Multi-photon entangled states play a crucial role in quantum informationapplications such as secure quantum communication, scalable computation, andhigh-precision quantum metrology. Quantum memory for entangled states is a keycomponent of quantum repeaters, which are indispensable in realizing quantumcommunications. Storing a single photon or an entangled photon has beenrealized through different protocols. However, there has been no reportdemonstrating whether a multi-photon state can be stored in any physical systemor not. Here, we report on the experimental storage of a two-photon NOON statein a cold atomic ensemble. Quantum interference measured before and afterstorage clearly shows that the properties of the two-photon NOON state arepreserved during storage. Our experiment completes the first step towardsstoring a multi-photon entangled state.
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