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Quantum Electronics and Laser Science Conference

Long-lived spin state of an atomic gas-a storage medium for a quantum state of light

Jens Lykke Sørensen, Brian Julsgaard, C. Schori, Eugene Simon Polzik

Published Jun 25, 2003• 1 citation
Publisher

Abstract

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.

DOI
10.1109/qels.2002.1031025

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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.
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.
A reversible quantum memory allowing us to store and retrieve quantum information serves as a key necessity for implementing many of novel quantum information protocols. As light serves as a reliable long-range carrier of quantum information, and atoms offer the possibility of long storage times, current attempts at creating quantum memories focus on the transfer of the quantum fluctuations of light onto atomic coherences. The work in this thesis focuses on the development of a quantum memory for squeezed light using an ensemble of cold Cesium atoms stored in a magneto-optical trap. Our two major milestones were the development of a source of nonclassical light, and the development of a suitable atomic medium for storage. We first present the results of our efforts to generate a source of squeezed vacuum states resonant with the Cesium D2 line using a PPKTP nonlinear crystal inside of an optical parametric oscillator. Additionally, we characterize these squeezed states by carrying out a quantum state tomography using an iterative maximum likelihood approach. Next we look at the development of a new experiment which would allow us to use cold Cesium atoms as a storage medium in our recently developed magneto-optical trap. As this requires an array of novel tools and experimental techniques, we will discuss the development of these elements, and how they have furthered our progress towards storing quantum states onto our Cesium atoms, and eventually entangling two atomic ensembles.
Light storage is the controlled and reversible mapping of light fields onto long-lived states of matter [1], forming the basis for quantum memories in optical quantum networks [2-6]. Prominent storage media are warm alkali gases due to their strong optical coupling and long-lived spin states [7-8]. In a dense gas, the random atomic collisions dominate the lifetime of the spin coherence, limiting the storage time to a few milliseconds [9-10]. Here we present and experimentally demonstrate a storage scheme that is insensitive to spin-exchange collisions, thus enabling long storage times at high atomic densities. This unique property is achieved by mapping the light field onto spin orientation, rather than onto higher spin moments. We report on a record storage lifetime for a warm system of 150 milliseconds (1/e) in cesium vapor. Furthermore, our scheme lays the foundations for hour-long quantum memories using rare-gas nuclear spins.
Light storage, the controlled and reversible mapping of photons onto long-lived states of matter, enables memory capability in optical quantum networks. Prominent storage media are warm alkali vapors due to their strong optical coupling and long-lived spin states. In a dense gas, the random atomic collisions dominate the lifetime of the spin coherence, limiting the storage time to a few milliseconds. Here we present and experimentally demonstrate a storage scheme that is insensitive to spin-exchange collisions, thus enabling long storage times at high atomic densities. This unique property is achieved by mapping the light field onto spin orientation within a decoherence-free subspace of spin states. We report on a record storage time of 1 s in room-temperature cesium vapor, a 100-fold improvement over existing storage schemes. Furthermore, our scheme lays the foundations for hour-long quantum memories using rare-gas nuclear spins.
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.