Koji Azuma

A quantum internet holds promise for achieving distributed quantum sensing and large-scale quantum computer networks, as well as quantum communication among arbitrary clients all over the globe. The main building block is efficient distribution of entanglement,entangled bits (ebits), between arbitrary clients in a quantum network with fixed error, irrespective of their distance. In practice, this should be accomplished across multiple quantum networks, analogously to what the current Internet does in conventional communication. Here we present a practical recipe on how to give arbitrary clients ebits with fixed error efficiently, regardless of their distance, across multiple quantum networks. This recipe is composed of two new concepts, minimum cost aggregation and network concatenation. Our recipe forms the basis of designing a quantum internet protocol for networking self-organizing quantum networks to make a global-scale quantum internet.

Time-bin encoding is more favorable in fiber-based implementation of quantum key distribution (QKD) than polarization encoding as it avoids issues inherent for polarization encoding, such as birefringence, caused by optical fibers. QKD only with passive devices is desirable to prevent side-channel attacks possible in the case of use of active devices such as modulators. The Bennett-Brassard 1984 (BB84) protocol is a strong candidate for an implementation with satisfying these; it can be implemented using time bins with a passive delayed interferometer that inevitably generates "satellite time bins", two pulses outside the phase-interference timing. Although time-bin encoding BB84 has been frequently demonstrated, there is no consensus whether satellite time bins can be used to extract a key. Besides, there is no security proof for either case. Here, we prove the security of time-bin encoding BB84 protocol with a passive delayed interferometer and threshold detectors. If satellite time bins are used for key generation, we show that an additional operation is necessary for security. The result is not limited only to BB84 but can be applied to Bennett-Brassard-Mermin 1992 and quantum conference key agreement based on time bins.

Linear-optical entanglement swapping works only probabilistically. Quantum repeater protocols based on it, classified in the first generation, inevitably need classical communication between non-adjacent nodes, which makes the protocols very slow and requires quantum memories with long coherence time. One way to realize faster quantum repeater protocols, classified in the second generation, is to use matter qubits, which enables deterministic entanglement swapping. However, such matter qubits are still experimentally challenging. Here we propose a linear-optical circuit that projects two input qudits of dimension d onto a Bell state defined in a two-qubit subspace with the probability of 1 − d−1, which can be used to realize almost deterministic entanglement swapping for large d. Based on it, we propose a quantum repeater protocol consisting of linear optical elements, photon detectors, high-dimensional quantum memories, and photonic three-qudit GHZ states. In the protocol, the classical communication between non-adjacent nodes becomes unnecessary thanks to the high success probability of the entanglement swapping, making the protocol be categorized into the second generation.