, M. Raha, C. M. Phenicie, S. Ourari, and J. D. Thompson, Science 370, 592 (2020). DOI: 10.1126/sci-ence.abc7821

Presentation #2

Title:

Rare-Earth Dopants

Abstract: Distributed quantum networks [1] will allow users to perform tasks and to interact in ways which are not possible with present-day technology. Their implementation is a key challenge for quantum science and requires the development of stationary quantum nodes that can send and receive as well as store and process quantum information locally. The nodes are connected by quantum channels for flying information carriers, i.e., photons. These channels serve both to directly exchange quantum information between nodes and to distribute entanglement over the whole network.
In order to scale such networks to many particles and long distances, an efficient interface between the nodes and the channels is required. While early work has focused on atoms trapped in vacuum [2], solid state systems may offer advantages with respect to scalability. This talk will therefore introduce rare-earth doped crystals as an emerging hardware platform for quantum network nodes [3]. Such crystals can serve as efficient quantum memories [4] with exceptional coherence times, with the current record of 6 h achieved in Eu:YSO [5]. Furthermore, ensemble memories can maintain a high bandwidth and multiplexing capacity, and store entanglement between photons [6]. Recent approaches have demonstrated the spectroscopy and multiplexed control of individual dopants by integrating them into optical resonators [7–9]. This may allow for the implementation of local deterministic quantum gate operations and thus pave the way for a realization of the seminal quantum repeater protocol [10] in order to scale quantum networks to global distances.
References:
[1] S. Wehner, D. Elkouss, and R. Hanson, Science 362, Issue 6412, eaam9288 (2018). DOI: 10.1126/sci-ence.aam9288
[2] A. Reiserer and G. Rempe, Rev. Mod. Phys. 87, 1379 (2015). DOI: 10.1103/RevModPhys.87.1379
[3] C. W. Thiel, T. B ̈ottger, and R. L. Cone, J. Lumin. 131, 353 (2011). DOI: 10.1016/j.jlumin.2010.12.015
[4] M. Afzelius, N. Gisin, and H. de Riedmatten, Phys. Today 68, 42 (2015). DOI: 10.1063/PT.3.3021
[5] M. Zhong, M. P. Hedges, R. L. Ahlefeldt, J. G. Bartholomew, S. E. Beavan, S. M. Wittig, J. Longdell, andM. J. Sellars, Nature 517, 177 (2015). DOI: 10.1038/nature14025
[6] C. Clausen, I. Usmani, F. Bussires, N. Sangouard, M. Afzelius, H. de Riedmatten, and N. Gisin, Nature 469,508 (2011). DOI: 10.1038/nature09662
[7] J. M. Kindem, A. Ruskuc, J. G. Bartholomew, J. Rochman, Y. Q. Huan, and A. Faraon, Nature 580, 201(2020). DOI: 10.1038/s41586-020-2160-9
[8] S. Chen, M. Raha, C. M. Phenicie, S. Ourari, and J. D. Thompson, Science 370, 592 (2020). DOI: 10.1126/sci-ence.abc7821
[9] B. Merkel, A. Ulanowski, and A. Reiserer, Phys. Rev. X 10, 041025 (2020). DOI: 10.1103/Phys-RevX.10.041025
[10] H.-J. Briegel, W. D ̈ur, J. I. Cirac, and P. Zoller, Phys. Rev. Lett. 81, 5932 (1998). DOI: 10.1103/Phys-RevLett.81.5932

Title: Optical Fiber-Cavity Quantum-Network Nodes

Abstract: Quantum-optical systems consisting of modular and individually programmable information-processing modules [ Daiss et al., Science 371, 614 (2021)] will be essential tools of an advanced quantum backbone for long-distance quantum communication and distributed quantum computation. Incorporating optical fiber technology has additional advantages such as miniaturization that increases photon confinement and, moreover, allows for novel optical architectures. Against this backdrop, the talk reports on recent achievements including a passive heralded quantum memory for photonic qubits, an ideal quantum-network end node [ Brekenfeld et al., Nature Physics 16, 647 (2020)], and a nondestructive photonic qubit detector, a promising quantum-network middle node that has the potential to speed up a plethora of quantum communication protocols, e.g., entanglement distribution and teleportation as required for a quantum repeater [Niemietz et al., Nature (in press)].

Title: Achieving high-data-rate communication on optical quantum channels: a theoretical implementation-oriented perspective

Abstract: Quantum information science is shifting from theory to practice. In communication, key theoretical results of the past have shown that long-distance communication on optical quantum channels, such as optical fiber and free space, can attain the channel capacity by using simple coherent-state alphabets. Unfortunately, the receiver end is where quantum mechanics plays a crucial role, requiring the implementation of joint quantum measurements on multiple signals, which proves challenging with current technology.

Title: Quantum repeaters based on concatenated bosonic and discrete-variable quantum codes

Abstract: We propose a novel architecture of quantum-error-correction-based quantum repeaters that combines the techniques used in discrete and continuous-variable quantum information. Specifically, we propose to encode the transmitted qubits in a concatenated code consisting of two levels. On the first level we use a continuous-variable GKP code which encodes the qubit in a single bosonic mode. On the second level we use a small discrete-variable code, encoding a logical qubit in as few as four or seven physical qubits. Such an architecture introduces two major novelties which allow us to make efficient use of resources. Firstly, our architecture makes use of two types of quantum repeaters: the simpler GKP repeaters that need to only be able to store and correct errors on a single GKP qubit and more powerful but more costly multi-qubit repeaters that additionally can correct errors on the higher level. We find that the combination of using the two types of repeaters enables us to achieve performance needed in practical scenarios with a significantly reduced cost with respect to an architecture based solely on multi-qubit repeaters. Secondly, the use of continuous-variable GKP code on the lower level has the advantage of providing us with the information about the success probability of the specific GKP correction round. This analog information, unique to bosonic codes, provides significant boost in performance when used to correct second level errors in the multi-qubit repeaters.

Background: https://arxiv.org/pdf/2011.15076.pdf

Title: When are Adaptive Strategies in Asymptotic Quantum Channel Discrimination Useful?

Abstract: We present a broad investigation of asymptotic binary hypothesis testing, when each hypothesis represents asymptotically many independent instances of a quantum channel, and the tests are based on using the unknown channel and observing its output. Unlike the familiar setting of quantum states as hypotheses, there is a fundamental distinction between adaptive and non-adaptive strategies with respect to the channel uses, and we introduce a number of further variants of the discrimination tasks by imposing different restrictions on the test strategies. The following results are obtained: (1) The first separation between adaptive and non-adaptive symmetric hypothesis testing exponents for quantum channels, which we derive from a general lower bound on the error probability for non-adaptive strategies; the concrete example we analyze is a pair of entanglement-breaking channels. (2) We prove that for classical-quantum channels, adaptive and non-adaptive strategies lead to the same error exponents both in the symmetric (Chernoff) and asymmetric (Hoeffding, Stein) settings. (3) We prove, in some sense generalizing the previous statement, that for general channels adaptive strategies restricted to classical feed-forward and product state channel inputs are not superior in the asymptotic limit to non-adaptive product state strategies. (4) As an application of our findings, we address the discrimination power of quantum channels and show that adaptive strategies with classical feedback and no quantum memory at the input do not increase the discrimination power of entanglement-breaking channel beyond non-adaptive tensor product input strategies.

arXiv: 2011.06569

Title: Interconnecting nodes of an trapped-ion quantum processor

Abstract: Quantum processor architectures are limited in scalability. While we see clear plans that will lead us to fully controlled 50, up to 100 ion qubits, it may be hard to scale up further. However, scalability above this limits may be reached when processor nodes of that type are interconnected, such that quantum entanglement is built up and qubits are exchanged between nodes. I discuss different options and sketch experimental studies for networks between trapped-ion processor nodes.

Title: Towards high-dimensional quantum communication in space

Abstract: Entanglement is a key resource in quantum information processing and its distribution between distant parties is a key challenge in quantum communications. In this presentation, I will review ongoing technology developments for efficient generation and manipulation of photonic entanglement in various degrees of freedom, as well as ongoing engineering challenges in the development and integration of field-ready quantum communication systems in fiber networks, long-distance free-space and, ultimately, satellite links.

Title: Scaling and delivering diamond quantum solutions

Abstract: A brief review of diamond technology for quantum applications summarizing some of the current research areas. Presentation will also briefly discuss the needs to develop an ecosystem of solutions and how road mapping can be a useful tool to get alignment to deliver on scale.

Title: Entanglement-assisted communication beyond first-order asymptotics

Abstract: Arguably the most fundamental communication task involving entanglement is entanglement-assisted classical communication. It is thought to be well understood, at least since Bennet et al.’s work establishing its single-letter capacity formula. Nonetheless, in this talk I will present some progress and an open problem concerning the second-order asymptotics of this problem, showing that it still retains some secrets.

Title: Quantum repeaters: protocols, basic elements, and rate analysis [cancelled]

Abstract: Direct quantum communication through optical fibers is hard to realize on a large, global scale, because either the quantum states' arrival probability or their fidelity decay exponentially with distance. We give an overview of protocols and basic elements to circumvent this complication with the focus on memory-based quantum repeaters
and their quantitative assessment.

Title: QuISP: the Quantum Internet Simulation Package

Abstract: Simulating a Quantum Internet is a challenging task: of course, thenumber of network nodes can reach the thousands; non-Pauli errorssuch as loss and relaxation are important physical processes to model;advancing generations of quantum repeaters may create and useentangled states of hundreds of qubits; and distributed controlprotocols and software must be modeled accurately. Our QuantumInternet Simulation Package aims to scale to simulate a completeQuantum Internet on all these fronts. I will review the progress andcurrent state of our open source software and discuss how others canbe involved.

Title: Tools for interdisciplinary training on the topic of quantum physics

Abstract: In this talk, we will give an introduction to the Entanglement Demonstrator (quED) and available add-ons for the instrument. The quED is designed for educational purposes, fits on any lab desk, and can be set up in minutes. The easy-to-use system helps to explain the complex phenomena of quantum mechanics through the demonstration of several different quantum experiments. During this webinar, you will also get a brief overview of the quNV (quantum sensing) and the Quantenkoffer (their versatile quantum science kit) which complete qutools' educational portfolio.
Qutools GmbH provides tools for quantum research and educational outreach. Founded in 2005, qutools' aim is to enable the better understanding of quantum physics on one hand and to advance technology through this understanding on the other hand. That is why qutools focuses on innovation while addressing the needs in the lab, making it possible to concentrate on the didactics, not on the measurement tools.

Title: Elementary quantum repeater cell with atoms in optical resonators

Abstract: Entanglement between stationary quantum memories and photonic channels is the essential resource for future quantum networks. Together with entanglement distillation it will enable for efficient distribution of quantum states. Here we report on the generation and observation of entanglement between a Rb-87 atom and a photon at telecom wavelength over 20 km optical fiber.
To overcome the strong absorption of spontaneously emitted photons from Rb we use polarization-preserving quantum frequency conversion transforming the wavelength of the photon entangled with the atomic spin state from 780 nm to the telecom S-band at 1522 nm. We give an update on the prospects for the next step, i.e., to the observation of entanglement between two distant atomic quantum memories.

Title: Zero-error communication via channels - entanglement assistance and beyond

Abstract: We put Shannon's theory of zero-error communication via (classical) channels into a broader perspective by allowing the sending and receiving parties to share additional resources. Indeed, while Shannon already considered the case of instantaneous feedback, work by various authors over the past years has shown that entanglement or more abstractly nonlocal correlations can enhance both one-shot and asymptotic zero-error capacities. The talk will survey the known results on this quantum and nonlocal advantage, focusing especially on upper bounds. Among them the most important is the Lovász number and its refinement by Schrijver, which continue to be upper bounds on the entanglement-assisted zero-error capacity. Haemers' rank bound on the other hand is broken by entanglement-assisted zero-error codes.

Title: QKD@DT - We are building the security of the future.

Abstract: Quantum computers enable attacks on established key exchange methods, which form the basis for all digital communication. Quantum key distribution is based on physical laws of nature and, if implemented correctly, also provides security against currently unknown quantum algorithms. We show how Deutsche Telekom is dealing with the threat scenario, what developments we expect in the coming years and provide an insight into the architecture of the planned QKD platform.

Title: Advanced diamond quantum materials for photonic integration

Abstract: Color centers in nanofabricated diamond devices have demonstrated highly-efficient spin-photon interfaces, which have a direct impact on long-distance quantum communication and distributed quantum computing [1].
Key towards system scalability is reproducible engineering of advanced diamond quantum materials for photonic integration.
In my talk I will summarize the challenges for cavity quantum electrodynamics with optically active spins in diamond, with a particular focus on material and fabrication.
[1 ] M.K. Bhaskar et al. Nature 580, 60 (2020). DOI: 10.1038/s41586-020-2103-5

Title: Entanglement-Assisted Communication: Experiment to Surpass the Ultimate Classical Capacity

Abstract: The seminal work by Bennett, Shor, Smolin, and Thapliyal showed that pre-shared entanglement between communication parties can be harnessed to increase the rate of reliable classical communication over noisy and lossy channels, known as entanglement-assisted communication (EACOMM). Despite the advances of quantum technology in the last a few decades, EACOMM surpassing the ultimate classical channel capacity has never been experimentally demonstrated. We report the construction of an efficient entangled-photon source and a nontraditional quantum phase-conjugate receiver to realize EACOMM over a lossy and noisy bosonic channel. We show that EACOMM beats the classical capacity of the channel, quantified by the Holevo-Schumacher-Westmoreland formula, by up to 14.6% even though the initial entanglement is completely destroyed by loss and noise. As a practical performance benchmark, a classical communication protocol without entanglement assistance is implemented over the same bosonic channel, showing that EACOMM reduces the bit-error rate by up to 69%. Our work opens a route to provable quantum advantages in a wide range of quantum information processing tasks.

Title: Entanglement-assisted communication: theory and protocol design

Abstract: Entanglement offers substantial advantages in quantum information processing, but loss and noise hinder its application in practical scenarios. Although it has been well known for decades that the classical communication capacity of lossy and noisy bosonic channels can be significantly enhanced by entanglement, no practical encoding and decoding schemes are available to realize any entanglement-enabled advantage.
In this talk, we will present the theory and protocol design of entanglement-assisted classical communication. Our first result is a capacity theorem of entanglement-assisted classical communication for multiple-access channels (MAC), where we show that the capacity region enjoys a similar advantage to the known single-sender case. Then we provide structured encoding and decoding schemes for to achieve entanglement advantage in both the single-sender case and the MAC case. The protocol design has recently led to the first experimental demonstration of entanglement-assisted communication surpassing the ultimate classical capacity. We also discuss how the absence of a shared phase reference will affect the entanglement advantage.

References:
Phys. Rev. Applied 13, 034029 (2020). DOI: 10.1103/PhysRevApplied.13.034029
arXiv: 2010.11974 (2020), to appear on Phys. Rev. Lett.
arXiv: 2101.07482 (2021)
arXiv: 2101.12173 (2021)