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.

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.

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.

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."

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.

Late in 2016 a small number of pilot projects were 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.

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.

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 does not 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 will 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 focusing Germany, but as well the USA, the UK and more countries.

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).

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.

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.

The BMBF has opened the following call:
https://www.bmbf.de/foerderungen/bekanntmachung-3346.html

In this panel discussion we will look for ideas and collaborations to create proposals to submit to this call.

In order to bring quantum networks into the real world, we would like to determine the requirements of quantum network protocols including the underlying quantum hardware. Since sufficiently detailed architecture proposals are generally too complex for mathematical analysis, it is natural to employ numerical simulation. Here, we introduce NetSquid, a generic discrete-event based platform for simulating all aspects of quantum networks and modular quantum computing systems, ranging from the physical layer hardware and the control plane all the way to the application level. We study several use cases to showcase the power of NetSquid. First, we perform a detailed physical layer simulation of repeater chains using processing nodes based on NV centres in diamond, as well as repeaters based on atomic ensembles. Second, we study the control plane of a recently introduced quantum switch beyond the analytically known regime. Use of the novel discrete-event paradigm for simulating networks allows the study of time-dependent noise in an entirely modular simulation, predicting the combined performance of physical hardware and control. Finally, we showcase Netsquid's ability to investigate large networks by simulating entanglement distribution over a chain of up to one thousand nodes using simplified physical models in less than three seconds.

Simulating a Quantum Internet is a challenging task: of course, the number of network nodes can reach the thousands; non-Pauli errors such as loss and relaxation are important physical processes to model; advancing generations of quantum repeaters may create and use entangled states of hundreds of qubits; and distributed control protocols and software must be modeled accurately. Our Quantum Internet Simulation Package aims to scale to simulate a complete Quantum Internet on all these fronts. I will review the progress and current state of our open source software and discuss how others can be involved.

As quantum internet technologies develop, the need for simulation software and education for quantum internet rises. QuNetSim aims to fill this need. QuNetSim is a Python software framework that can be used to simulate quantum networks up to the network layer. The goal of QuNetSim is to make it easier to investigate and test quantum networking protocols over various quantum network configurations and parameters. The framework incorporates many known quantum network protocols so that users can quickly build simulations and beginners can easily learn to implement their own quantum networking protocols.

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

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

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.

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.

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.

References
[1] M.K. Bhaskar et al. Nature 580, 60 (2020). DOI: 10.1038/s41586-020-2103-5

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

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).

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.

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.

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.

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.

What do you see as the potential use of quantum effects in the field of communication networks in 5, 10, and 20 years, and what are the current biggest challenges for this use?

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.

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.

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.

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 independent interest for designing future protocols.

Quantum architectures may indeed play a significant role in future classical communication network, but in what precise way seems less clear. In this talk, we will have a look at in what way genuinely multi-partite features may come into play in this endeavour, beyond quantum-assisted point-to-point protocols. We ask how multi-partite routing in quantum networks can be deviced [1]. Along the way, we address the old question of the manipulation of multi-partite entangled states, a question for which we can offer technical progress [2]. Turning to more practical aspects, it has long been suggested that multi-partite schemes and entangled resources may offer advantages in multi-partite cryptographic protocols, but it remains a challenge to pinpoint specific network coding advantages. We will investigate a family of secret sharing protocols in which such an advantage can be proven [3]. If time allows, we will hint at even more practically minded classical simulations of quantum repeater schemes for 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

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.

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.

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.

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.

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.

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

We consider a generalization of the quantum state redistribution task, where pure multipartite states from an ensemble source are distributed among an encoder, a decoder and a reference system. The encoder, Alice, has access to two quantum systems: system $A$ which she compresses and sends to the decoder, Bob, and the side information system $C$ which she wants to keep at her site. Bob has access to quantum side information in a system $B$, and wants to decode the compressed information in such a way to preserve the correlations with the reference system on average.

As figures of merit, we consider both block error (which is the usual one in source coding) and per-copy error (which is more akin to rate-distortion theory), and find the optimal compression rate for the second criterion, and achievable and converse bounds for the first. The latter almost match in general, up to an asymptotic error and an unbounded auxiliary system; for so-called irreducible sources they are provably the same.

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.

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)

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 $N_S \ll 1$ 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 {\em square-root law} for covert communications, with entanglement assistance, is discussed.

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.

TBA