If a world-wide quantum network is established only with optical devices, it leads to a cost-efficient high-speed quantum internet in the future. It is natural to imagine that such an all-optical network is composed of various protocols specialized in intracity, intercity and intercontinental quantum communication. Here I will talk about recent rapid progress on this kind of all-photonic approach towards the quantum internet.
A quantum internet is regarded as the holy grail of quantum information processing, enabling the deployment of a broad range of quantum technologies/protocols on a global scale. However, a quantum version of the current Internet Protocol (to make a network of networks) is missing, despite the necessity to control large-scale self-organizing quantum networks. In this talk, we present a practical recipe to give entangled bits (ebits) efficiently to arbitrary two points in a given quantum network with arbitrary topology, by networking its subnetworks with minimum cost. This recipe forms the basis of designing and controlling a global-scale quantum internet.
A quantum internet holds promise for accomplishing 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 over a quantum network. This could be achieved by aggregating quantum repeater protocols. However, the existing protocol requires point-to-point entanglement generation not only to suppress the error, depending on the size of the whole network, but also to be run more than necessary. Here we present an aggregated quantum repeater protocol which works with minimum cost. We also introduce the concept of concatenation of it to achieve arbitrary long-distance communication with fixed error over the network, independently of its size.
Entanglement is a fundamental resource not only for quantum communication but also for distributed quantum computation. Especially, entanglement including only one type of error is favorable, compared with one including multiple types of noise. In this talk, we consider protocol that presents single-error-type entanglement for distant qubits via coherent-state transmission over lossy channels. This protocol is considered to be a subroutine to serve entanglement for larger protocol to yield a final output, such as ebits or pbits. A protocol based on remote non-destructive parity measurement (RNPM) [K. Azuma, H. Takeda, M. Koashi, and N. Imoto, Phys. Rev. A 85, 062309 (2012)] is identified as a subroutine which achieves the global optimal for typical yield functions monotonically non-decreasing with respect to the singlet fraction, such as an arbitrary convex function of a singlet fraction and two-way distillable entanglement/key.
The private capacity of a lossy bosonic channel shows that there remains not much room to improve known point-to-point quantum key distribution (QKD) protocols further, in terms of the key rate versus distance. The current question in our field is how to overcome this fundamental limit with the help of a single station in the middle between communicators. In this talk, we explore recent trials to achieve this goal, such as adaptive measurement-device-independent QKD and twin-field QKD. These trials are good milestones towards the realization of quantum repeater networks.
A quantum internet holds promise for achieving communication tasks that seem to be intractable by the current internet. The required functionality of a physical layer for such a quantum internet is to distribute entanglement efficiently to clients over a quantum network. A fundamental building block to design such efficient distribution of entanglement is to bound capacities of such quantum internet protocols. In this talk, we present a set of efficient linear programs to bound quantum/private capacities of quantum internet protocols, as well as their analytic upper/lower bounds. Our linear program is applied to bipartite cases, multi-pair cases, and a multi-partite case, covering almost all known situations.
It has long been known that quantum networks will enable a whole new range of communication tasks to be undertaken. The simplest is quantum key distribution (QKD) and are commercially available but currently only operate securely over distances around 100 km. A significant advance has been the development of mdiQKD, a scheme where Alice and Bob send one photon at a time to an intermediate node where a Bell measurement is performed. This Bell measurement can only succeed when both Alice and Bob photons arrive at the same time and so the key rate is limited by the exponential losses in both fibres. It limits the practical distance keys can be generated to less than 400km. Spatial or temporal multiplexing is a natural solution to this where one stores the photons that independently arrive from Alice and Bob. Only when the immediate node has both does it perform the Bell measurement. This means we are effectively only limited by fibres losses in one half of the channel. It however means one requires quantum memories at this immediate node, a technically challenging feat and one that changes the general resources used in QKD schemes. In our spatial multiplexed approach, we propose the use of an “all photonic non-destructive measurement (QND)” to herald whether the photon has arrived successfully from either Alice or Bob. Optical switches can them be used to route these photons to the Bell measurement, meaning that we are only limited by the channel loss between either Alice and the immediate node or Bob and the intermediate node, but not both. Further this can achieved without the use of quantum memories at all. Only optical switches, single-photon sources, photon detectors, and passive feed-forward techniques are required. Our approach can be applied naturally to entanglement distribution and so has applications beyond QKD.
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