Efficiency of Quantum Memory Gets a Boost

Quantum memory efficiency, an essential component for quantum computers, is the focus of joint research from the Hong Kong University of Science and Technology (HKUST), South China Normal University (SCNU), and Nanjing University. The research team said it has found a way to boost the efficiency of photonic quantum memories to over 85% with a fidelity of over 99%.

Photonic quantum memories allow for the storage and retrieval of flying single-photon quantum states. However, production of such highly efficient quantum memories remains a challenge as it requires a perfectly matched photon-matter quantum interface. Meanwhile, the energy of a single photon is too weak and can be easily lost in the noise of stray light. Until now, these challenges have kept quantum memory efficiencies to below 50%, which is considered the threshold value for practical applications.

Experimental setup and energy level scheme of the single-photon quantum memory. Courtesy of the Hong Kong University of Science and Technology.

To create a more efficient quantum memory, the team trapped billions of rubidium atoms in a tiny space and cooled the atoms to nearly absolute zero (about 0.00001 K) using lasers and a magnetic field. The team also found a way to distinguish a single photon from the noise of background light.

The researchers said that using their approach, for a single-channel quantum memory, the optimized efficiency for storing and retrieving single-photon temporal waveforms could be as high as 90.6%. This result pushes the photonic quantum memory closer to practical applications in quantum information processing. The researchers said that more efficient quantum memory could also be used as repeaters in a quantum network, laying the foundation for a quantum-based internet.

“In this work, we code a flying qubit onto the polarization of a single photon and store it into the laser-cooled atoms,” said professor Shengwang Du. “Although the quantum memory demonstrated in this work is only for one qubit operation, it opens the possibility for emerging quantum technology and engineering in the future.”

The research was published in Nature Photonics (https://doi.org/10.1038/s41566-019-0368-8).