Quantum Information in Spain ICE-8

May 29, 2023, 9:00 AM → Jun 1, 2023, 7:00 PM Europe/Madrid

Santiago de Compostela

The 9th edition of the conference Quantum Information in Spain (ICE), supported by the Spanish Network on Quantum Information and Quantum Technologies, aims to provide a gathering for discussion to researchers working in quantum information science and technologies in Spain to bring together academia and industry.

ICE covers the fields of quantum communication and cryptography, quantum computation and simulation, quantum metrology and sensing, quantum information theory, quantum thermodynamics, photonics, trapped ions, and enabling technologies.

We strongly encourage junior researchers to contribute.

REGISTRATION IS NOW OPEN

*Note we have a limited number of affordable participants of 150. After reaching this number registrations will be closed.

 

1stBulletin_ICE-82023.pdf

abstract submission guidelines ICE-8.pdf

Monday, May 29

9:00 AM → 9:15 AM Registration

9:15 AM → 9:30 AM Welcome and conference opening

9:30 AM → 10:50 AM Morning sesion 1: (TBA, Fully passive quantum key distribution)

Convener: Javier Mas Sole

9:30 AM TBA (Valerio Pruneri)

Speaker: Valerio Pruneri (ICFO)

10:20 AM Fully passive quantum key distribution

In recent years, quantum key distribution (QKD) has become a fully fledged application of quantum information science, and QKD services are being supplied by different companies/institutions around the world. However, the practical security of QKD is not well-established yet, mainly due to the difficulty of guaranteeing that real QKD implementations stick to the assumptions and models on which theoretical security proofs rely.
A particularly conflictive assumption present in most QKD security proofs is that no information is undesirably leaked outside the users’ locations. In fact, optical QKD systems typically rely on the use of active modulators to encode the key information, and these modulators may be a source of side-channels in different ways. For instance, an eavesdropper may actively tamper with a QKD module to gain information about the protocol settings or, more simply, the information can be inadvertently encoded in undesired degrees of freedom.
A candidate solution to overcome this problem is to consider passive (rather than active) state preparation, which rules out all possible modulator side-channels by avoiding the use of active modulation of any kind. Precisely, a passive QKD transmitter generates the quantum states prescribed by a QKD protocol at random, combining a fixed quantum mechanism and a post-selection step. Putting the security upgrade aside, getting rid of all actively driven elements could be very appealing for QKD in practice, because it may allow to boost the frequency of operation of QKD systems while reducing the complexity (and thereby the cost) of QKD infrastructures. Needless to say, this would entail an advantage in many practical situations, for instance, when it comes to deploying QKD on a satellite. Notably though, these advantages come at the price of decreasing the key generation rate because of two main reasons. On the one hand, in a passive transmitter, additional sifting is required to discard those protocol rounds where the randomly generated settings do not lie in certain acceptance intervals. On the other hand, the quantum states post-selected in a passive transmitter are in a mixed polarization state. This represents an inherent source of noise not present in the active case, where one typically considers perfectly prepared pure states.
In a recent collaboration, we presented the first linear optics scheme suitable for fully passive QKD, and analyzed its expected performance within two sharply different approaches for polarization encoding and secret-key-rate estimation. However, these analyses addressed the asymptotic limit of infinite signals, and in both cases the distillable key rate was limited by the inherent noise of the mixed polarization states. Here, we report on a novel parameter estimation technique that surpasses this limitation ---in so reaching tighter bounds on the secret key rate--- and address the practical scenario where a finite number of signals is exchanged. Furthermore, the developed techniques for the estimation of the secret key parameters might be of independent interest for the field of quantum cryptography.

Speaker: Víctor Zapatero (Vigo Quantum Communication Center)

10:50 AM → 11:20 AM Coffee Break

11:20 AM → 1:00 PM Session 2

Convener: Marcos Curty (Universidad de Vigo)

11:20 AM KeySight

11:40 AM Fast single-photon detectors and real-time key distillation enable high secret-key-rate quantum key distribution systems

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 fabricated NbTiN detector consists of 14 nanowires which are arranged in an interleaved pattern. Together with the in-house made readout electronics, the detector shows a jitter below 60 ps and simultaneously an efficiency of 64\% at a count rate of 320 Mcps, which represents the operating point of the detector for our shortest-distance key exchange. We performed real-time error correction with a low-density parity check algorithm implemented on a dedicated field-programmable gate array. This algorithm has a leakage of 17% at the highest quantum bit error rate found in our experiment, which was 0.4%. The privacy amplification was performed in real time on a consumer-grade GPU. We achieved a secret key rate of 64 Mbps over a distance of 10.0 km of ultra-low-loss (ULL) single-mode fiber (0.16 dB/km) and 3.0 Mbps over 102.4 km of ULL single-mode fiber . Additionally, we monitored the secret key rate over a longer time over 10.0 km ULL SMF, showing that the secret key rate can be maintained at a similar value for more than 1000 consecutive privacy amplification blocks.

Speaker: Dr Fadri Gruenenfelder (University of Vigo)

abstract.pdf

presentation_fadri_gruenenfelder.pdf

12:00 PM Distributing circuits over heterogeneous, modular quantum computing network architectures

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 aforementioned type, minimising the number of Bell states required to do so.

We extend previous works on circuit distribution over fully connected networks to the case of heterogeneous networks. On one hand, we extend the hypergraph approach of [Andres-Martinez & Heunen. 2019] to arbitrary network topologies, making use of Steiner trees to find efficient realisations of the entanglement sharing within the network, reusing already established connections as often as possible. On the other hand, we extend the embedding techniques of [Wu, et al. 2022] to networks with more than two modules. Furthermore, we discuss how these two seemingly incompatible approaches can be made to cooperate. Our proposal is implemented and benchmarked; the results confirm that, when orchestrated, the two approaches complement each other's weaknesses.

Speaker: Pablo Andres-Martinez (Quantinuum)

12:20 PM Topologically robust network nonlocality

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 such independent sources. In particular, in networks without inputs, parties connected with a common classical source can be interpreted as malicious parties working together. We explore the first steps in this framework and show that there exist nonlocal distributions which are robust against the topology of the network, i.e. in the spirit of decentralized protocols, the overall distributions are nonlocal even if one is unsure which parties are collaborating. In fact, we show that in large random networks this is quite typical. We further examine the relation to randomness generation and highlight some remaining challenges in developing protocols based on network nonlocality.

Speaker: Tamás Kriváchy (TU Wien)

12:40 PM Guaranteeing non-classicality in experimental quantum networks without assuming quantum mechanics

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 of quantum cryptographic protocols, a necessary condition is to guarantee that the observations of the parties cannot be reproduced when classical systems are distributed instead (i.e., to observe non-locality). However, in contrast with traditional bipartite scenarios, the standard notion of multipartite non-locality only guarantees that something quantum is happening somewhere in the network. In contrast, recently a new notion of non-locality in networks has been introduced, called full network nonlocality, that allows to guarantee that non-classical behavior is present everywhere in the network. Moreover, this notion does not assume quantum mechanics. Therefore, proofs of security based on full network nonlocality will not break if, in the future, we ever find physical systems that go beyond quantum mechanics.

In this talk I will describe several experimental observations of full network nonlocality in scenarios that are specially relevant as building blocks of large-scale quantum communication networks. The first one is a star-shaped network where three branch parties share each a bipartite quantum state with a central node that performs tripartite entanglement swapping. This is an important scenario to realize multipartite quantum cryptographic protocols mediated by a central authority. The second one is a quantum repeater scenario where we strictly enforce the network structure via space-like separation of its components. The fact that full network nonlocality is observed in a significant manner, in demanding scenarios, and with state-of-the-art technology, strongly motivates the development of multipartite quantum cryptographic protocols in networks and proofs of security based on full network nonlocality.

Speaker: Alejandro Pozas-Kerstjens (Institute of Mathematical Sciences (ICMAT-CSIC))

41-APozasKerstjens.pdf

1:00 PM → 2:20 PM Lunch

6:20 PM → 7:10 PM Inertial sensing using matter-wave interferometry

In a Mach-Zehnder-type light pulse atom interferometer, matter waves are split, mirrored, and recombined using coherent atom optics. With the leading order phase shift scaling with the enclosed space-time area, the momentum transfer induced by the atom optics light pulses as well as the free evolution time are key to significantly enhanced sensitivity to inertial forces and motivate ground-based 10m-scale facilities as well as space-borne experiments. Beyond fundamental physics, the ability to provide long-term stable, accurate measurements gives rise to applications in inertial navigation.
In this talk we will introduce the field of atom interferometric inertial sensing, provide an overview of state-of-the-art experiments, and present current activities at Leibniz University Hannover. In particular, we will provide an overview on the Hannover VLBAI facility (Very Long Baseline Atom Interferometry). With shot noise-limited instabilities better than 10−9 m/s2 at 1 s at the horizon it may compete with state-of-the-art superconducting gravimeters, while providing absolute instead of relative gravity measurements. Operated with rubidium and ytterbium simultaneously, tests of the universality of free fall at a level of parts in 1013 and beyond are in reach. We will finally discuss strategies mitigating vibration noise, which is the dominant noise source in absolute acceleration sensing using atom interferometry. To this end, we report on hybrid inertial sensing by correlating novel opto-mechanical accelerometers with atom interferometers and describe a path towards state-of-the-art inertial sensing in the field without the need for seismic attenuation.

Speaker: Dennis Schlippert (1Leibniz Universität Hannover, Institut für Quantenoptik)