Long-lived and high-fidelity memory for a photonic polarization qubit (PPQ) is crucial for constructing quantum networks. We present a millisecond storage system based on electromagnetically induced transparency, in which a moderate magnetic field is applied on a cold-atom cloud to lift Zeeman degeneracy and, thus, the PPQ states are stored as two magnetic-field-insensitive spin waves. Especially, the influence of magnetic-field-sensitive spin waves on the storage performances is almost totally avoided. The measured average fidelities of the polarization states are 98.6% at 200  μs and 78.4% at 4.5 ms, respectively.

The light-matter quantum interface that can create quantum correlations or entanglement between a photon and one atomic collective excitation is a fundamental building block for a quantum repeater. The intrinsic limit is that the probability of preparing such nonclassical atom-photon correlations has to be kept low in order to suppress multiexcitation. To enhance this probability without introducing multiexcitation errors, a promising scheme is to apply multimode memories to the interface. Significant progress has been made in temporal, spectral, and spatial multiplexing memories, but the enhanced probability for generating the entangled atom-photon pair has not been experimentally realized. Here, by using six spin-wave-photon entanglement sources, a switching network, and feedforward control, we build a multiplexed light-matter interface and then demonstrate a ∼sixfold (∼fourfold) probability increase in generating entangled atom-photon (photon-photon) pairs. The measured compositive Bell parameter for the multiplexed interface is 2.49±0.03 combined with a memory lifetime of up to ∼51  μs.

We report an experiment in which long-lived quantum memories for photonic polarization qubits (PPQs) are controllably released into any one of multiple spatially-separate channels. The PPQs are implemented with an arbitrarily-polarized coherent signal light pulses at the single-photon level and are stored in cold atoms by means of electromagnetic-induced-transparency scheme. Reading laser pulses propagating along the direction at a small angle relative to quantum axis are applied to release the stored PPQs into an output channel. By changing the propagating directions of the read laser beam, we controllably release the retrieved PPQs into 7 different photonic output channels, respectively. At a storage time of δt = 5 μs, the least quantum-process fidelity in 7 different output channels is ~89%. At one of the output channels, the measured maximum quantum-process fidelity for the PPQs is 94.2% at storage time of δt = 0.85 ms. At storage time of 6 ms, the quantum-process fidelity is still beyond the bound of 78% to violate the Bell’s inequality. The demonstrated controllable release of the stored PPQs may extend the capabilities of the quantum information storage technique.

The generation and storage of entangled photons play important roles in quantum information technique. Spontaneous Raman scattering (SRS) in atomic ensembles provides a promising method to generate entangled photons capable of storage. In the past experiments, a spin-wave-photon entangled state is produced via SRS in an atomic ensemble, with which a pair of entangled photons is obtained. Here, we report a scheme of simultaneously generating two spin-wave-photon entangled states in an atomic ensemble by collecting Stokes photons at two different directions. Based on the obtained two atom-photon entangled sources, we generate a three-photon GHZ polarization-entangled state and conditionally prepare a polarization-entangled photon pair, respectively. 2The entangled photon pairs are the crucial resources in linear optical quantum computations (LOQC) and quantum communications (QC) [1][2][3][4][5].However, the probabilistic generations of entangled photons limit their applications in the real world [1,[6][7]. For solving this problem, a promising scheme is to effectively store the entangled photons in atomic or solid-state ensembles for a desired time [1][2][3][4][5].Spontaneous Raman scattering (SRS) in an atomic ensemble can emit a single photon and simultaneously create a single spin-wave excitation [8][9][10][11][12][13][14][15][16][17][18][19][20].The emitted photon can be directly stored in atomic ensembles via EIT with a larger storage efficiency up to 20-50% [11][12][13]. The correlation between the emitted photons and the spin-wave excitations forms the physical fundament of generating the spin-wave-photon (atom-photon) entanglement [3]. In past experiments [13][14][15][16][17], an atom-photon entangled state has been generated from a cold atomic ensemble, with which a pair of entangled photons is obtained.However, in some quantum information protocols, the three-qubit GHZ state[18] and on-demand entanglement source [19] are required, whose preparation relies on the simultaneous generations of two or more entangled photon pairs capable of storages.On the other hand, although the quantum repeater node [17] has been experimentally demonstrated by using two atom-photon entangled states simultaneously generated from two cold atomic ensembles, the storage 2 3 3 lifetime of the entanglement is very short (~ 6 µs), which is not enough for the long-distance quantum communication [17]. For solving this problem, we can firstly produce a pair of polarization-entangled photons in a heralded or conditional manner, and then mapped the entanglement into atomic-ensemble memories via the long-lived (1-ms) EIT quantum storage scheme [20].In this letter, we present an experimental demonstration of the simultaneous generation of two atom-photon entangled states in a cold atomic ensemble via SRS induced by a write laser pulse. In contrast to past experiments [13,[16][17], which achieved the high-fidelity atom-photon entanglement by encoding the photonic qubit in two spatial modes of a single photon, the presented experiment empl...

The storage and retrieval efficiency (SRE) and lifetime of optical quantum memories are two key performance indicators for scaling up quantum information processing. Here, we experimentally demonstrate a cavity-enhanced long-lived optical memory for two polarizations in a cold atomic ensemble. Using electromagnetically induced-transparency (EIT) dynamics, we demonstrate the storages of left-circularly and right-circularly polarized signal light pulses in the atoms, respectively. By making the signal and control beams collinearly pass through the atoms and storing the two polarizations of the signal light as two magnetic-field-insensitive spin waves, we achieve a long-lived (3.5 ms) memory. By placing a low-finesse optical ring cavity around the cold atoms, the coupling between the signal light and the atoms is enhanced, which leads to an increase in SRE. The presented cavity-enhanced storage shows that the SRE is ∼30%, corresponding to an intrinsic SRE of ∼45%.