This year’s Nobel Prize in Physics celebrated the fundamental interest in quantum entanglement and also imagined the possible applications in “the second quantum revolution,” a new era in which we can manipulate the weirdness of quantum mechanics, including quantum superposition and the entanglement. A fully functional, large-scale quantum network is the holy grail of quantum data science. It will open a new frontier of physics, with new possibilities for quantum computing, communication and metrology.
One of the most important challenges is to extend the distance of quantum communication to a practically useful scale. Unlike classical signals that can be amplified without noise, superposition quantum states cannot be amplified because they cannot be perfectly cloned. Therefore, a high-performance quantum network requires not only ultra-low-loss quantum channels and quantum memory, but also high-performance quantum light sources. There has been exciting recent progress in satellite-based quantum communications and quantum repeaters, but the lack of suitable single photon sources has hampered progress.
What is required of a single photon source for quantum network applications? First, you have to emit one (only one) photon at a time. Second, to achieve brightness, single-photon sources must have high system efficiency and high repetition rate. Third, for applications like quantum teleportation that require interference with independent photons, the individual photons should be indistinguishable. Additional requirements include a scalable platform, narrowband and tunable linewidth (favorable for time synchronization), and interconnectivity with matter qubits.
One promising source is quantum dots (QDs), semiconductor particles a few nanometers in size. However, in the last two decades, the visibility of quantum interference between independent QDs has rarely exceeded the classical limit of 50%, and distances have been limited to a few meters or kilometers.
As reported in advanced photonics, an international team of researchers has achieved high visibility quantum interference between two independent QDs connected with ~300 km fiber optics. They report efficient, indistinguishable single-photon sources with ultra-low-noise, tunable single-photon frequency conversion and low-dispersion long-fiber transmission. Single photons are generated from individual resonance-driven QDs deterministically coupled to microcavities. Quantum frequency conversions are used to remove QD inhomogeneity and shift the emission wavelength to the telecommunication band. The visibility of the observed interference is up to 93%. According to lead author Chao-Yang Lu, a professor at the University of Science and Technology of China (USTC), “feasible improvements may further extend the distance to ~600 km.”
Lu comments: “Our work jumped from previous QD-based quantum experiments at the scale of ~1 km to 300 km, two orders of magnitude larger, and thus opens up an exciting prospect of state-of-the-art quantum networks.” solid”. With this reported leap, the dawn of solid-state quantum networks could soon begin to dawn into the day.