New Directions in Quantum Information

1 Apr 2019, 09:00 → 26 Apr 2019, 18:00 Europe/Stockholm

122:026 (Nordita, Stockholm)

Erik Aurell, Janet Anders, Mikael Skoglund, Mohamed Bourennane, Pawel Horodecki

Venue

Nordita, Stockholm, Sweden

Scope

Quantum Information Science is a major frontier of modern science and technology, exploring physical situations that are classically impossible. An important technological application, already available today, is secure quantum key distribution realized by spatially separated entangled quantum states. This program will be centered around fundamental physical questions arising from the current development of new quantum technologies. The focus will be on effects and phenomena that appear already in quantum systems with a modest number of degrees of freedom, and which are already (or may soon be) experimentally realized.

[Timetable - available from start of the program]

Format

The program will be organized around four focus topics: Quantum Channel Capacity (C), Quantum Thermodynamics (T), Synchronization (S), and Future (F). Participants are encouraged to self-organize so that the focus will be mainly on one of the topics in each week.

One of the most striking differences between quantum and classical information is that two quantum channels can transmit more information together than the sum of what they can do separately, provided that the inputs are entangled. It has even been shown experimentally (Nawareg et al "Superadditivity of two quantum information resources" Science advances 3 e1602485 (2017)) that two zero-capacity channels can be super-activated to form a quantum channel with positive capacity. Focus topic C will provide a format to explore these new scenarios and applications. It will preliminarily take place in the working week April 1-5, 2019 .

Quantum thermodynamics stands for efforts to extend standard thermodynamics and non-equilibrium statistical physics to ensembles of sizes well below the thermodynamic limit, in non-equilibrium situations, and with the full inclusion of quantum effects. Recently there has been a convergence of tools from non-equilibrium thermodynamics and open quantum systems such that it is possible to consider (experimentally and theoretically) heat and work in single quantum systems (Vinjanampathy and Anders "Quantum thermodynamics" Contemporary Physics 57, 545-579 (2016)). A better understanding of quantum work and quantum heat can be expected to impact on fundamental limits to quantum information processing. Focus topic T will preliminarily take place in the working week April 8-12, 2019 .

In multiprocess networks individual processes need to have clocks that must be synchronized. In the classical domain the fraction of faulty process that can be tolerated is limited ("Byzantine Generals" problem), but in the quantum domain this is not so. Focus topic S will be centered on such issues and extensions to active replication, medium access control, position verification and causality verification and other possible new applications of quantum resources to fault-tolerant networks and distributed systems. It will preliminarily take place in the working week April 15-18, 2019 . Good Friday (April 19) is a public holiday in Sweden.

The overall focus of the program is on new areas where using quantum features can surpass classical bounds already for a finite and small number of qubits (or other states), and new applied and fundamental physical problems that arise in this context. Focus topic F will provide a a format to consider forward-looking and open-ended issues. It will preliminarily take place in the working week April 23-26, 2019 . Easter Monday (April 22) is a public holiday in Sweden.

Throughout the program there will be one or at most two talks per day with plenty of time for discussions in a relaxed atmosphere.

Participants include

Antonio Acín, ICFO (Barcelona, Spain) Tapio Ala-Nissilä, Aalto University (Espoo, Finland) Konrad Banaszek, University of Warsaw (Poland) Ingemar Bengtsson, Stockholm University (Sweden) Gunnar Björk, KTH (Sweden) Časlav Brukner, University of Vienna (Austria) Dagmar Bruß, Universität Düsseldorf (Germany) Adán Cabello, Universidad de Sevilla (Spain) Ralf Eichhorn, Nordita (Sweden) Juan Garrahan, University of Nottingham (UK) Markus Hennrich, Stockholm University (Sweden)

Michał Horodecki, University of Gdansk (Poland) Alberto Imparato, Aarhus University (Denmark) Vladimir Krasnov, Stockholm University (Sweden) Ryoichi Kawai, University of Alabama (US) Jan-Åke Larsson, Linköping University (Sweden) Göran Lindblad, KTH (Sweden) Satya Majumdar, Université de Paris-Sud (France) Kirone Mallick, CE-Saclay (France) Jean-Marc Merolla, FEMTO-ST (Besancon, France) Klaus Mølmer, Aarhus University (Denmark) Juan Parrondo, Complutense University (Spain)

Jukka Pekola, Aalto University (Espoo, Finland) Anna Sanpera, UAB (Catalonia) Aditi Sen De, Harish-Chandra Research Institute (India) Ujjwal Sen, Harish-Chandra Research Institute (India) Sai Vinjanampathy, IIT Bombay (India) Mark M. Wilde, Louisiana State University (US) Dong Yang, University of Bergen (Norway) Marek Żukowski, University of Gdansk (Poland) Karol Życzkowski, Jagiellonian University (Poland)

Application

Registration to this event is now closed. If you believe to have a strong argument to participate anyway, contact the Organization Committee.

Travel Reimbursement

We generally expect all participants to cover their own travel expenses. Exceptions will be decided on a case-by-case basis by the Organizating Committee.

PhD students and young Postdoc fellows are eligible for travel grants to participate in the program. If you are interested in such a grant, please mark the corresponding field in the application form, briefly summarize your interest in the program in the comments field, and indicate an estimation of your expected travel expenses. Since only a limited number of grants is available, decision concerning the grants will be made on a case-by-case basis. Notification will be made shortly after the application deadline.

Accommodation

Nordita provides a limited number of rooms in the Stockholm apartment hotel BizApartments. For more imformation, see application form, or contact the organizers.

Please be aware that unfortunately, scammers sometimes approach participants claiming to be able to provide accommodation and asking for credit card details. Please do not give this information to them. For successful applicants, Nordita will be in touch via email regarding accommodation. If you are in any doubt about the legitimacy of an approach, please get in contact with the organisers.

Sponsored by:

This event is made possible by generous support from Nordita, from Quantum Technology Finland and from International Centre for Theory of Quantum Technologies.

Monday 1 April

1 Breakfast

2 Resource theory of POVM-based coherence

Dagmar Bruss

Resource theory of POVM-based coherence

Quantum coherence is a fundamental feature of quantum mechanics and an underlying requirement for most quantum information tasks. In the resource theory of coherence, incoherent states are diagonal with respect to a fixed orthonormal basis, i.e., they can be seen as arising from a von Neumann measurement. Here, we introduce and study a generalization to a resource theory of coherence defined with respect to the most general quantum measurement, i.e., to an arbitrary positive-operator-valued measure (POVM). We establish POVM-based coherence measures and POVM-incoherent operations which coincide for the case of von Neumann measurements with their counterparts in standard coherence theory. A semidefinite program allows to characterize interconversion properties of resource states. We exemplify our framework by means of the qubit trine POVM.

Speaker: Dagmar Bruss

Tuesday 2 April

3 Detection of quantum channel capacities with few local measurements

Chiara Machiavello

Detection of quantum channel capacities with few local measurements

We propose a method to detect lower bounds to quantum capacities of a noisy quantum communication channel by means of few measurements and by avoiding full process tomography. We show how the procedure can be easily applied to any finite dimensional system and unknown noisy channels. We test its efficiency by studying its performance for most well known single qubit noisy channels, for the generalised Pauli channel in arbitrary finite dimension and for correlated Pauli and amplitude damping channels acting on two qubits.

Speaker: Chiara Machiavello

4 One-shot Quantum Dense Coding

Aditi Sen De

One-shot Quantum Dense Coding

I will discuss the scenario of one-shot classical information transmission between multiple senders and a single receiver deterministically, when they a priori share a multipartite quantum state – an attempt towards building a deterministic dense coding network. I will then introduce the concept of conclusive dense coding in which the sender uses a single copy of pre-shared entangled state with the receiver to send complete classical messages unambiguously.

Speaker: Aditi Sen De

Wednesday 3 April

5 Quantum uncertainty relations: some further landscapes

Ujjwal Sen

Quantum uncertainty relations: some further landscapes

We wish to discuss about two related topics.
Based on the statistical concept of the median, we will propose a quantum uncertainty relation between spreads of the position and momentum distributions of arbitrary quantum states. The relation will be universal, unlike that based on the mean and standard deviation, as the latter may become non-existent or ineffective in certain cases. We will show that the median-based one is not saturated for Gaussian distributions in position.
In the second part, we will propose a quantum uncertainty relation for arbitrary quantum states in terms of Lipschitz constants of the corresponding position and momentum probability distributions. The Lipschitz constant of a function may be considered to quantify the extent of fluctuations of that function, and is in general independent of its spread. We find that the product of the Lipschitz constants is greater than the inverse square of the Planck constant.

Speaker: Ujjwal Sen

6 High dimensional frequency bin entanglement applications

Jean-Marc Merolla

High dimensional frequency bin entanglement applications

The use of high-dimensional entangled states is a key enabler for high- capacity quantum communications and key distribution [Dad11,Bar08], quantum computation [Bar08], information processing [Kre14]. For the success of these quantum photonic applications [Ali16], high visibility quantum interference and high integration is essential. Among the different degrees of freedom of photons, time-energy entangled photon pairs at telecommunication wavelengths allows the implementation of high visibility experiments and are especially well suited for integration with the current fiber optic infrastructure [Rog2016]. Many experiments exploits the concept of time bins, in which the photons are detected at discrete times [Dyn09], because the time-bin entangled states are robust with respect to the decoherence over large distances. Since 2010 [Oli10,Oli12,Oli14] our group has introduced the concept of frequency bin entanglement and, for the first time, we have demonstrated Bell inequality violations using high frequency electro-optic phase modulation. Several groups [Luk17, Cap10, Ima2017, Kue17] have proposed different potential applications using the same phase modulation techniques. These new works encourage to further work on frequency bin entanglement, which is a promising candidate to consolidate information processing solutions based on quantum photonics technology. Indeed the frequency domain is attractive, because (i) the frequency domain is naturally of high dimensionality, and (ii) building blocks such as frequency entangled source, modulators and filters involved in the manipulation method can be integrated on chip. This is why the frequency degree of freedom in quantum photonic undoubtedly offers a promising platform for scalable and robust quantum information processing. In this contribution, we will report on the different frequency bin entanglement architectures we have implemented using standard Telecom optoelectronic devices such as electro- optic phase modulators and filters. Proof of concept experiments will be presented demonstrating high reliability and high potentiality of the method.

[Ali16] O. Alibart et al., Journal of Optics 18, 104001 (2016). [Bar08] J T Barreiro, T C Wei, & P G Kwiat, P. G, Nature Physics, 4, 282-286 (2008).
[Cap10] J. Capmany and C. R. Fernandez-Pousa, J. Opt. Soc. Am., B 27, A119 (2010).
[Dyn09] F. Dynes,1, H. Takesue,Z. L. Yuan, A. W. Sharpe,1 K. Harada, T. Honjo, H. Kamada, O. Tadanaga, Y. Nishida, M. Asobe,& A. J. Shields, Opt. Express Vol. 17, 11440-11449 (2009)
[Dad11] A C Dada, J Leach, G S Buller, M J Padgett, & E Andersson, Nature Physics 7, 677-680 (2011).
[Ima2017] P.Imany, O. D. Odele, J. A. Jaramillo-Villegas, Daniel E. Leaird, and Andrew M. Weiner, arXiv:1709.05274v2 , 18 Sep 2017.
[Kre14] M Krenn, et al. Proceedings of National Academy of Sciences 111, 6243-6247 (2014).
[Kue17] M. Kues, C. Reimer, P. Roztocki1, L. Romero Cortés, S. Sciara, B. Wetzel, Y. Zhang, A. Cino, S. T. Chu, B. E. Little, D. J. Moss, L. Caspani, J. Azaña1 and R. Morandotti, Nature, 546, pp 622, 2017.
[Luk17] J. M. Lukens and P.Lougovski ,Optica, vol.4, No. 1 (2017).
[Oli10] L. Olislager, J. Cussey, A.T. Nguyen, Ph. Emplit, S. Massar, J.- M. Merolla, and K. Phan Huy, Phys. Rev. A, 82,1, 013804, 2010.
[Oli12] L. Olislager, I. Mbodji, E. Woodhead, J. Cussey, L. Furfaro, P. Emplit, S. Massar, K. P. Huy and J.- M. Merolla, New J. of Phys. 14, 043015, (2012).
[Oli14] L. Olislager, I. Mbodji, E. Woodhead, J. Cussey, L. Furfaro, P. Emplit, S. Massar, K. Phan Huy, J.-M. Merolla, Phys.Rev. A, 89,5, 052323, 2014. [Rog2016] S. Rogers, D. Mulkey, X. Lu, W. C. Jiang and Qiang Lin, ACS Photonics, 3 (10), pp. 1754-1761, 2016

Speaker: Jean-Marc Merolla

7 Reception

Thursday 4 April

8 Information-theoretic aspects of the generalized amplitude damping channel

Mark Wilde

Information-theoretic aspects of the generalized amplitude damping channel

The generalized amplitude damping channel (GADC) is one of the sources of noise in superconducting-circuit-based quantum computing. It can be viewed as the qubit analogue of the bosonic thermal channel, and it thus can be used to model lossy processes in the presence of background noise for low-temperature systems. In this work, we provide an information-theoretic study of the GADC. We first determine the parameter range for which the GADC is entanglement breaking and the range for which it is anti-degradable. We then establish several upper bounds on its classical, quantum, and private capacities. These bounds are based on data-processing inequalities and the uniform continuity of information-theoretic quantities, as well as other techniques. We also establish upper bounds on the two-way assisted quantum and private capacities of the GADC ("two-way" meaning that public classical communication is available for free). These bounds are based on the squashed entanglement, and they are established by constructing particular squashing channels. We compare these bounds with the known max- Rains information bound, the known mutual information bound, and another based on approximate covariance. For all capacities considered, we find that a large variety of state-of-the-art techniques are useful in establishing bounds.

This is joint work with Sumeet Khatri (LSU) and Kunal Sharma (LSU). https://arxiv.org/abs/1903.07747

Speaker: Mark Wilde

9 Nordita and AlbaNova Colloquium (S Reddy)

Speaker:Sanjay Reddy (University of Washington)
Link to further information

Friday 5 April

10 Fidelity in port based teleportation from irreducible representations of walled Brauer algebra

Port based teleportation is a varaiant of teleportation disovered by Ishizaka and Hiroshima, where state is transferred to the receiver without he need of unitary correction. Instead, Bob has several particles (ports) Alice sends to Bob message "which port" and Bob recevies the particle in that port. The fidelity is transmission is not equal to 1 but it can be made arbitrarily close to one, once the number of ports is increased. For qubits, fidelity was evaluate danalytically by use of SU(2) representation. Here I present development for higher dimension, where the problem can be recast in terms of so-called wall Brauer algebras. Based on joint work with Marek Mozrzymas, Michal Studzinski and Sergii Strelchuk

Speaker: Michal Horodecki

11 General mapping of multi-qudit entanglement conditions to non-separability indicators for quantum optical fields

We show that any multi-qudit entanglement witness leads to a non-separability indicator for quantum optical fields, which involves intensity correlation measurements and is useful for field states of undefined photon numbers. With the approach we get, e.g., necessary and sufficient conditions for intensity or intensity-rates correlations to reveal polarization entanglement. We also derive separability conditions for experiments with mutually unbiased multiport interferometers. Within the standard detector inefficiency model these entanglement indicators work for any detection efficiencies. For specific cases, we show advantages of using local intensity rates rather than intensities. The approach with rates allows a mapping of Bell inequalities for qudits to ones for optical fields. Our results may find applications also in studies of non-classicality of correlations in "macroscopic" many-body quantum systems of undefined or uncontrollable number of constituents. [arXiv:1903.03526 ]

Speaker: Prof. Marek Zukowski

Monday 8 April

12 Breakfast

13 Short talks

Participants of the week who want to give short talks should contact the organizers during the morning breakfast. A short talk is max 10 minutes + questions, max 3 slides. Whiteboard talks are also welcome.

14 Quantum heat transport by photons in superconducting circuits

I first review the physics and experimental status of quantized heat conductance in nanostructures. Then I move on to our recent experiments on photonic heat transport in superconducting quantum circuits, where different physics emerges depending on the hierarchy of various coupling strengths. I also present our recent observations of heat current rectification. Work done in collaboration with Bayan Karimi, Jorden Senior, Alberto Ronzani, Azat Gubaydullin, Yu-Cheng Chang and Joonas Peltonen.

Speaker: Jukka Pekola

Tuesday 9 April

15 Heat engines and batteries: two stories with lessons from quantum optics

Klaus Moelmer

Heat engines and batteries: two stories with lessons from quantum optics

We study a simple model of an autonomous heat engines using tools and methods from quantum optics to go beyond the master equation and steady state density matrix of the system. Heat and work are not state functions in classical thermodynamics. In the same way they are not observables of a quantum engine, but they are naturally defined and their fluctuations are readily calculated by the theory of measurements in quantum optics and the quantum regression theorem.
Collective effects and superadiance have been promoted as mechanisms leading to genuine quantum advantages in many-body heat engines and energy storage devices. We show that superradiant engines and batteries have a peculiar feature, which may be relevant for their practical use in experiments: They age. i.e., their output power and charging capacity decrease over time. Like household electronics, to regain their full capacity they should be fully discharged at frequent intervals.

Speaker: Klaus Moelmer

16 Interplay of dynamics and thermodynamics in nanoscale heat engines

Classical synchronisation (unrelated to clock synchronisation) is the study of the mutual adjustment of rhythms of coupled non-linear dynamical systems. Such dynamical systems exhibit a certain kind of stability known as the Arnold’s tongue. In this talk, i will discuss our recent study of the interplay of non-linear dynamical effects and performance of nanoscale thermal machines.

Speaker: Sai Vinjanampathy

Wednesday 10 April

17 Generation of multipartite entangled states by an autonomous thermal machine

Jonatan Bohr Brask

Generation of multipartite entangled states by an autonomous thermal machine

Abstract: Entanglement is a key resource for quantum information processing, and generating and maintaining entanglement is therefore a central challenge. While entanglement is fragile and generally degrades under unavoidable interactions of a system with its environment, it has been realised that dissipation may in some cases aid the generation and stabilisation of entanglement. In particular, heat gradients present in a multipartite system interacting with multiple thermal baths can induce entanglement in a steady state out of thermal equilibrium. Here, we describe a thermal machine capable of generating a large class of highly entangled, multipartite states of qubits, including GHZ-, W-, and cluster states. Entanglement is obtained from the steady state of the machine in a heralded manner. Apart from the heralding step, the machine is fully autonomous, requiring only incoherent couplings to thermal baths and energy-preserving, always-on interactions between parts of the machine. The final heralding step needs only local, projective measurements, which do not by themselves generate any entanglement.

Speaker: Jonathan Bohr Brask

18 Dynamically Induced Heat Rectification in Quantum Systems

Anna Sanpera Trigueros

Dynamically Induced Heat Rectification in Quantum Systems

Heat rectifiers are systems that conduct heat asymmetrically for forward and reversed temperature gradients. We present an analytical study of heat rectification in linear quantum systems. We demonstrate that asymmetric heat currents can be induced in a linear system only if it is dynamically driven. This asymmetry emerges when the driving frequency favors the non-symmetric heat exchange processes in front of the symmetric ones. Finally, we demonstrate the feasibility of such driven harmonic network to work as a thermal transistor, quantifying its efficiency through the dynamical amplification factor.

Speaker: Anna Sanpera Trigueros

19 Certifying the non-classicality of fluctuations

Patrick Potts

Certifying the non-classicality of fluctuations

Fluctuations have an important role in quantum thermodynamics. For instance, the output of a quantum thermal machine will in general be a fluctuating quantity. Due to the unavoidable measurement back-action in quantum mechanics, a measurement of multiple observables can in general not be described by a probability distribution that does not include the measurement apparatus explicitly. An example which has received considerable attention is provided by work fluctuations. In a classical scenario, a measurement might also act back on the measured system. However, this is in principle avoidable. Nevertheless, it is non-trivial to infer from measured data if the underlying observables can be described by a positive probability distribution or not. In this talk, I will show how one can rule out a description using only positive probability distributions (i.e., a classical description) by making a few natural assumptions on the measurement apparatus. This is achieved by obtaining an experimentally accessible inequality. In quantum theory, this inequality can be violated if and only if the Keldysh quasi-probabiliy distribution, a distribution that has been used before to describe work fluctuations, becomes negative. Importantly, all assumptions on the detectors can be fulfilled in quantum theory, allowing for the conclusion that the measured data cannot be obtained from any classical system.

Speaker: Patrick Potts

20 Reception

Thursday 11 April

21 Quantum duets as autonomous thermal motors

Alberto Imparato

Quantum duets as autonomous thermal motors

I will discuss how directed transport can emerge in two-temperature autonomous motors with broken spatial symmetry.
I will first consider the case of two quantum Brownian particles in contact with two heat baths at different temperatures and moving on shifted periodic potentials. The model exhibits a non vanishing center-of-mass average velocity, as a consequence of the temperature gradient and of the broken spatial symmetry. Such velocity can be calculated exactly in the limit of small corrugation. This model represents the extension to the two-temperature case of the ”Quantum Brownian motion in a periodic potential“ considered in M. P. A. Fisher and W. Zwerger, Phys. Rev. B 32, 6190 (1985). I will then show that the Quantum Molecular Dynamics (QMD) numerical algorithm can be successfully used to evaluate the properties of such a non- equilibrium, non-linear system.
I will finally consider the discrete counterpart of the previous autonomous motor, composed of two 2D rotators that interact through the clock model Hamiltonian. In this case the dynamics is of Lindblad form, and one can derive an explicit formula for the probability current.

Speaker: Alberto Imparato

22 Nordita and AlbaNova Colloquium (J Parrondo)

Speaker: Juan MR Parrondo (Universidad Complutense de Madrid)
Link to further information

Friday 12 April

23 Quantum Work and Energy Exchange from Quantum Trajectories and Conditional Wave Functions

I will discuss our recent efforts to circumvent the no-go theorem for quantum work [1] by using quantum trajectories from the Hamilton-Jacobi (Bohm) formalism. Quantum trajectories allow us to define work for a closed system that is in close analogy to the classical definition, and which recovers the standard results in the appropriate limits [2]. Time permitting I will also discuss how the concept of a conditional wave function can be used to calculate energy exchanges between strongly coupled quantum systems [3].
1. M. Perarnau-Llobet, E. Bäumer, K. V. Hovhannisyan, M. Huber, and A. Acin, Phys. Rev. Lett. vol. 118, 070601 (2017).
2. R. Sampaio et al., Phys. Rev. A vol. 97, 012131 (2018).
3. R. Sampaio et al., arXiv:1809.03273.

Speaker: Tapio Ala-Nissila

Monday 15 April

24 Breakfast

25 Short talks

26 The covariance of physical laws in quantum reference frames

Every observation in physics is made with respect to a frame of reference. In practise, we use real physical systems as reference frames, and as such they obey quantum mechanical laws. I will introduce a general method to quantise reference frame transformations, which generalises the usual reference frame transformation to a "superposition of coordinate transformations". I will describe how states, measurements, and dynamical evolutions transform between different quantum reference frames. While entanglement and superposition will be shown to be frame-dependent features, the form of the dynamical physical laws (e.g. the Schrödinger equation) remain the same in all frames, which generalises the notion of covariance of physical laws to quantum reference frames. I will end with two applications of our results: a definition of the rest frame for a quantum particle in a superposition of velocities, and a resolution of an old problem of identifying the qubit for a relativistic spin particle.

Speaker: Caslav Brukner

Tuesday 16 April

27 Autonomous thermal machine for amplification and control of energetic coherence

Juan Parrondo

Autonomous thermal machine for amplification and control of energetic coherence

We present a model for an autonomous quantum thermal machine comprised of two qubits that are capable of amplifying the coherence in a non-degenerate system by using only thermal resources. This novel method of coherent control allows for the interconversion between energy, both work and heat, and coherence. This model opens up new possibilities in the generation and manipulation of coherence by autonomous thermal machines.

Speaker: Juan Parrondo

28 Entropy production in a small quantum system strongly coupling with an environment: computational experiments

Many theoretical expressions of dissipation along non-equilibrium processes have been proposed. However, they have not been fully verified by experiments. Especially for systems strongly interacting with environments the connection between theoretical quantities and standard thermodynamic observables are not clear. We have developed a computer simulation based on a spin-boson model, which is suitable for testing the proposed theories. Measuring the dissipation with conventional thermodynamic quantities, we check the deviation from the standard thermodynamic behavior presumably due to the strong coupling. Then, we attempt to construct empirical expressions that are consistent both with the experiments and the theories.

Speaker: Ryoichi Kawai

Wednesday 17 April

29 Simple explanation of quantum nonlocality and contextuality

Adán Cabello, University of Seville
Simple explanation of quantum nonlocality and contextuality

Considerable efforts have been devoted searching the principle that explains why some forms of Bell nonlocality and Kochen-Specker (KS) contextuality are possible and others are forbidden. Here, we show that the assumptions that statistically independent experiments exist and that every behavior that is not forbidden is compulsory, single out the same set of behaviors that quantum theory predicts for any Bell and KS contextuality scenario. As a byproduct, our result explains why the so-called almost quantum correlations for Bell scenarios are forbidden.

Speaker: Adán Cabello

30 Distributed private randomness distillation

Dong Yang, University of Bergen
Distributed private randomness distillation

We develop the resource theory of private randomness extraction in the distributed and device- dependent scenario. We begin by introducing the notion of independent random bits, which are bipartite states containing ideal private randomness for each party, and motivate the natural set of free operations. As the main tool of our analysis, we introduce Virtual Quantum State Merging, which is essentially the flip side of Quantum State Merging, without communication. We focus on the bipartite case and find the rate regions achievable in different settings. Perhaps surprisingly, it turns out that local noise can boost randomness extraction. As a consequence of our analysis, we resolve a long-standing problem by giving an operational interpretation for the reverse coherent information in terms of the number of private random bits obtained by sending quantum states from one honest party (server) to another one (client) via the eavesdropped quantum channel.

Speaker: Dong Yang

31 What is this program about?

This is the event in the program where one of the organizers explains with a blackboard talk the main research scope and scientific challenges addressed by the program, aimed to PhD students who have just arrived at Nordita.
The event is open to all Nordita faculty, postdocs, students and staff, and will be followed by a reception and mingling event.

Speaker: Mohamed Bourennane

32 Reception

Thursday 18 April

33 Unscheduled

Tuesday 23 April

34 Breakfast

35 The Future of Quantum Mechanics

I was asked to talk about the future. I don't know much about it, but I will discuss whether quantum mechanics will be superseded by a better theory in the future. Perhaps because this is needed to handle gravity? Or when the domain of application is extended in some other way? In retrospect it is easy to see that the formalism of classical mechanics has some features that point directly to quantum mechanics. Does the formalism of quantum mechanics have some features that point beyond it?'

Speaker: Ingemar BENGTSSON

36 Why photon counting is great

Konrad Banaszek

Why photon counting is great

Speaker: Konrad Banaszek

Wednesday 24 April

37 Large deviation methods in open quantum systems

In this talk I will describe a perspective on the dynamics of Markovian open quantum systems based on the study of the statistical properties of ensembles of quantum trajectories. This "thermodynamics of quantum trajectories" approach is to dynamics what the standard equilibrium configuration ensemble method is for statics. Just like in the static case, this is based on the formalism of large deviations. I will go through the main ideas, and discuss applications to problems including trajectory phase transitions, how to optimally realise rare events, so-called prediction-retrodiction, fluctuation bounds, and catching and reversing quantum jumps.

Speaker: Juan P. Garrahan

38 Space-to-Ground Quantum Key Distribution

Quantum key distribution (QKD) can in principle offer unconditional security by making use of the fundamental laws of quantum mechanics. In practice, this is typically achieved by preparing individual photons in quantum superposition states and sending them to a remote receiver. To date, most commercially available QKD systems rely on the transmission of the photons via optical fibers, which, due to channel loss and detector noise, limits the distance over which QKD is feasible to a few hundred kilometers. Alternatively, satellite-based QKD facilitates low photon loss and negligible signal disturbance and offers a viable solution for establishing a global scale quantum network. Recently, a quantum science mission of the Chinese Academy of Sciences (CAS) in collaboration with the Austrian Academy of Sciences (AAS) and the University of Vienna aimed at a satellite-based intercontinental quantum-key relay. Using the Chinese Quantum Science Satellite “Micius” as a trusted relay, a quantum network consisting of three optical ground stations located in China (Xinglong, Nanshan) and Austria (Graz-Lustbühel) has been demonstrated successfully. Here we report on the development of a quantum receiving module, installed at the Satellite Laser Ranging Station in Graz (Austria) capable of implementing the so-called decoy-state QKD protocol in a downlink scenario from the LEO satellite “Micius”. Furthermore, we will present the experimental results obtained during several downlinks from the Chinese satellite focusing in particular on the performance of the Austrian receiving station.

Speaker: Prof. Rupert Ursin (IQOQI - Vienna, Austrian Academy of Sciences)

39 Rotating trapped fermions in 2d and the complex Ginibre ensemble

Satyanarayan Majumdar, LPTMS, Universite Paris-Sud, Orsay
Rotating trapped fermions in 2d and the complex Ginibre ensemble

We establish an exact mapping between the positions of N noninteracting fermions in a 2d rotating harmonic trap in its ground-state and the eigenvalues of the NxN complex Ginibre ensemble of Random Matrix Theory (RMT). Using RMT techniques, we make precise predictions for the statistics of the positions of the fermions, both in the bulk as well as at the edge of the trapped Fermi gas. In addition, we compute exactly, for any finite N, the Renyi entanglement entropy and the number variance of a disk of radius r in the ground-state. We show that while these two quantities are proportional to each other in the (extended) bulk, this is no longer the case very close to the trap center nor at the edge. Near the edge, and for large N, we provide exact expressions for the scaling functions associated with these two observables.

Speaker: Satyanarayan Majumdar

40 Reception

Thursday 25 April

41 Decoherence in the space of quantum operations: how to coherify a classical map?

Karol Zyczkowski

Decoherence in the space of quantum operations: how to coherify a classical map?

The theory of quantum information processing meets experiment: in view of the emerging field of quantum technologies future theoretical studies will be closer linked to the physical world by considering more realistic systems and putting even more emphasis on interaction of the system in question with environment and the effect of quantum decoherence.
In this talk I will shortly review the standard notion of decoherence and its formal inverse: coherification of a classical probability vector denotes the search of all its preimages with respect to the coarse graining channel, which induces the decoherence.
A similar problem can also be posed for channels: For a given classical map corresponding to a stochastic transition matrix T we look for quantum channel Phi, which induces the same classical transition matrix T, but is "more coherent". To quantify the coherence of a channel Phi we measure the coherence of the corresponding Jamiolkowski state J. We show that a classical transition matrix T can be coherified to a reversible unitary dynamics if and only if T is unistochastic. Otherwise the Jamiolkowski state J of the optimally coherified channel is mixed, and the dynamics must necessarily be irreversible.

References:
[1] K. Korzekwa, S. Czachorski, Z. Puchala and K. Zyczkowski, N.J.Phys. vol 20, 043028 (2018).
[2] K. Korzekwa, S. Czachorski, Z. Puchala and K. Zyczkowski, preprint arXiv:1812.09083

Speaker: Karol Zyczkowski

42 The experiment paradox

The foundations of quantum mechanics are haunted by the notorious measurement paradox, caused by the invasive character of projective measurements. I will show that it is in fact an instance of the `experiment paradox' lurking in the very core of the scientific method. It turns out that any experiment performed on a physical system is - by necessity - invasive and thus establishes inevitable limits to the accuracy of any mathematical model. I will highlight the dramatic consequences of the paradox for both classical and quantum systems. The talk is based on a recent preprint with Pawel Horodecki: https://arxiv.org/abs/1904.04117

Speaker: Prof. Michał Eckstein (University of Gdańsk)

43 Nordita and AlbaNova Colloquium (M Berry)

Speaker: Michael Berry (H H Wills Physics Laboratory, University of Bristol, UK)
Title: Geometric phases and the separation of the world
Link with more information

Friday 26 April

44 The power of quantum networks

We discuss the power of current and near-future quantum networks for quantum information processing. We start in the classical regime and review the concept of causal networks, designed to understand when a given cause pattern is able to reproduce some observed correlations. We then move to the quantum case and explain how Bell’s theorem can be interpreted as a gap between the correlations observed in a network when using classical or quantum causes. We present other classical-quantum separations and discuss about their possible use for the design of novel quantum information protocols with no classical analogue.

Speaker: Antonio Acin

45 Quantum computation and the additional degrees of freedom in a physical system

The speed-up of Quantum Computers is the current drive of an entire scientific field with several large research programmes both in industry and academia world-wide. Many of these programmes are intended to build hardware for quantum computers. A related important goal is to understand the reason for quantum computational speed-up; to understand what resources are provided by the quantum system used in quantum computation. Some candidates for such resources include superposition and interference, entanglement, nonlocality, contextuality, and the continuity of state-space. The standard approach to these issues is to restrict quantum mechanics and characterize the resources needed to restore the advantage. Our approach is dual to that, instead extending a classical information processing systems with additional properties in the form of additional degrees of freedom, normally only present in quantum-mechanical systems. In this talk, we will have a look at these additional degrees of freedom and how quantum computers make use of them to achieve the so-called quantum speedup. We will also discuss whether the additional degrees of freedom can be viewed as a ”side channel,” a term often seen in cryptology, and whether quantum parallelism rather should be viewed as computation performed in some additional degree of freedom.

Speaker: Prof. Jan-Åke Larsson (Linköping University)