In the quest to realize a scalable quantum network, semiconductor quantum dots (QDs) offer distinct advantages, including high single-photon efficiency and indistinguishability, high repetition rate (tens of gigahertz with Purcell enhancement), interconnectivity with spin qubits, and a scalable on-chip platform. However, in the past two decades, the visibility of quantum interference between independent QDs rarely went beyond the classical limit of 50%, and the distances were limited from a few meters to kilometers. Here, we report quantum interference between two single photons from independent QDs separated by a 302 km optical fiber. The single photons are generated from resonantly driven single QDs deterministically coupled to microcavities. Quantum frequency conversions are used to eliminate the QD inhomogeneity and shift the emission wavelength to the telecommunication band. The observed interference visibility is 0.67 ± 0.02 (0.93 ± 0.04) without (with) temporal filtering. Feasible improvements can further extend the distance to ∼600 km. Our work represents a key step to long-distance solid-state quantum networks.
Quantum walks are a well-known powerful technique to perform quantum search algorithms, quantum simulations, and universal quantum computation. They have been extensively explored in the optical regime. In our work we have realized an 8x8 two-dimensional square superconducting qubit array with 62 functional qubits. We have used this processor to demonstrate high fidelity multi-particle quantum walks. The programmability of our processor also allows us to implement a Mach-Zehnder interferometer where quantum walkers can coherently traverse both paths of the interferometer before interfering and exiting it. Our work shows an alternate approach for information processing on these NISQ processors.
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