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arXiv:1906.01710 · Physical Review A

Comment on “Fully device-independent conference key agreement”

Timo Holz, Daniel Miller, Hermann Kampermann, Dagmar Bruß

Published Aug 26, 2019• 11 citations
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Abstract

In this Comment we discuss the device-independent conference key agreement (DICKA) protocol [Phys. Rev. A 97, 022307 (2018)]. We show that the suggested honest implementation fails, because perfectly correlated measurement results and the required Bell-inequality violation cannot be achieved simultaneously, in contradiction to what is claimed. We further show via semidefinite programming that there cannot exist any suitable honest implementation in the tripartite setting, rendering the DICKA protocol incomplete.

DOI
10.1103/physreva.100.026301

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Conference key agreement aims to establish shared, private randomness among many separated parties in a network. Device-independent conference key agreement (DICKA) is a variant in which the source and the measurement devices used by each party need not be trusted. So far, DICKA protocols largely fall into two categories: those that rely on violating a joint Bell inequality using genuinely multipartite entangled states and those that concatenate many bipartite protocols. The question of whether a hybrid protocol exists, where a multipartite Bell inequality can be violated using only bipartite entanglement, was asked by Grasselli et al. [Quantum 7, 980 (2023)2521-327X10.22331/q-2023-04-13-980]. We answer this question affirmatively, by constructing an asymptotically secure DICKA protocol achieving the same rate as the concatenation of bipartite device-independent quantum key distribution, yet relying on a single joint Bell violation. Our results prompt further discussion on the benefits of multipartite entanglement for DICKA over its bipartite alternative, and we give an overview of different arguments for near-term devices.

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Device-independent conference key agreement (DI-CKA) enables $N > 2$ distant parties to establish a common secret key without detailed characterization of the source or measurement devices. Many existing protocols rely on full-body Bell correlators, whose measurement can be demanding for a large number of parties. Here, we show that two-body correlators alone suffice for DI-CKA using balanced Dicke states for every even $N \geq 4$. We construct two-body Bell inequalities and derive a tight analytical lower bound on the conditional von Neumann entropy by comparing their Bell operators with those of asymmetric Clauser-Horne-Shimony-Holt inequalities. In the ideal setting, certified key rates can approach $1-h[(N-2)/(4(N-1))]$, where $h$ is the binary entropy function. This rate remains bounded away from zero as $N \rightarrow \infty$, tending to $1-h(1/4) \approx 0.189$ bits per key-generation round. We show that the honest implementation of our protocol is invariant under collective phase noise. Moreover, a complementary full-body Bell construction certifies positive conference-key rates from correlations arbitrarily close to the local set.

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Fully device-independent conference key agreement

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We present a security analysis of conference key agreement (CKA) in the most adversarial model of device independence (DI). Our protocol can be implemented by any experimental setup that is capable of performing Bell tests [specifically, the Mermin-Ardehali-Belinskii-Klyshko (MABK) inequality], and security can in principle be obtained for any violation of the MABK inequality that detects genuine multipartite entanglement among the $N$ parties involved in the protocol. As our main tool, we derive a direct physical connection between the $N$-partite MABK inequality and the Clauser-Horne-Shimony-Holt (CHSH) inequality, showing that certain violations of the MABK inequality correspond to a violation of the CHSH inequality between one of the parties and the other $N\ensuremath{-}1$. We compare the asymptotic key rate for device-independent conference key agreement (DICKA) to the case where the parties use $N\ensuremath{-}1$ device-independent quantum key distribution protocols in order to generate a common key. We show that for some regime of noise the DICKA protocol leads to better rates.

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Reply to “Comment on ‘Fully device-independent conference key agreement' ”

Jérémy Ribeiro, Gláucia Murta, Stephanie Wehner

In this Reply we correct a mistake that we made in the correctness proofs of our protocol. Specifically, the Bell inequality we used ensures security but does not allow us to produce a key. In this Reply we explain and correct this mistake by adjusting the Bell inequality we used in the proof. Incidentally, this correction leads to slightly better asymptotic key rates. Importantly, none of the conclusions of the article are affected.

Physical Review A•Aug 26, 2019•18 citations

Device-independent Quantum Key Distribution and the Role of Bell Nonlocality

Timo Holz

Quantum entanglement is a unique property of quantum particles. It can correlate them in such a way, that further correlations to an additional unwanted party is prevented. This renders quantum theory as the prime candidate to implement a secure distribution of an encryption key. Quantum key distribution (QKD) is dedicated to this task. A striking feature of correlations in a statistical experiment is, that they can, in principle, always be approximated. Even more so, correlations can be obtained without a detailed knowledge about the underlying physical process that generated the experimental data. This no-characterization approach is at the heart of the device-independent (DI) paradigm. Here, Bell inequalities are an imperative tool to detect nonlocal correlations, which are necessary for a DI secret key. One central subject of this thesis is multipartite DIQKD and the implications of Bell nonlocality in this context. Any DIQKD protocol involves test rounds in which the violation of a Bell inequality is checked. For multipartite DIQKD, however, not all Bell inequalities are suitable. We identify the crucial properties a Bell test requires to be a viable option. In this light, a published protocol for multipartite DIQKD is examined. We establish an incompatibility which is inherent to the proposed Bell test in combination with the required quantum states. This leads us to the conclusion, that the proposed protocol necessarily aborts. In a subsequent work, we develop a family of multipartite Bell inequalities, specifically tailored to the task of DI conference key agreement (CKA). Several features of this Bell inequality are analytically characterized. In addition, we prove its usefulness for the purposes of DICKA. To this end, semidefinite programming techniques are employed and extended to the multipartite scenario which allows us to quantify asymptotic DI conference key rates. In a second part, we explore the challenges of implementing (bipartite) long-distance DIQKD with quantum repeaters. For two repeater protocols, we describe how experimental parameters manifest themselves in DI secret-key rates. In doing so, we shed light on the fundamental differences between the usual and the DI scenario and we benchmark the threshold requirements of quantum devices to make profitable DI secret-key rates feasible. Finally, we develop a general method to describe in a non-DI setting the propagation of an important class of errors through quantum circuits in arbitrary dimensions. With this, we discuss the potential of error-corrected quantum repeaters to overcome fundamental point-to-point limitations.

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Reply to “Comment on ‘Device-independent entanglement-based Bennett 1992 protocol’ ”

Giuseppe Vallone, Giovanni Di Giuseppe, Paolo Mataloni +2 more

Our paper [Phys. Rev. A 86, 032325 (2012)], setting forth a previously unknown connection between the Bennett 1992 protocol and a Bell inequality, has recently received a Comment [Phys. Rev. A 93, 066303 (2016)] about a possible flaw in its potential device-independent (DI) implementation. We point the authors of the Comment to prior works showing that what they assume to be specific to our protocol's DI implementation is actually a standard assumption in DI quantum key distribution. Therefore there is no need to revise any of the conclusions drawn in Lucamarini et al. [Phys. Rev. A 86, 032325 (2012)].

Physical Review A•Jun 27, 2016•0 citations
In this Comment we discuss the device-independent conference key agreement (DICKA) protocol [Phys. Rev. A 97, 022307 (2018)]. We show that the suggested honest implementation fails, because perfectly correlated measurement results and the required Bell-inequality violation cannot be achieved simultaneously, in contradiction to what is claimed. We further show via semidefinite programming that there cannot exist any suitable honest implementation in the tripartite setting, rendering the DICKA protocol incomplete.
Conference key agreement aims to establish shared, private randomness among many separated parties in a network. Device-independent conference key agreement (DICKA) is a variant in which the source and the measurement devices used by each party need not be trusted. So far, DICKA protocols largely fall into two categories: those that rely on violating a joint Bell inequality using genuinely multipartite entangled states and those that concatenate many bipartite protocols. The question of whether a hybrid protocol exists, where a multipartite Bell inequality can be violated using only bipartite entanglement, was asked by Grasselli et al. [Quantum 7, 980 (2023)2521-327X10.22331/q-2023-04-13-980]. We answer this question affirmatively, by constructing an asymptotically secure DICKA protocol achieving the same rate as the concatenation of bipartite device-independent quantum key distribution, yet relying on a single joint Bell violation. Our results prompt further discussion on the benefits of multipartite entanglement for DICKA over its bipartite alternative, and we give an overview of different arguments for near-term devices.
Device-independent conference key agreement (DI-CKA) enables N>2N > 2N>2 distant parties to establish a common secret key without detailed characterization of the source or measurement devices. Many existing protocols rely on full-body Bell correlators, whose measurement can be demanding for a large number of parties. Here, we show that two-body correlators alone suffice for DI-CKA using balanced Dicke states for every even N≥4N \geq 4N≥4. We construct two-body Bell inequalities and derive a tight analytical lower bound on the conditional von Neumann entropy by comparing their Bell operators with those of asymmetric Clauser-Horne-Shimony-Holt inequalities. In the ideal setting, certified key rates can approach 1−h[(N−2)/(4(N−1))]1-h[(N-2)/(4(N-1))]1−h[(N−2)/(4(N−1))], where hhh is the binary entropy function. This rate remains bounded away from zero as N→∞N \rightarrow \inftyN→∞, tending to 1−h(1/4)≈0.1891-h(1/4) \approx 0.1891−h(1/4)≈0.189 bits per key-generation round. We show that the honest implementation of our protocol is invariant under collective phase noise. Moreover, a complementary full-body Bell construction certifies positive conference-key rates from correlations arbitrarily close to the local set.
We present a security analysis of conference key agreement (CKA) in the most adversarial model of device independence (DI). Our protocol can be implemented by any experimental setup that is capable of performing Bell tests [specifically, the Mermin-Ardehali-Belinskii-Klyshko (MABK) inequality], and security can in principle be obtained for any violation of the MABK inequality that detects genuine multipartite entanglement among the NNN parties involved in the protocol. As our main tool, we derive a direct physical connection between the NNN-partite MABK inequality and the Clauser-Horne-Shimony-Holt (CHSH) inequality, showing that certain violations of the MABK inequality correspond to a violation of the CHSH inequality between one of the parties and the other $N\ensuremath{-}1$. We compare the asymptotic key rate for device-independent conference key agreement (DICKA) to the case where the parties use $N\ensuremath{-}1$ device-independent quantum key distribution protocols in order to generate a common key. We show that for some regime of noise the DICKA protocol leads to better rates.
Quantum entanglement is a unique property of quantum particles. It can correlate them in such a way, that further correlations to an additional unwanted party is prevented. This renders quantum theory as the prime candidate to implement a secure distribution of an encryption key. Quantum key distribution (QKD) is dedicated to this task. A striking feature of correlations in a statistical experiment is, that they can, in principle, always be approximated. Even more so, correlations can be obtained without a detailed knowledge about the underlying physical process that generated the experimental data. This no-characterization approach is at the heart of the device-independent (DI) paradigm. Here, Bell inequalities are an imperative tool to detect nonlocal correlations, which are necessary for a DI secret key. One central subject of this thesis is multipartite DIQKD and the implications of Bell nonlocality in this context. Any DIQKD protocol involves test rounds in which the violation of a Bell inequality is checked. For multipartite DIQKD, however, not all Bell inequalities are suitable. We identify the crucial properties a Bell test requires to be a viable option. In this light, a published protocol for multipartite DIQKD is examined. We establish an incompatibility which is inherent to the proposed Bell test in combination with the required quantum states. This leads us to the conclusion, that the proposed protocol necessarily aborts. In a subsequent work, we develop a family of multipartite Bell inequalities, specifically tailored to the task of DI conference key agreement (CKA). Several features of this Bell inequality are analytically characterized. In addition, we prove its usefulness for the purposes of DICKA. To this end, semidefinite programming techniques are employed and extended to the multipartite scenario which allows us to quantify asymptotic DI conference key rates. In a second part, we explore the challenges of implementing (bipartite) long-distance DIQKD with quantum repeaters. For two repeater protocols, we describe how experimental parameters manifest themselves in DI secret-key rates. In doing so, we shed light on the fundamental differences between the usual and the DI scenario and we benchmark the threshold requirements of quantum devices to make profitable DI secret-key rates feasible. Finally, we develop a general method to describe in a non-DI setting the propagation of an important class of errors through quantum circuits in arbitrary dimensions. With this, we discuss the potential of error-corrected quantum repeaters to overcome fundamental point-to-point limitations.
Our paper [Phys. Rev. A 86, 032325 (2012)], setting forth a previously unknown connection between the Bennett 1992 protocol and a Bell inequality, has recently received a Comment [Phys. Rev. A 93, 066303 (2016)] about a possible flaw in its potential device-independent (DI) implementation. We point the authors of the Comment to prior works showing that what they assume to be specific to our protocol's DI implementation is actually a standard assumption in DI quantum key distribution. Therefore there is no need to revise any of the conclusions drawn in Lucamarini et al. [Phys. Rev. A 86, 032325 (2012)].