Clément Meignant | LIP6 - Équipe QI

Outcome determinism in measurement-based quantum computation with qudits

Fri, 24 Feb 2023 00:00:00 +0000

In measurement-based quantum computing (MBQC), computation is carried out by a sequence of measurements and corrections on an entangled state. Flow, and related concepts, are powerful techniques for characterising the dependence of the corrections on previous measurement outcomes. We introduce flow-based methods for MBQC with qudit graph states, which we call Zd-flow, when the local dimension is an odd prime. Our main results are proofs that Zd-flow is a necessary and sufficient condition for a strong form of outcome determinism. Along the way, we find a suitable generalisation of the concept of measurement planes to this setting and characterise the allowed measurements in a qudit MBQC. We also provide a polynomial-time algorithm for finding an optimal Zd-flow whenever one exists.

Multipartite communications over quantum networks

Fri, 17 Dec 2021 00:00:00 +0000

The field of quantum networks is currently a major area of investigation in quantum technologies. One of the simplest acts of quantum communication, the distribution of a single bipartite entangled state, has been highly studied as it is a simple problem to characterize, simulate and implement. It is also useful for a prominent quantum network application: the secured distribution of a cryptographic key. However, the use of quantum networks goes far beyond. We need to study the simultaneous distribution of multipartite states over quantum networks. In this manuscript, we report on several works of progress in the domain. We first study the recycling of previously distributed resources in the asymptotic regime by the use of entanglement combing and quantum state merging. Then, we characterize the distribution of quantum states using the tensor network formalism. We also characterize a broad class of classical distribution protocols by the same formalism and use this similarity to compare the distribution of classical correlations over classical networks to a the distribution of quantum state over quantum networks. We also build protocols to distribute specific classes of states over quantum networks such as graph states and GHZ states by using the graph state formalism and a bit of graph theory. Finally, we implement the previous protocols in a more realistic setting and participate in the elaboration of multipartite features for a quantum network simulator: QuISP. We also aimed to popularize the notions of quantum information to a broad audience. We report on the creation of a video game based on quantum optics, adding to the existing popularization ludography.

Classical-quantum network coding: a story about tensor

Tue, 30 Nov 2021 00:00:00 +0000

We study here the conditions to perform the distribution of a pure state on a quantum network using quantum operations which can succeed with a non-zero probability, the Stochastic Local Operation and Classical Communication (SLOCC) operations. In their pioneering 2010 work, Kobayashi et al. showed how to convert any classical network coding protocol into a quantum network coding protocol. However, they left open whether the existence of a quantum network coding protocol implied the existence of a classical one. Motivated by this question, we characterize the set of distribution tasks achievable with non zero probability for both classical and quantum networks. We develop a formalism which encompasses both types of distribution protocols by reducing the solving of a distribution task to the factorization of a tensor with complex coefficients or real positive ones. Using this formalism, we examine the equivalences and differences between both types of distribution protocols exhibiting several elementary and fundamental relations between them as well as concrete examples of both convergence and divergence. We answer by the negative to the issue previously left open: some tasks are achievable in the quantum setting, but not in the classical one. We believe this formalism to be a useful tool for studying the extent of quantum network ability to perform multipartite distribution tasks.

Rates of Multipartite Entanglement Transformations

Mon, 17 Aug 2020 00:00:00 +0000

The theory of the asymptotic manipulation of pure bipartite quantum systems can be considered completely understood: the rates at which bipartite entangled states can be asymptotically transformed into each other are fully determined by a single number each, the respective entanglement entropy. In the multipartite setting, similar questions of the optimally achievable rates of transforming one pure state into another are notoriously open. This seems particularly unfortunate in the light of the revived interest in such questions due to the perspective of experimentally realizing multipartite quantum networks. In this Letter, we report substantial progress by deriving simple upper and lower bounds on the rates that can be achieved in asymptotic multipartite entanglement transformations. These bounds are based on ideas of entanglement combing and state merging. We identify cases where the bounds coincide and hence provide the exact rates. As an example, we bound rates at which resource states for the cryptographic scheme of quantum secret sharing can be distilled from arbitrary pure tripartite quantum states. This result provides further scope for quantum internet applications, supplying tools to study the implementation of multipartite protocols over quantum networks.

Distributing Graph States Over Arbitrary Quantum Networks

Wed, 27 Nov 2019 00:00:00 +0000

Multipartite entangled states are great resources for quantum networks. In this work we study the distribution, or routing, of entangled states over fixed, but arbitrary, physical networks. Our simplified model represents each use of a quantum channel as the sharing of a Bell pair; local operations and classical communications are considered to be free. We introduce two protocols to distribute respectively Greenberger-Horne-Zeilinger (GHZ) states and arbitrary graph states over arbitrary quantum networks. The GHZ states distribution protocol takes a single step and is optimal in terms of the number of Bell pairs used; the graph state distribution protocol uses at most twice as many Bell pairs and steps than the optimal routing protocol for the worst case scenario.

Super- and subradiance of clock atoms in multimode optical waveguides

Tue, 01 Jan 2019 00:00:00 +0000

The transversely confined propagating modes of an optical fiber mediate virtually infinite range energy exchanges among atoms placed within their field, which adds to the inherent free space dipole-dipole coupling. Typically, the single atom free space decay rate largely surpasses the emission rate into the guided fiber modes. However, scaling up the atom number as well as the system size amounts to entering a collective emission regime, where fiber-induced superradiant spontaneous emission dominates over free space decay. We numerically study this super-and subradiant decay of highly excited atomic states for one or several transverse fiber modes as present in hollow core fibers. As particular excitation scenarios we compare the decay of a totally inverted state to the case of π/2 pulses applied transversely or along the fiber axis as in standard Ramsey or Rabi interferometry. While a mean field approach fails to correctly describe the initiation of superradiance, a second-order approximation accounting for pairwise atom-atom quantum correlations generally proves sufficient to reliably describe superradiance of ensembles from two to a few hundred particles. In contrast, a full account of subradiance requires the inclusion of all higher order quantum correlations. Considering multiple guided modes introduces a natural effective cutoff for the interaction range emerging from the dephasing of different fiber contributions.