Networking quantum networks with minimum cost aggregation

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, entangled bits (ebits), across quantum networks. This could be achieved by aggregating quantum repeater protocols. However, the existing protocol is not practical as it requires point-to-point entanglement generation, the first step of the protocol, not only to suppress the error, depending on the whole size of the networks, but also to be run more than necessary. Here we present a practical recipe on how to aggregate quantum networks in order to present ebits to clients with minimum cost. This is combined with a conception of concatenation to enable arbitrary clients to have arbitrary long-distance communication with fixed error across quantum networks, regardless of the overall size. Our recipe forms the basis of designing a quantum internet protocol to control a self-organizing large-scale quantum network.Comment: 7 pages, 3 figure

A versatile design for resonant guided-wave parametric down-conversion sources for quantum repeaters

Quantum repeaters - fundamental building blocks for long-distance quantum communication - are based on the interaction between photons and quantum memories. The photons must fulfil stringent requirements on central frequency, spectral bandwidth and purity in order for this interaction to be efficient. We present a design scheme for monolithically integrated resonant photon-pair sources based on parametric down-conversion in nonlinear waveguides, which facilitate the generation of such photons. We investigate the impact of different design parameters on the performance of our source. The generated photon spectral bandwidths can be varied between several tens of MHz up to around $1\,$GHz, facilitating an efficient coupling to different memories. The central frequency of the generated photons can be coarsely tuned by adjusting the pump frequency, poling period and sample temperature and we identify stability requirements on the pump laser and sample temperature that can be readily fulfilled with off-the-shelve components. We find that our source is capable of generating high-purity photons over a wide range of photon bandwidths. Finally, the PDC emission can be frequency fine-tuned over several GHz by simultaneously adjusting the sample temperature and pump frequency. We conclude our study with demonstrating the adaptability of our source to different quantum memories.Comment: 10 pages, 8 figure

Assisted Entanglement Distillation

Motivated by the problem of designing quantum repeaters, we study entanglement distillation between two parties, Alice and Bob, starting from a mixed state and with the help of "repeater" stations. To treat the case of a single repeater, we extend the notion of entanglement of assistance to arbitrary mixed tripartite states and exhibit a protocol, based on a random coding strategy, for extracting pure entanglement. The rates achievable by this protocol formally resemble those achievable if the repeater station could merge its state to one of Alice and Bob even when such merging is impossible. This rate is provably better than the hashing bound for sufficiently pure tripartite states. We also compare our assisted distillation protocol to a hierarchical strategy consisting of entanglement distillation followed by entanglement swapping. We demonstrate by the use of a simple example that our random measurement strategy outperforms hierarchical distillation strategies when the individual helper stations' states fail to individually factorize into portions associated specifically with Alice and Bob. Finally, we use these results to find achievable rates for the more general scenario, where many spatially separated repeaters help two recipients distill entanglement.Comment: 25 pages, 4 figure