### Conveners

#### Session 4.2

- Chair: Marcos Curty (Universidad de Vigo)

#### Session 4.2

- Chair: David Zueco (Instituto de Nanociencia y Materiales de Aragon)

#### Session 4.2

- Chair: Andres Gomez Tato

#### Session 4.2

- Chair: Alba Cervera Lierta (Barcelona Supercomputing Center)

We implemented a simplified time-bin BB84 quantum key distribution protocol with the purpose of achieving the highest possible secret key rate at short distances. The sender Alice emits signals at a rate of 2.5 GHz. In the key-generating basis, we use a superconducting nanowire single photon detector (SNSPD) with a novel design optimized for fast count rates. The in-house designed and...

We consider a heterogeneous network of quantum computing modules, sparsely connected via Bell states. Operations across these connections constitute a computational bottleneck and they are likely to add more noise to the computation than operations performed within a module. We introduce several techniques for transforming a given quantum circuit into one implementable on a network of the...

In recent years, the study of Bell-type nonlocality on networks has led to an array of intriguing foundational results. Nonetheless the field still faces difficulties in finding a justified application. One of the key barriers for this is the assumption of independent sources in network nonlocality, which is difficult to enforce. In our work we examine a possible operational interpretation for...

Quantum technologies promise interesting new approaches to areas such as computing and communication. A branch that is becoming increasingly interesting is that of quantum networks. The technological assets for quantum networks have been developing rapidly in recent years and many implementations, often geared towards quantum cryptography, have been reported. In order to demonstrate security...

We present a method to detect bipartite entanglement based on number-phase-like uncertainty relations in split spin ensembles. First, we derive an uncertainty relation that plays the role of a number-phase uncertainty for spin systems. It is important that the relation is given with well-defined and easily measurable quantities, and that it does not need assuming infinite dimensional systems....

While current research in quantum theory focuses on the exploitation of quantum effects in communication and computation scenarios, quantum systems are also known to be advantageous for some mechanical tasks. The most known effect is that of tunneling, but there are other less well known effects. One of those is quantum backflow [1], a phenomenon in which a free quantum particle with positive...

**Variational Quantum Algorithms** (VQAs) [1] use a classical optimizer to train a parametrized quantum circuit (PQC). These have emerged as a practical way to exploit state-of-the-art quantum computers. Currently, most VQAs have been designed for fully digital approaches, in which the error ends up accumulating for circuits with many parameters. A possible way out is the use of analogue...

Classical simulation of quantum dynamics from many-body systems with tensor networks is hindered by the exponential growth of entanglement contained at the bonds of a chosen wavefunction factorization (typically Matrix Product States). Modern algorithms try to overcome this entanglement barrier by folding and contracting transversely the network [1], or optimizing schemes to exploit only...

The field of cavity qed materials seeks to modify the properties of bulk materials by coupling them to an electromagnetic cavity at equilibrium. When the material is, e.g., composed of magnetic dipoles, the resulting system is described by a generalized Dicke model. Under certain conditions, the cavity modes can be traced out, leaving a spin Hamiltonian with cavity-mediated (effective)...

Quantum computers in the NISQ era (noisy, intermediate-scale, quantum) still offer a relatively small amount of qubits. The largest quantum computers so far, dedicated to binary optimization, do not surpass a few thousands qubits. We nevertheless are willing and able to probe such computers in real-life tasks with their high demand in number of variables to optimize over.

We tackle a binary...

Quantum walks (QWs) play an important role in quantum computing. On the one hand, some algoritmical problems can be recast as a QW. On the other hand, many physical phenomena can be simulated with the help of a QW. Here we concentrate on discrete-time QWs, and we discuss quantum circuits that can implement discrete-time quantum walks having an arbitrary position-dependent coin operator [1]....

Hybrid quantum-classical algorithms emerge as one promising approach to improve the performance of current quantum computers. In this work, we consider the method to execute general quantum algorithms on two different QPUs connected via classical communication. The optimal protocol for such computation consists of two steps: First, a quasi-probabilistic simulation scheme generates the required...