# Quantum Optics and Quantum Information - Quantum Memory

**Quantum Memory
in Cold Atomic Ensemble**

**Deterministic
and Storable Single-Photons from Cold Atomic ensemble**

A single-photon source is realized with a cold atomic ensemble (Rb 87 atoms). A single excitation,written in an atomic quantum memory by Raman scattering of a laser pulse, is retrieved deterministically as a single photon at a predetermined time. It is shown that the production rate of single photons can be enhanced considerably by a feedback circuit while the single-photon quality is conserved. Such a singlephoton source is well suited for future large-scale realization of quantum communication and linear optical quantum computation. Details are in the reference below.

Reference:

Deterministic
and Storable Single-Photon Source Based on a Quantum Memory

Phys. Rev. Lett. 97, 173004 (2006)

**Entanglement
of independent single photons**

We have created independent, synchronized single-photon sources with built-in quantum memory based on two remote cold atomic ensembles. The synchronized single photons are used to demonstrate efficient generation of entanglement. The resulting entangled photon pairs violate a Bell's inequality by 5 standard deviations. Our synchronized single photons with their long coherence time of 25 ns and the efficient creation of entanglement serve as an ideal building block for scalable linear optical quantum information processing. Details are in the reference below.

Reference:

Synchronized Independent Narrow-Band Single Photons and Efficient Generation
of Photonic Entanglement

Phys. Rev. Lett. 98, 180503 (2007)

Hong-Ou-Mandel dip in time and frequency domain respectively

**Quantum connection
of two remote atomic ensembles: torwards a quantum repeater**

We have demonstrated a novel way to efficiently and very robust create an entanglement between an atomic and a photonic qubit. A single laser beam is used to excite one atomic ensemble and two different spatial modes of scattered Raman fields are collected to generate the atom-photon entanglement. With the help of build-in quantum memory, the entanglement still exists after 20.5 micro second storage time which is further proved by the violation of CHSH type Bell's inequality. Our entanglement procedure is the building block for a novel robust quantum repeater architecture [Zhao et al, Phys. Rev. Lett. 98, 240502 (2007)]. Our approach can be easily extended to generate high dimensional atom-photon entanglements. Details are in the reference below.

References:

A Robust Atom-Photon Entanglement Source for Quantum Repeaters, arXiv:0706.2327.

Memory-built-in quantum teleportation with photonic and atomic qubits, arXiv:0705.1256.

**Long Lifetime
of Spin Wave**

We are working on optical dipole trap and operating the atoms at clock state