Makoto Ishihara

Device-independent conference key agreement (DI-CKA) enables secure multipartite communication without relying on the internal structure or reliability of the devices used. However, realizing DI-CKA protocols is extremely challenging because observing loophole-free Bell violations demands exceptionally high detection efficiency. In this study, we incorporate noisy preprocessing—a technique that deliberately introduces noise into raw key bits to reduce an eavesdropper's information—into a DI-CKA protocol based on the Holz inequality to relax the required detection efficiency. We evaluate the performance of the protocol by calculating a lower bound on the key rate using a numerical optimization method, alongside an analytical upper bound derived from a specific eavesdropping attack. Our results demonstrate that the introduction of noisy preprocessing successfully relaxes the required detection efficiency even in the Holz-type DI-CKA. Furthermore, we find that the analytical upper bounds coincide with the numerical lower bounds with very high precision, demonstrating the tightness of the evaluated key rates.

Quantum network sensing has a potential to enhance the estimation precision of multiple distributed parameters. In this context, several papers recently considered privacy of local parameters of such sensing. However, it is known that some kinds of noises such as bit-flip noise affect the privacy. In this work, we propose a private quantum network sensing scheme using an efficient photon-number GHZ state distribution protocol for the linear combination of local parameters in a lossy network. We show that our protocol using the photon-number GHZ states satisfies the privacy condition in the noisy distribution environment.

Device-Independent quantum key distribution (DI-QKD) enables the distribution of secret keys over an untrusted network with uncharacterized devices1, whose security is guaranteed by certification of quantum correlations between remote, legitimate parties through violation of Bell inequalities2. However, implementations of DI-QKD protocols in practice are impeded by the detection loophole, imposing stringent detection efficiency thresholds, preventing practical realizations of DI-QKD. To overcome this limitation, the novel concept of routed Bell tests was recently introduced3,4,5. Here, we propose a DI-QKD protocol based on the routed Bell tests with only standard quantum optical tools, namely two-mode squeezed states, displacement-based measurements and on/off detectors. Fig. 1(a) illustrates this in more detail. Two honest, distant parties, Alice and Bob, each receive one mode of a two-mode squeezed state, and perform displacement-operations, D(α) and D(β_L ), on their received mode and detect it with an on/off detector with detection efficiencies η_A and η_(B_L ), where Alice has her input choices x∈{0,1}, and Bob has his input choices y∈{0,1,2}, obtaining classical outputs a,b∈{0,1}. In addition, Bob can route his mode via a switch with input z∈{S,L} towards another displacement-based measurement device with displacement operation D(β_S ), with input choices and classical outputs denoted by y ̂∈{0,1} and b ̂∈{0,1} respectively, and detection efficiency η_(B_S ), where η_(B_S )≥η_(B_L ). It is crucial that Bob’s routing choice z should not have an influence on Alice’s measurement input and outcomes. We denote (x,y,z)=(0,2,L) as key generation rounds where some rounds are used for estimating error correction cost, and others to construct their keys, and all other input combinations are used to certify their correlations. We optimize for Alice and Bob’s displacement operation D(α),D(β_S ) and D(β_L) and compute the lower bounds on the key rate using numerical optimization6, setting η_A=η_(B_L ). In Fig. 1(b), we observe that our protocol allows us to relax the detection efficiency requirements and see improved key rates against an unrouted protocol, facilitating the possibility of realizing long-distance DI-QKD in the future.

Device-independent conference key agreement (DI-CKA) realizes information-theoretically secure key distribution among more than two remote parties without any assumptions on the inner workings of the devices, relying instead on the violation of Bell inequalities. While several DI-CKA protocols based on Greenberger-Horne-Zeilinger states have been proposed, it remains an open question whether W states can also be used for DI-CKA. In this study, we affirmatively answer this open question by constructing Bell inequalities that are maximally violated by W states.

We propose a long-distance device-independent conference key agreement (DI-CKA) protocol. We use an efficient GHZ state distribution protocol based on entanglement swapping. We calculate a key rate of our protocol from violation of a multipartite Bell inequality and show that our protocol can distribute a secret key over longer distance than a direct transmission DI-CKA protocol. We also consider practical displacement-based measurement and show experimental feasibility of our protocol.