# Best Poster Award

SFB BeyondC encourages young researchers to present their results during poster sessions at the SFB events. Twice per year the “Best Poster Award” of 200 Euros will be granted.

On this page we have listed **all the winners of the "Best Poster Award". **

## Entangling Single Atoms Over 33 km Telecom Fibre

### Pooja Malik

##### Ludwig Maximilian University of Munich

##### SFB BeyondC Conference 2022

Quantum repeaters will allow scalable quantum networks, which are essential for large scale quantum communication and distributed quantum computing. A crucial step towards a quantum repeater is to achieve heralded entanglement between stationary quantum memories over long distances. To this end, we present results demonstrating heralded entanglement between two Rb-87 atoms separated by 400 m line-of-sight, generated over telecom fibre links with a length up to 33 km [1]. To entangle the two atoms, we start with entangling the spin state of each atom with the polarisation state of a photon in each node via synchronised excitations during the spontaneous decay. The emitted photons (780 nm) are then converted to the low loss telecom S band (1517 nm) via a polarisation preserving frequency conversion to overcome high attenuation loss in optical fiber [2]. The long fibre links guides these photons to a middle station where a Bell-state measurement swaps the entanglement to the atoms. Finally, the atomic states are analysed after a delay that allows for two-way communication between the nodes and the BSM over the respective fibre length. We observe loss in fidelity for longer fibre links due to the limited atomic coherence time.

## Reassessing the computational advantage of quantum-controlled ordering of gates

### Martin Johannes Renner

##### University of Vienna

##### SFB BeyondC Autumn Workshop 2021

In quantum computation, indefinite causal structures allow to perform certain tasks more efficiently than any conventional (causal) quantum algorithm. For example, the quantum switch can decide whether two unitary gates commute or anticommute with a single call to each gate, while in any causal quantum algorithm at least one gate has to be called twice. A generalization of this task to n unitary gates, can be solved with the quantum-n-switch and a single call to each gate, while it was expected that the best causal algorithm calls O(n2) gates. We present more efficient causal lgorithms for this task and conclude that this advantage is smaller than expected so far.

## Towards NPT bound entanglement: computational complexity and field extensions

### Mirte van der Eyden

##### University of Innsbruck

##### SFB BeyondC Winter Workshop 2021

One of the classic open problems in quantum information is the existence of bound entangled states with a non-positive partial transpose (NPT). This problem is related to the positivity of linear maps under tensor powers. In our ongoing work we prove undecidability of the following problem: is a given linear map positive under tensor powers when the input are neighbouring Bell pairs? Proving undecidability of the same question without a specified input would prove existence of NPT bound entangled states. However, we show that the most natural reduction to this more general problem is not possible. In a second line of research we prove existence of NPT bound entangled states considered on the hyper-complex field.

## Macroscopicity of Matter Wave Interference

### Björn Schrinski

##### Universität Duisburg-Essen

##### International Conference on Quantum Optics 2020

One driving force to motivate interference experiments involving high masses,long interference times, and large path separations is to verify the validity of theSchrödinger equation on ever larger scales. The degree of macroscopicity can beassessed by the amount of falsified modifications of quantum mechanics (collapsemodels [1]) which may be quantified with help of the underlying parameter space [2]. We focus on the fundamental measurement outcomes by applying Bayesianparameter estimation [3] and discuss state of the art experiments [4,5,6].

[1] Bassi et al., Rev. Mod. Phys. 85 (2013) [2] Nimmrichter et al., Phys. Rev.Lett. 110 (2013) [3] Schrinski et al., Phys. Rev. A 100 (2019) [4] Kovachy et al.,Nature 528 (2015) [5] Fein et al., Nat. Phys. (2019) [6] Xu et al., Science 366 (2019)

## Quantum Computing with Graphene Plasmons

### Irati Alonso Calafell, J. D. Cox, M. Radonjić, J. R. M. Saavedra, F. J. García de Abajo, L. A. Rozema, P. Walther University of Vienna

##### University of Vienna

##### Austrian Quantum Information Conference 2019

Among the various approaches to quantum computing, all-optical architectures are especially promising due to the robustness and mobility of single photons. However, the creation of the two-photon quantum logic gates required for universal quantum computing remains a challenge. Here we propose a universal two-qubit quantum logic gate, where qubits are encoded in surface plasmons in graphene nanostructures, that exploits graphene's strong third-order nonlinearity and long plasmon lifetimes to enable single-photon-level interactions. In particular, we utilize strong two-plasmon absorption in graphene nanoribbons, which can greatly exceed single-plasmon absorption to create a “square-root-of-swap” that is protected by the quantum Zeno effect against evolution into undesired failure modes. Our gate does not require any cryogenic or vacuum technology, has a footprint of a few hundred nanometers, and reaches fidelities and success rates well above the fault-tolerance threshold, suggesting that graphene plasmonics offers a route towards scalable quantum technologies.