nonlinear photonic crystal. The state is favorable for existing Gaussian entanglement distillation schemes, since local squeezing operations can enhance the final entanglement and the success probability. The crystal is designed for enabling three concurrent quasi-phase-matching parametric-down conversions, and hence relieves the auxiliary on-line bi-side local squeezing operations. The compact source opens up a way for continuous-variable quantum technologies and could find more potential applications in future large-scale quantum networks.
In this article we obtain results for the task of converting a single
N-qubit
W-class state (of the form $\sqrt{{x}_{0}}|00...0\ensuremath{\rangle}+\sqrt{{x}_{1}}|10...0\ensuremath{\rangle}+\ensuremath{\cdots}+\sqrt{{x}_{N}}|00...1\ensuremath{\rangle}$) into maximum entanglement shared between two random parties. Previous studies in random distillation have not considered how the particular choice of target pairs affects the transformation, and here we develop a strategy for distilling into general configurations of target pairs. We completely solve the problem of determining the optimal distillation probability for all three-qubit configurations and most four-qubit configurations when
x0=0. Our proof involves deriving new entanglement monotones defined on the set of four-qubit
W-class states. As an additional application of our results, we present new upper bounds for converting a generic
W-class state into the standard
W state $|{W}_{N}\ensuremath{\rangle}=\sqrt{\frac{1}{N}}(|10...0\ensuremath{\rangle}+\ensuremath{\cdots}+|00...1\ensuremath{\rangle})$.
Quantum repeaters are essential to realizing long-range entanglement distribution networks. To achieve enhanced rates of high-fidelity entanglement distribution, we investigate how entanglement distillation can be used on trapped-ion-based quantum repeater networks. Entanglement distillation is the process of distilling from a large number of copies of low-fidelity entangled qubits a fewer number of copies of higher-fidelity entangled qubits. It has been shown that quantum error-correcting codes (QECCs) can be used to devise protocols for entanglement distillation. In this paper, we consider entanglement distillation based on three lifted-product (LP) quantum low density parity check (QLPDC) codes ([[544, 80, 12], [[714, 100, 16]], and [20, 10, 136, 20]) on trapped-ion repeater networks with spatial and time multiplexing over various total distances with different inter-repeater spacing to calculate the end-to-end entanglement rates that they enable. The reported rates assume entanglement over each elementary link succeeds synchronously and do not assume any constraint on the number of ions present in each trap. We furthermore assume that distillation occurs at every elementary link and that the entanglement swaps are ideal. Our findings can be considered as groundwork for implementing more efficient distillation and communication protocols on trapped ion networks.