Broadband waveguide quantum memory for entangled photons

TL;DR

This paper demonstrates a broadband, integrated quantum memory in a solid-state device that can reversibly store and transfer entanglement between photons and atomic excitations, advancing quantum communication technology.

Contribution

It introduces the first reversible transfer of photon-photon entanglement into photon-atom entanglement using a lithium niobate waveguide with a 5 GHz spectral acceptance.

Findings

Achieved entanglement preservation after storage and retrieval

Increased spectral acceptance from 100 MHz to 5 GHz

Demonstrated perfect mapping within statistical error

Abstract

The reversible transfer of quantum states of light in and out of matter constitutes an important building block for future applications of quantum communication: it allows synchronizing quantum information, and enables one to build quantum repeaters and quantum networks. Much effort has been devoted worldwide over the past years to develop memories suitable for the storage of quantum states. Of central importance to this task is the preservation of entanglement, a quantum mechanical phenomenon whose counter intuitive properties have occupied philosophers, physicists and computer scientists since the early days of quantum physics. Here we report, for the first time, the reversible transfer of photon-photon entanglement into entanglement between a photon and collective atomic excitation in a solid-state device. Towards this end, we employ a thulium-doped lithium niobate waveguide in conjunction with a photon-echo quantum memory protocol, and increase the spectral acceptance from the current maximum of 100 MHz to 5 GHz. The entanglement-preserving nature of our storage device is assessed by comparing the amount of entanglement contained in the detected photon pairs before and after the reversible transfer, showing, within statistical error, a perfect mapping process. Our integrated, broadband quantum memory complements the family of robust, integrated lithium niobate devices. It renders frequency matching of light with matter interfaces in advanced applications of quantum communication trivial and institutes several key properties in the quest to unleash the full potential of quantum communication.