Amillisecond quantummemory for scalable quantum networks

Scalable quantum-information processing requires the capability of storing quantum states1,2. In particular, a longlived storable and retrievable quantum memory for single excitations is of key importance to long-distance quantum communication with atomic ensembles and linear optics3–7. Although atomic memories for classical light8 and continuous variables9 have been demonstrated with millisecond storage time, lifetimes of only around 10μs have been reported for quantum memories storing single excitations10–13. Here we present an experimental investigation into extending the storage time of quantum memory for single excitations. We identify and isolate distinct mechanisms responsible for the decoherence of spin waves in atomic-ensemble-based quantum memories. By exploiting magnetic-field-insensitive states— so-called clock states—and generating a long-wavelength spin wave to suppress dephasing, we succeed in extending the storage time of the quantum memory to 1 ms. Our result represents an important advance towards long-distance quantum communication and should provide a realistic approach to large-scale quantum information processing. The quantum repeater with atomic ensembles and linear optics has attracted broad interest in recent years, as it holds promise to implement long-distance quantum communication and the distribution of entanglement over quantumnetworks. Following the protocol proposed in ref. 3 and the subsequent improved schemes4–7, significant experimental progress has been accomplished, including the coherent manipulation of the stored excitation in one10,11 or two14–16 atomic ensembles, the demonstration of memory-built-in quantum teleportation17 and the realization of a building block of the quantum repeater13,18. In these experiments, the atomic ensembles serve as the storable and retrievable quantum memory for single excitations. Despite the advances achieved in manipulating atomic ensembles, long-distance quantum communication with atomic ensembles remains challenging owing to the short storage time of the quantummemory for single excitations. For example, for direct generation of entanglement between twomemory qubits over a few hundred kilometres, we need a memory with a storage time of a few hundredmicroseconds. However, the longest storage time reported so far is of the order of only 10 μs (refs 10–13). It has long been believed that the short coherence time is mainly caused by the residual magnetic field19,20. Thereby, storing the collective state in the superposition of the first-order magnetic-field-insensitive states21, that is, the ‘clock states’, is suggested to inhibit this decoherence mechanism19. A numerical