Interfacing a two-photon NOON state with an atomic quantum memory

Multiphoton entangled states play a crucial role in quantum information applications such as secure quantum communication, scalable computation, and high-precision quantum metrology. Quantum memory for entangled states is a key component of quantum repeaters, which are indispensable in realizing quantum communications. Storing a single photon or an entangled photon has been realized through different protocols. However, there has been no report demonstrating whether or not a multiphoton state can be stored in any physical system. Here, we report on the experimental storage of a two-photon NOON state in a cold-atomic ensemble. Quantum interference measured before and after storage clearly shows that the properties of the two-photon NOON state are preserved during storage. Our experiment completes a step towards storing a multiphoton entangled state.