Device-independent quantum key distribution (DI-QKD) enables two remote parties to share an information-theoretically secure key without any assumptions on the inner workings of the devices used. Device-independent conference key agreement (DI-CKA) is multipartite DI-QKD in which more than two parties share a common secure key. The performance of DI-CKA, however, is strictly limited because of its susceptibility to losses due to, e.g., imperfect detection efficiency and channel transmission. Here, we propose a DI-CKA protocol which reduces this limitation by using a heralding scheme to distribute multipartite entanglement. We analyze key rates of our protocol for two different measurement scenarios, and we show that our protocol outperforms a previous DI-CKA protocol even with an experimentally feasible measurement.
Multipartite device-independent quantum key distribution (DI-QKD), also known as device-independent conference key agreement, enables more than two remote parties to share a common key with information-theoretic security even without trusting the devices. So far, several multipartite DI-QKD protocols have been proposed where Greenberger-Horne-Zeilinger (GHZ) states are used as multipartite entanglement. A natural question is then whether one can construct multipartite DI-QKD with the other type of multipartite entanglement. W state is of particular interest since it is intrinsically different from GHZ state and in some cases, easier to optically implement. In this paper, we show that multipartite DI-QKD is possible with W states. To this end, we construct Bell inequalities largely violated by W states, which can be used for the multipartite DI-QKD. Furthermore, we consider several different implementation scenarios. First, we analyze the minimum required detection efficiencies to extract finite amount of keys. Then we propose a long-distance multipartite DI-QKD protocol with single-photon interference and make detailed analyses with several physical implementation scenarios. We show that the protocol enables secret key distribution over longer distances than the existing multipartite DI-QKD protocols based on GHZ states. This study provides new insight about the relationship between multipartite entanglement and device-independent quantum information processing as well as opens an alternative path toward long-distance multipartite DI-QKD.
Device-independent conference key agreement (DI-CKA) lets a group of parties establish a shared secret key from untrusted quantum devices, with security certified by non-locality. Existing DI-CKA protocols are each built around a single Bell inequality, typically a multiparty variant of the CHSH game. DI-QKD protocols, in contrast, have been built from a much richer landscape of non-local games, and it has remained unclear how to carry this landscape over to the conference setting. We introduce $\textit{Parity-$G$ games}$, which extend any two-player game
G to
N players, for every
N, provided
G has an optimal strategy in which one player measures Pauli observables. The extension preserves the quantum and classical values of
G, and the security of the resulting
N-party protocol follows from an analysis of the two-player game alone. Our framework recovers the Parity-CHSH game of Ribeiro, Murta and Wehner (Phys. Rev. A, 2018) as a special case. Applied to the Mermin--Peres Magic Square Game, it yields a new
N-player pseudo-telepathy game, the
Parity Magic Square Game, which ideal devices win in every round. We use it to construct the
first DI-CKA protocol based on a pseudo-telepathy game. We prove the protocol secure against coherent attacks. It produces up to two key bits per round, and at low noise its key rate exceeds that of the DI-CKA protocol based on the Parity-CHSH game.
Device-independent quantum key distribution (DI-QKD) enables information-theoretically secure key exchange between remote parties without any assumptions on the internal workings of the devices used for its implementation. However, its practical deployment remains severely constrained by the need for loophole-free Bell inequality violations, which are highly susceptible to losses and detection efficiencies. In this paper, we propose two long-distance DI-QKD protocols based on a heralding scheme using single-photon interference. Our protocols consist of only standard quantum optics tools such as two-mode squeezed states, displacement operations, and on-off detectors, making them experimentally accessible. To further enhance robustness against realistic imperfections, we integrate a classical noisy preprocessing technique during post-processing. We calculate key rates of the protocols by numerical optimization and show the supremacy of this implementation over existing protocols in terms of communication distances.
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≥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→∞, tending to
1−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.
Abstract Conference key agreement (CKA), or multipartite key distribution, is a cryptographic task where more than two parties wish to establish a common secret key. A composition of bipartite quantum key distribution protocols can accomplish this task. However, the existence of multipartite quantum correlations allows for new and potentially more efficient protocols, to be applied in future quantum networks. Here, the existing quantum CKA protocols based on multipartite entanglement are reviewed, both in the device‐dependent and the device‐independent scenario.
Abstract Quantum key distribution (QKD) is a technique enabling provable secure communication but faces challenges in device characterisation, posing potential security risks. Device-independent (DI) QKD protocols overcome this issue by making minimal device assumptions but are limited in distance because they require high detection efficiencies, which refer to the ability of the experimental setup to detect quantum states. It is thus desirable to find QKD protocols that are based on realistic assumptions on the devices as well as implementable over long distances. In this work, we consider a one-sided DI QKD scheme with two measurements per party and show that it is secure against coherent attacks up to detection efficiencies greater than 50.1% specifically on the untrusted side. This is almost the theoretical limit achievable for protocols with two untrusted measurements. Interestingly, we also show that, by placing the source of states close to the untrusted side, our protocol is secure over distances comparable to standard QKD protocols.