Pan's team makes breakthrough in long-range solid-state quantum networks from 1 to 300 km

Recently, Jianwei Pan, Chaoyang Lu, and Qiang Zhang of the University of Science and Technology of China (USTC) and collaborators achieved high-visibility quantum interference between two independent quantum dots (QDs) connected by more than 300 km of optical fiber. The paper was published in Advanced Photonics [1] on December 27.

Quantum dots are a promising technology for single-photon sources, but in the past two decades, the visibility of quantum interference between independent quantum dots has rarely exceeded the classical limit of 50% and the distance has been limited to a few meters or kilometers. Therefore, this work is a critical step for long-range solid-state quantum networks.

01The key technology for quantum networks: high-performance quantum light sources

This year's Nobel Prize in Physics was awarded for the important research on quantum entanglement and its potential application in the "second quantum revolution": when we can manipulate quantum mechanics, a new era of quantum superposition and entanglement will be ushered in, and a large-scale and fully functional quantum network will open up a new field of physics, offering new possibilities for quantum computing, communications, and metrology.

One of the most important challenges here is to extend the distance of quantum communication to a practically useful scale. Unlike classical signals that can be amplified without noise, superimposed quantum states cannot be amplified because they cannot be cloned perfectly. Therefore, a high-performance quantum network requires not only ultra-low-loss quantum channels and quantum memories, but also high-performance quantum light sources.

What are the requirements of a single photon source for quantum network applications? First, it should emit one (and only one) photon at a time; second, to achieve brightness, the single-photon source should have high system efficiency and a high repetition rate; and third, for applications such as quantum transport that require interference with independent photons, the single photon should be indistinguishable. Other requirements include a scalable platform, a tunable narrow-band linewidth (which facilitates time synchronization), and interconnectivity with matter quantum bits.

One source with other single-photon sources is quantum dots (QDs), which are semiconductor particles of only a few nanometers. However, in the last two decades, the visibility of quantum interference between independent QDs rarely exceeds the classical limit of 50% and distances are limited to a few meters or kilometers or so.

02Achieving fiber-connected, high-visibility quantum interference

On December 27, Pan's team achieved high-visibility quantum interference between two independent QDs, connected by a fiber of about 300 km. They report efficient and undifferentiated single-photon sources with ultra-low noise, tunable single-photon frequency conversion, and long fiber transmission with low dispersion.

Quantum interference between two solid-state QD single-photon sources.

Quantum interference experimental configuration between two independent solid-state QD single-photon sources separated by a 302 km fiber.

The single photons are generated from resonantly driven single QDs that are deterministically coupled to a microcavity. Quantum frequency conversion was used to remove inhomogeneities in the QDs and shift the emission wavelength to the telecom band, with observed interference visibility of up to 93%. According to author Lu Chaoyang, a professor at the Chinese University of Science and Technology [2], "feasible improvements could further extend the distance to about 600 km."

Professor Lu said, "Our work opens up exciting prospects for solid-state quantum networks by extending the scale of previous QD-based quantum experiments from about 1 km to 300 km, two orders of magnitude larger."

With this technological advancement, solid-state quantum networks are poised for a new breakthrough.

Reference links:

[1]https://www.spiedigitallibrary.org/journals/advanced-photonics/volume-4/issue-06/066003/Quantum-interference-with-independent-single-photon-sources-over-300km-fiber/10.1117/1.AP.4.6.066003.full?SSO=1

[2]https://phys.org/news/2022-12-high-visibility-quantum-independent-semiconductor-dots.html