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Quantum Electronics and Laser Science Conference

Entanglement and communication via teleportation between multi-atom quantum memory cells

Eugene Simon Polzik, Brian Julsgaard

Published Nov 13, 2002• 0 citations
Publisher

Abstract

Summary form only given. A method for generating entanglement between atomic samples necessary for teleportation protocols using this interaction has been recently proposed. We have implemented this idea experimentally using glass cells filled with Cs at room temperature. The cells are coated with paraffin from inside providing the life time of the ground state spin polarization up to 30 milliseconds. About 10/sup 10/ Cs atoms in each of the two cells are spin polarized.

DOI
10.1109/qels.2001.962043

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Summary form only given. A method for generating entanglement between atomic samples necessary for teleportation protocols using this interaction has been recently proposed. We have implemented this idea experimentally using glass cells filled with Cs at room temperature. The cells are coated with paraffin from inside providing the life time of the ground state spin polarization up to 30 milliseconds. About 10/sup 10/ Cs atoms in each of the two cells are spin polarized.
Summary form only given. A new approach to the problem of the quantum interface between light and atoms has been recently developed. The method utilizes free space dispersive interaction between pulses of light and spin polarized atomic ensembles. Entanglement between the polarization state of light and the collective spin state of atoms is established by measurement, more precisely by detection of light in a certain polarization basis. In the first demonstration of this approach we have generated a long-lived entangled state of two separate macroscopic atomic samples by a polarization measurement on light transmitted through the samples. We then have shown that this approach also works for mapping of a quantum state of light onto long-lived atomic spin state paving the road towards realization of the quantum memory for light. Recently we have also extended the free space light-atoms interface beyond the continuous variables in the proposal, where a coherent superposition "cat" state of an atomic ensemble is generated by a measurement sequence. Progress with other communication protocols such as atomic state teleportation and multiparty networks will be presented.
Summary form only given. The spin polarized ground state of a cesium gas in a paraffin coated glass cell at room temperature has been recently used for generation of a long lived entangled state of atoms. In this paper we report on the first results towards using this medium as a quantum memory for polarization states of a light beam. The possibility of a reversible transfer of polarization states between light and atoms via interspecies teleportation protocols has been previously proposed. A narrowband tunable source of entangled light needed for such a transfer has been constructed before. We demonstrate that our set-up allows detection of the quantum state of light propagating through an atomic sample by means of observing the state of the long lived collective spin of the sample. For this purpose a polarization squeezed optical probe is prepared by mixing orthogonally polarized squeezed vacuum and coherent light with a variable phase on a polarizing beamsplitter.
Abstract In this thesis, different experiments on spin squeezing and entanglement involving room temperature ensembles of Cesium atoms are described. The key method is the off-resonant Faraday interaction of spin-polarized atomic ensemble with a light field. And the key component is the micro-fabricated vapor cell coupled into an optical cavity. Quantum backaction evading measurement of one quadrature of collective spin components by stroboscopically modulating the intensity of probe beam at twice Larmor frequency is used to generate the spin-squeezed state. A projection noise limited optical magnetometer at room temperature is reported. Furthermore, using spin-squeezing of atomic ensemble, the sensitivity of magnetometer is improved. Deterministic continuous variable teleportation between two distant atomic ensembles is demonstrated. The fidelity of teleportating dynamically changing sequence of spin states surpasses a classical benchmark, demonstrating the true quantum teleportation.
Quantum communication between two remote locations often involves remote parties sharing an entangled quantum state. At present, entanglement distribution is usually performed using photons transmitted through optical fibers. However, the absorption of light in the fiber limits the communication distances to less than 200 km, even for optimal photon telecom wavelengths. To increase this distance, the quantum repeater idea was proposed. In the quantum repeater architecture, one divides communication distance into segments of the order of the attenuation length of the photons and places quantum memory nodes at the intermediate locations. Since the photon loss between intermediate locations is low, it is possible then to establish entanglement between intermediate quantum memory nodes. Once entanglement between adjacent nodes is established, one can extend it over larger distances using entanglement swapping.\n\nThe long coherence time of a quantum memory is a crucial requirement for the quantum repeater protocol. It is obvious that the coherence time of a quantum memory should be much longer that the time it takes for light to travel between remote locations. For a communication distance l = 1000 km, the corresponding time is t = l/c = 3.3 ms. One can show that for a simple repeater protocol and realistic success probabilities of entanglement generation, the required coherence time should be on the order of many seconds, while for the more complicated protocols that involve multiplexing and several quantum memory cells per intermediate node, the required coherence time is on the order of milliseconds.\n\nIn this thesis, I describe a quantum memory based on an ensemble of rubidium atoms confined in a one-dimensional optical lattice. The use of the magnetically- insensitive clock transition and suppression of atomic motion allows us to increase coherence time of the quantum memory by two-orders of magnitude compared to previous work. I also propose a method for determining the Zeeman content of atomic samples. In addition, I demonstrate the observation of quantum evolution under continuous measurement. The long quantum memory lifetime demonstrated in this work opens the way for scalable processing of quantum information and long distance quantum communication.
Optically thick spin polarized atomic ensembles have emerged as a new medium optimal for the exotic quantum state engineering via quantum interface with light. The exciting feature of this medium is that quantum atom optics can be carried out with atoms in the range of temperatures from nano-Kelvin to room temperature. Recent theoretical and experimental developments, such as light-atoms entanglement, quantum memory, quantum teleportation, and quantum nondemolition detection of strongly correlated systems will be reviewed in this talk.
Quantum communication aims at generating and distributing entangled pairs of photonic qubits (or flying qubits) at a telecom wavelength between distant sites, in order to implement quantum information protocols, such as quantum key distribution, quantum teleportation and quantum processing. For many quantum protocols, and especially for quantum repeaters, which make quantum communication over long distances much more efficient, storing of entangled states is necessary and represents a crucial step towards the realization of a real quantum network. A current outstanding challenge is thus to implement quantum memories for entangled qubits, carried by telecom photons, that combine high efficiency, long storage time and near-perfect fidelity between the input and output states. With this project, our goal is to merge the techniques of telecom wavelengths qubit distribution with quantum storage at shorter wavelength (typically for atomic transitions). Our idea is to map, via a quantum interface, polarization entangled qubits at a telecom wave- length onto a cold atomic ensemble, serving as a quantum memory device.